Details, Details
We're getting closer to having the information we need to decide what we need from a battery management solution. This time around, we'll dig a little deeper.
The datasheets or manufacturer's instructions will give us the limits for our battery pack. How hot and how cold? How much load? How fast can we charge? High and low voltage. Approximate cycle life in different conditions. And the point - how much of this do we need to actively manage?
Tesla Motors published an excellent paper on the Roadster's battery pack. This sealed pack contains 6831 cells (18650 sized - a bit larger than an AA battery). The pack has liquid heating and cooling and multiple layers of battery management. The pack is large, complex, and expensive - and can feed enough power to accelerate this 2700 lb car from zero to 60 MPH in under four seconds.
But what about our smaller packs? What do we need? Let's start at the far 'outer limits' and move in. We'll continue to use numbers for PSI's 40138 sized 10Ah cells.
Min/Max Voltage
LiFePo4 chemistry appears to have an absolute minimum voltage of 2.0V per cell and a maximum of 4.3V per cell. Moving outside these limits won't just shorten cell life - it's likely to cause immediate damage.
Min/Max Temperature
PSI recommends a pair of temperatures for their cells - one for discharge and one for charging. This is actual cell temperature. The cell can be discharged when it's between -20°C and 55°C (-4°F to 151°F). It can be charged between 0°C and 45°C (32°F to 113°F).
Charge/Discharge Current
Maximum discharge current is 10C / 100A continuous and 130A for up to 18 seconds. Maximum charging current is 4C - 40A.
These values define the 'box' that we'll work within. Using these limits will work, but we'll want to select more conservative limits to achieve longer pack life.
Temperature... Since we're working with relatively small air-cooled packs, we have a much simpler thermal management problem than the folks at Tesla. Chances are good that we won't be riding much when it's colder than -20°C/-4°F. And even if we do, we'll probably keep the pack inside where it's warmer. The pack will warm a bit as it's discharged, so once we begin our cold ride we're in good shape. On the warm end of the range - like riding in South Texas in the summer - we should still have some flexibility. Its 100°F as I write this and the pack is stored outside. Even with some pack heating we have 50°F before we reach max temperature for the pack. Most users won't need a BMS to monitor pack temperature.
Maximum Current... We can manually manage our charge and discharge current by selecting components during vehicle design. As we saw last time, a 10Ah pack won't give us much range when feeding a 100A controller, but it's within the design limits of the battery. Max discharge for a 60Ah pack with the 100A controller is only 1.6C - this is a very conservative number. Likewise, the 15A charger is well below the 40A limit for recharging. We don't need the BMS to manage discharge current, but we will ask it to help during cell balancing. More on this later...
Maximum Cell Voltage... While the upper limit of the chemistry appears to be 4.3V per cell, manufacturers agree that reducing that limit will add cell life. Thunder Sky recommends charging to 4.25V to 4.3V per cell for a 2000 charge-cycle life. PSI and A123 Systems recommend a more conservative 3.65V per cell. There are a couple of ways to charge the pack.
One method is to use one charger per cell. The up-side is that each cell can charge at it's own rate, voltage is controlled automatically, and the pack is balanced after each charge. The downside is expense - 21 15A chargers is considerably more expensive than one. Charger selection takes care of cell management - no BMS support is necessary for single cell charging.
The more commonly used method is a single 'bank' charger. Charging thru the entire pack means that the charger does not know the charge status of any single cell - it only knows the average or pack voltage. Cells that finish charging a bit earlier will likely be overcharged while the slower cells might not completely fill. We'll need our battery management system to monitor each cell's voltage level and make sure that cells are not allowed to overcharge.
Minimum Cell Voltage... This is probably the most critical value to manage in order to keep our cells alive. Most controllers have a low-voltage detection system built in. But like bank charging, the controller doesn't know the condition of any single cell - it only knows the average. For example: Let's look at a three-cell pack with a 2.1V low voltage limit. The controller will be set for a 6.3V lower limit and will shut down to 'protect' the pack. If the cells are perfectly matched, perfectly charged, and at exactly the same temperature, they might all reach the 2.1V point at the same time. We'll have 2.1V, 2.1V, and 2.1V for our cells - the controller will shut down and all is well. But what happens if one cell didn't quite fully charge? The next time the controller shuts down at 6.3V our cells are at 2.15V, 2.0V, and 2.15V. The pack is still in acceptable condition. On recharge, this cell will take even longer to charge. Say we just charge 1/2 way because we need to make a quick ride to the store. 100 feet from home we deplete the pack and the controller shuts down. Cell voltages are 2.4V, 1.5V, and 2.4V. Uh oh. The middle cell is damaged...time to shop for a replacement. As with charging, pack average management isn't enough to protect the cells. We need our BMS to monitor individual cells and take action when necessary.
