Battery backup runtime calculation

[Eon]

At the end of 2020 I bought a 3000W 24V Growatt inverter and paired it with 2X 200Ah Allgrand batteries just to use as a load shedding cure. According to the initial calculations, I would've been able to get +- 9 hours out of this system while using about 200 - 250W. The formula according to the inverter salesperson was as follows:

1) Take the Ah of the batteries and multiply it by the system voltage to get the total Watt hour, so 200Ah X 24V = 4800W/hour.

2) Take the total W/hour and multiply it by the desired battery Dod percentage, so 4800 X 50% = 2400 usable W/hour.

3) Then devide the usable W/hour by the load to het the backup time. 2400/250W = 9.6 hours. The 250W is made up of my fridge, TV, router and phone charger.

Is this calculation correct?

In my tests I've only been getting about an hour and a half before the low voltage cut off (24V) is reached. So either my inverter/battery settings are way off, my batteries are damaged, or I've been fed a fart by the salesperson. 

Any help would be great. 

 

Thanks.

Edited March 6, 2021 by Eon

[Richard Mackay]

The 250W sounds way too low. Also there is a current that the inverter uses just to keep it running.

Get hold of a multimeter and measure the battery current without a load and then with a load on the inverter

[Lynne B]

I seem to be having a similar challenge. I’m running a 5Kva inverter hooked up to 4 x 100Ah batteries. We are currently being old shed twice a day for 2,5hrs. The inverter indicates that the batteries are running down by half and inverter has started switching off. Do I need to add another battery bank or are the batteries not getting enough time to charge up ?

[Dougiedanger]

Hi, what sort of load are you drawing on the inverter, ie how many Watts? 

Do you have panels attached to this setup? 

How old are the batteries? 

Unfortunately in my limited experience it is usually a sign of the batteries being on their way out. 

[GTP]

Out of curiosity, are the batteries using BMS? Or set per voltages?

The calculations seem ok. I use the same calc and my battery performs pretty darned close to the calcs. 

I did find on a previous non BMS battery that I got some wild changes in voltages during chatge and discharge cycles. If the settings are per voltage, could it be a settings  issue?

Am I understanding that the cutoff (low) on the inverter is set to 24v? Isnt that high for 24v batteries?

Edited June 7, 2021 by GTP
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