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Forum Index : Other Stuff : 48 v battery bank discharge

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windman1000
Regular Member

Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 01:49pm 25 Apr 2020
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hi guys /gals it been a long time since iv been here and i think i have a problem with my 48 v battery bank , i have 8 AGM maint free 225AH 6 Volt VMAX batteries and have had them for 2 years now i charge them to 58.4v and discharge to 48v.
this is kinda like a power wall, after the sun goes down and my main solar system would shut down i would start the power wall, at that time i didn't have a monitoring system on the batteries and it seem like i was getting approx 3 hours at 1000 watts.
now i have a monitoring system for it and i notice that the batteries would drop from 58.4v to 48v and shut down after an hour.
so i decided to give the batteries a break and just charge them and float them at 58.4v  for a couple of days and watch my monitoring system.
now i don't know what the discharge of the battery bank was when new but now i think there is a problem,
i charge/float to 58.4 and in the morning the battery bank is at 51.6v with no usage the night before.

58.4 to 51.6v in 12 hours not using them just sitting is that ok or is my bank used up??
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 3261
Posted: 11:49pm 25 Apr 2020
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When you take batteries off charge, the voltage will fall away significantly, even with zero external load.
It will then gradually settle at some resting voltage and not fall below that unless there is some kind of internal self discharging leakage going on.

A drop to 51.6v overnight from full charge is not serious, a drop to overnight 50.0v with zero load would be borderline, and anything less than 50.0v a problem.
Its more a judgement thing, these numbers are not written in stone. Battery temperature comes into it as well.
Cheers, Tony.
 
windman1000
Regular Member

Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 03:18am 26 Apr 2020
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thanks as soon as i get a chance il measure each battery, maybe 1 is in bad shape
 
Davo99
Guru

Joined: 03/06/2019
Location: Australia
Posts: 493
Posted: 05:17am 26 Apr 2020
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  windman1000 said  

i have 8 AGM maint free 225AH 6 Volt VMAX batteriesi charge them to 58.4v and discharge to 48v.
it seem like i was getting approx 3 hours at 1000 watts.


I have no experience with off grid systems and my mathematics is poor.

Seems to me though you have around 8Kwh total capacity and about 4kw at 50% DOD.
As you say you are/were getting 3 hours @ 1000W, IE 3 KHW, that doesen't seem that far away to me.

What is your inverter set at for shut down? If it's 48V, then I could well see a 1000w Load may pull the pack down to that after 3 hours.
You could also have a bad connection, under gauge wire etc. If the batteries are they type that need to be topped up, has that been taken care of?

You need to methodically go through and check things first before getting too concerned and assure yourself there are no obvious faults that have nothing to do with the batteries.
 
johnmc
Senior Member

Joined: 21/01/2011
Location: Australia
Posts: 197
Posted: 05:51am 26 Apr 2020
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Good Day all

I have used AGM batteries I have found that they are limited by the
duty cycle.

For example Depth Of Discharge  100% DOD about 300 cycles.
                                50% DOD about 600 cycles.
When I used AGM batteries 24/7, about  2 Years useful life  was all I expected.

I feel AGM batteries are meant for stand by systems, or mobile systems where flooded
lead acid, FLA batteries, causes problems .

cheers john
johnmc
 
renewableMark

Guru

Joined: 09/12/2017
Location: Australia
Posts: 1564
Posted: 05:57am 26 Apr 2020
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I didn't look up your batteries but I doubt the specs would say to float at 58v.

Sounds like your batteries are getting a little tired.

If/when you replace them look into lipo batteries or lead batteries that you can maintain (top up electrolyte).

Lead batteries like an equalise charge but it's not really a great idea on a sealed maintenance free battery.

If you are going to toss them, then you may as well try it though.

It's a fairly small battery bank, so you can't expect a huge amount from it.

After a full charge the at rest voltage of 51.6 is fine, but it's draw capacity may have diminished over time.

Sounds like they are a bit sulphated.

What charger are you using?
Cheers Caveman Mark
Off grid eastern Melb
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 3261
Posted: 06:39am 26 Apr 2020
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  Quote  
You need to methodically go through and check things first before getting too concerned and assure yourself there are no obvious faults that have nothing to do with the batteries.