Charge Management... This will affect both maximum cell voltage and maximum charge current. A charger programmed to charge LiFePO4 will reduce the output current as the pack fills - the transition usually happens between 3.65 and 3.7V. The charge profile is known as constant current/constant voltage - CC/CV. This is automatically managed by single cell chargers, but our bank charger cannot sense individual cell voltages - so it changes modes based on pack voltage. As we've already seen, this will result in some cells being overcharged and others left less than full. We're going to ask our management solution to monitor cell voltage and control the charger so that our cells are charged and balanced.
We have the information we need to select our battery management solution. We'll manage pack temperature manually thru a combination of current control, pack size, and storage/use environment. We'll manage current in and out by design - pack capacity, controller and charger size. We'll need our BMS to monitor maximum cell voltage and control the charge current, and we'll need it to monitor minimum cell voltage and remove the load on the pack when any one cell reaches our selected low voltage point.
Next time we'll look at a couple of real-world solutions and compare their strengths and weaknesses.
Sunday, August 9, 2009
Saturday, April 18, 2009
LiFePO4 Battery Pack Management - Part 3
What's my application and what's my goal?
Let's look at some trade-offs for a typical application. Let's look at a pack for an electric motorcycle/scooter. This device has a 5000 watt brushless hub motor and a controller limited to 100A. The controller has an 80 volt maximum.
Long pack life is our first goal for this example, followed by range. We'll use 10Ah PSI 40138 cells for the example, with numbers pulled from PSI's tech page referenced in the last post. This will be just an overview - we won't be getting into absolute numbers. Though we seldom hit the streets at full throttle trying to climb a constant hill, we'll assume constant max current for our quick and dirty worst case comparison.
We'll use 3.65V per cell as the end of charge point. That means we need 21 cells in series for our 80V maximum. Let's look at a 21 cell string of 10Ah cells.
Series and Parallel
Cells in series gives us higher overall voltage. Cells in parallel cuts the load each cell has to carry.
For example, a single cell under a moderate load will spend most of its time at 3.2V. Two cells in series with the same load will give us 6.4V. Twenty-one cells in series at the same load will give us 67.2V.
A single cell subjected to a 100A load will have to do all the work itself. We can cut the workload in half by putting two cells in parallel - two cells will cut the load for each cell in half (50A for two cells is 100A). Four cells in parallel means each cell only has to deliver 1/4 of the load - 25A. More cells in parallel will weigh more but make life easier on each cell - and give us longer cell life and better range.
Other benefits of reducing the load on any single cell is that the cell will deliver more energy (from 9Ah to 10Ah) and pack voltage will stay much higher - from 2.5V part way thru at 100A to about 3V at 25A per cell.
Quick Overviews
21S1P
A single string of 10A cells in series (21S1P) will give us a maximum 100A continuous discharge. The pack will be fairly lightweight (about 7.7kg ignoring mounting and connectors) and should give us pretty quick acceleration, at least initially. Pack voltage will drop fairly quickly, as the cells drop thru 2.5V with a 100A load. The pack will heat internally - The pack will need some fresh air. This will be a pretty hard life for the cells. While PSI shows that we can get 2500 cycles if we manage our low voltage limit and keep the discharge current to 1C (10A), a constant diet of 10C (100A) discharges will not guarantee us a long pack life.
Quick summary: 100A load, pack voltage at the half-way point will be about 50V. We'll get about 8Ah from the pack - our ride will be over in about 5 minutes. Not good for pack life.
21S2P
Let's add more cells and see if we can make life a bit easier for the cells. We'll add a second set of 21 cells for a 'two cells in parallel (2P)' configuration.
