Has anyone seen current issues with paralleled balance taps

[John in CR]

I'm changing up my battery pack to mix two 20s strings of old cells with 2 strings of new packs. Since I had a dieing cell in the strings the new packs are replacing, I want to help support the older cells by paralleling all 4 strings at the cell level. It will also make balancing and monitoring easier, because the new strings are much more accessible.

My one concern is whether or not there's risk of significant current flowing through those thin balance tap wires. The strings will be paralleled at the ends, so that tells me that voltage differentials between strings while in use will flow primarily through the very low resistance connections at the ends.

 

[Hillhater]

I use paralleled balance taps successfully.
My main concern is at the end of the discharge curve.
If one cell "falls of the cliff" before its paralleled twins, there may be a tendency to pump high amps over to the "low" cell via the balance taps , and over heat them before any voltage monitoring can detect it.
But i suspect that would only occur if you were running a high discharge amps at low pack voltage.

 

[oatnet]

If you lose a cell, it will kill the cells that are in parallel with it.

If it degrades slowly, it will just drain the parallel cells until they are flat too. If it degrades rapidly or completely fails, the other cells will try to charge it up to an equilibrium point, thereby pushing a lot of current through the balance wires, and then yep you have a fire risk. When I parallel cells, I do it through a buss bar instead of balance wires to mitigate that risk.

 

[liveforphysics]

Throwing in a few high current ties, or even just one at half pack can do a lot to keep voltages from getting too out of wack from eachother. It's not considered good practice to parallel various conditions of cells, but I think you can make something work.

 

[John in CR]

Thanks guys. Now I'm having a debate about just going all new with 12 of those 10s 65-130c Nanotech on sale. I wonder how much sag I can expect drawing just 245A from 20ah of those puppies. The almost 20V of sag I get now with 30s really sucks. I think I can fit 6 of those in my swingarm, and then I can make the other half of the pack removable, leaving my bike clean and light for everyday use. 8)

 

[Hillhater]

. If it degrades rapidly or completely fails, the other cells will try to charge it up to an equilibrium point, thereby pushing a lot of current through the balance wires, and then yep you have a fire risk. When I parallel cells, I do it through a buss bar instead of balance wires to mitigate that risk.

It need not fail, just have a little less capacity than its parallel cells such that it hits the "Voltage cliff" first.
Even if it just drops to 3.0v whilst the other cells are still at 3.7v , then im guessing, you could have 50+ A in those balance taps on a LiPo pack
It not practical to install buss bars on the 4-6s RC type lipo packs, so the best thing to do is ensure you have well matched cells and dont run them down too low.

 

[John in CR]

I don't do deep discharges, period. (a accurate use of that punctuation unlike you know who). I've done 3 full discharges on Konions, but rarely over 50% with my RC lipo and I have chargers in my backpack for long rides.

After more thought though I think I should do the same thing I did with my hardpacks, which is build a permanent pack with 14ga wire paralleling at the cell level. I'll do that after proving the packs by first running them in a temporary arrangement paralleled only at the ends to identify weak cells, if any. Getting away from what I've proven to work well would be kinda stupid, especially for a pack to be mounted in a swingarm. There's just no shortcuts for that.

 

[dogman dan]

Paralell with bigger wires sounds like the best solution.

I just don't parallel them, since adding more flaky small wire seems dodgy to me, if you rely on jst plugs for the contact. Once well sorted, they need less balancing, and low dod makes it even more of a non issue.

 

[John in CR]

Paralell with bigger wires sounds like the best solution.

I just don't parallel them, since adding more flaky small wire seems dodgy to me, if you rely on jst plugs for the contact. Once well sorted, they need less balancing, and low dod makes it even more of a non issue.

Once I prove all of the new cells are good, the parallel structure will serve 3 purposes, since the pack is unlikely to come off the bike fore at least a year, so packs will never get connected individually to an RC balance charger.
1. Averaging multiple good cells together by paralleling them helps them support each other and helps maintain better balance.
2. I don't run any cell level alarms or BMS, so if they are paralleled then checking cell level voltages will be 1/4th the work since I have 4 strings. Making it simple means it will get done more often, so more safety.
3. I have some tiny and cheap BMS's for testing, and I'll need paralleled structure to easily connect a BMS, which will give me a low cell alarm and balance charge including a direct connection for solar panels with no charge controller needed. That's a huge savings at our high ebike voltages, making a solar balance charger comparably priced with a moderate power bulk charger.

 

[oatnet]

1. Averaging multiple good cells together by paralleling them helps them support each other and helps maintain better balance.

This is a double-edged sword... If one cell goes bad, it will drain the good cells in the other packs, and kill them too. I've seen this happen in paralleled cells in my VW bus conversion (8p), and in my Vectrix (5p); because I use LIfePO4, the paralleled cells simply went to 0v and kept on passing current, no big deal. However, since you are using LiPo, the consequences of draining to 0v could be more severe, and because you series before parallel, multiple packs will be impacted. You may well have standards in place to mitigate this risk, and maybe the consequences will be the same be it 1 cell or 4 cells, but I thought I'd mention that.

-JD

 

[John in CR]

Thanks for the input Oaknet, and while I can see your point, I recently had a cell failure. Had that cell not been paralleled to another cell with a 2" long 14ga wire, I believe the odds are pretty good that instead of 1 dead cells and 1 damaged cell, I would have lost the entire bike due to the dead cell going into thermal runaway.

It turns out that the dead cell self discharged at less than 1A, so unless more current flowed across the parallel wire while running, then regular balance tap wires could have handled the issue.

 

[dogman dan]

I agree, the way you handle your packs, you would want them paralleled at cell level.

I just meant that trusting thin cheap wires and cheap plugs in a large pack could result in other problems that you would avoid by using solder and larger wire. I think you are doing the right thing for how you operate.

To clarify my comment about I don't parallel at cell level to others, I don't build big packs, I do 10 ah blocks. Mostly I use just one, but on long rides I can add more if I have them. This gives me flexibility to run various bikes, and the mower.

When I do need to check every cell, it's very tedious. This is a big effort when the packs are new, and I don't know which one is dodgy yet.

Later on, not so bad. Mostly I'm just keeping an eye on the newest pack that I use the most. And the use I give it keeps it pretty well balanced for months at a time. There are 6 jst plugs to check on a 14s 10 ah pack, so not that tedious to check it with a cellog. To balance, I can pop on a paralleling jst wire set, and then run a balance charge, or just single cell charge one cell if only one cell is out on the entire pack.

 

[parajared]

Working awesome for me so far; way easier than balancing each and every pack. I like to make 20ah bricks out of four 5ah 6s hobby lipos.