scosha72 avatar image
scosha72 asked

Steady amps, sinking voltage

Something is not adding up. I run an 880Ah leisure battery bank on a barge through a new Victron Multiplus 3000/120, and 400w of solar through a Victron MPPT Controller 100/15. Putting amps back in using either engine or genny, when i reach a full state of charge i watch as the voltage drops from 12.60 down to 12.20v where the voltage stays but only if the inverter and every appliance it powers is switched off. With inverter on, the voltage can drop to 11.95v at 99% full. In short, newish batteries are discharging fast, according to the BMV700 monitor, without any significant load on them (other than a few 1.8w downlights) .

Something is wrong. Before i infer that the batteries are shot to bits after only 6 months, is there something i can do to maintain a healthier voltage, not that the batteries are working hard at all?


voltageinverter current draw
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2 Answers
JohnC avatar image
JohnC answered ·

Hi SS. More questions than answers from me I'm afraid. Not much adding up for me either..

1/ 880Ah @ 12V? Then a /15 mppt to charge them the full batt rating would take 58 hours. Of good sun. And that's just spec levels, in practice longer than that. With no loads.

2/ 12.6V isn't a charge V. What V's do you see under charge?

3/ 12.2V standing suggests maybe as little as 25% charged, and any load at all added after that will show serious V sag (like that 11.95V).

4/ The BMV showing 99% may mean it's just not set up correctly. Ignore it for now.

So far, seems a classic case of undercharging. Maybe add some more info so we can comment further.

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scosha72 avatar image
scosha72 answered ·

Hi JohnC.

Thanks for taking the time out both to read and reply to my message, and to attempt to solve my predicament.

1) Here's how the setup is: 8 x 110Ah 12v leisure batteries, designed for motor homes but in this case housed in the engine bay of a large boat. Don't know if they are in series or parallel, but they are wired up as one.

2) on the BMW700 monitor the charged voltage is set to a default reading of 13.2v. The fluctuating voltage reading is at this moment 12.38v and i supect it's jumped because the sun came out for a minute in an otherwise dismal run of weather here in Somerset. Inverter is off. No load at all.

3) 25% oh dear me! I run the genny or engine each day for an hour or two. When the monitor hits 100% i often ignore that, and continue to charge.

4) The monitor's present readings are: 0.27A, 4W, - 22.8Ah, 98.4%, 1NF hrs, 12.30v.

I have a feeling I've ruined the batteries by running the inverter/240v fridge almost throughout summer. Can you advise me on any life-saving rescucitation for the poor batteries? I read that i can equalise them by hitting the on/off switch on the Multiplus. Perhaps a couple of them have deposits on wet lead cells?

P. S. Havong just swotched on the inverter, the V reading has plummetted to 12.06.



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JohnC avatar image JohnC ♦ commented ·

Yeh Scott. You have issues.

1/ 12V batts wired 8x parallel isn't nice, even if 'perfectly' wired. Once they age a little, imbalance could kill them off, one by one. Longer term issue.

2/ The BMV settings have nothing whatsoever to do with your charging (maybe very minor if set to do so). By all means use it to read your V for now. Forget it's soc. And dang, 12.38V under charge is what flat batts do.

3/ Forget the % monitor (until you tune it), just watch V. You should see (say) 14.4V in Absorb for a couple hours, then a Float 13.8V for maybe a couple more.

4/ Believe the 12.3V, not the %. Your batts are nowhere near charged. You must fix this..

Resuscitation aside, just try to get them charged for now. Your genny/Multi should be able to wind up to 14.4V fairly smartly, but there may be issues there too if the Multi is rejecting the genny or even not set up right. Separate issue, but come back if this is happening.

*Watch your V*.

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scosha72 avatar image scosha72 JohnC ♦ commented ·

Hi JohnC, and thanks for the forensics. This issue is giving me mild anxiety disorder. 240v appliances are effectively off now.

I'll watch out for 14.4v on absorption for 2hrs and 13.8v on float for similar period. It's the steep and rapid drop without serious load on the batteries that worry me. Before condemning the batteries, I'll try charging each individual one on mains power overnight as soon as i can get to a proper national grid hook-up.

Being designed for these spacious, powerful American motorhomes, common sense tells me that the batteries are used to getting a good intake of mains when they're hooked up overnight in trailer parks, etc. My boat is totally off-the-grid and has been for a good six months now. The batteries might have been critically overworked as a consequence of this.


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