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Old 05-02-2021, 08:50 AM   #1
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House Battery Drains in 3 Hours

My apologies for the length of the post, but I wanted to include as much relevant details as possible.

I have a 2018 MADP which is now 3.5 years old. It has 8 Harris 200 Ah AGM 6V batteries (800AH at 12V). The coach is currently parked in my driveway and is typically plugged in. I took delivery of the coach at the factory just after it was built and have never let the batteries die, although there were times when boondocking that the voltage got down to the low-mid 11V range by morning.

The other day after unplugging the coach, the house battery voltage dropped from 13.5V to around 12.9V after about 30 minutes. This is typically as the battery voltage is stabilizing after the charging has stopped. The voltage stayed there for about three hours and then the voltage dropped to around 10.8V within 30 minutes.

Iím trying to figure out whether this is an indication that the batteries need to be replaced or if I have some other issue. My understanding is that AGM batteries typically last around 6 years, but is also affected by the number of charge cycles. I havenít found any indication of something draining the battery.

I conducted a test with the inverter breaker box MAIN breaker turned off and a separate test with ďsalesmanĒ switch off which shuts off most but not all of the 12V power. In both case after about 3 hours the battery voltage took a dive.

Iíve attached an annotated graph showing the battery voltage and amps drawn over time. You can see that once the voltage started to drop, it took about 30 minutes to drop from 12.8 to 10.8. 
Also of note is that when I plugged the coach back in to charge the batteries, it only charged the batteries for about 2 hours before switching to float. At first the batteries pulled up to 80 amps of power and as the batteries got full, the amps draw reduced. The two hour period seems too short and shorter than normal.

I have purchased a BMV-712 but I have not yet installed it as I still need to purchase two additional battery cables. This just moved up to the top of my list. This would obviously tell me if something starting consuming lots of power at that 3 hours mark.

There are three cables connected to the positive side of the battery bank. One goes to the charge bridge. Using a clamp meter, it measuring 0 amps as expected. The inverter cable was pulling 3.5 amps which is consistent with the Xantrax data sheet for internal power consumption. The other cable feeds the house disconnect. It was measured at around 5 ADC. Obviously this is only a reading at one point in time.

Itís usually pretty obvious when a starting battery needs to be replaced, but I donít have any experience with determining when house batteries are shot. Please let me know what your thoughts are. Thanks.

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Old 05-02-2021, 09:19 AM   #2
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I can only offer a guess.


What I keyed on in your post was the time the charger took to move to "float." With batteries drawn down to 10.8v, that seems way too short a time for a recharge.


I would suspect one or more bad batteries which may be drawing down the others over the three hour period you mention. AGM batteries are more stable and hardier, but they can still go bad. I would have each battery load tested before moving on.
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Old 05-02-2021, 09:25 AM   #3
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I think I would fully charge the batteries and then disconnect them from the coach.

If they drop the voltage in the same way again you probably have a bad battery.

If they remain charged then you need to start troubleshooting.
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Old 05-02-2021, 09:40 AM   #4
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House Battery Drains in 3 Hours

Hello Flyboy013,
You are obviously employing the right tools and you know the right questions. Iíll contribute some thoughts that might trigger some ideas for you. IMHO, your overnight, with that much battery capacity, should not have dropped that low. (Running just a fridge through the inverter?) I am wondering if there is a bad connection in the parallel or series cabling. Do you have a clamp on DC ammeter? They are so useful for diagnosing just these kinds of situations. If one bank in the parallel lineup is bad, your whole 800 ah bank will act like, not only a 600 ah bank, but a 600 with a heater connected. A quick and valuable check with just a digital voltmeter, would be to put a good load on the bank and go down the line checking the voltage on each positive battery post.
Also check to see if you see any significant voltage between the first battery negative post and every other one. Do the same with the positive posts. Place the probe directly on a post and the adjacent batteryís post. This will test the quality of the series or parallel strapping cable connect. Hope this makes some sense.
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Old 05-02-2021, 09:40 AM   #5
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Great details, I would suggest that the batteries are bad, however that suggestion would be based on a couple of thoughts, first is to be sure that there is no other load on the batteries than indicated (for example something not going through the shunt), second would be that the batteries are reaching full charge. Your graph shows that it is only at 14.4 volts for a little more than an hour which I would have suspected it to be a longer period of time. Most multi-stage chargers hold ~14.4 at a constant voltage until the charge amps drop to a low value (~5 amps or so, depends on the charger) and some also start a timer regardless of the charge amps at 14.4v which will drop the 14.4v down to ~13.8 as in your graph.

