Battery Charge Rates

[tt90lrb]

I'm trying to calculate the charging time required for a UPS battery bank.

I have a 260 Amp/hr battery bank (120VDC - sealed lead acid type) connected to a 6 ohm, 2.5KW resistor bank for 2 hours. This test is used to simulate the maximum foreseeable load and to chart voltage and current of each cell.

At the end of the test, the battery bank is connected to a 5 amp charger.

Is there a calculation for determining the time required to recharge the battery bank?

 

[IRstuff]

To the first order, you've used 20*2=40 amp hrs and you're replacing it at 5 amp hrs, so you'll need at least 8 hrs to recharge. TTFN

 

[tt90lrb]

Which is what I would have expected. The actual re-charge took over 15 hours. Somehow I'm off by a factor of roughly two.

 

[Crowbar1]

DId you measure the charging current? IF you are only drawing 2.5 amps then you are not replacing at 5 am hrs. That could be why it takes twice as long to charge.

 

[tt90lrb]

No, I haven't. Took the info out of the user's manual and didn't confirm with a meter. The configuration of the batteries is 2 - 10 battery strings. That would divide the charging current by half, but the output would also be half from each string.

I'll take a messurement next time to verify. Any other ideas would be greatly appreciated.

 

[IRstuff]

Is the charger designed for this specific configuration?

Is it designed for constant current output, so you'd expect the output voltage to increase as the batteries got charged? TTFN

 

[tt90lrb]

Follow up. Problem with our test procedure rather than our math. Compairing data from a full battery discharge with objectives based on normal load.

Found a good info source in the Exide website. An equation listed for lead acid type batteries:

Charge Amp = ((Battery Amp x Battery Hrs x 1.10 / Specified Charge Hrs) + L) x 1/K1 x 1/K2

Where:
L = other load on batteries during charging
K1 = derate for temp (1 to 122F / .8 to 131f / .6 to 140F)
K2 = derate for altitude (1 to 3300ft / .8 to 5000ft / .6 to 10000ft)
1.10 factor for charger conversion lead acid batteries.

This should work for most lead acid batteries regardless of manufacturer. This should get you to the 90% charge range - which, to my understanding, is an industry standard. The last 10% can take quite a bit of time.

Very close to IRstuff's post. Thanks!!

Just a quick note ... the info stated that some newer chargers have output current curves that will give more accurate time.