Each cell is required to provide 50A. Voltage sag is better - the pack will spend much of its time around 60V. The cells will heat less - temperatures in the battery box will drop. Range will approximately double to about 10 minutes of use. The pack is a bit heavier at 15.3kg but we should feel better overall performance due to the higher pack voltage.
21S4P
Now we have a 40Ah pack of 21 cells. Voltage sag is lessened again - about 3V per cell for a 63V pack. We'll get more from the cells now that our demand is down to about 25A (2.5C) - closer to 10Ah. The approximately 40Ah will give us about 24 minutes at full load. Though the pack now weighs about 30.7kg, speed should still be good due to the higher overall voltage.
21S6P
Things keep looking better as we increase the size of the pack. We're at 126 cells now for a weight of about 45.9kg. Each cell is only asked to provide 16.7A. Pack voltage will stay above 3V per cell longer. There won't be any noticeable pack temperature rise. Our time at full load will increase to about 36 minutes. While we're still over 10A per cell at 1.6C, we're much closer to our 2000 discharge cycle goal. Overall, life has gotten much better for the pack solely by reducing the amount of current each cell must provide.
Let's look at some trade-offs for a typical application. Let's look at a pack for an electric motorcycle/scooter. This device has a 5000 watt brushless hub motor and a controller limited to 100A. The controller has an 80 volt maximum.
Long pack life is our first goal for this example, followed by range. We'll use 10Ah PSI 40138 cells for the example, with numbers pulled from PSI's tech page referenced in the last post. This will be just an overview - we won't be getting into absolute numbers. Though we seldom hit the streets at full throttle trying to climb a constant hill, we'll assume constant max current for our quick and dirty worst case comparison.
We'll use 3.65V per cell as the end of charge point. That means we need 21 cells in series for our 80V maximum. Let's look at a 21 cell string of 10Ah cells.
Series and Parallel
Cells in series gives us higher overall voltage. Cells in parallel cuts the load each cell has to carry.
For example, a single cell under a moderate load will spend most of its time at 3.2V. Two cells in series with the same load will give us 6.4V. Twenty-one cells in series at the same load will give us 67.2V.
A single cell subjected to a 100A load will have to do all the work itself. We can cut the workload in half by putting two cells in parallel - two cells will cut the load for each cell in half (50A for two cells is 100A). Four cells in parallel means each cell only has to deliver 1/4 of the load - 25A. More cells in parallel will weigh more but make life easier on each cell - and give us longer cell life and better range.
Other benefits of reducing the load on any single cell is that the cell will deliver more energy (from 9Ah to 10Ah) and pack voltage will stay much higher - from 2.5V part way thru at 100A to about 3V at 25A per cell.
Quick Overviews
21S1P
A single string of 10A cells in series (21S1P) will give us a maximum 100A continuous discharge. The pack will be fairly lightweight (about 7.7kg ignoring mounting and connectors) and should give us pretty quick acceleration, at least initially. Pack voltage will drop fairly quickly, as the cells drop thru 2.5V with a 100A load. The pack will heat internally - The pack will need some fresh air. This will be a pretty hard life for the cells. While PSI shows that we can get 2500 cycles if we manage our low voltage limit and keep the discharge current to 1C (10A), a constant diet of 10C (100A) discharges will not guarantee us a long pack life.
Quick summary: 100A load, pack voltage at the half-way point will be about 50V. We'll get about 8Ah from the pack - our ride will be over in about 5 minutes. Not good for pack life.
21S2P
Let's add more cells and see if we can make life a bit easier for the cells. We'll add a second set of 21 cells for a 'two cells in parallel (2P)' configuration.
Each cell is required to provide 50A. Voltage sag is better - the pack will spend much of its time around 60V. The cells will heat less - temperatures in the battery box will drop. Range will approximately double to about 10 minutes of use. The pack is a bit heavier at 15.3kg but we should feel better overall performance due to the higher pack voltage.
21S4P
Now we have a 40Ah pack of 21 cells. Voltage sag is lessened again - about 3V per cell for a 63V pack. We'll get more from the cells now that our demand is down to about 25A (2.5C) - closer to 10Ah. The approximately 40Ah will give us about 24 minutes at full load. Though the pack now weighs about 30.7kg, speed should still be good due to the higher overall voltage.
