Testing Batteries Wired in Parallel

[PawleysDude]

I'm using two identical wet batteries connected in parallel to run my 12V trolling motor, depthfinder, etc. It's a small boat with no other propulsion. I charge the batteries with a NOCO permanent mount single-bank charger.

 

It works fine for me but recently it feels like run time on the batteries has decreased. I want to test the charge level of each battery independently after fully charged and then again after time on the water. I assume I will need to disconnect one of the jumper cables between the two batteries while I'm testing, right? Does it matter if it's positive or negative? I'll be testing with a multimeter. Anything I'm overlooking? Anything else I should do that would be helpful? Thanks for your help. ?

[Scott F]

The only way to tell which one of the batteries is weak is to disconnect them from each other. 

[Woody B]

I'd recommend disconnecting one of the jumper cables (either wire like you mentioned) letting them sit for 24 hours or so then get them load tested.   

[MN Fisher]

Personally - I'd disconnect both cables and have them load tested independently...most local auto-stores will do it for free.

[PawleysDude]

26 minutes ago, MN Fisher said:

Personally - I'd disconnect both cables and have them load tested independently...most local auto-stores will do it for free.

Ugh…. Thanks for the feedback. I figured that was the best thing to do but it’s an old Tracker that has been “reconfigured” and getting the batteries in and out is a royal PIA. I was hoping to avoid that ordeal.  Guess I just need to suck it up.

[Tennessee Boy]

You can always buy one of these.

 

https://www.harborfreight.com/100-amp-612v-battery-load-tester-61747.html

[casts_by_fly]

If your boat/trailer will allow for it, put the trolling motor down so that you can turn it on.  Hook up each battery individually and test voltage before and after turning the TM on.  Its a poor man's load test but if there is something very wrong with one of them you'll know without having to pull them.

[Fishingmickey]

Hello Pawley,

     Batteries eventually die. If they are old batteries, out of warrenty and the charge isn't holding like it used too.  It is possible they are reaching that point. The battery companies have the science down to a art form.  At least in car batteries, you get to that three year (or two or one) pro-rated point depending on battery and it dies. I've been through several batteries for the boat/s too, both trolling motor and starting.  If they don't get used and maintained regularly, I think they die sooner.

FM

 

p.s. Not much help in the post, but considerable commiseration.

[Kirtley Howe]

Testing a battery with a multimeter won't tell you much. A battery can have a "surface charge" that shows 12.6 to 13 volts, and still be just about worthless. A multimeter cannot measure the batteries ability to hold a charge or supply amps. You need to load test them individually. As casts by fly mentioned you can do a poor mans load test using your trolling motor, but the only definitive test is to have them individually checked with a load meter.

[PawleysDude]

5 hours ago, casts_by_fly said:

...Hook up each battery individually and test voltage before and after turning the TM on.  Its a poor man's load test but if there is something very wrong with one of them you'll know without having to pull them.

Thanks, that should be doable. A couple questions... Would I see a voltage drop right away or do I need to let the TM run a bit? How much voltage drop, more or less, would you consider acceptable? Thanks!

3 hours ago, Kirtley Howe said:

Testing a battery with a multimeter won't tell you much...

Thanks. Yeah, I was afraid of that. I know that replacing the batteries is a part of life, but this boat is permanently docked at the bottom of a steep hill and when the time comes I'll probably be looking for some younger, more able help, so I'm trying to avoid that step until I know it's time. ? With the poor man's test, how much voltage drop would be acceptable? Appreciate your help.

[Kirtley Howe]

I think a drop down to 12.3 volts or less after a couple of minutes of running is an indicator that the batteries are shot. A drop from 12.6-13 volts after a couple of minutes, but the battery "recovering" on its' own after a few minutes when you quit running the trolling motor would make me think the problem is not the batteries, but more likely a bad connection somewhere that is acting like a vampire and drawing voltage away from the trolling motor/batteries. Just for kicks, have you tried disconnecting the batteries from everything else and letting them sit for a few days? A good battery should still have very close to a full charge after only a few days. If the voltage/amps drop much after a couple of days, then your batteries are definitely starting to die. Good luck.

[casts_by_fly]

21 minutes ago, PawleysDude said:

Thanks, that should be doable. A couple questions... Would I see a voltage drop right away or do I need to let the TM run a bit? How much voltage drop, more or less, would you consider acceptable? Thanks!

Thanks. Yeah, I was afraid of that. I know that replacing the batteries is a part of life, but this boat is permanently docked at the bottom of a steep hill and when the time comes I'll probably be looking for some younger, more able help, so I'm trying to avoid that step until I know it's time. ? With the poor man's test, how much voltage drop would be acceptable? Appreciate your help.

 

You're not looking at a hard drop number per se, you're looking at how they compare to each other.  How much it drops will depend on the battery itself (capacity, health, etc) but since they are roughly the same battery if one drops a lot more than the other then you know that one is struggling.  And you don't need to leave the motor on long.  With the motor off, the battery will be resting state voltage.  It will drop immediately in turning the motor on.

 

You don't say what capacity of batteries you're using now, but if it is two group 27's in parallel then instead of buying two new ones you can just get one lithium and not worry about the weight anymore.  For $400 (cost of two lead acid wet cells) you can probably get the same total capacity as two 27's.  

[WRB]

To disconnect 2 batteries to test them without removing the battery you need a Perco battery switch.

The old school method that still works is a battery hydrometer to test each battery wet cell to determine the charge level or a poor / dead cell. You want a hydrometer with a rubber tube tip to get into some hard to reach cell.

Tom

 

[FranklinSantee]

How old are the batteries?

Have you checked the water levels?

I'd also make sure the terminals are clean.

 

Then I'd charge each battery independently before doing the TM load test.