Playing with A123 20Ah lithium cells

[Daox]

I picked up some lead terminals for when I assemble the pack. I'm not sure how well these will work over time, but they'll do for now.

[Daox]

I finished up testing out the 4th cell last night. The capacities are as follows:

1) 18.6 Ah
2) 17.8 Ah
3) 17.8 Ah
4) 18.0 Ah

Cells 2-4 were fully charged at 20A, fully discharged @ 20A, and then recharged again @ 20A. Cell #1 was charged and discharged at least 4 times which may explain its higher capacity. In any case this is what I have. As you can see the batteries are below their 20Ah specification, which is probably why I have them (A123 rejected cells) and they were so inexpensive.

The next step is to throw them together into a pack for the Paseo.

[bwilson4web]

We're in the same ditch, I've ordered 30 of these puppies from Eric and plan to build a two-stack, pack for our eBike:


It currently has a 48V, lead-acid battery and rated at 10 miles, one-way. I've gone 6-7 miles but my work is 10 miles away. My plan is to build two, 15 cell, stacks. I'll drive out on one and when it is exhausted, flip to the second and drive home.

I will still need a minimal BMS, more of a reverse voltage detector and shut-off. But if I have an opportunity to add a safe, minimal charge capability . . . carpe diem. I don't need 'fast' charge, 12 hours is good enough for my purposes. But that leads to my follow-up question.

Any recommendations on do-it-yourself, BMS chips and circuits? I know there are at least two around, Linear and TI if my memory serves me right. From what I remember the biggest glue piece for these ICs are the power MOSFETs which with a longer charge rate, can be 1-2A, fairly inexpensive. BTW, I had to build my own 48V battery charger from my 'junk box' when the original failed:

Crude, it didn't cost much from my 'junk box' but has turned out to be 'good enough.'

I got a note from Eric that my cells will take awhile to arrive. Then I will have to measure all 30 cells to normalize them and assemble balanced strings. I am interesting in any results you might have about the Ahr change after cycling the cells.

Now our NiMH batteries charge exothermic. Do you have any info about whether the temperature change you saw was charge or discharge?

I appreciate your efforts on cell interconnect. Once I get mine, I'll investigate some alternatives. They won't 'fit' exactly in the existing battery space, not without some clever work, but I have options.

Bob Wilson

[Daox]

I'm glad to see someone else tinkering around with these cells.

The temperature I saw was during charge and discharge. It stayed pretty constant.

IMO you really need a BMS unless you plan to babysit the heck out of your pack. I did this for a while with my Prius' PHEV kit and it gets to be a real pain.

I'm actually almost done designing a BMS for my Prius' PHEV kit. It uses an Arduino and celllog8 devices for voltage monitoring. However, my next version of the BMS will likely be switching to the TI or similar chip to reduce cost and increase flexibility. It would also allow you to reuse that charger assuming you can adjust the voltage for the lithium cells.

[Ryland]

Have you looked at buying a low voltage disconnect like one of these? Elite Power Solutions

I thought about buying one, but I'm most likely going to end up with GBS batteries and Elite power solutions makes a BMS that is designed for those cells and that seems like where someone might find a good market, if you can build a BMS for the A123 system batteries that fit with the batteries.

I've had a few people tell me that if you have a low voltage cut out and a charger that cuts off at the right high voltage that you don't need a BMS but that at that point wiring it up so you can charge each cell to bring them up to a balanced charge should work just fine, the people who have suggested a set up like this of course said that if you check the full or empty voltage of each cell after a year or two that you should find that they don't drift out of balance.
I of course like the idea of a BMS that can talk to the charger and cut out at low voltage.

[Daox]

My very first version of my BMS was going to operate quite similar to the way that LVD worked. The problem is you can not use voltage as an indicator of when a lithium battery is discharged, at least not if you want to put any resonable load on the battery. Voltage sag varies depending on the load and temperature. You'd constantly have to change the detection voltage based on both of these variables which would be quite difficult.

There are people out there with EVs and no BMS. They basically become the BMS by monitoring things all the time and making darn sure they don't discharge too far. They set their chargers to be very conservative on charging as well. They also normally oversize their pack quite a bit to give the the buffer on the charge/discharge they need to avoid damage.

So, it is doable, but its a PITA. I did it for a while with the PHEV kit on my Prius and the cost of the BMS is worth it to me.

[redpoint5]

Apologies for the thread-jack, but these cells look very interesting and I'm considering using them to replace my conventional group 51R battery in the TSX. Would these cells be ideal for an alternator kill switch setup?

If I assume a 5% increase in FE using an alternator kill switch, I would save approximately $5/month in fuel. This would be a long payback when considering the batteries, BMS, charger, etc. Still, it would be fun to reduce a little weight and play with the technology.

My conventional battery has an 85min reserve, which I haven't figured out how to convert to A/hr rating yet. I'd need to size my battery to run my car for up to 1.5 hrs at night. Thoughts?

[bwilson4web]

I'm starting to look for A123 charging protocols. I didn't find a clean description at their web sites. So these are my initial thoughts.

I'm looking at two, battery management chips:

• LTC6803 (Linear) - includes integrated MOSFETs for discharge balancing. However, discharge balancing limit appears to be 80 ma for a single cell, 0.25W. With up to 12 cells, two are needed.
• BQ76PL536 (TI) - external balance MOSFETs, current is design selectable. With a limit of 3-6 cells, three are needed configured 5 cells each.

Near as I can tell, these are series, voltage measurement chips with modest temperature monitoring. The LTC6803 has a modest cell balancing, up to 80 ma, one cell at a time, 0.25W capability. The BQ76PL536 drives an external MOSFET which increases the discharge current that can be used to discharge a cell.

