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Old 12-08-2016, 10:41   #1
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Battery @ 11.75v under sustained 8 amp load

My basic question is: is this ok, or do I have a problem?

I have recently installed three new deep cycle Group 31 batteries maintenance-free, each claiming 120 amp-hr capacity. Single bank, so 360 amp-hr capacity. This is matched with solar/wind charger system that is rated at 400 watts each (800 total).

In the evening I can start with a fully charged bank (according to my Victron meter), but after ~ 8 to 10 hrs I'm measuring 11.75 volts while under 8 amp load. Lighten the load to 1 amp and the voltage rises to 11.9v fairly quickly, and might get up to 12v after a few minutes.

I understand that I need to look at the batteries at rest (fully charged, then no load/charge for hours), but we're out cruising right now, and that is a PITA to do.

All the readings I can find talks about at rest voltage measures. I'm hoping the gurus here can tell me if I have a problem, or should just enjoy my cold beer.


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Old 12-08-2016, 11:35   #2
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Battery @ 11.75v under sustained 8 amp load

Before I proclaim them dead, make real sure all your terminal connections are bright and clean and tight, then ignore anything you have that tells you SOC, get on a shore power charger and charge them at absorption voltage until they are at very low amps, like it's not going down any more, off hand I'd say 1% of rated capacity, then see what you have.
My theory is your not charging them up but using capacity during the night and gradually walking the charge down, loosing a little each day.


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Old 12-08-2016, 11:42   #3
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Re: Battery @ 11.75v under sustained 8 amp load

Did you verify the voltage readings at the batteries with a good meter?
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Old 12-08-2016, 12:07   #4
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Re: Battery @ 11.75v under sustained 8 amp load

Cold beer is such a drain. I'v seen charts that show turning off the fridge at night and back on in the morning uses less power than leaving it on all night.
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Old 12-08-2016, 12:09   #5
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Re: Battery @ 11.75v under sustained 8 amp load

I will note that my battery's (440AH GC2's) after 15 hours of running (all night to next morning), I still read 12.3 to 12.4V for 2.5 year old batteries. That's with batterys under load. From experience, I know my battery's at rest for 15-20 minutes would recover another 0.15 V or so.

I suspect your 330ah set is a bit light for the fridge load. I use to have 12V batteries. But the golf cart batterys are just superior in every way.

When I had 12V batteries, they would be at 12.1V or 12.2V the next morning. 6V GC2's are the way to go. That was two group 29's after 8 hours.

Is your Victron meter calibrated for the new batteries???
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Old 12-08-2016, 12:25   #6
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Re: Battery @ 11.75v under sustained 8 amp load

Those voltages sound way too low to me, Mike.

The first things I would check are:
  • The voltage reading. Is this accurate?
  • Look for any bad connections.
  • Isolate each of the batteries and see if one is defective.
  • Is the charging indication correct? The best way is to look at battery return amps.
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Old 12-08-2016, 13:02   #7
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Re: Battery @ 11.75v under sustained 8 amp load

Mike, as another point of comparison my house bank is nominally 460 ah of four GC batteries that are 4 years old. No solar. In the morning after being at anchor I'm still reading 12.6 under load from the fridge. After three days and 2 nights with no charging I'm still reading 12.3-4. At this point my amp hour monitor reads about 75 percent. Then I start up the Honda eu2000 and recharge for a few hours.


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Old 12-08-2016, 13:36   #8
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Re: Battery @ 11.75v under sustained 8 amp load

My 460ah bank with 40ah out and about 4-5 amp load in morning normall reads in the 12.4 - 12.5 range. They have done this since new 5 years ago
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Old 12-08-2016, 14:46   #9
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Re: Battery @ 11.75v under sustained 8 amp load

Anything below 12v even with a load on it is a flat battery (below50% ). As someone mentioned probably been pulled down a bit day by day without being totally replaced


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Old 12-08-2016, 17:06   #10
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Re: Battery @ 11.75v under sustained 8 amp load

Thanks guys & gals. All really helpful. Here's some other facts which may help. We've been off shorepower for over a month. If it weren't for my voltage readings I would never suspect I had an issue. Everything keeps working, including all our LED lighting, which I know is voltage sensitive. Beer stays cold. We're even making ice.

Batteries are down 70ish amp-hrs by morning, and voltage shows below 12 under load. Over the day my charging system brings them us to full. Starts slowly, but once showing around 20 amp-hr deficit, seems to rise to full quickly.

Here's another part of the puzzle; my solar/wind controller consistently shows from 1/3rd to 1/2 volt higher than the victron or the measure. B/c of this the regulator has been cutting off at a voltage of 13.8, which seems too low for bulk charge levels. I've now upped the cutoff voltage to 15v on the controller, which seems to allow a bulk charge level of around 15.5v as measured at the victron.

Have checked voltage measures at the battery terminals and they correspond with Victron display. We reset the Victron for the new amp-hr battery measures, but I view it as an approximation of reality. Amp-hrs in and out (not % soc) and voltage measures are really what I use it for.


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Old 12-08-2016, 17:20   #11
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Re: Battery @ 11.75v under sustained 8 amp load

It's always good to check the voltage with a good VOM too. Could be a bad volt meter. I have found that the volt meters or the wiring to it go bad some times.
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Old 12-08-2016, 17:25   #12
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Re: Battery @ 11.75v under sustained 8 amp load

Sustained continous 8 amp load is extremely very extremely high.
What the hell are you running?

If its a beer fridge you can look at moi, as i like all to do... Turn the bloody thing off! Mine is on for 10 minutes every second hour (timer) untill 11am then its on for 30 mins/hour till 2pm then hour on, off till 6pm then back to 10 mins per second hour.

Beer is only cold when it needs to be
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Old 12-08-2016, 17:54   #13
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Re: Battery @ 11.75v under sustained 8 amp load

Quote:
Originally Posted by MarkJ View Post
Sustained continous 8 amp load is extremely very extremely high.
What the hell are you running?

If its a beer fridge you can look at moi, as i like all to do... Turn the bloody thing off! Mine is on for 10 minutes every second hour (timer) untill 11am then its on for 30 mins/hour till 2pm then hour on, off till 6pm then back to 10 mins per second hour.

Beer is only cold when it needs to be
So does this mean you have drunk all your beer by 6pm?
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Old 12-08-2016, 18:01   #14
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Re: Battery @ 11.75v under sustained 8 amp load

Mike a 11.7 volt battery is basically dead. I recognize you have an 8 amp load but if its continuous then is too high for your battery bank. If you run 8 amps continuous that is close to 200 amp hours per day and you are going to have to have a very good charging system to keep replenishing the batteries. Keep in mind you have to add closer to 220 amps back in to the batteries to make up for the 200 you took out.
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Old 12-08-2016, 18:08   #15
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Re: Battery @ 11.75v under sustained 8 amp load

I just read another note from you Mike and you say you use around 70 amp hours overnight. I'm not that far behind you and my battery voltage is around 12.6 volts in the morning.
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