21S6P
Things keep looking better as we increase the size of the pack. We're at 126 cells now for a weight of about 45.9kg. Each cell is only asked to provide 16.7A. Pack voltage will stay above 3V per cell longer. There won't be any noticeable pack temperature rise. Our time at full load will increase to about 36 minutes. While we're still over 10A per cell at 1.6C, we're much closer to our 2000 discharge cycle goal. Overall, life has gotten much better for the pack solely by reducing the amount of current each cell must provide.
LiFePO4 Battery Pack Management - Part 2
So... You're sitting there looking at a stack of cells. They look pretty 'happy' siting there - and you'd like to find a way to keep them happy and give them a long life once you put them to work. One way to look at battery management is a way to ensure you have a good return on investment.
I use the Robert Kiyosaki definition of 'investment' here - financially, an investment is something that puts money into your pocket on a regular basis. I put a fair amount of money down to purchase the cells - I want to guarantee that they'll repay me each time I ask them to work, and I want a guarantee that they'll have a very long life.
The Datasheet is your Friend
The first place to start, since you can't interview the cells on the table, is to spend some 'quality time' with the datasheet for your cells. Some of you are using cells from A124-Systems, some from PSI, some from Thunder Sky, and others. Each company gives high and low temperature limits, voltage limits for charge and discharge, and current limits. Some companaies will also list a range of voltages - absolute maximum before damage is certain, along with more conservative limits that give longer life. You may have to put on your detective hat for some of this - because quality of information will vary just as cell quality varies from manufacturer to manufacturer.
The Thunder Sky Battery Manual is available for their cells. A123-Systems publishes data sheets for their cells, as do the folks at Phoenix Silicon International (PSI).
Important Numbers
Some of the things we'll need to think about when deciding how to manage our battery is what our goal for pack is. If long life is the number one choice, we'll manage the pack differenly than one that's builting a pack for a drag racer. Basically we'll have to look at all the possibilities then decide what we want to trade to get our goal. It will be fairly easy to manage the cell once we have our goal in mind.
For example - take a look at the low voltage levels. For a maximum discharge, we can take the cell down to 2.1V. But for max cycle life, we may choose to follow PSI's lead and stop when the cell reaches 2.3V. Thunder Sky lists 2.5V for their long-life choice.
In general, the more conservative we are with the cell, the longer it will last. For example - if we move from using 100% of the energy to using only 70% we can move from 1500 recharges to more than 2500. In this example, we choose to trade range each trip for battery pack life. Worst case is operating the cell at max temperature, with max discharge rate, after charging the cell to 100% and discharging to 2.1V.
Do note, though, that the 2000 or so cycle 'end of life' doesn't mean the cell is dead - it just means that the cell is down to 80% of it's new power capacity. The cell that starts at 10Ah will be down to about 8Ah after 2000 well-managed cycles (or 1500 more difficult cycles). The cell will continue to work beyond that point.
I use the Robert Kiyosaki definition of 'investment' here - financially, an investment is something that puts money into your pocket on a regular basis. I put a fair amount of money down to purchase the cells - I want to guarantee that they'll repay me each time I ask them to work, and I want a guarantee that they'll have a very long life.
The Datasheet is your Friend
The first place to start, since you can't interview the cells on the table, is to spend some 'quality time' with the datasheet for your cells. Some of you are using cells from A124-Systems, some from PSI, some from Thunder Sky, and others. Each company gives high and low temperature limits, voltage limits for charge and discharge, and current limits. Some companaies will also list a range of voltages - absolute maximum before damage is certain, along with more conservative limits that give longer life. You may have to put on your detective hat for some of this - because quality of information will vary just as cell quality varies from manufacturer to manufacturer.
The Thunder Sky Battery Manual is available for their cells. A123-Systems publishes data sheets for their cells, as do the folks at Phoenix Silicon International (PSI).
Important Numbers
Some of the things we'll need to think about when deciding how to manage our battery is what our goal for pack is. If long life is the number one choice, we'll manage the pack differenly than one that's builting a pack for a drag racer. Basically we'll have to look at all the possibilities then decide what we want to trade to get our goal. It will be fairly easy to manage the cell once we have our goal in mind.
For example - take a look at the low voltage levels. For a maximum discharge, we can take the cell down to 2.1V. But for max cycle life, we may choose to follow PSI's lead and stop when the cell reaches 2.3V. Thunder Sky lists 2.5V for their long-life choice.
