# Is it possible to power two 300 amp alternators with one 20HP Motor?

Discussion in 'General Electronics Chat' started by ptownbro, Oct 9, 2013.

1. ### ptownbro Thread Starter New Member

Sep 28, 2013
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Let's say you have 1 motor that drew 20 hp or approx 15kW from some power source. Could you use that 15kW of power to turn 2 alternators to put out 300 Amps each? If so, what would be the voltage needed to do that? (If I'm thinking about it right).

Ultimately what I'd like to do then is use the 600 amps to recharge a battery.

Motors => Alternator => Battery

(Feel free to tell me if my math or anything else is wrong. Much appreciated!)

2. ### #12 Expert

Nov 30, 2010
16,685
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Let's run some math on it. 15Kw into a motor, guessing 70% efficiency puts 10500 watts into 2 alternators that put out 600 amps. If the alternators are 100% efficient, 10500 watts/600 amps is 17.5 volts.

The formula is: P = IE
Power is current times voltage. Just keep running numbers through the formula to get the shape of answer you need.

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3. ### crutschow Expert

Mar 14, 2008
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3,373
What battery do you have that can tolerate a 600A charge current?

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4. ### ptownbro Thread Starter New Member

Sep 28, 2013
17
0
Great question. I'm glad you asked because this helps in my understanding. Note, that this is all hypothetical. I'm using an example to learn.

In my example, I was assuming this would be a standard car battery.

From your question I assume that it wouldn't accept the full 600 amps. I would have to split it among a number of batteries? If so, any idea how to determine how many?

Jul 18, 2013
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Let me know when and where, I may be away that day.
Max.

Jul 18, 2013
10,839
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If you are referring to automotive alternators, for those typically rated at 100-150 amps, don't expect it, at least for not more than a VERY short period.
Max.
.

7. ### #12 Expert

Nov 30, 2010
16,685
7,325
People are having fun with the absurdities, but there are things to be learned.

A car battery might be rated for 100 amp hours. A nice, conservative rate would be called C/10. That means 10 hours for a full charge. 600 amps would need 60 batteries to accept that charge and not suffer any damage. If you wanted to push pretty hard, you could get away with a C/3 rate, but that kind of energy causes strange things like warping internally. You could assume that some of the batteries will fail in just a few charges.

Consider that you would be pushing 17.5 volts at 10 amps and immediately see that you are inflicting 175 watts on the battery. Obviously, that causes it to heat up. Try to throw 600 amps in and it will explode. Meanwhile, we're still working with P = IE.

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8. ### shortbus AAC Fanatic!

Sep 30, 2009
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People forget that a car battery only is in the system for starting. After the engine starts only a very small amount of the alternator output is used for battery charging. The rest of the output is used to supply all of the electric use of the running car. The battery is effectively not supplying any energy to the operation of the car.

9. ### crutschow Expert

Mar 14, 2008
13,496
3,373
A large percentage (about 70%) of that power is used to charge the plates so 30% would generate heat (and some electrolysis out-gassing for a non-sealed battery). That's still significant of course.

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10. ### PackratKing Well-Known Member

Jul 13, 2008
850
216
Considering only the mechanical load induced by both alternators, it would be advisable to employ a soft-start circuit on either the loading or the output of those alternators, because both kicking in 100% all at once, is likely to stall the engine, or smoke the drive belt at the very least.