Simple elctronics help

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

Quote:

Originally Posted by AcidHell2

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.

Quote:

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.

Quote:

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.

phykell · 17-02-2009, 22:07

Quote:

Originally Posted by AcidHell2

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

Quote:

Originally Posted by AcidHell2

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.

17-02-2009, 21:52	#1
Glaucus Absinthe Join Date: Jun 2008 Posts: 1,855	Simple elctronics help 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? __________________

17-02-2009, 22:11	#4
Glaucus Absinthe Join Date: Jun 2008 Posts: 1,855	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.. __________________ Last edited by Glaucus; 17-02-2009 at 22:14.

18-02-2009, 06:15	#7
Glaucus Absinthe Join Date: Jun 2008 Posts: 1,855	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. __________________

18-02-2009, 11:35	#8
phykell Vodka Martini Join Date: Sep 2007 Posts: 833	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. __________________

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17-02-2009, 23:19	#6
Mark Screaming Orgasm Join Date: Jul 2006 Location: Newbury Posts: 15,194	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.