batteries

[roboninja93]

hi
even i had a doubt regarding battery ratings
if a battery is rated 12V 2200mAh that means it can supply 2.2A for an hour or 4.4A for 30 min... but how do we get to know what maximum current can be drawn? i mean using this logic can we get 8.8A for 15min or 22A for 6min? if not so then how do get to know the max output a rechargeable battery can  supply?
Thanks
 

[Soeren]

Hi,

if a battery is rated 12V 2200mAh that means it can supply 2.2A for an hour or 4.4A for 30 min...

No, it means that it can supply 440mA for 5 hours!
The higher the current draw, the less the capacity.
If you draw 2.2A from it,it won't last more than 30..45 minutes (depending on manufacture parms).

Lead-acid/car/starter batteries are a little different that way - they're rated by a 20 hour discharge (i.e. a 100Ah lead-acid can give 5A for 20 hours, but not 100A for 1 hour).

[...] but how do we get to know what maximum current can be drawn? i mean using this logic can we get 8.8A for 15min or 22A for 6min? if not so then how do get to know the max output a rechargeable battery can  supply?

By reading the datasheet for that particular battery. The Ri (internal resistance), which is a composite of electrical and chemical resistance, will determine the max current vs. a the lowest tolerated voltage.

[roboninja93]

Thanks....

[absentwizard]

The actual performance is typically lower than that suggested by the internal resistance. You should find a set of discharge curve on the datasheet which shows voltage over time at different constant discharge rates. That's the ticket.

[Soeren]

Hi,

The actual performance is typically lower than that suggested by the internal resistance. You should find a set of discharge curve on the datasheet which shows voltage over time at different constant discharge rates. That's the ticket.

The internal resistance (Ri) not only follows the curve of the discharge, it's the reason behind it, you may say, so when reading a curve of "voltage under load", you read the direct consequence of Ri, making your first sentence a bit self-contradicting

[absentwizard]

For example, the Quallion QL038KM is nominally 38 Ah and 25.9 V (maximum 135A continuous discharge). However, if you look at its cell (not battery) discharge curve by rate vs discharge capacity @ room temperature, a 0.2C-rate discharge nets you about 1.95 Ah, a 0.5C-rate nets you about 1.9 Ah, and a 5.0C-rate nets you barely 1.75 Ah. I'm eyeballing it, of course.

[Soeren]

Hi,

For example, the Quallion QL038KM is nominally 38 Ah and 25.9 V (maximum 135A continuous discharge). However, if you look at its cell (not battery) discharge curve by rate vs discharge capacity @ room temperature, a 0.2C-rate discharge nets you about 1.95 Ah, a 0.5C-rate nets you about 1.9 Ah, and a 5.0C-rate nets you barely 1.75 Ah. I'm eyeballing it, of course.

Yes, this is perfectly normal behaviour for all battery cells, no matter their chemistry or capacity - higher drain results in lower capacity, so I'm not sure what your point is here?


The only thing I can imagine that anyone might find vaguely related to our previous posts in this thread is that the difference between 2C and 5C is much larger than the difference between C5 and C2, but that's just because the limit of the cell is reached (or rather crossed). If the curves were shown eg. each C10 and all the way to say 10C (or even more, everybody loves destructive testing ), it would be obvious how the capacity accelerates with increased load.

The exact point (and hence the maximum continuous discharge rate) is up to the manufacturers interpretation of his results and how he wants to spec it, even within DIN9xxx etc.

[absentwizard]

My point is essentially what you said, but geared to point out to roboninja93 that you can't expect to discharge at 10C and use a simple calculation to figure out how long it'll last (before it... umm.. kaputs).