Lithium batter voltages different in parallel.

[Pat911]

I agree in principle there Volt_Ampare but the OP had his batteries connected in parallel for "several days" whilst discharging at a measly 2A and all it proceeded to do was put his cells further out of balance.

Because of the flat LiFePO4 voltage curve, the lower the resistance between the cells the better the balance. I'm specifically talking about initial balancing via parallel connected cells, not balancing via the BMS while in series connected pack configuration.

[Volt_Ampere]

If it were me doing this, I would charge each battery to the same voltage separately before hooking them together. I made sure my Leaf modules were all well balanced befor I built my pack so that the BMS did not have to work hard to balance a big pack with small currents.

[Pat911]

Yes, so would I. Parallel connection was for a set and forget for a month or so.

[Silenttrouble]

Originally Posted by Pat911

I agree in principle there Volt_Ampare but the OP had his batteries connected in parallel for "several days" whilst discharging at a measly 2A and all it proceeded to do was put his cells further out of balance.

Because of the flat LiFePO4 voltage curve, the lower the resistance between the cells the better the balance. I'm specifically talking about initial balancing via parallel connected cells, not balancing via the BMS while in series connected pack configuration.

Man u guys are the best, thanks for helping.

You were absolutely right that I made it worse, one by pulling from one end of the chain and 2nd with these alligator small gage wire.

What I have managed to do is use all the bus bars I have in the middle of the chain, Then 22ga wire between the bus bars and the other terminals. The wire is 0.023ohm from bar to each terminal so it shouldn't be additive in series if that makes sense.

Like I said the cells were low to begin with so bringing them up to match isn't the end of the world with charger, So far I am within 0.015v, once they match should I rebuild the pack and let bms do its thing with charging and balancing? For reference the bms canbus runs the elcon charger.

Dan

[Pat911]

The true test of voltage difference will be when you disconnect the cells.

You really need to charge up to 3.65v now. Simply connecting them in parallel and leaving them like that will not balance the cells if they are in the flat part of the discharge curve. You need to get them to their upper voltage.

No BMS is needed for this if you have a fixed voltage power supply.

[Silenttrouble]

Originally Posted by Pat911

The true test of voltage difference will be when you disconnect the cells.

You really need to charge up to 3.65v now. Simply connecting them in parallel and leaving them like that will not balance the cells if they are in the flat part of the discharge curve. You need to get them to their upper voltage.

No BMS is needed for this if you have a fixed voltage power supply.

So how does lifepo4 deal with full charge storage? I'm not sure if I know how to discharge them back to a safe storage lol, I need to drive like 10miles in my cart freezing my *** off

[Pat911]

It is similar to any other Lithium Ion chemistry. It is best to store at 40% to 60% but since you're storing it in a cold ambient temperature the degradation by storing at 100% will be less pronounced.

Long term storage at high SOC in high temperatures (above 25 Celsius) is where degradation begins to become measurable.

You could also leave them as they are and begin the charge up process one month prior to use. BUT REMEMBER, DO NOT CHARGE THE BATTERIES IF TEMPERATURE IS BELOW 0 CELCIUS.

[augiedoggy]

What type of bms are you using? I'm not sure what kind of cells your using but I use one of these balancers and it works very well to keep my cells in balance. https://www.ebay.com/itm/31390533980...Bk9SR4qGmd-NYQ

I top balanced mine in parallel on a bench power supply to 3.650 v but now days im reading over and over that its not necessary as the bms should take care of it.

[Pat911]

That’s a good balancer Audgiedoggy but it should only be activated once the cell voltage is above 3.45v. It will balance your pack quite quickly. You’ll risk putting the pack out of balance if using it when in the flat discharge area of the voltage curve though.