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BYD Battery - Charge low cell

steviep19

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Dec 26, 2019
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I have several BYD battery packs. Cell number 8 on one of the packs has always been out of balance.

If I disconnect the battery pack, is it safe to charge Cell Number 8 directly from a CV CA charger. I'd set the target voltage to the voltage of the other cells in the pack. I'm not able to disconnect the individual cells from series.
1608833382634.png

I'm thinking this is OK because essentilay isn't this what an active balancer does? Just in the case of an active balancer it takes energy from high cells, and puts it in lower.
 
Yes, you can charge that cell directly. I have a few BYD packs too, and cell 8 is the runner, but evens out by 3.35V and another cell takes over as the runner, so you should keep an eye on all of the cells as you reach your voltage target.
 
Pjones,

Thanks I just hate to loose like 3/4 of a kWh when all the need is a balance.
 
I have had similar issue and charged them as you mentioned. I have not had much success. Cycle the battery a couple of times and it is all over the place again.
 
Some have paralleled smaller cell. Have you had any luck with anything like that?
 
Some have paralleled smaller cell. Have you had any luck with anything like that?
I have some 5000 mah cells and a plan. I have all of the cells of two of the BYD batteries in parallel but I didn't bring out a lead for piggyback cells. No easy access to the wiring right now.
 
I have some 5000 mah cells and a plan. I have all of the cells of two of the BYD batteries in parallel but I didn't bring out a lead for piggyback cells. No easy access to the wiring right now.
I have been considering paralleling all the cells, but I left the covers on when built my "power-wall" Unfortunately, access to the cells on the back is blocked. I think it's going to be worth doing but those BYD packs are so heavy. I considered using the balance leads, but I don't think the traces on that PCB could take the current. One of these weekends I'm sure I will. I'm in Massachusetts, so I'm luck to pull in 12 KWH, of solar on a sunny day, so why do today what you can put off till tomorrow lol
 
I have been considering paralleling all the cells, but I left the covers on when built my "power-wall" Unfortunately, access to the cells on the back is blocked. I think it's going to be worth doing but those BYD packs are so heavy. I considered using the balance leads, but I don't think the traces on that PCB could take the current. One of these weekends I'm sure I will. I'm in Massachusetts, so I'm luck to pull in 12 KWH, of solar on a sunny day, so why do today what you can put off till tomorrow lol
There are holes in the bus bars that will take a pop rivet. This is how I made my parallel cell connections.Rivets.jpg
 
Slightly off-topic, on cells that are low, you can parallel a cell to help it along, but what do you do with runners--how do you slow them down?
 
Because paralleling those cells is a pain, I would just stick a power supply on that one cell and charge it up to match the others.

But that cell probably has lowest capacity, so at low soc it might be at higher voltage than others when you charge it up.

Do a capacity test after you balance them to see if it made any difference at all.
 
Those holes will also take a small screw for those that may need disassembly. I have been getting pretty snug connections and have not stripped a hole yet even with repeated use.
2020-12-24 19.45.44.jpg
 
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I used the same screws as above, works really well.. I added headways to the cells that are lower in capacity, then paralleled the cells with fused interconnections. After all that I still only charge to 95% which my batteries hit at 54.4v at 2-5hrs at absorption , if the batteries are fully discharged it can take a full 5hrs at 54.4v to hit the 95% SOC.. if the batteries are only been discharged 30% they will get to 95% SOC with 2hrs absorption at 54.4v. I basically float them all day at 54.4v...

I bet that if you were to put a say 10amp load on the pack at the 27.2v that it is showing the cells would fall in line to 20mv within 5 minutes... and if you try charging any one the cells individually at the 27.2v pack voltage that it would take take less then 10amps to push that individual cell to the 3.65v cell max. I have done alot of random tests of individual cell capacities, of the 208 byd cells I have they all tested between 110-130ah, only one of the 208 cells showed 67ah and the cell next to it was the best at 143ah..
.
I ran my packs from 3.1v-3.36v with no bms for 4 months and they stayed stable at these voltages, this was providing 85% of the total capacity of the packs. Now with the interconnected fused cells now pull 95% of the packs total capacity and now run the cells from 3v-3.4v per cell.

I guess my point is you can run these packs and try to run them from the 2.8v-3.65v range but its going to take a large effort to see an additional 10%, alot easier to add one more pack and stick to the 15%-85% SOC range.. I put in the 60hrs+ and $300.00 in wire , qnbbm balancers($400.00) when I could of just added a couple packs for the same results..

In the end these packs are great, these days with the limited solar they have spent a couple weeks around 3v per cell.. we had the first sunny day in weeks yesterday and I was able to charge the Powerwall from 4-40% and the cells ended up the day 6mv out..
 

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