In general, the more conservative we are with the cell, the longer it will last. For example - if we move from using 100% of the energy to using only 70% we can move from 1500 recharges to more than 2500. In this example, we choose to trade range each trip for battery pack life. Worst case is operating the cell at max temperature, with max discharge rate, after charging the cell to 100% and discharging to 2.1V.
Do note, though, that the 2000 or so cycle 'end of life' doesn't mean the cell is dead - it just means that the cell is down to 80% of it's new power capacity. The cell that starts at 10Ah will be down to about 8Ah after 2000 well-managed cycles (or 1500 more difficult cycles). The cell will continue to work beyond that point.
Monday, December 22, 2008
Torque and Assembly Blocks
We have a couple of updates for you. The first is a 'demonstration' of why torque matters when attaching hardware to 40138 terminals. The second is one way to connect cells into a road-ready battery.
As the manufacturer of all 40138 cells currently on the market, Phoenix Silicon International (PSI) 'gets' to tell us how best to use our cells. One point they make clear is that it's not good to overtighten the nuts when wiring cells. Yes - here comes the 'why not?' part!
The terminal studs are made of different metals. The negative pole is plated copper. The positive - and the subject of our demonstration - is aluminum. And it's hollow. And it's threaded inside and out.
PSI gives us a torque limit of 6.9 in/lb or .78 nm and highly recommends using a torque driver or small torque wrench when installing hardware.
There are a number of decent (and better than decent!) torque drivers on the market and one can spend $157 and more - like this from SK. We found a fully functional yet frugal torque driver at Harbor Freight (online item 65397) for $34.99.
It's easy to set, 7 in/lb is far enough off the bottom of the scale to be reasonably accurate, and it just plain works. The driver accepts standard hex screwdriver bits, so you'll either need a specialty 10mm nutdriver bit or an adaptor for your 1/4 inch drive socket. Either way, it's cheap, easy, quick, brainless, and won't destroy the cells.
Ok - so you want to do more than string a bunch of cells together and make your living room lamp glow. Now what? Remember from an earlier post that the plastic seals on the cells are not designed to be pothole, expansion joint, or rock-hopping proof. A leaking cell is not a happy cell. We need a way to hold the cell bodies securely in the battery case so that the cells aren't hanging by the wiring - then we can hit the road.
PSI has one solution - the molded plastic assembly block.
Fit and finish of these is excellent! The mold part lines are accurate and there's no flash. The blocks have 'T' shaped locking tabs on two sides and can be assembled into many shapes. They don't have permanent locks - they're a friction fit - so the pack can be reconfigured easily.
Here's a pretty boring 12S pack. The block has .6 inch deep extensions cable-side to protect the wiring from the sides of the battery box. The extensions have pre-molded locations for fastening the pack into a hard box.
The blocks fit tightly enough that one can handle the pack, twist it around, and suspend it by one block and everything stays locked together.
One feature designed into the blocks is the gap between cells. This allows the center cells in larger packs to get cooling air. This isn't an issue for these cells until you start pulling 60+ amps from a cell, but it's one less thing to have to worry about when filling a box for a plug-in hybrid or pure EV.
For more information or to order assembly blocks or connecting straps, visit us at www.rechargeablelithiumpower.com
As the manufacturer of all 40138 cells currently on the market, Phoenix Silicon International (PSI) 'gets' to tell us how best to use our cells. One point they make clear is that it's not good to overtighten the nuts when wiring cells. Yes - here comes the 'why not?' part!
The terminal studs are made of different metals. The negative pole is plated copper. The positive - and the subject of our demonstration - is aluminum. And it's hollow. And it's threaded inside and out.
PSI gives us a torque limit of 6.9 in/lb or .78 nm and highly recommends using a torque driver or small torque wrench when installing hardware.
There are a number of decent (and better than decent!) torque drivers on the market and one can spend $157 and more - like this from SK. We found a fully functional yet frugal torque driver at Harbor Freight (online item 65397) for $34.99.
It's easy to set, 7 in/lb is far enough off the bottom of the scale to be reasonably accurate, and it just plain works. The driver accepts standard hex screwdriver bits, so you'll either need a specialty 10mm nutdriver bit or an adaptor for your 1/4 inch drive socket. Either way, it's cheap, easy, quick, brainless, and won't destroy the cells.
