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Old 16-05-2018, 08:10   #1
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Which battery bank is defective?

Hi,

I am coastal cruising for two weeks and off grid right now and scratching my head on this one.

I have two identical battery banks. Each has two 100+aH 6 volt flooded batteries in series and I run the house with the selector on 'both'. This has worked flawlessly for me for the last 4 years. We charge with a solar array through a Morning Star controller set to deliver 50% to each bank. Each morning battery bank 2 is showing yellow light on the controller remote and the voltage reads .2 to .3 volts less than bank 1. (eg. if bank 1 reads 12.5v and is green light bank 2 reads 12.3 and shows yellow).

Charging doing the sunny day shows each bank receiving equal aH until bank 1 gets about 10-12 aH and then bank 2 starts to get the majority of aH. By late evening, bank 2 has received the lion's share of aH (eg.if bank 1 got 15aH total, bank 2 got 65 aH). Bank 2 never seems to come up to the voltage of bank 1 and shows yellow the next morning. I tried running the house on bank 1 alone but it showed in the red the morning at 12.1v.

Question is: do I have a bad bank and if so which one is bad? I do not have a functioning hydrometer and only have the voltage readings.

I would appreciate any guidance if anyone has ideas.

Dan
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Old 16-05-2018, 08:41   #2
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Re: Which battery bank is defective?

Hve you checked the connections at both ends of the wires?
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Old 16-05-2018, 08:58   #3
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Re: Which battery bank is defective?

Yes, clean your terminals even if they look clean. Wet corrosion may be on the post. If you have a short in one battery it will be warmer after charging, but solar charging may not be enough to identify that.
After 4 years it's possible one battery is just starting to fail.
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Old 16-05-2018, 09:07   #4
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Re: Which battery bank is defective?

Not sure if this will work but I would first disconnect bank 2 from the solar array and take the voltage reading. Let bank 1 get fully charged as it may be able to keep up if it is getting full ahs. If bank 2 is failing it will not be able to hold its charge check voltage on it after 4 or 5 hrs of resting. If it has dropped likely it is your culprit. Hope this helps. PS The other advice is right on also. I would start with it first then go to mine if nothing changes.
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Old 16-05-2018, 09:13   #5
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Re: Which battery bank is defective?

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Originally Posted by PDScott View Post
Not sure if this will work but I would first disconnect bank 2 from the solar array and take the voltage reading. Let bank 1 get fully charged as it may be able to keep up if it is getting full ahs. If bank 2 is failing it will not be able to hold its charge check voltage on it after 4 or 5 hrs of resting. If it has dropped likely it is your culprit. Hope this helps. PS The other advice is right on also. I would start with it first then go to mine if nothing changes.
^^ Yes, charge, isolate, test hours later for voltage drop.
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Old 17-05-2018, 03:34   #6
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Re: Which battery bank is defective?

Thanks all for the guidance. I did clean the battery side connections but not the other side and will address that. I can isolate bank 2 and test but we are moving for the next three days so I will have to wait until we come to a stretch of days at anchor. I will report back when complete.

Thanks so much.

Dan
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Old 17-05-2018, 09:54   #7
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Re: Which battery bank is defective?

1. if you're using the boat overnight a 100AH battery bank is too small.
2. you should set up your battery banks to avoid running on both, you will wake up with dead batteries and no way to start the engine.
3. set up asymmetric battery banks, 200+ deep cycle for house and 80-100AH for starting battery.
4. Test battery cells with a hydrometer.
5. Test connections with a volt meter.

Good luck
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Old 17-05-2018, 10:54   #8
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Re: Which battery bank is defective?

A volt meter is about as useless as tits on a boar hog by itself. You may want to disconnect your solar. Turn on as much draw of power as you have. Measure a banks voltage before and after applying the load one bank at a time. The on with the largest drop is likely the one with a problem. I think someone else mentioned it. Combining both to supply your house needs may leave you dead in the water.
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Old 17-05-2018, 11:03   #9
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Re: Which battery bank is defective?

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Originally Posted by guyrj33 View Post
1. if you're using the boat overnight a 100AH battery bank is too small.
2. you should set up your battery banks to avoid running on both, you will wake up with dead batteries and no way to start the engine.
3. set up asymmetric battery banks, 200+ deep cycle for house and 80-100AH for starting battery.
4. Test battery cells with a hydrometer.
5. Test connections with a volt meter.

Good luck
The OP did not say the size of the boat or the loads on the battery. If there is refrigeration I would agree with point 1 but for just lighting there is plenty of power. The OP also did not say if there was an engine start battery.

A one-two-both battery switch is an obsolete concept. All four batteries should be hard wired together (series-parallel) with a simple disconnect switch. Having two battery banks and switching between them will shorten the life of both. With a single bank the depth of discharge and the cycle life is better. Having said that the OP says he has them combined at the switch. The switch contacts are a possible point of a voltage drop and some of the so called "marine" battery switches were made poorly.
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Old 17-05-2018, 11:11   #10
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Re: Which battery bank is defective?

I'm gonna agree with all the thoughts noted above and add a couple: (like you don't have enough to do already) . . anyway, consider that you should have a "House" bank of batteries and a "Start" bank of batteries depending on the wiring. Install a simple ACR (automatic charging relay) and make sure that you're starting batts are up and charged first, then let whatever is left over go to your house bank. As others have stated, you don't want to be dead in the water! The wiring is fairly simple and easy and adding solar to the equation is the best deal going. Send all your solar to one side of the ACR and let it send it to whichever batt needs it most . . . one and done!
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Old 17-05-2018, 11:21   #11
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Re: Which battery bank is defective?

Yes a voltmeter by itself isn't very useful, but when it's turned on and used to measure voltage drop in an active circuit it can be very informative.
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Old 17-05-2018, 11:39   #12
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Re: Which battery bank is defective?

A voltmeter in the hands of a reasonably knowledgeable user is indispensable for troubleshooting DC circuits.
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Old 17-05-2018, 11:58   #13
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Re: Which battery bank is defective?

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A voltmeter in the hands of a reasonably knowledgeable user is indispensable for troubleshooting DC circuits.
A knowledgeable person knows in battery troubleshooting it's drop not measured voltage. And yes a VOM is indispensable for DC or AC troubleshooting just not for batteries, on their way out, without a load. A battery is a big capacitor.
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Old 17-05-2018, 16:45   #14
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Re: Which battery bank is defective?

""". I tried running the house on bank 1 alone but it showed in the red the morning at 12.1v. ...."""

Sounds like what you have feeding off bank2 is a lot more than off bank1? Instead of switching everything, maybe shift a few of the 'bleeders'?
Do you have a clamp meter to test what each item is drawing?
If you have the switch set to 'both' then wouldn't both be at the same voltage? Or maybe it's LiFepo4 batteries we're talking about?
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Old 17-05-2018, 19:20   #15
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Re: Which battery bank is defective?

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If you have the switch set to 'both' then wouldn't both be at the same voltage? Or maybe it's LiFepo4 batteries we're talking about?
OP mentioned "flooded 6V in series", so no LFPs. But yes, if set to "both" they have to show the same voltage, unless something else is wrong.

Since he's checked the contacts and cables, it's most likely that one cell on his second battery is shorting or highly sulfated. That explains why one batt is taking 5x more current but is not giving anything back.

Keep them separate and run off batt 1 as the faulty one will just drag the better one down.


Two 6V batts in series make a 12V batt, not really a "bank". Batts or cells in series would make a bank, but that's just semantics.

The OP might even have a complete separate starter batt. One would hope so if the voltage is around 12.1-12.3 in the morning.
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