Batteries in series problem

[SonomaScott]

I've got two 12 volt batteries wired in series to run my 24 volt trolling motor. Something weird has been happening recently-

Battery B (its pos is wired to the neg of Battery A) has been draining very rapidly. For example, this morning I went to charge my batteries and found that Battery A was 82% charged while Battery B was at 2%. This happened a couple of months ago and I thought I had a bad battery. I replaced it but now the same thing is happening again. Is there any explanation for why the second battery in a series would be depleting like this?

 

[bruceb58]

How are you charging them?

They should discharge identically assuming you don't have other loads on that one battery.

Out of curiosity, how can you tell a battery is 2% charged? What are you using to come up with that number?

 

[alldodge]

I've got two 12 volt batteries wired in series to run my 24 volt trolling motor. Something weird has been happening recently-

Battery B (its pos is wired to the neg of Battery A) has been draining very rapidly. For example, this morning I went to charge my batteries and found that Battery A was 82% charged while Battery B was at 2%. This happened a couple of months ago and I thought I had a bad battery. I replaced it but now the same thing is happening again. Is there any explanation for why the second battery in a series would be depleting like this?

Got a pic of your wiring setup?

 

[SonomaScott]

Thanks for the replies.

bruceb58-
Yes, they should discharge the same, but they are not. I am using a Schumacher 15A charger which gives you a remaining percentage when you begin charging. Battery A showed 82% charged and Battery B showed 2%. This was about a week after I had charged both to 100%. I give them a charge every 7-10 days when I'm not fishing. Also, when this happened before, it wasn't just when the boat was sitting- the battery was discharging very quickly when I was trolling. Normally I would get a good 8 to 10 hours before my onboard battery meter would drop below 50%, but it was dropping below 50% in about 2 hours. I thought the battery was bad but it checked out fine when I took it in.

AllDodge-
I've attached a pic. This was taken with my old batteries which I replaced a few months ago. The battery that is depleting is on the left. It's negative runs to the trolling motor and it's positive is connected to the negative post on Battery A. If you would like to see a more detailed photo of the wiring, let me know and I'll take some more pics.

I should also add that these batteries are dedicated to the trolling motor and they don't run anything else on the boat. The only other thing connected is the onboard batter meter.

Thanks for any help you can give.

 

[alldodge]

Have you pulled the tops off the battery and checked the water level?
I take it this is a 12V charger and your charging the batteries separately?
Are you using deep cycle marine batteries?

 

[SonomaScott]

Yes, it's a 12V/6V charger and I charge the batteries separately. The water level in both batteries is good. Both batteries are deep cycle marine, group size 27 as recommended by Minn Kota.
I just can't figure out why one battery is discharging so quickly, whether in use or just sitting the driveway, while the other battery holds a charge. And like I said, this has happened with 2 brand new batteries in a row. Something is depleting just the second battery, even while just sitting.

 

[alldodge]

I just don't see how one battery would be depleted and not the other with a series circuit. Try this, charge them up and swap the battery connects around, turn bat a to b and vise versa

Edit: Is there anything which sits or lays on top the batteries?

 

[SonomaScott]

That sounds like a good idea, AllDodge. I'll swap the positions and see what happens. (Actually, my wife says she suggested that but I was listening. :facepalm

I'll repost here and let you know how it goes over the next few days.

No, there is nothing on top of the batteries. Nothing touches them that could be drawing a charge.

Thanks for the advice.

 

[bruceb58]

Have you ever let these batteries go completely dead?

Are these deep discharge batteries? Have a picture of the side of the case so we can see what type they are?

 

[alldodge]

The Admiral gave me a beer huggy with "My wife says I never listen to her or something like that", still don't understand why :noidea: glad she doesn't read my post

 

[SonomaScott]

AllDodge- My reply should have said "I wasn't listening to my wife" but I typed "was listening". I guess I don't pay attention to my typing either.

bruceb58- Both batteries are Deep Cycle Marine and both are group size 27. They are the batteries recommended by Minn Kota. I don't ever let them completely drain and when I return from fishing, I immediately charge both. However, yesterday I went to do my regular weekly charging and found that the one battery had depleted to 2% while sitting in the drive way for about 7 days. It's a repeat of what happened a few months ago. Just can't figure out why.

 

[Silvertip]

This may sound a bit "out there" but thoroughly clean the tops of the batteries (they seem a bit dirty but it may just be the picture quality) and then slide a piece of cardboard or some other insulating material between them. If that doesn't reduce the drain, disconnect the series jumper and see if the battery still drains. If it does, you drew the unlucky card and got a bad replacement battery. Is the troller plugged in during storage time? If it is, then unplug it and see if the drain stops. It may be the motor is the culprit.

 

[bruceb58]

Instead of sticking the charger on them to see what their state of charge is, measure the voltage with a voltage meter.

Charge them both up and wait a few days and measure the voltage of each.

May also be worth investing in a cheap load tester as well.

 

[gm280]

WOW, I've read everyone of the comments and this is amazing to hear. Current HAS to flow equally through both batteries the same. There has to be something we are not seeing for this to happen. I've hear of batteries discharging at different rates, but this is his second battery doing the exact same thing. The statistical probabilities are so low that it is basically non-existent. I have to watch this to see what really is going on.

 

[SonomaScott]

Silvertip-
The batteries in the picture are my old ones. The new batteries are clean and were manufactured in May of this year. You might be right about the motor drawing power when not in use, but it should be draining both batteries, not just the one. I'm gonna swap battery positions and see what happens. If the problem persists, I'll take your advice and just disconnect the cables when not in use.
I'll update this in a week after trying these ideas. Thanks for all the help. Much appreciated.

 

[SonomaScott]

gm280-
You've grasped my dilemma. I replaced both batteries earlier this year. Everything seemed fine for a few weeks until one day on the lake, my battery meter showed 40% remaining after only a couple hours of trolling. I went home and checked my batteries: Battery A was at about 80% but Battery B was almost completely depleted. I charged both to 100% and then checked them a few days later. Battery A was at 94% and Battery B was at around 2%. That's when I figured Battery B was defective. The store gave me a new replacement.

Now the same thing is happening. Battery A is fine, but Battery B is depleting very quickly, even when the boat is just sitting in the driveway. There must be something that is causing this to happen, but I have now idea what it could be.

I just got back to town so I'm gonna try AllDodge's idea and swap the batteries around. Thanks for following and I'll post an update when I have some results.

One question-
Could the problem be the wires connecting the batteries together? Should I replace the wires connecting them?

This is a real head-scratcher!

 

[alldodge]

Could the problem be the wires connecting the batteries together? Should I replace the wires connecting them?

In theory no, don't see how it could happen

 

[Fed]

Your funny readings could easily be coming from the wiring or the lugs or connections.
Test it right on the lead battery posts.
I bet if you took all the wires off & charged the batteries individually then tested them they would be fine.

 

[SonomaScott]

Fed, you might be right. From everything I'm hearing, there is really no way that with two new batteries operating in series, one would be depleting far more quickly than the other. I'm gonna explore the idea that -for some reason- I'm getting false readings on the second battery.

 

[gm280]

Fed, you might be right. From everything I'm hearing, there is really no way that with two new batteries operating in series, one would be depleting far more quickly than the other. I'm gonna explore the idea that -for some reason- I'm getting false readings on the second battery.

What are you using to read the voltages on the batteries? Is it a hand held meter or a panel meter with a switch?