Ok - so you want to do more than string a bunch of cells together and make your living room lamp glow. Now what? Remember from an earlier post that the plastic seals on the cells are not designed to be pothole, expansion joint, or rock-hopping proof. A leaking cell is not a happy cell. We need a way to hold the cell bodies securely in the battery case so that the cells aren't hanging by the wiring - then we can hit the road.
PSI has one solution - the molded plastic assembly block.
Fit and finish of these is excellent! The mold part lines are accurate and there's no flash. The blocks have 'T' shaped locking tabs on two sides and can be assembled into many shapes. They don't have permanent locks - they're a friction fit - so the pack can be reconfigured easily.
Here's a pretty boring 12S pack. The block has .6 inch deep extensions cable-side to protect the wiring from the sides of the battery box. The extensions have pre-molded locations for fastening the pack into a hard box.
The blocks fit tightly enough that one can handle the pack, twist it around, and suspend it by one block and everything stays locked together.
One feature designed into the blocks is the gap between cells. This allows the center cells in larger packs to get cooling air. This isn't an issue for these cells until you start pulling 60+ amps from a cell, but it's one less thing to have to worry about when filling a box for a plug-in hybrid or pure EV.
For more information or to order assembly blocks or connecting straps, visit us at www.rechargeablelithiumpower.com
Monday, November 3, 2008
Lithium VS Lead
Is a picture worth a thousand words?
In this corner, weighing in at 24 lbs, is a 36V 12Ah pack of AGM lead acid. And in this corner, weighing in at 10 lbs, is a 36V 10Ah pack of LiFePO4.
The LiFePO4 pack consists of 12 40138 cells in series. The lead acid pack is three Werker 12V 12Ah batteries from Batteries Plus. In each pack, cells were fully charged and balanced. They were both discharged thru a 10A load until reaching the pack cut-off of 30V for the lead and 29V for the LiFePO4. Ambient temperature was 21°C.
The lead acid pack started just above 36V but voltage started to drop early on and kept falling until hitting a 'shoulder' at around 6.6Ah. The LiFePO4 pack started above 36V and stayed until 8.4Ah.
Voltage Support: Advantage Lithium.
The 12Ah lead pack provided just under 7Ah (58% of rated capacity), while the 10Ah LiFePO4 pack provided just over 9.6Ah (96% of rated capacity).
Power output: Advantage Lithium.
Discharge cycles: Lead acid should give somewhere between 300 and 500 cycles at 100% depth of discharge. Our LiFePO4 should give about 1500 cycles at 100% depth of discharge.
Discharge cycles: Advantage Lithium
Weight: 10lbs VS 24lbs Advantage Lithium
Overall value: Lead was $44.95 each for a total of $134.85. If we get 400 cycles of 6.93Ah, we're paying $.0486 per Ah. (A more realistic 300 cycles costs us $.0649 per Ah.) LiFePO4 at $48 each was $576. If we get 1500 cycles of 9.63Ah, we're paying $.0399 per Ah.
Overall value: Advantage Lithium
Smaller, lighter, more power for a longer period, no hazardous chemicals - what's not to like?!
In this corner, weighing in at 24 lbs, is a 36V 12Ah pack of AGM lead acid. And in this corner, weighing in at 10 lbs, is a 36V 10Ah pack of LiFePO4.
The LiFePO4 pack consists of 12 40138 cells in series. The lead acid pack is three Werker 12V 12Ah batteries from Batteries Plus. In each pack, cells were fully charged and balanced. They were both discharged thru a 10A load until reaching the pack cut-off of 30V for the lead and 29V for the LiFePO4. Ambient temperature was 21°C.
The lead acid pack started just above 36V but voltage started to drop early on and kept falling until hitting a 'shoulder' at around 6.6Ah. The LiFePO4 pack started above 36V and stayed until 8.4Ah.
Voltage Support: Advantage Lithium.
The 12Ah lead pack provided just under 7Ah (58% of rated capacity), while the 10Ah LiFePO4 pack provided just over 9.6Ah (96% of rated capacity).
Power output: Advantage Lithium.
Discharge cycles: Lead acid should give somewhere between 300 and 500 cycles at 100% depth of discharge. Our LiFePO4 should give about 1500 cycles at 100% depth of discharge.
