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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Just to be awkward; the math is correct but many battery types are rated at 20 hour discharge rate (gentle) at heavier discharge rates the capacity is less.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Patrick wrote ...
if i have a battery rated for 6 Ah how long can i run a 5.6 amp load.
3600 seconds per hour X 6 Ah = 21,600 21,600 / 5.6 A = 3,857 As 3,857 / 3,600 = 1.07 x 60 min = 64 min of operation.
is this right?
No
And it all depends on what 'rated' means, and the battery chemistry.
If your rated capacity is at the 1C rate, that is, specified for a one hour discharge, then without doing any sums, it's obviously more or less right, as Shrad points out.
If however you have a lead acid battery which is rated at the 20 hour rate, or 0.05C, then drawing the 1C rate would give you nothing like that capacity.
All batteries lose deliverable capacity as the discharge rate rises, some chemistries lose it faster than others.
Registered Member #4465
Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145
"Peukert's formula" = the formula for calculating how long a battery will really last. T = C / ( I exp n ) where C is theoretical capacity (in amp-hours, equal to actual capacity at one amp) I is current (in amps) T is time (in hours) and n is the Peukert number for the battery.
The Peukert's exponent range from 1.1 to 1.3 shows how well the battery holds up under high rates of discharge. details here: Online calculator:
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
must admit, I'd never heard of "Peukert's formula" ... interesting. Problem is, battery manufacturers do not give Peukert's values.
The BEST thing to do is get a datasheet for your battery and look at the graphs for your specific discharge rate. Most manufacturers publish in-depth datasheets.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Hmm! According to that link, Peukert's formula does not account for temperature, is inaccurate at low and high discharge rates, and has to be derived from the curves in data sheets. By the time you've avoided all the caveats, you're better off just interpolating between the curves directly by eye.
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Please check my math
