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Old 05-24-2016, 03:10 PM   #1
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Battery Bank Question

My house bank of 4 8d AGMs will charge up to 100% SOC on a Balmar Smart Gauge and when charge is removed will stay at or above 13 volts for several hours with a 10 amp draw. After about 10-11 hours with a comparable draw it will be down to 92% with a voltage under load of 12.5 volts. The math says that is about right for 1000A house bank. However in the next 2 hours it will plummet to 60% SOC with the same load and voltage drop below 12 to around 11.6 and by then I start the main or generator.

Any ideas? I have isolated loads to see if some device has gone south and don't see any. I am suspecting a battery is acting up and plan to isolate each side to see if I can see which one it is. This morning I isolated all batteries and measured 12.4 volts on the starboard bank of 2 and 12.5 volts on the port bank. Plan is today to charge both banks to 100% and then run on only one bank to see what happens.

Tom
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Old 05-24-2016, 03:58 PM   #2
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What type of batteries are in the banks? How old are they? What type of charger have you been using? A modern 3- or 4-stage charger?

Sounds like they're getting weak...maybe sulfated. Does your charger include a periodic equalization stage?
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Old 05-24-2016, 04:15 PM   #3
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What type of batteries are in the banks?
4x8D AGMs. We all love ya', but try and pay attention, Al.
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Old 05-24-2016, 04:58 PM   #4
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4x8D AGMs. We all love ya', but try and pay attention, Al.
How did I miss that one?
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Old 05-24-2016, 05:03 PM   #5
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How did I miss that one?
Too many times at high altitude w/o sufficient oxygen?
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Old 05-24-2016, 05:29 PM   #6
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The ONLY way to accurately measure SOC is with a specific gravity meter and since you can't do that with AGMs you really don't know the SOC. Voltage measurements are highly misleading.


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Old 05-24-2016, 05:34 PM   #7
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How about load testing?
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Old 05-24-2016, 06:45 PM   #8
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Load testing with each bank isolated is my next step. At anchor tonight I will take the starboard bank of two off line and see how the port bank fares. Over time, various owners installed three different ways for the batteries to be in parallel making the battery switch irrelevant. I've eliminated one. It will take some work to eliminate the other two.

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Old 05-24-2016, 06:52 PM   #9
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The ONLY way to accurately measure SOC is with a specific gravity meter and since you can't do that with AGMs you really don't know the SOC. Voltage measurements are highly misleading.


David
Not exactly true with the modern battery monitors, which can be set to battery type as well as starting capacity. As a side note my Magnum monitor charging my inverter bank lined up with the hydrometer (SG) reading almost perfectly, over years of bank wear and tear, user error using the SG device not withstanding.
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Old 05-24-2016, 08:11 PM   #10
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On an earlier boat I had, a 33' wood Monk cruiser, there was a battery meter monitor installed that with the push of a button I could see how each batter was doing. Now in that boat I only had a Group 27 for starting and an 8D for house but it seemed to do pretty well and was easy to keep an eye on.
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Old 05-25-2016, 04:33 PM   #11
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You say you have two banks. Is there a combiner in the circuit?
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Old 05-27-2016, 01:31 AM   #12
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Both banks are paralleled three ways, which is possibly part of the problem. I intend to fix that tomorrow. The only way to shut off the batteries is removing cables. I am pretty sure the batteries are on their last legs. One bank will give me about 11 hours before it falls off the cliff. The other will give me about 5 hours. Falling off the cliff means it gets to about 12.7 volts under a 15 amp load and then starts rapidly dropping in voltage with the same load.

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Old 05-27-2016, 05:19 AM   #13
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Have you calibrated the unit recently? Not familiar with the Balmar unit, but my Victron unit needs to be periodically calibrated. Simply, with the battery at 100%, you are telling the meter to read 100%. Your meter can only read a maximum of 100%, but it may think the battery is at 125% and not show a decline in the meter reading for quite a while.

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Old 05-27-2016, 07:49 AM   #14
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I think you have a problem w your wiring. Possible dueling combiners/isolators. Unlikely that the bank under load stays at 13v for any length of time after a charge. My guess is you are reading from a battery that is not under load. Suggest you return to that SOC and put a meter on each battery individually. Do they all read13 v under load? For hours?
Monitor individual batteries over time as the SOC drops. Are they all still at the same voltage as the Balmar says thy are, even below 12.5? And for safety's sake, you need to be able to switch the batteries off.
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Old 05-27-2016, 12:00 PM   #15
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Interesting subject as I am learning battery management with my house banks. 2 banks with 4 golf cart batteries each! I think I don't have a big enough chargers.
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Old 05-27-2016, 01:39 PM   #16
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Each bank has a sweet spot of just the right amount of charge. Too much and the battery cannot accept it or, as in my case, the small generator cannot handle the load. Too small and the battery takes too long to recharge and lives much of its life in a partial state of charge (PSOC). If gen load is not an issue, better to err on the oversized than the undersized charger current.
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Old 06-04-2016, 11:41 PM   #17
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How are you doing. I have 8D GELS, not ATM. Had a similar problem. Load testing did not highlight what was going on. I will admit I do not have a high class tester but even so all 3 held up fine.

Finally figured out which battery was the problem one using my clamp on ammeter.

Loaded all three and watch the current balance. All started the same and held like that for a while. Almost said phooey but realized one battery current had dropped and the other two had risen. As I continued to watch the one dropping battery actually reversed polarity
thus becoming a load to the other two.

I double checked my mete, that I hadn't physically reversed it, but realized one battery has reversed.
Disconnected the bad one and removed it. Got almost two more years out of the remaining pair. Just had to be more careful about recharging a V levels

Clean up what you have and then pay attention to the wiring.

Errors I made were to use different lead lengths and not pay enough attn. To the stacking of connections. I knew better but ignored some details. I did pretty well as the bad one was 11 yrs and the others were 13 yrs when I installed three new spring 2015.
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Old 06-05-2016, 09:57 PM   #18
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I think 80% of the problem was the three parallel connections. Things seem to be working they way they should based on measured loads and remaining capacity over time. The Balmar Smart Gauge seems to the most accurate measurement compared to the Link 2000. I do need to get the length of cables between each bank and the battery switch comparable in length. The port bank is probably 12 feet and the starboard bank is maybe 5.

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Old 06-05-2016, 10:11 PM   #19
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Just had 4 new AGM's installed and new Blue Sea isolators. One of the original AGM's was split wide open. They were 6 years old. I remember recently smelling rotten eggs for a couple days on the boat.
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Old 06-06-2016, 12:23 AM   #20
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The Balmar Smart Gauge seems to the most accurate measurement compared to the Link 2000.

Tom
The Balmar unit provides a very accurate indication of SOC, but no current or cumulative AH readings. To do so would complicate their 'simple installation' claim. IMO, the best combination is Balmar's SOC and a Victron or Xantrex AH data. I wish one gauge could combine both.
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