Discharge cycles: Advantage Lithium
Weight: 10lbs VS 24lbs Advantage Lithium
Overall value: Lead was $44.95 each for a total of $134.85. If we get 400 cycles of 6.93Ah, we're paying $.0486 per Ah. (A more realistic 300 cycles costs us $.0649 per Ah.) LiFePO4 at $48 each was $576. If we get 1500 cycles of 9.63Ah, we're paying $.0399 per Ah.
Overall value: Advantage Lithium
Smaller, lighter, more power for a longer period, no hazardous chemicals - what's not to like?!
Saturday, November 1, 2008
LiFePO4 Battery Pack Management - Part 1
Welcome Back!
You just know that any day that starts with a good cup of coffee and new toys to play with is heading in the right direction! We’ve had a couple of these days in a row – here’s how they’ve unfolded so far:
The first part of the fun began when UPS brought the battery management board kit that we ordered from Gary Goodrum at TPPacks.com. He and others on the Endless Sphere battery forum designed a battery management board tailored to PSI and A123 LiFePO4 cells used in electric bicycle-sized battery packs.
It took a couple of minutes to remember which box the soldering irons were in, then the fun started. The BMS went together quickly and worked first time. Always a good sign!
You just know that any day that starts with a good cup of coffee and new toys to play with is heading in the right direction! We’ve had a couple of these days in a row – here’s how they’ve unfolded so far:
The first part of the fun began when UPS brought the battery management board kit that we ordered from Gary Goodrum at TPPacks.com. He and others on the Endless Sphere battery forum designed a battery management board tailored to PSI and A123 LiFePO4 cells used in electric bicycle-sized battery packs.
It took a couple of minutes to remember which box the soldering irons were in, then the fun started. The BMS went together quickly and worked first time. Always a good sign!
The next piece of the puzzle happened along last week when our friends at UPS brought us a couple of boxes. Now we have a new batch of 40138 cells to use – and some new battery chargers to test.
Stay tuned for part two - the fun is just getting started!
Thursday, October 2, 2008
Energy Plans and Our Future
Welcome to October!
The tests continue on our guinea pig cells. We've completed 50 discharge cycles on 'old silver'. It's in the middle of a slow discharge test with periodic internal resistance samples - 30 minutes down, 4 1/2 hours to go. We'll post internal resistance results for our most experienced cell once we've worked thru the numbers.
We've also gotten requests for a look at the equipment we're using to perform these tests. Thank you for the requests - your reviews are on their way!
We'd like to depart from tech for a couple of minutes and talk about one of the reasons we're interested in LiFePO4 cells and their support. Thanks for your indulgence as we editorialize a bit.
We've just finished watching a couple of clips from C-SPAN. The latest is a short discussion of the connectedness of energy, food, and water in both the global and US economy. Here's a link to the main C-SPAN energy page. Scroll down to the 'Recent Programs' area and look for the "Clinton Global Initiative Conference" from September 25th. This talk, moderated by Tom Brokaw, was a conversation with Bob Zoellick, President of the World Bank, Shimon Peres, President of Israel, T. Boone Pickens, Chairman & CEO of BP Capital Management, Helle Thorning-Schmidt, leader of the Danish Social Democratic Party, and Gavin Newsom, Mayor of San Francisco.
One point we came away from was the reminder that it's very difficult or impossible in a complex and interconnected system to solve only one problem at a time. It's sort of like medical triage in a way. Basic first aid reminds us to scan the area for immediate threats first, then move quickly to check airway, breathing, and circulation. We can get to broken bones a bit later.
If an accident victim is found unconscious, not breathing, and bleeding from an artery, we have to work on controlling the bleeding and do something about getting oxygen into the system in short order. Focus on either problem while ignoring the other virtually ensures 'mission failure'.
Put another way - if you find yourself in a hole, the first step is to stop digging.
As we prepare this, our representatives in Washington DC are working on the "Emergency Economic Stabilization Act of 2008". One of the numbers we hear is that it will cost the US Government at least 700 billion dollars to stabilize things and restore confidence in the economy, credit system and stock market.
$700 billion is an interesting number and one we've heard before - from Texas oilman Boone Pickens. He reminds us in his advertisements and on the Pickens Plan website that we're spending around $700 billion each year for foreign oil. This money leaves the country and doesn't provide jobs or tax income.
