# Battery bank requirements

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1. Jul 21, 2016

### micuklein

Hello,

I have a bit of an issue and can't figure out how to find out the answer. I want to understand how to find out what battery I would need if I want to scrap the diesel generator I have and go for a battery bank.

I have a 1250 kVA rated generator. PF is 0.85, Rated voltage is 400V, rated current 1804 A and rated freq is 50Hz.

If I want to replace this generator with a battery bank, how do I go about trying to size the battery bank needed? It is assumed that the battery bank would have to provide power for 4 days, discharge to 85% max and only be used tops once a year.

Thank you!

Looking forward to hearing from you!

Kind regards,

mk.

2. Jul 21, 2016

### Staff: Mentor

3. Jul 21, 2016

### gleem

To deliver that power for 96 hours would require thousands of batteries. Assuming lead acid batteries because they are cheaper you need to limit the amount of current from each battery to about 2 amps so that they last 96 hours. So each battery will need to produce 24 watts of power. Your power requirement is 720,000 W. So on this info you would need 30,000 deep cycle 12V batteries (e.g. Surrette 12-CS-11PS ) with quantity discount ($800 ea) you might be talking$24 M(US). FYI the total wgt of the batteries is about 4000 tons (272lbs x 30000)

This does not include the space to put them about 81,000 cu-ft (10x90x90) The cable to connect them (over 15,000 ft), the DC to AC converter(s), the chargers since the batteries may loose up to 10% of their capacity per month plus possible other equipment and expenses.

Battery technology just isn't up to snuff yet at a reasonable price. To produce the amount of energy from a mechanical generator talks only about 70 gal of fuel. Stick with the generator. If you needed to provide backup power for an hour then batteries would be doable but still expensive.

4. Jul 21, 2016

### micuklein

Thanks a lot guys for the answer.
Let assume money and footprint are no issue.

How can I get a number of how big of a battery I need from the generator figures?
Anorlunda.....you mentioned a battery bank of 100MWh, how did you get that number?

Thanks a lot!

Kind regards,

mk.

5. Jul 21, 2016

### OmCheeto

I'm guessing, maths?

400V * 1804 A * 4 days * 24hrs/day = 69 million watt hours

6. Jul 21, 2016

### gleem

the energy in watt*hours is gotten by multiplying the output voltage by the current by the hours you needed to provide the power. Now the voltage and current that you gave was not specified as peak or average since you have an AC generator.. This will produce a difference in the calculation of power by a factor of two. Poweraverage = 0.5*peak voltage *peak current. Average power is what you need to consider. I assumed it was average and I got 69 M-watt-Hrs.of energy Each battery I used for the calculation can store about 2000 W-hrs of energy at a nominal voltage of 12.3 V which will reduce the full charge voltage from about 12.7V to 11.9V. after 4 days using a current 1.9 amps each. So we get about 30,000 batteries. The energy that you are producing (720,000 W-hrs) will power a house like mine for a about two weeks.

7. Jul 22, 2016

### Staff: Mentor

$1250 KVA * 0.85 PF = 1062 KW \approx 1 MW"$
$4 days = 96 hours \approx 100 hours$
$1 MW * 100 hours = 100 MWH$

But listen, for something like this, you need to hire a consulting engineer rather than asking questions on an Internet forum. ( speaking as one who spent most of his life as a consulting engineer ). There is much more than price and footprint to consider. For example protection. Suppose the whole thing experienced a short circuit and exploded in a ball of fire the first day.

Then tell the consultant to work with one of the big four suppliers (ABB, Siemens, GE, Westinghouse) who can design, deliver, install, and test a turnkey solution). Most important, they can warrant the adequacy and suitability of the solution and their pockets are deep enough to do that without involving insurance companies.

Be prepared for some push back and arguments that you should stick with a diesel generator. Your reasons for wanting to switch may be perfectly valid, but they are unusual.

8. Jul 24, 2016

### gleem

I will second that, this forum can give you an overview of possibilities. But based on the cost of a commercial 1500 Kw diesel generator about 310,000 $(US) I still would recommend the diesel genset over a battery bank. Gensets are reliable and I think are used in all large scale mission critical backup systems. You could reduce the number or batteries to 13,000 by by draining the batteries to 80% of their capacity over the 96 Hours of use which might be OK if you do this once a year and not decrease their life too much. But still your talking about$10M for the batteries. And these batteries require maintenance periodically keeping them charged, checking water levels and specific gravity. Maintenance free batteries do not require watering and specific gravity checks but still must be charged periodically but are more expensive.

Good Luck.

9. Jul 26, 2016

### micuklein

Thanks a lot guys!