I've been thinking about cell balancing and two approaches come to mind:

1. Discharge cells to a lowest dV uniform state and then when all are at the same low, state, let charging continue to when the first cell achieves a limiting, dV. To achieve reasonable times, the discharge needs to handle a hefty current, 1-5A. Once initially balanced, the discharge draw-down would be fairly short so 1A may be feasible.
2. Top limit, uniform using a shunt regulator. This regulator can quickly become pretty complex if one tries to keep it efficient. A simple, series Zener stack would work but the total charge load would remain constant until the last cell reaches the Zener limit. Again, as the cells become balanced, the time during which individual cells complete their charge becomes smaller but at the end, the Zeners are handling the total heat load. Tapering the charge current at the cell voltages increase could mitigate the Zener load at the end.

You've had a lot more experience with balancing LiON cells. Any insights to share?

Also, looking at the solder tabs, have you considered spot welding? It would help if we had test articles but spot welding two or three 'dots' per tab would both block air and ensure metal-to-metal bonding. It would be mechanically simpler and only intra-tab insulation would be needed.

Thanks,
Bob Wilson

[bwilson4web]

Originally Posted by Daox

I've pretty much finished up the first round of testing on the first cell. . . .

Well done! Love your charts which I can't find from the vendor.

Just a couple of questions. I've re-ordered them to look at charge and discharge:

Originally Posted by Daox

Looking at these excellent discharge curves, I get the impression regardless of the starting voltage, the bulk of the energy starts at ~3.278-3.302V. Charging over this voltage does not appear to give any corresponding increase in total energy. So I'm wondering what is the smallest peak charge voltage that would give a starting discharge of 3.278-3.302V? Would a peak charge of 3.5V still put the cells in a state that they still start their discharge at 3.278-3.302V?

At the end, it looks like 3.017-3.055V is also where the available energy pretty well falls off. You went ahead and stopped it at 2.813V but I'm thinking of cell powered, battery management. I might stop it at 3.0V only to make sure the BMS logic continues to run without external power ... for a long time. My rough estimate from your charts is about 2Ahr, 10% of the rated capacity so I might go a little lower depending upon BMS load.

Originally Posted by Daox

What first attracted me was the initial 3.431V on both charge cycles. Given the discharge went to 2.813V, we're looking at ~0.62V 'recovery' after the discharge ended. After the cells are discharged to 2.813V, have you measured the unloaded, recovery voltage, what they float to once the discharge ends?

The reason I ask is the open circuit voltage after discharge, what ever it float up to AFTER discharge to 2.813V, helps us understand the initial V*I, power needed to start the charge cycle. It also gives an idea of the dV needed to force charge into the cell.

In a similar fashion, what happens to the cell open voltage once the charge completes? Does it droop?

Thanks,
Bob Wilson

[Daox]

Originally Posted by redpoint5

Apologies for the thread-jack, but these cells look very interesting and I'm considering using them to replace my conventional group 51R battery in the TSX. Would these cells be ideal for an alternator kill switch setup?

If I assume a 5% increase in FE using an alternator kill switch, I would save approximately $5/month in fuel. This would be a long payback when considering the batteries, BMS, charger, etc. Still, it would be fun to reduce a little weight and play with the technology.

My conventional battery has an 85min reserve, which I haven't figured out how to convert to A/hr rating yet. I'd need to size my battery to run my car for up to 1.5 hrs at night. Thoughts?

You could certainly replace your lead acid battery with these cells. I don't think you'll ever come out ahead financially though, if you do you'll at least be on your next vehicle by then. Thankfully lithium batteries last longer than lead acid, so this might actually be possible.

I wouldn't go at all by the reserve capacity on your current battery which is just a starting battery and not designed at all to be deep cycled. What you really need to figure out is what your current draw is going to be so you can size your battery accordingly. With the TSX being a bit more of a luxury vehicle, I'd assume it gobbles up more electricity than the average econobox. The best thing to do would be to get a clamp ammeter (or other ammeter) and see how much power your car draws while running with the headlights on.

As an example, I'm going to guess and say that it draws 40A. So, in one hour you use 40Ah (40 amps per hour). In 1.5 hrs you use 60Ah. These cells are rated at 20Ah, and as you can see, at least these ones aren't quite 20Ah. Also, you never discharge a battery completely. This extends the battery's life by quite a bit. For lithium you can use 70-80% of its capacity. 70% will give you longer life, and some additional buffer in case you need more than 1.5 hrs, so lets use that. Now, we have ~18Ah batteries (per my testing), and we can only use 70% of that. So, we actually have 12.6Ah of usable capacity. As we calculated before you need 60Ah of capacity. So, we can wire the cells in parallel to add capacity. To get the minimum required 60Ah we would need to parallel 5 cells to get 63Ah of usable capacity. So, you have 5 cells in parallel, and 4 cells in series for a total of 20 cells @ $27.50 (the price I paid) each. That comes out to a $550 battery (~10 year ROI based on $5/mo savings). If you go with the 80% depth of discharge (vs 70%) you could get 57.6Ah and only parallel 4 batteries and reduce your cost by ~$110. If your car only pulls 30A it would drastically change these numbers, so you need to know how much power the car pulls.

Alternatively you could buy a group 31 lead acid battery for about $100 and have close to the same capacity. Most group 31 batteries have just over 100Ah (~105-115Ah). But, with lead acid batteries you shouldn't take them down below 50% depth of discharge to get good life, so you would have around 55Ah of usable capacity. However, you'd need to replace the lead acid more often and it would of course weigh more.

IMO its probably not worth it ($ wise) for an alternator delete unless you take into consideration using it in your next vehicle. Its definitely not worth it for a starting battery because all you're saving is weight. I did this to start messing around with these batteries, figure out an assembly technique, and get comfortable working with the cells so I can use them on a larger project.