The pack consists of 192 A123 2.3Ah cells, configured as 16s12p (nominally 52.8V/27.6Ah/1457Wh). Lately, the pack is empty after ~530Wh, which is 3x less capacity than nominal! I'd noticed that a few particular banks were consistently underperforming, so today I rode around until the first cell hit 2.5V (0% full)(10.4Ah), then I used a 300W programmable load (BK8500) to pull the other 15 taps down to 2.5V, recording the additional ampacity of each individual bank. The results were sub-optimal, with the lowest bank at 10.4Ah (37% of nominal), and the highest bank at 22.9Ah (83% of nominal; acceptable). Overall, 11 of 16 banks exceeded 20Ah (72% of nominal). The remaining 5 banks were 19.9,19.4,16.4,12.9,10.4Ah... pretty bad.
At this point, I thought "well shit, that's what you get for not running a battery management system on the cells... over/undercharged all the time and this is the price you pay." But then I noticed a broken tab on the 10.4Ah cell... then another broken tab, etc. All said, there were 11 broken tabs (as noted):
A1: 22.4Ah
A2: 22.4Ah
A3: 19.4Ah - 2x broken tabs
A4: 21.4Ah
A5: 19.9Ah
A6: 20.4Ah
A7: 12.9Ah - 4x broken tabs
A8: 21.0Ah
B1: 10.4Ah - 5x broken tabs
B2: 16.4Ah - 0x broken tabs :(
B3: 20.1Ah
B4: 21.4Ah
B5: 21.9Ah
B6: 22.4Ah
B7: 22.9Ah
B8: 21.4Ah
What does this mean? A broken tab means the particular cell isn't connected. For example, B1 had five broken cells, which means that only 7 of the 12 cells were actually connected. Thus, one would expect the measured nominal ampacity to decrease to 58% of nominal (16Ah). Since our measured ampacity was 10.4Ah, B1 hits 65% of nominal (7 cells) instead of 37% (with 12 cells). 65% is still lower than the other 'healthy' cells, but hints that fixing the broken tabs could yield nearly twice the distance travelled on a single charge. Doing some quick math estimating all cells connected, the expected ampacity becomes:
A1: 22.4Ah
A2: 22.4Ah
A3: 23.0Ah - expected
A4: 21.4Ah
A5: 19.9Ah
A6: 20.4Ah
A7: 19.4Ah - expected
A8: 21.0Ah
B1: 17.8Ah - expected
B2: 16.4Ah
B3: 20.1Ah
B4: 21.4Ah
B5: 21.9Ah
B6: 22.4Ah
B7: 22.9Ah
B8: 21.4Ah
What does this mean? Simply rewelding the broken tabs (aside: weld != solder; you must use a high current battery tab welder!) would increase total usable energy from ~530Wh to ~840Wh... a 58% increase! So I'd be crazy not to just do this, right?
Wrong... For the past 3 years I've internally loathed the fact that the existing battery pack construction didn't allow for any method to implement a high current, active battery management system. In order to add a high current BMS, I'd need to be able to switch each bank in and out, such that once a cell hits empty or full, it is removed from the stack. Since both the charge and discharge use cases are current controlled, the actual stack voltage isn't relevant; decreasing voltage limits maximum power, but otherwise doesn't matter.
Instead of fixing this flawed design, I've decided to take this opportunity to iterate the design. I am literally going to tear the battery pack down to individual cells and start over. It's a tough decision, but I've had three years to mull it over. One major contributing factor is that I don't actually own a battery tab welder, which means I can't fix the pack as-is. Soldering to cells is difficult and damages the chemical structure, since the entire cell acts as a heat sink (and must heat up enough to flow solder).
In a previous post, I mulled over and then ultimately decided against using/building a battery tab welder for three reasons:
1). Turn-key battery tab welders are expensive
2). DIY battery tab welders are a science, and can melt through tabs, etc.
3). The original A123 DeWalt drill packs were already tab welded in series together... I simply soldered 12 packs together across the existing leads to ensure there were only 16 voltages (instead of 192 in the case that the 12 parallel packs were each only connected at the stack plus and minus).
(1) is still true, but now that (3) is no longer true (tabs have separated), (2) seems like the logical solution. Plus - and this is a big one - now I get a chance to start all over again and implement a BMS, without having to purchase 192 new cells (at $13/each = $2500). An even better benefit is that once I get all the cells apart, I can test the cells in the weaker banks individually, and replace just those that need replacing, thus yielding 20+Ah on all banks, which would equate to 1000+kWh (a 93% improvement).
Of course, this requires I either purchase/borrow a turn-key battery tab welder (1), or build my own (2). Conceptually, a battery tab welder is simply a power supply, a large capacitor, a silicon controlled rectifier (a switch you can turn on, but not off) and two solid cables with copper tabs. The magic is in how much energy - and at what voltage - you store in the caps. I'm going to look around and if I can't find something cheap, I'll begin experimenting with the tab welder.
The new design should ideally fit in the existing mechanical enclosure. I think this is possible. Of course, another option is to remove half the cells, which would continue to give me the range I'm getting now, but I'd rather have a much larger range and complete idiot-proof batteries. More thoughts to follow.