If yours was mine, what I would likely do is photograph the battery wiring and then check the water in each cell and take a flashlight and take note if any cells look overly cloudy (in the water\acid) as that is a sign that the cell is bad, then refill each cell as needed. Then I would disconnect the batteries from each other and either test them individually at 6v, but if you don't have a 6v charger then I would keep them in pairs and charge them with another "smart" charger and see if they come back up, let them rest several hours after charge and check the voltages again. If during the charge a cell or two or more start bubbling way more than others, that is also a sign of a bad cell (battery). Once they are charged back up, and I am not sure how easy this is for you if you don't have a load tester, but that is what would be needed next to but a load on them and determine how many amps you can pull out of them before the voltage drops to an unusable level. I suppose you could simply connect them back into the RV circuit a pair at a time (may require jumper cables or similar to reach) and use the system you used earlier to measure the amps and voltage. I would also continue to use the dc clamp amp meter to verify that you don't have any current going around your shunt that is measuring the current.

As a last thought, if you carefully inspected each cell and found a few (or more) cells looking cloudy and those bubbled a lot during the charge, I would simply replace the batteries, further testing could find if it was just one or two batteries that are very bad, but I would be hesitant to replace only one or two if all of the batteries are at the same age and instead I would replace them all. ~Craig
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Old 05-02-2021, 09:47 AM   #6
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One more thought...
The fact that you recharge so fast and the batteries are AGMs which canít go dry, makes me even more suspicious that there is a missing or very bad strapping cable. Wouldnít that be great if youíve been living with only a 200 AH bank!
Best of luck to you and keep us posted.
Tom
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Old 05-02-2021, 09:48 AM   #7
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craigay...he has AGM batteries.... no water to check.
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Old 05-02-2021, 09:48 AM   #8
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Quote:
Originally Posted by craigav View Post
Great details, I would suggest that the batteries are bad, however that suggestion would be based on a couple of thoughts, first is to be sure that there is no other load on the batteries than indicated (for example something not going through the shunt), second would be that the batteries are reaching full charge. Your graph shows that it is only at 14.4 volts for a little more than an hour which I would have suspected it to be a longer period of time. Most multi-stage chargers hold ~14.4 at a constant voltage until the charge amps drop to a low value (~5 amps or so, depends on the charger) and some also start a timer regardless of the charge amps at 14.4v which will drop the 14.4v down to ~13.8 as in your graph.

If yours was mine, what I would likely do is photograph the battery wiring and then check the water in each cell and take a flashlight and take note if any cells look overly cloudy (in the water\acid) as that is a sign that the cell is bad, then refill each cell as needed. Then I would disconnect the batteries from each other and either test them individually at 6v, but if you don't have a 6v charger then I would keep them in pairs and charge them with another "smart" charger and see if they come back up, let them rest several hours after charge and check the voltages again. If during the charge a cell or two or more start bubbling way more than others, that is also a sign of a bad cell (battery). Once they are charged back up, and I am not sure how easy this is for you if you don't have a load tester, but that is what would be needed next to but a load on them and determine how many amps you can pull out of them before the voltage drops to an unusable level. I suppose you could simply connect them back into the RV circuit a pair at a time (may require jumper cables or similar to reach) and use the system you used earlier to measure the amps and voltage. I would also continue to use the dc clamp amp meter to verify that you don't have any current going around your shunt that is measuring the current.

