Negative Charge on Battery

[Ed Majors]

I have a 3 bank onboard charger connected to my 3 trolling motor batteries. They are connected in series to run a 36V Fortrex TM. Suddenly, the trolling motor would not run and upon checking I found one oy my three batteries was showing - 12.84 volts(as in minus negative voltage). I had not changed any connections and verified all battery and charger connections were correct. I took the battery to Interstate and they agreed it read negative volts and had no idea what caused it. But they replaced it under warranty. My first thought was the charger was bad and took the boat to a dealer to replace the charger. He checked it and said the charger was working fine!This was all back in February. Everything was fine with the new battery. I didn't use the boat alot this summer, just an occasional fishing trip or boat ride. I kept it charged up regularly. Then today the TM would not work again. I checked and the same (new) battery shows negative volts again. What would be shorting this battery out? Thanks for any help!

[Russ E]

you should always change all 3 batteries at the same time on a 36 volt system. more than likely one of the other batteries are bad and taking that one down. Has anybody put a battery load tester on the other two batteries?

 

[MN Fisher]

^ What he said. If you're using a 3-bank, 12v system - disconnect the batteries from each other and charge them separately. If you want to keep them connected, you need a 36v charging system.

[Russ E]

You don't need a 36 volt charger and you don't have to disconnect the batteries. Nearly all bass boats have multi bank chargers.  They charge each battery individually with 12 volts. They are designed to charge the batteries while they are still connected. If you have a smart trolling motor it is wise to disconnect it while charging.

One possible issue I have seen on bass boats with 24 or 36 volt systems is  a jump start circuit that connects to one of the trolling motor batteries. It allows you to jump start the outboard with a trolling motor battery. There is a switch that disconnects the power wire, but the grounds of the trolling motor battery and outboard battery are always connected.

This extra circuit can only be connected to the battery that is connected to the trolling motor ground.

If it is connected to any other battery in the circuit it can ruin the batteries and possibly damage electronics.

   Unfortunately I found this out the hard way after a shop installed a new onboard charger and hooked them up wrong. It boiled one of my trolling motor batteries dry.

 

[Rocketvapor]

One possible cause of a battery going negative is leaving the series connected batteries in a low state of charge.  

Like running them down and not putting at least a partial charge when you get home.  

Even when using Deep Cycle batteries, which have a better chance of recovering when put on charge after a deep discharge. 

 

Starting out with 3 batteries in series when one has a little less total capacity, say 100ah, 100ah, 90ah, maybe one is just weaker than the others or self discharge or one has a 12 volt drain/load applied can result in a large difference in state of charge after a boating trip.  

Deep discharged to 10%, 20%, 15% remaining capacity for each individual battery after running the TM.  It would not take much further discharge to take the 10% battery to zero % and then negative.  It is important to start the charger and put at least a partial charge as soon as possible.  

 

In this poor sketch, the batteries/load on the left side is how we usually view a 36v stack.  

One battery at a low state of charge will still contribute to the load voltage. 

When multiple devices are connected in series and current is flowing, the position in the circle of current flow doesn't really matter.  A weak source can be at any position, it's just drawn per the diagram on the left. 

If you look at the source/load in the right diagram you can see how a low/weak battery can see voltage applied in reverse exceeds the voltage of the weak battery.  

 

Smart bank chargers will detect and try to correct imbalance between batteries connected in series.  

IF, you hook up the charger before the weak battery goes too low.

A single cell in a battery stack (18 cells in a 36v stack) can cause that battery to be weak and susceptible to complete discharge under load. 

 

Even a Deep Cycle rated battery will not recover if one of it's six cells goes negative.  That causes a permanent chemical change in the cell.  If a 12v battery ever goes below approximately 9v, 5 cells may be  1.8 volts and one cell may have made it to zero volts.  It may be forever damaged. 

 

 

 

 

[Tennessee Boy]

Very interesting problem here.  I'm not an expert on batteries and I think this question requires an expert.  I've never encountered a battery with a negative charge but I've heard that it can happen and understand how it can happen.  I don't see how a battery could have a negative 12.8 voltage without being reversed charged and even then it's hard to believe it could be that high (or low to be mathematically correct).  Personally,  I would suspect that you've got a bad charger or a neighbor that reversing the leads on your battery when you're not looking just to mess with you. Good luck. 

 

Oh and welcome to the forum.

[WRB]

Always keep batteries in series equal. The weakest drain down the stronger shorting the positive cells to ground = negative voltage.

To solve the problem you need 3 new equal batteries and clean inspect all connections.

Tom

PS, at least 1 newer fried battery is under warranty.

[moguy1973]

Batteries are DC.  If it's reading negative, swap your leads on your voltmeter.  It'll read +12.84

[Rocketvapor]

1 hour ago, moguy1973 said:

Batteries are DC.  If it's reading negative, swap your leads on your voltmeter.  It'll read +12.84

You sure that will fix his problem??

[moguy1973]

12 hours ago, Rocketvapor said:

You sure that will fix his problem??

Well, if he has his voltmeter hooked up properly (red to positive and black to negative) and it's still reading a negative value then his battery may have have experienced "Pole Reversal" which can happen if you use old and new batteries in series.

 

https://www.panasonic-batteries.com/en/faq/why-does-battery-show-minus-voltage-when-checked-voltmeter#:~:text=If the poles of your,phenomenon called “polarity reversal”.&text=In this case there is,old and new batteries together.

[TnRiver46]

13 hours ago, moguy1973 said:

Batteries are DC.  If it's reading negative, swap your leads on your voltmeter.  It'll read +12.84

Mine does that everytime I accidentally cross the voltimeter

[fissure_man]

I'm no electrician but if the issue was only that the batteries are non-identical age/condition, I would not expect the newly replaced battery to be the first to die the second time around. 

 

Seems to me that something is killing that battery specifically, could be a slow short or charger problem, need to check all the wiring connected to it including the charger. 

 

I think if you just replace all 3 batteries there's a high chance you will find the same problem down the road.   

 

OP is fortunate the battery was replaced under warranty as I doubt this is a problem with the battery itself (happened twice in a row now).

[Ed Majors]

Thanks for all the responses! Some great advice. I have tried most of it and no change. I don't trust my middle bank on the charger since it seems to have ruined 2 new batteries. I have it disconnected and charge that battery with a separate charger. I will see what happens before replacing the onboard charger. Thanks again!

[GaryH]

Ed. I would take your rig to a battery dealer. They will hook them up and test them for shorts and draw. I know most places up here don't charge anything to test your system. 

[Ed Majors]

30 minutes ago, GaryH said:

Ed. I would take your rig to a battery dealer. They will hook them up and test them for shorts and draw. I know most places up here don't charge anything to test your system. 

Thanks, I have done that. All other batteries check out fine and the charger tests fine. Somehow over time, the middle bank somehow reverses polarity on that one middle battery. It works fine for 2 or 3 months, then without warning the middle battery starts showing negative volts. This negative 12v subtracts from the positive 24v of the other 2 batteries. This leaves 12v going to a 36v trolling motor. So it quits working altogether.