We must have an energy plan in this country that helps us transition to renewable energy and helps us wean ourselves - the quicker the better - from petroleum imports. We think groups such as Pickens' and the WE campaign are helping bring energy awareness into the US presidential campaign. We think it's all connected - energy, food supply, water, the economy, jobs, climate change, transportation - and that the ability to solve these challenges is ours. We've proven in our past that with a plan and desire we can climb great heights.
There's discussion by some that we need a "Manhattan Project" of sorts for energy and the economy. Tom Brokaw referenced the '100,000 garages' concept attributed to author Tom Friedman while talking with Mayor Newsom in the interview referenced in paragraph four. I found that the reference is from Friedman's book Hot, Flat, and Crowded. Here's a quote from WIRED magazine:
Enough of this for now - back in the garage!
The tests continue on our guinea pig cells. We've completed 50 discharge cycles on 'old silver'. It's in the middle of a slow discharge test with periodic internal resistance samples - 30 minutes down, 4 1/2 hours to go. We'll post internal resistance results for our most experienced cell once we've worked thru the numbers.
We've also gotten requests for a look at the equipment we're using to perform these tests. Thank you for the requests - your reviews are on their way!
We'd like to depart from tech for a couple of minutes and talk about one of the reasons we're interested in LiFePO4 cells and their support. Thanks for your indulgence as we editorialize a bit.
We've just finished watching a couple of clips from C-SPAN. The latest is a short discussion of the connectedness of energy, food, and water in both the global and US economy. Here's a link to the main C-SPAN energy page. Scroll down to the 'Recent Programs' area and look for the "Clinton Global Initiative Conference" from September 25th. This talk, moderated by Tom Brokaw, was a conversation with Bob Zoellick, President of the World Bank, Shimon Peres, President of Israel, T. Boone Pickens, Chairman & CEO of BP Capital Management, Helle Thorning-Schmidt, leader of the Danish Social Democratic Party, and Gavin Newsom, Mayor of San Francisco.
One point we came away from was the reminder that it's very difficult or impossible in a complex and interconnected system to solve only one problem at a time. It's sort of like medical triage in a way. Basic first aid reminds us to scan the area for immediate threats first, then move quickly to check airway, breathing, and circulation. We can get to broken bones a bit later.
If an accident victim is found unconscious, not breathing, and bleeding from an artery, we have to work on controlling the bleeding and do something about getting oxygen into the system in short order. Focus on either problem while ignoring the other virtually ensures 'mission failure'.
Put another way - if you find yourself in a hole, the first step is to stop digging.
As we prepare this, our representatives in Washington DC are working on the "Emergency Economic Stabilization Act of 2008". One of the numbers we hear is that it will cost the US Government at least 700 billion dollars to stabilize things and restore confidence in the economy, credit system and stock market.
$700 billion is an interesting number and one we've heard before - from Texas oilman Boone Pickens. He reminds us in his advertisements and on the Pickens Plan website that we're spending around $700 billion each year for foreign oil. This money leaves the country and doesn't provide jobs or tax income.
We must have an energy plan in this country that helps us transition to renewable energy and helps us wean ourselves - the quicker the better - from petroleum imports. We think groups such as Pickens' and the WE campaign are helping bring energy awareness into the US presidential campaign. We think it's all connected - energy, food supply, water, the economy, jobs, climate change, transportation - and that the ability to solve these challenges is ours. We've proven in our past that with a plan and desire we can climb great heights.
There's discussion by some that we need a "Manhattan Project" of sorts for energy and the economy. Tom Brokaw referenced the '100,000 garages' concept attributed to author Tom Friedman while talking with Mayor Newsom in the interview referenced in paragraph four. I found that the reference is from Friedman's book Hot, Flat, and Crowded. Here's a quote from WIRED magazine:
"Twelve guys and gals going off to Los Alamos won't solve this problem...We need 100,000 people in 100,000 garages trying 100,000 things — in the hope that five of them break through."So...here we are, typing and listening to the battery analyzer's cooling fan cycle on and off as our test cell is poked and prodded. Which of you, of the interconnected 'us', will be one of Friedman's five to find solutions? What part, if any, will LiFePO4 play in our renewable future?
Enough of this for now - back in the garage!
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