Battery run time calculators

[Makalu]

Can anyone help me out with finding a battery run time calculator?

I've searched the web but can't seem to find one that I understand (maybe because I'm just too technically under developed!).

I'm trying to estimate a run time for powering a 10W Vistalight on a 6V 4aH Sealed Lead Acid battery. The best I can calculate is an estimate of 1h 45 minutes.

Any help appreciated.

[Binaural]

OK, it's not too hard to get a basic estimate of runtime, but since manufacturers can be a bit, er, "optimistic" don't bother trying to do it too carefully ;)

Your lamp is 10W, the battery voltage is 6V. Since for DC electricity P = VI, you can rearrange to calculate current I --> 10/6 = 1.66A in continuous running. Since you have a 4Ahr battery pack, you should be able to get 4/1.66 = 2.4hrs running time. Hope that helps!

[arpit]

Binaurals way is pretty decent... But, a more accurate method would be just to look at the datasheet for the battery on the net. Also, note that as the voltage drops, your bulb will become yellower, emit a greater proportion of its energy as heat, and generally be less useful.

Also, unless they it's a deep cycle battery, don't discharge it all the way.

[Binaural]

Binaurals way is pretty decent... But, a more accurate method would be just to look at the datasheet for the battery on the net. Also, note that as the voltage drops, your bulb will become yellower, emit a greater proportion of its energy as heat, and generally be less useful.

Also, unless they it's a deep cycle battery, don't discharge it all the way.

You're right - ideally you would integrate the voltage vs time discharge curve to give you total energy expended by the battery. But the effort to find a discharge curve then estimate whether your globe is consuming 9.7W or 10W plus resistive losses is really not worth your time when you can get an safe estimate then discover the rest by testing. An important part of design is knowing when you can get away with using a rule of thumb ;)

[Makalu]

Thanks for the advice. That's sounding good.

Provided I can estimate a run time of between 1.5 and 2 hours for this battery then I'm satisfied, as for each double night lap at next week's Scott I have an NiMH power source to get me 1.2 hours (first lap) + the SLA battery (second lap) to get me through.

Cheers