Batteries not discharging the same

[alhelton1942]

I have 2-12v, 60ah lithium batteries running a 80# Minn Kota Ultrex trolling motor. After running the trolling motor all day I checked the batteries and one of them had 56% charge left but the other only had 23% left. I can't understand how they could be different since they are wired in series to run the trolling motor. Looks like they should have equalized since they are in series. How can this be?

[smoken86]

Check and make sure nothing else is being powered on that one battery. I had the same issue when I first put my lithiums in. A previous owner had wired my front graph to a trolling battery and I had no idea.

[haus9393]

Are you sure they were both at 100% when the day started? If not recharge to 100% and keep a keen eye on the app as the day goes on. Set an alarm to remeber every 2-3 hours of fishing time.

[alhelton1942]

They were both charged to 100% when I started fishing. I checked them with the blue tooth before I left. There is nothing else hooked to the batteries. I am a retired electrical engineer and I can't figure how this can be since they are in series.

[mactlman]

might post in the trolling motor area also as the battery guys are always there

https://www.bbcboards.net/forumdisplay.php?f=128

[SteezMacQueen]

They were both charged to 100% when I started fishing. I checked them with the blue tooth before I left. There is nothing else hooked to the batteries. I am a retired electrical engineer and I can't figure how this can be since they are in series.

Not knocking on your career credentials…..but….just because they are series wired and identical specifications, doesn’t mean the batteries are identical. It could just be that one of the batteries is not quite as well made as the other.

I have identical batteries and one is always .2-.3 volts lower than the other at the end of the day.

[Bjxds]

Did you contact tech support for your batteries?
What type of battery?

I recently switched to lithiums, and had many questions about interpreting the info in the app. SOC percentage may not be as important as voltages and individual cell data.

[billnorman1]

They were both charged to 100% when I started fishing. I checked them with the blue tooth before I left. There is nothing else hooked to the batteries. I am a retired electrical engineer and I can't figure how this can be since they are in series.

If they were in Parallel they would most likely have the same degree of charge in use. Current is equal in a series circuit and imbalances are probably caused by state of charge or manufacturing or age differences. No two items are totally identical especially the more complicated they get. Lithium batteries are multitudes of cells and a BMS, unlike the simple lead/acid 6 cell 12 volt battery.
Also, the common person now has a meter that will read in the hundreths of volts which gives one data that used to be unavaiable. When I was in aviation we hated to have a plane that gave the pilot two or more gauges that could measure the same parameter, he/she would always write the system up as defective even though it worked perfectly.

[Coastal Mountaineer]

My two 60ah batteries do the same thing. After about 6 hours on the water, one battery will be about 15% lower than the other. I have a delta volt battery gauge hooked to them, but I've checked for parasitic drain and there is none.

I haven't called the battery retailer yet. I want to get before and after numbers using a volt meter rather than the app.

[DrewFlu33]

Lots of explanations above, but one simple one to check I haven't seen mentioned: Is your jumper wire that forms the series connection smaller than your main wire run?

Had a buddy who fought with uneven discharge for years before he found his jumper was like 10 AWG. Switched that out to 6 AWG to match the rest of the setup and he was good as new.

[alhelton1942]

I have a 6 AWG jumper wire, the same as the troll motor wires.
I talked to Kevin and he said it was because the individual cells have slightly different charges and they would tend to equalize after about 6 charge cycles. He said that was normal.

[lpugh]

Batteries in series do not ever equalize. Be sure the charger is bringing them up to 100%, if so, it is a bad battery assuming nothing else is connected to the battery in question.

[cadman]

but I've checked for parasitic drain and there is none.

How do you check for parasitic drain?

[lpugh]

amp meter connected in series in the circuit being tested.

[alhelton1942]

Went fishing today and when I got through one of the batteries had 74% left and the other had 72% left. Looks like they are starting to equalize after a few charges like Kevin said. This was the 4th charge.