Battery bank against load and time

Hi All,

I’m having a little challenge with a quotation I’m having from my supplier over a battery bank and standby time they’re promising.
The load profile of these two installation average 4.5kW and 3kW respectively. And the battery banks 12V 250Ah x 35pcs(8750Ah) and 12V 250Ah x 26pcs(6500Ah) all for at least 10hrs with batteries DOD of 50%.
I raised raised doubts about how that would be possible and their response was that, I should consider the battery’s voltage of 12v and multiply it with the ampere-hour to get the total KWh of the bank which then becomes 105kwh and 78kWh respectively.

Will I be safe going by these method of estimating the bank please? I will not want to embarrass myself after the installation. Thanks.

King Alloy

1 Like

Looks like you are buying something prehistoric, 12 v battery bank. Look for lithium. 48v , 120v or better over 200v

I don’t understand all that but I do understand that if you calculate the cost of the batteries and what production you will get out of them, you won’t be investing you money well if all you want to do is reduce your grid costs. Batteries are still far to expensive to provide a pay back.

I’m more concerned about the standby time when there is no solar production?

My understanding about this is very simple but
Power (Watts) = Volts x Current (Amps) or Energy Storage = Volts x Ah, so they seem to have calculated this correctly to get 105 kWh and 72 kWh, however if your DOD is 50% then the available energy is only half of this (52.5 kWh and 39 kWh respectively).
If your load for the two sites are 4.5 kW and 3 kW continuously (on average) for 24 hours then to last a day you need 4.5 kW x 24 h = 108 kWh for one site and 3 kW x 24 h = 72 kWh for the other.
There will be losses into and out of your batteries, and this is only a back-of-the-envelope calculation, however it looks like your current solution will last about 1/2 a day, which seems to roughly agree with the 10 hours you mentioned.


1 Like

Thanks Jim. You gave me the answers I needed.

You’re welcome Alloy. Hopefully you can share with us if it works out as calculated.

I’ll certainly do that, Jim. Thanks.

Hi Alloy,
Just a warning - using multiple strings of 12V batteries is very bad practice. Almost all large capacity battery banks above 20kWh only use single cell (2V) batteries in series.
For example use 24 x 2V 1000Ah batteries to build a 48V 1000Ah battery bank = 48kWh total capacity.

The problem with multiple strings of 12V batteries in parallel is balancing the strings. Any slight difference in cable length, cell temperature or internal resistance will cause an imbalance and lead to over-charging some batteries and under-charging others. This will kill all batteries in only a few years.

I recommend you try to source single cell 2V batteries in 1000Ah or larger. The tubular Gel deep cycle batteries are the most reliable and have the longest life. Also for a system this size you should be using 48V battery inverters.

Hope this helps.

Hi Alloy,
I recommend you purchase Nickel Iron (NiFe) battery bank, as they cost approx. US$370kWh.
I would suggest you stay away from cave-man LAB and invest in NiFe battery Bank, as they commonly last 7~12+ years before you may only need to replace the electrolyte, for a like new battery again.

Replacing the electrolyte commonly is only be 5% of the battery cost.
NiFe Batteries are the most proven energy storage technology in the last 100 years !

People saying Battery technology is to expensive, are Obviously excessively marking-up the cost or are purchasing through a long chain of agents.

If you really want a battery that will out-live you, I would only look at NiFe or Super Capacitor (SC) Energy storage modules.
NB: Quality Dry-cell SC modules are US$8’000 for a 15kWh module, approx US$535kWh Wholesale.

We ship world wide if you are interested.

Hi P2P,

Glad to hear your view on this though I would rather you chose your description of others who hold other opinions more respectfully.

Re:Energy storage modules.

NB: Quality Dry-cell SC modules are US$8’000 for a 15kWh module, approx US$335kWh Wholesale.
Could you please elaborate on the last figure US$335kWh?

1 Like

Sorry, Typo, $8000 divided by 15 =$535

hello sir , please send me mail for getting load calculation workbook or offgrid solar system design excel sheet.