adding a small kicker cell to a 16s battery? 16s+1

[scott harris]

I have sixteen 280AH cells which I plan on using to make a 16s 48v battery. My problems is that one of the cells is weak, 265 AH. But I also have a spare 60 Ah cell. Can I just add that in parallel to the weak cell to make up the difference. Sort of a 16s with a small kicker?

Thanks,

 

[FilterGuy]

Interesting proposition. I can't think of a reason for it not to work..... but I have never done anything like that.

You may find the biggest issue is getting the smaller cell physically hooked into the rest of the bank. The terminal spacing and terminal height will be different.

If you try it, please let us know how it works out.

 

[scott harris]

So the plan would be to not include it in the compression box, but just to put it to the side and wire it in parallel to the weak cell using heavy wire and ring terminals. No bus bars. good plan? bad plan?

 

[codfish]

So the plan would be to not include it in the compression box, but just to put it to the side and wire it in parallel to the weak cell using heavy wire and ring terminals. No bus bars. good plan? bad plan?

If you have some sort of active balancer, such as a Heltec capacitive balancer, I bet it will work just fine.

Without active balancing the differences in IR and Ah capacity across the 16s series would probably cause serious imbalance over time.

 

[scott harris]

hmm, now that you mention balance, I'm wondering if balance would matter at all. Since the compound cell has a larger capacity it would neither be the first cell to reach the top nor the bottom. Is this right?

 

[FilterGuy]

hmm, now that you mention balance, I'm wondering if balance would matter at all. Since the compound cell has a larger capacity it would neither be the first cell to reach the top nor the bottom. Is this right?

It all depends on the charge of the various cells when you put it all together, If the compound cell is 90% charged and all the other are 10% (Yes, an exaggeration), the compound cell is probably going to hit the knee and run away long before the other cells are charged.

 

[scott harris]

good point. But if I started out with the compound cell at 20% and the others at 10% I guessing that I would not have any problems.

 

[codfish]

hmm, now that you mention balance, I'm wondering if balance would matter at all. Since the compound cell has a larger capacity it would neither be the first cell to reach the top nor the bottom. Is this right?

In a series configuration the same amps flow thru all the cells. Stay in the flat portion of the voltage curve and it doesn’t matter. But... if you try to fully charge the pack, say to 3.5 v per cell ( 56 v for 16s), then your super cell will be lazing along at something like 3.3v, but the pack is headed to 56v. So... 56v - 3.3v = 52.7v spread across the remaining 15 cells. Or about 3.513 per cell. Still OK. If you push the pack voltage higher then most of the voltage increase will just be in the 15 cells until the super cell gets into the “knee” as well. This all can work, but you need the balancer and keep the pack voltage reasonable.

 

[FilterGuy]

In a series configuration the same amps flow thru all the cells. Stay in the flat portion of the voltage curve and it doesn’t matter. But... if you try to fully charge the pack, say to 3.5 v per cell ( 56 v for 16s), then your super cell will be lazing along at something like 3.3v, but the pack is headed to 56v. So... 56v - 3.3v = 52.7v spread across the remaining 15 cells. Or about 3.513 per cell. Still OK. If you push the pack voltage higher then most of the voltage increase will just be in the 15 cells until the super cell gets into the “knee” as well. This all can work, but you need the balancer and keep the pack voltage reasonable.

That is why top balancing will be important.

They all start top balanced and charged to the knee. They all discharge X amp-hours and then charge back up X amp hours and they are all back at about the same place as they started.

You are correct though, there is probably a reasonable concern (but not a guarantee) the super-cell will drift out of balance with the others over time (Or perhaps the mega cell does not drift but the others do)..... How quickly and how badly they drift apart is really hard to say. If the battery is not driven hard, it might not be for several years, if at all...... but it might happen quickly. I would be tempted to wait and see how it does before I added an active balancer.

 

[OffGridInTheCity]

I do this with DIY 18650 packs. I've built 84 x 260ah packs so far in my solar life. They are arranged as 6 x 14s batteries in parallel in my powerwall. These packs have between 88 and 120 cells each - depends on the cell capacity used. I've found that about 10% of the packs are just 'weak' (sag during the charge/discharge curve) compared to the others when I put them online. No they are not self-discharging - just sag 'a bit' for some reason. So I leave room in my packs to add up to 10% more cells to 'touch these up'. Some packs have 3 extra cells and some 10extra. This has worked very well and once 'touched up' the overall 84 packs stay healthy with touch-up-balance needed every 6 months to reign in 30mv worth of drift.

The same principle should apply to LifePo4 - except as noted above, the physical placement could be a bit difficult.