Battery calculation please?

[greensec]

Hi, im looking for a little advice on batteries... im after the calculation for working out how long a battery will last. for example if i have a 17Ah battery and the draw is 900mAh how many hours will the battery last?

Also, if i charge the battery off a 5A PSU how long will it take to fully charge from flat?

Many thanks for any help

 

[telectrix]

if the load was 1000mA then it would theoretically last 17 hours. in practice, a bit less. charging at 5A would take 17/5 hours.

 

[Des 56]

In theory the 17 Ah battery will supply 17 amps for one hour,so the basics maths will give the time for the 900m/amp load
However that is assuming the battery is actually 100% efficient,a capacity test would be required to determine the percentage battery capacity before the actual figure could become accurate

Charging will depend on the type of battery that is being charged,different battery types require different procedures for determining charging rate and when full charge is achieved
Some rely on temperature rise.others like lead acid use a 10 hour charge rate
If your 17 Ah battery were a lead acid it would be 1.7 amps for 10 hours
If it were a alkeline ,it would be temperature rise

 

[amlu]

make sure you dont discharge them more than around 20% capacity, it will kill the battery

 

[firespark]

I know it was only a theoretical question, but what would be the application of said battery and charger?

As others have previously mentioned, If you have a 17Ah battery and your load is 1000ma (1Ah) then the battery should last for 17 Hours. (based on a fully charged battery in perfect health)

So in your case 17000mAh (17Ah) divide by 900mAh gives you 18.88 hours. However running a battery like that in real life would run the risk of putting it into a state of deep discharge and killing it.
As for charging it Des 56 is right on the money!
If you’re really interested in charging lead acids I refer you to this.

One last thing to bear in mind (in a real world situation). Depending on the application of said battery, (say they were stand by batteries for a fire alarm system) you cannot rely on the simple math as laid out above.
A different equation taking into account equipment load in both quiescent and alarm state as well as battery degradation would be required.

 

[lister101]

If the battery is from a decent manufacturer, you should be able to download charge & discharge graph as well as concise charging guidance to guarantee the life of the battery.

 

[Inteificio]

There is a lot more to batteries than most suppose.

The battery itself will have an internal resistance that needs to be taken in to account.
The mAh will be at a specific discharge rate, normally 0.1A. If you draw at 1A the efficiency will be far lower and you will not get close to the rated capacity.
The voltage of the cell will vary over the discharge as well.
The temp of the cell will also have an effect.

Get the manufacturers data sheet, that will give most info.
If you charge/discharge faster than what the cell is tested at you will lose efficiency.
If you do it slower you will gain efficiency.

I would also take in to account self discharge of cells (especially NiCd, or high current NiMh)

hope that helps