Battry Backup Time

Discussion in 'General Electronics Discussion' started by [email protected], Oct 29, 2012.

1. [email protected]

24
0
Mar 26, 2012
Plz guide me a formula to calculate the battery backup time
for example i have a battery of 12V 7Ah and a load of 12V 1Ah
what is the formula to calculate the backup

2. donkey

1,298
56
Feb 26, 2011
in that case it would be 7 BUT

no device I know of has an AH rating, they either have amps or watts. so finding an AH rating on a battery is common on a device its not. I am noobish and I always assume that amps rating is amps used in an hour but I may be corrected there.

as for the wattage http://lib.store.yahoo.net/lib/sports-imports/BatterydrainCalc.html has a calculator you can use

3. CocaCola

3,635
5
Apr 7, 2012
Do note that is in a perfect world, in the real world when all factors are considered the answer is probably in the 3-4 range, possibly a little more or a little less...

4. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
First of all, the battery is rated in Ah (that's a measure of energy storage), the load draws a current in A (not Ah).

Secondly, you don't tell us if that is just what is written on (say) a power adapter, or if it is something you have measured.

Thirdly we don't know if you're powering the device directly from the battery or if you're running an inverter and then connecting the power adapter to that.

Let's assume that this is the actual load, and that you're connecting it directly.

The simple answer is 7Ah/1A = 7h (see how the units cancel out)

The real answer may be different. For a start, you are probably best NOT discharging the battery (lead acid, I presume) all the way. 50% is a good amount. So the discharge time drops to 3.5h.

Another factor is that the Ah rating may be predicated on a load that will discharge the battery in 20 hours. You're discharging it faster, so it may not last as long. You may get 10% less time.

So, the answer is "between 3 and 7 hours". (and 7 hours means that you will possibly damage the battery, increasing to certainty if you don't start charging it again almost immediately)

If the power adapter is rated at 1A, then the device may never draw 1A, and may only approach that amount occasionally. If this is the case it may last MUCH longer.

If you're running an inverter, the opposite will be the case. You have inefficiencies and losses which may reduce the time to less than an hour.

5. [email protected]

24
0
Mar 26, 2012
plz tell me the formula

what is the formula

6. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
Glad to have helped. No problems.

(You did read the replies, right?)

7. donkey

1,298
56
Feb 26, 2011
if its straight ah and amps then
life of battery =ah/a
but you should at most use half that number and allow a further 10% for innefficiencies

for watts amps = watts divide voltage then calculate as above. once again do not drain below 50% and add 10% for inefficiencies