Adding a single parallel cell to boost a weak cell

Batteries, Chargers, and Battery Management Systems.
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mattjrogers   1 mW

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Joined: Apr 15 2017 2:47pm

Adding a single parallel cell to boost a weak cell

Post by mattjrogers » Apr 29 2017 6:24pm

I have a 48v 16s1p 45Ah LiFePo4 prismatic battery with what looks to be one weak cell that drains faster than the rest. Simple question - can I add a single parallel cell to the weak one to increase its capacity and allow the rest of the pack to continue working without tripping the BMS low voltage cell cutoff? For example, adding a 10Ah cell to the weak one in parallel to give it the extra capacity to work like the rest. This would save having to purchase a more expensive larger cell as a replacement.

These are generic cells from china. Supposed to be 45ah, but have yet to fully test the capacity and they could be 40ah. I did just run the pack down 35Ah and found the one cell at 2.40v with the rest at around 3.00v. This was immediately after the ride and obviously the voltage was slowly rising, so I don't know what voltages the cells would have settled at as I put on the charger after the ride. My gauges read that the minimum voltage drop was 42v on the pack under full load at the end of the run, but the overall pack voltage under no load was around 48v. What is the currently accepted minimum cell or pack voltage for prismatic LiFePo4 to set the BMS low voltage cuttoff for the battery?

Anyone know where to get a 45ah prismatic cell?
If I can't find an exact match, would the new cell possibly be not well matched with the pack by being too high capacity if it is better quality? What would you do if you had to replace a single cell if you could not find a perfect match and had to guess on what to purchase? Batteryspace has a 40ah single cell that could be a match. electriccarparts.com has 20ah cells that could be paralleled to match.

Right now I have a BMS that is hooked up as a charge-only-bms. I am getting a new BMS from Aliexpress BMS with bluetooth connection that I can monitor during riding and charging on my phone. It was $58 and allows me to program all values and see individual cell voltages on an app.

https://www.aliexpress.com/item/16S-50A ... 0.0.EsK98k

James Broadhurst   10 W

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Location: Oxford, England

Re: Adding a single parallel cell to boost a weak cell

Post by James Broadhurst » Apr 30 2017 1:46am

Very interested if the Bluetooth/rs232 interface works. Seems to be a more efficient way of monitoring a pack than using a logger. The loggers I've seen can only handle up to 8S.

Interesting that this stuff is beginning to cost a significant proportion of the battery build and, if truth be told, I never answer my phone anyway when riding the bike and what I really want is a buzzer/klaxon/bell telling me the pack's getting too low.

This warning is beginning to appear on these and other BMS
"Note: this BMS with balance function. your battery need match, battery should have good consistency, if not sure, please use new brand battery. "
Not very happy with this. When I built my pack I needed to use an iCharger to pull them into balance otherwise it took successive cycles with a BMS like this one.

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nutnspecial   1.21 GW

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Re: Adding a single parallel cell to boost a weak cell

Post by nutnspecial » Apr 30 2017 4:27am

Adding a single parallel cell to boost a weak cell (?)
I don't see why not? If it was lipo I'd retire it, but it makes sense for your big capacity lifepo. I wouldn't think you'd need much more than 5ah (15%), and do think you'd want to have it fairly close to cell average on account of maintaining balance.

mattjrogers   1 mW

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Joined: Apr 15 2017 2:47pm

Re: Adding a single parallel cell to boost a weak cell

Post by mattjrogers » Apr 30 2017 5:45am

Yes, I will report on how the bluetooth connection works out. I just got the link to the cell phone app yesterday. You do have to have a little trust in that it is not solely designed to steal your data as it does have some permissions on your phone, but a battery BMS monitoring app is not he type of high volume program that hackers are looking for (I hope).

I will probably wait until I get the new BMS and let it charge and balance the pack a few times, when I can watch each cell voltage and see some info about how low the one cell actually is. The pack is not very old, got it end of last summer and only has a few dozen rides on it so far. Hopefully it will eventually balance out or I can just add a small boost cell parallel to the weak one.

Looking at Batteryspace, I got very excited when they had a 40 amp lifepo4 charger for $159, until I saw that it was only 3.65v for one cell. My dreams got crushed of a one hour charge time for this pack, so I am back to overnight.

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