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Battery Expectations.

LDVV

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Feb 26, 2022
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I'm messing around with a first set up, and...

A slightly cloudy, but sunny day, and my two panels are making ~500 watts).

My batteries are at 100%, at least according to my Inverter. There are four 12V 120Ah AGM, arranged as 24V.

I try to use a 1,200watt grill.

Within minutes (<5), my battery level warning comes on. I watched as they were depleted down to ~25%.

Given my Battery Bank, I'd expect that I should have a capacity of just under 2.9Kw/h, so even without the panels, I should've been able to run the grill for an hour, and still be at ~50%.

Right?

What am I overlooking? Or am I expecting too much from the batteries (i'm confident that they're in good condition)?
 

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My batteries are at 100%, at least according to my Inverter.
Your battery pictured indicated that they should be 14.4V to 15V "cyclic".
What are the individual voltage of each of your 4 batteries?

Does your inverter recognize your battery chemistry? If it thinks you are using lead acid for example, a 13V battery will show as 100%.
 
You are running into the Puekert Effect with lead acid batteries. At high amps discharging, its significantly less power in the battery.

Try THIS calculator and change the puekert exponent from 1.1 to 1.5. If unknown, I kept the setting at 1.25.

EDIT: 5 minutes to 25% is a bit quick. Could be something else. If the Depth of Discharge is determined from a volt meter and not a shunt, voltage sag could account for the rest.
 
I try to use a 1,200watt grill.
If this is rated like a microwave for example, that watt rating is the cooking watts. The number of watts that it pulls can be significantly higher, i dunno how much.

Do you have a clamp meter or any way of measuring how many amps you are actually pulling from your batteries?
 
Your battery pictured indicated that they should be 14.4V to 15V "cyclic".
What are the individual voltage of each of your 4 batteries?

Does your inverter recognize your battery chemistry? If it thinks you are using lead acid for example, a 13V battery will show as 100%.
I charged each one before creating this little bank. If I remember right, they were all charged at 13.8v.

The Inverter does have an AGM setting (which has been selected).
 
You are running into the Puekert Effect with lead acid batteries. At high amps discharging, its significantly less power in the battery.

Try THIS calculator and change the puekert exponent from 1.1 to 1.5. If unknown, I kept the setting at 1.25.

EDIT: 5 minutes to 25% is a bit quick. Could be something else. If the Depth of Discharge is determined from a volt meter and not a shunt, voltage sag could account for the rest.
If I'm right, the grill is 6A.

I out that into the appropriate field in that calculator, and the results were way off what I experienced:

Down to 50% discharge, it would take ~12.7 hours.

(according to that calculator)

I was 4/5 depleted in a matter of minutes.

Maybe I do have a battery problem...
 

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If this is rated like a microwave for example, that watt rating is the cooking watts. The number of watts that it pulls can be significantly higher, i dunno how much.

Do you have a clamp meter or any way of measuring how many amps you are actually pulling from your batteries?
At the moment I have no way of getting an actual measurement, unfortunate.
 
It's probable the batteries are not fully charged. The AGM will need an absorbtion voltage of 14.7 volts and an absorbtion duration of around 3 hours. The voltage of 13.8 volts is the float voltage.

The 1200 watt load at 24 volts will take around 60 amps from the batteries, cables should be suitablefor this current.

Don't expect the full battery capacity at 60 amps.

Mike
 
If I'm right, the grill is 6A.
Yes 6Amp at 240V at 24V is 58Amp draw from your battery bank. also that battery states 15V charge for cyclic use and not the 13v you have considered full. lastly each battery is rated for 30amp max draw, so two in parallel only give you 60amp max discharge youre pulling 58A which is way close and probably why you see voltage sag and early cut off.
 
Take the AC amp draw (6amps) and multiply by 10 for a "seat of your pants" estimate of draw on a 12v battery. I have found this little advice to be very useful and easy to remember.
 
Take the AC amp draw (6amps) and multiply by 10 for a "seat of your pants" estimate of draw on a 12v battery. I have found this little advice to be very useful and easy to remember.
This works for 120 volts, but his grill is 240 volts, so should multiply by 20 to get 120 amps from a 12 volt system(the OP actually had a 24 volt system).

That kind of works with inverter inefficiencies, but with voltage sag to an inverter cutoff off 10 volts, it could be as high as 140 amps from the battery on a 12 volt system.

The OP is seeing voltage sag, just not sure how much. The OP also has a 24 volt system, so you do multiply by 10 for that. Also, with voltage sag and an inverter cutoff of 10 volts, the max draw would be around 70 amps.
 
Thanks for the previous messages.

So, if I add another two batteries into the bank, I could have a potential of 90 useable amps? Obviously I'm not actually going to approach those limits again after this experience. I'm just wanting a bit more capacity.

I topped up the Batteries (via the mains) to 100%, last night. I switched back to Batteries for the night, while the freezer was running (@ ~60 watt), and the batteries dropped to 87% within seconds.

I have ten of these batteries, so I was going to try fully charge six, then take out the existing four, and put in a freshly charged bank.

Problem is, my Charger (which I thought was quite an elaborate one), doesn't seem to be properly charging the batteries.

It does have an AGM setting, and a lengthy Absorption charge, but it only seems to take them up to ~13.8V, then reads CHARGED.
 
...also, I'll sometimes leave the garage at night, and the batteries are ~65%.

The freezer will be running intermittently through the night.

I get up in the morning, and the batteries are only down in the 50% range.

It seems as though they will deplete quickly (when near 100%), but then hold their ground a lot longer when lower.

I'm at a loss.

I haven't messed with any of there parameters for the Inverter. Maybe the default Voltage ranges are amplifying my perception of things.
 
My batteries are at 100%, at least according to my Inverter.
It seems as though they will deplete quickly (when near 100%), but then hold their ground a lot longer when lower.

I'm at a loss.
What inverter do you have? Getting battery SoC from an inverter is unusual because it only has voltage to go by and it probably does not know the battery type and the discharge curve (very flat for LiFePO4).
 
Ah, inverter/charger, that's different.

What battery settings are you using?
Right, *Inverter* vs *Inverter/Charger*.

I'll keep that in mind for explaining myself more clearly next time. ?

I have not changed any Battery parameters yet (apart from assign Battery type). Everything is as set at Default.
 
If I'm right, the grill is 6A.

I out that into the appropriate field in that calculator, and the results were way off what I experienced:

Down to 50% discharge, it would take ~12.7 hours.

(according to that calculator)

I was 4/5 depleted in a matter of minutes.

Maybe I do have a battery problem...
Label reads 50 HZ only are you in europe or US.
 
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