As a last thought, if you carefully inspected each cell and found a few (or more) cells looking cloudy and those bubbled a lot during the charge, I would simply replace the batteries, further testing could find if it was just one or two batteries that are very bad, but I would be hesitant to replace only one or two if all of the batteries are at the same age and instead I would replace them all. ~Craig
No caps to look in or water to check on AGM batteries.

The rest of your testing seems sound.
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Old 05-02-2021, 09:51 AM   #9
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Check if anything is on, electric hot water heater will drain it fast.
Or Aqua hot on electric. Engine block heater connected to Aqua Hot
Otherwise I would do a battery load test
I bought one of these on Amazon works great
https://www.amazon.com/OTC-3181-Heav...%2C172&sr=8-22
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Old 05-02-2021, 09:53 AM   #10
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craigay...he has AGM batteries.... no water to check.
I should have caught that, Thanks, in lieu of my previous suggestion, I would use a temp gun (or my hand) and determine if any cells where much warmer than the rest (or hot) during the charge cycle (after at least 15~30 minutes of charging) as they should all be at a similar temp. If you find some of the cells much warmer (hot) then that would also tell you that the battery has a bad cell (or internal short). ~Craig
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Old 05-02-2021, 10:24 AM   #11
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Thanks. Great ideas.

So, given that the batteries have been charging overnight, they should now be fully charged. The Xantrex inverter/charger is reporting via Silverleaf a constant 5ADC output to the batteries. Iím guessing that most (if not all) of that is to take care of the draw from the various Silverleaf and KIB modules that are using 12V power.

I checked the power draw using a clamp meter on the three cables connected to the batteries (inverter, house disconnect, charge bridge), but I did not check the connections between the batteries. If I understand correctly, I could have minimal power draw to the coach but lots of power between the batteries if there is a bad battery. I will check this and also check the battery temps.

So it sounds like the next test is to disconnected the batteries from the coach and to disconnect the 4 parallel string from each other. I assume I can just disconnect all the negative cables from the coach to the batteries and that as long as they are all disconnected there is no need to also disconnect the positive cables?

Also, to disconnect the 12 pairs from each other, can I just disconnect the negative jumpers between each set? If there is no need to pull the positive cables, it would make the job a little bit easier.

Once the four parallel sets have been disconnect from one another, I should check the voltage of each 12V pairs and continue to check them every so often (say 30 minutes). I should be looking for a voltage drop in any one (or more) pair. If I find one, I can then test the two batteries independently to see if one of the two 6V batteries is bad.

Prior to disconnecting any cables, I will check the voltage of each battery / pair, but since the batteries are connected to each other and the charger, I would expect the voltage to read whatever voltage the charger is putting out?
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Old 05-02-2021, 10:34 AM   #12
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First of all...TAKE PICTURES!

I would remove only the negative cable and then see what the voltages do.

No need to disconnect all the cables just yet, wait and see what the voltage does while that one cable is disconnected.

I'm trying to isolate the problem to either the batteries have a problem or the house load is drawing them down.
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Old 05-02-2021, 10:44 AM   #13
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Yes be sure to take pictures, in fact you could share a pic here if you would like to do so. What you are describing sounds correct, as long as you keep the batteries in 12v pairs. The (a) thought I have is that you could have another current draw outside of the inverter\converter, in other words I suspect your 12v lighting may not be going through the Xantrex inverter\converter (although the lighting wouldn't be a lot of current, perhaps something else is though). If there is a single point (or cable) that all of the battery banks current goes through, then that is where to clamp the dc amp meter to and see if how much current is being measured coming in or out of the entire bank of batteries. Isolating the bank into 12v pairs is simply to determine which (if any) batteries are bad. (which is what I suspect if they have been good for years and you made no changes to the electrical system and now they don't hold a charge for very long).
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Old 05-02-2021, 10:47 AM   #14
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I would say, leave them in pairs, and separate them from the others. If one is bad, you'll quickly narrow it down to a pair and then a single.
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