How to calculate manually Battery AH rating

[L3mu]

Hello Guys,

I am new in DC field so i could use some help perhaps with regards to my question.

I have a 125Vdc battery charger with a constant output current of 100 Amperes, i am planning to use 60 cells of Lead-Acid batteries.

How do i calculate manually the Ah(ampere-hour) rating of each cell at C10 (10 hours discharge time)?

A simple direct formula will be much appreciate.

 

[sibeen]

L3mu, I'm not exactly sure what you're asking.

If you want a 100 amps for ten hours then you'd require a 1000 A/hr battery at the C10 rate. A 1000 A/hr at C10 means that the battery will provide 100 amps for ten hours (100A x 10hr = 1000 A/hr).

 

[eeprom]

If you are trying to manually test your batteries, as in a controlled discharge for a period of time, the manual for the batteries should come with instructions for how to perform a 3 hour test. I don't remember ever seeing a 10 hour test. Perhaps your batteries have a 10 hour rating.

Another (manual) way of doing this is to discharge the batteries for 10 hours using a representative load. The size of the load should produce a reasonable amount of current. You would have to monitor the current at frequent intervals, perhaps one minute. If you are using a correctly sized load, the current will go down through the test. Each reading could be calculated as AH = (amps)/60. With a data collector, this would be time consuming but very easy. If you had to do this for each battery, this would take a long time.

Why can't you do the whole battery array at once?

 

[L3mu]

Thanks sibeen, thats what i thought as well however the supplier delivered to us together with the Battery charger, a battery with a nominal capacity of 680Ah rating at C10 and this is what i am trying to confirm.

any help would be much appreciated.
Thanks

 

[oldagpe]

The standard you want is IEEE 450.

Also, the charger should have several stages of charging. The bulk charge might be about 10 percent of capacity, but the finish charge is a fraction of that.

Lead acid batteries need maintenance and equalization charging.

Please be careful as those batteries will act as an arc welder.

You have a lot of homework ahead.

 

[ThePunisher]

L3mu, you may also want to check IEEE 485.

The standard has a worksheet for sizing lead acid storage batteries. The storage battery ampere-hour rating should be adequately sized to handle your loads. You may have to determine your battery load profile, obtain battery Kt factors, apply temperature derating and use the worksheet to determine your battery size. If your 125Vdc loads are applied to switchgears, then you should consider both continuous DC loads and momentary (or transient) DC loads like breaker closing/trip coils.

 

[rcw retired EE]

Look at battery supplier's on-line tools. Alcad for one has the IEEE 485 sizing program on line, YUSA and others probalby do also.

You will need to specify the battery load profile.

For example a substation load profile might be:
continuous load = all lights and relays
0-1 minute - all breakers trip and recharge closing springs
1 minute to 8 hours - continuous loads only
8 hours - all breakers close and recharge.

Loads are always listed with a minimum one minute duration, even if the trip coil is only energized for 1/2 second.

Battery temperature, end volts per cell, and aging factors all affect the sizing calculations.

 

[ElSparky]

What you need is a load bank that can be loaded with 68A for 10hours.

Measure the discharge current in regular intervals of maybe 15min.

Calculate your average ampere multiplied by the time in hours.
Discharge until you have discharged 680Ah if you want to prove that the Battery bank has more that what has been specified.

Discharge until you reach the final voltage of normally 1.8V per cell.
If you go all the way keep a very close eye on the voltage towards the end.The voltage will decrease fast at the end of discharge.

Study the battery data sheet so you understand when to stop or you will damage some of your cells.

If your discharge current is different from the 68A you will need to correct your resulting Ah discharged accordingly.
For this you need again the curve from the battery supplier.

And last please note that a new battery bank does not need to deliver the full rated Ah. Some standards define as 95% being enough to declare your test as passed.