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Glaucus
17-02-2009, 21:52
It's been a fair few years since I've done any electronics. I was never brilliant. But I can't work this out.

If I have a device that is rated at 50A at 14.8V How many mAh would I need to run it for 30mins.

If I linked two in parallel, the voltage would stay the same but I would need double the mAh?


and if a battery is rated as a maximum discharge of 30A would that mean 10 batterys linked in series would be capable of 300A maximum discharge?

Also is any one into rc planes?
Mark
17-02-2009, 22:01
It's been a fair few years since I've done any electronics. I was never brilliant. But I can't work this out.

If I have a device that is rated at 50A at 14.8V How many mAh would I need to run it for 30mins.
Logic would suggest 25 Ah (25,000 mAh) assuming the battery was also rated at 14.8V. It also assumes 100% efficiency which never happens, so allow more.

If I linked two in parallel, the voltage would stay the same but I would need double the mAh?
Correct. Parallel affects amps, series affects volts.

and if a battery is rated as a maximum discharge of 30A would that mean 10 batterys linked in series would be capable of 300A maximum discharge?
No - see above. They'd need to be in parallel, and again assuming 100% efficiency which never happens.

I think, anyway. :dunno: :)
phykell
17-02-2009, 22:07
If I linked two in parallel, the voltage would stay the same but I would need double the mAh?
Yep, due to the resistance being halved if they're in parallel and the resistance being doulbe if they're in series.
Glaucus
17-02-2009, 22:11
Ohh thats good so i would need 50,000mAh which would be 12 high powered rechargeable batterys. which would also equal 14.4V.

Now I just need to clarify maximum discharge rates.
Humm looking at battery packs, you do indeed seem to be right about discharge staying at 30amps..
kaiowas
17-02-2009, 23:01
Ohh thats good so i would need 50,000mAh which would be 12 high powered rechargeable batterys. which would also equal 14.4V.

Now I just need to clarify maximum discharge rates.
Humm looking at battery packs, you do indeed seem to be right about discharge staying at 30amps..

Capacity doesn't sum. 1 1.2v 2000mAh battery will give you 2000mAh at 1.2V, 10 of them will give you 2000mAh at 12V

I reckon that whatever you've got in mind your power requirements are too high for a few rechargable batteries.
Mark
17-02-2009, 23:19
I concur. 50A @ 14.8V is roughly equivalent to the power required by 15 laptops, or 4 desktops, or 12 old-fashioned 60W light bulbs. It is possible - my UPS is rated at somewhere near double that power draw. All I can say is that it's bloody heavy. :)
Glaucus
18-02-2009, 06:15
Yeah. I think it's just me getting confused. It's an electric motor for rc planes. I found the normal packs they take are 14.8v at 4400mAh. So the 50A must be maximum current, not what they actually use.
phykell
18-02-2009, 11:35
A DC motor takes a relatively large amount of current to start it, which corresponds to its relatively flat torque curve; and batteries are usually designed to be able to supply a correspondingly large amount of "cranking" current. If you look at a typical car battery, you'll see that cranking current is often quoted alongside the normal rating. A car's starter motor is typically DC and requires a lot of starting current - the fairly flat torque curve allows it to accelerate a relatively large mass (the engine's crank shaft) relatively quickly.