Unequal Discharge Rate

[flboy]

I just paired (in parallel) a brand new 280aH LifePo4 (3.2V Prismatic Cells) and a similar 230aH bank each with their own 150A BMS (batteries are the same chemistry and made by EVE). Both were top balanced and fully charge when paired.

In testing my battery bank with the 1500W inverter and running a 800W heater, I noticed (Via Bluetooth Monitor on BMS's) that the 280aH battery was drawing ~39A while the 230ah bank is drawing ~21A. It is roughly 2/3 to 1/3.

This is the first time I have done this where I have had the capability to monitor individual banks, and I am assuming it is not right. Further, I am assuming this may be due to an internal resistance difference (to include leads and connections).

Any advise on where I should start... I'd think the battery drawing only 1/3? May move the ground lead first to the other battery to see if that changes anything, then check all connections and leads (which I crimped).

Just hoping to save some time and only tear apart one battery to check it out.

Help?

[featherliteCT1]

What size AWG wire do you have between the two banks?

Did you check if any of the terminals and/or connections are getting warm to the touch?

[flboy]

4 gage, external battery connections not hot. No way to check inside as it is installed.

ANL fuses were very warm.. and borderline hot, but both batteries are presented with those fuses.


I am assuming from your comments that the discharge should be balanced and that would make sense? Will focus on the 230aH battery since it seems like it would be the issue if it has lower current.



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[troubleScottie]

Not having tried it myself, the general opinion (read google results and reading on this site) is that you should not do that -- have two batteries with different capacity connected in parallel. The power draw will be uneven as you noticed.

It is not clear if they will recharge in the same manner, thus both return to fully charged or end up uneven.

You might consider have a switch to use one or the other battery at a time. Obviously not the best idea -- you will not be able to fully drain the batteries automatically AND you would need two meters/shunts to determine the charge on each AND you will have to charge one then switch to charge the other.

Most people aim for same voltage, same capacity, same chemistry eg Pb acid, AGM, Li and deep cycle or general, same manufacturer with about the same manufacturing date for all batteries. Often replacing all of them at once.

[flboy]

I do not think this is true for LiFePo4 banks of the same voltage (assuming top balanced and fully charged) that have separate BMS. What you are saying is applicable to SLA or like batteries with no BMS. I get that.

In the same manner as discharging, the BMS will handle the individual Bank as it charges and turn it off when limit is reached independent of the other bank in parallel. They just need as regulated 14.6VC with a constant current source and the BMS will handle the charge. That is my understanding.

Given that one battery is delivering less current (both are well below their delivery capacity of 150A (BMS limit) each when pushing 800W) , I believe there may be an imbalance in the resistance? but I am new to the DIY LiFeP04 scene here and I am looking for someone with first hand knowledge of what I am seeing. I didn't think it would be so sensitive to small resistance (like one cable longer than the other), unless I just have a really bad lead or connection somewhere.

I have never had the ability to monitor each battery bank in the parallel configuration before so this caught my attention. I don't know why this would be "normal".. but if it is, it seems one battery (in this case the larger capacity) will deplete faster than the smaller battery.

I was expecting them to deplete at the same rate with the 230aH shutting off before the the 280aH which would get left holding the whole load for the last 50aH. Is my thinking wrong?

[Pmullen503]

What you are seeing is not necessarily bad. You should not expect the smaller battery to discharge first. Batteries in parallel will "balance" each other, that is, they must all be at the same voltage.

For RC plane use, you run at very high currents, completely exhausting a lipo in as little as four minutes. I those cases I run real time telemetry to monitor cell imbalances. At those high current rates you can see differences due to the condition of each cell. You'd never see them at 1 or 2C discharge rates.

[flboy]

I was able to get the discharge equalized. It was the 230aH battery as suspected. I found a łoose connection on one cell and shortened up the positive lead and put the B- lead for the bank on the 230aH battery. That did it.


Edit... having the BT Monitoring on the Smart BMS is very handy. The Power Monitor reading from the shunt would never pick this up, but after I removed the load... you could literally see one battery discharging 1.2A into the other battery which was charging until they equalized. They were not that far off because I do have the discharge very close...


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