Very sound advice
Edited 2020-04-26 16:40 by Warpspeed
Cheers, Tony.
 
windman1000
Regular Member

Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 12:25pm 26 Apr 2020
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thanks for the advice by all

i have a 10,000 watt On Grid system and i wasn't thinking or planning on a power wall system at the time i was installing the on grid system so the on grid inverter is not the right kinda inverter for a power wall.
but after the 10,000 watt install i had 2000 watts of panels left over from my old solar sun tracker so that's when i had this brain fart to build a small power wall system.
so i bought 2 1000W MPPT Solar Grid Tie Inverter With Power Limiter(CT) and a Morningstar TS-60 TriStar-60 amp 48 volt Solar Charge Controller and the batteries.
i ask VMAX what was the best charge and float for these batteries and they said 58.4 and 48 max discharge and i would get (???) cycles and that the batteries should last approx 5 years.
the 1000W MPPT Solar Grid Tie Inverter With Power Limiter are programmable so i program both of them for 500 watts output max so i could get longer life out of these china inverters. ( NOTE i have 220 split phase so i needed 2 110 volt inverters)
after all installed i didn't like the way these inverters were always on sitting at idle until power was needed by its Power Limiter ( CT clamp on current transformer).
so i took a arduino mega 2560 and some SSR 100amp relays and 2 CT's and made a power monitor to turn power off the inverters until needed.
yes that is what the inverter does all by them self but i wanted all power off so when sitting at idle on hot days the fans wouldn't run.
well i had another brain fart after 2 years of this system up and running.
i rebuilt my arduino mega 2560 and added in 8 CT's and 6 SSR 100amp relays and a battery bank voltage monitor, that's when i noticed maybe battery bank problems.
im going to redo my arduino mega 2560 to monitor all 8 batteries separate and if the battery bank is on its last leg im thinking on buying 48v Nissan Leaf Lithium ion Mega Power Pack Battery 28 kwh 528 ah Storage G1

and yes i know this all is a lot of watts for 1 house and during the day im producing way over what i need unless i run both ac units.

but i really use a lot of power at night , i have two young adults MY KIDS still living here and they both are game programmers so they stay up all night when there is no solar power and sleep all day when there is all that extra power to be used.

plus i just like tinkering with all this stuff.
 
renewableMark

Guru

Joined: 09/12/2017
Location: Australia
Posts: 1564
Posted: 04:38am 27 Apr 2020
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Chargers normally have a bulk charge (57.6 in my case), then an absorbtion charge that keeps them at that voltage for a specified time (2 hours in my case), after that they go to float voltage (54.00 in my case)

Am I reading correctly that you set it to 58v float?
Cheers Caveman Mark
Off grid eastern Melb
 
Bazz55
Newbie

Joined: 26/04/2020
Location: Australia
Posts: 1
Posted: 10:30am 27 Apr 2020
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Without looking at the specs for the batteries, I suggest 58v too high for float. I think you have cooked a couple. Check each battery after taking off charge and allowing voltage to settle. Each battery should be very close to the same voltage. If not, there you are.
 
windman1000
Regular Member

Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 12:48pm 27 Apr 2020
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lm sorry yes the bulk charge is 58.4 and float is 54.0 and stupid me made a maintenance  free battery box so i can get to 4 batteries no problem but the other 4 i need to take the battery box apart, the 4 i can get to have 6.5 volts each so i took 6.5 X 8 = 52 and my monitor said 51.6 so i figure the other 4 batteries were approx  6.5v.
if i remember right they were 6.5v each new but lasted longer under load.
i should have my newer version of the arduino mega 2560 monitoring system built by this next week and il see what each battery is doing during bulk and float and under load, just wish i had this in the beginning then i could really see what is going on with the batteries.
 
windman1000
Regular Member

Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 02:41pm 16 May 2020
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ok looks like all you guys are right and im really really stupid.
after making newest arduino monitoring system , i would watch the voltage on all 8 batteries and the final voltage of 51.xx volts drop to 48 volts and turn off inverters
very quick,10 minutes.

ok here is where i was really stupid, yes i had it set to turn off at 48 volts to save battery life.
and it look to me that the bat bank was all used up so i changed my min voltage to 46 volts just to get a little more time out of the bat bank before i scrap them, that is when every thing got better, now the voltage drops from 51.xx to 48 very quick but it goes back up to 49.xx,50.xx,47.xx,49.xx,50.xx , in approx 2 hours i never see bat bank drop below 47.xx and most of the time it is 48.xx,49.xx,50.xx.

and the inverts shut off after the 2 hour time limit in the program not from low voltage and the bat voltage would regenerate it self back to high 50.xx to 51.xx with in 5-10 minutes after shut down of 2 hours running.

so i look threw my old programs of the very first arduino monitoring  that only monitor the whole battery bank and it was set at 47 volts turn off NOT 48 volts and i bet the it wasn't shutting down from low voltage at the beginning but from the time limit i had set back then.

bottom line here is it sucks getting old and not remembering anything.
 
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