Cranking current draw battery/starter

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Cranking current draw battery/starter

Post  JBR-3 on June 10th 2016, 10:22 am

Trying to get a little better handle on this.

Suppose it takes 2-HP to crank an engine ( I don't know how accurate
this guesstimate is, it may be more or less than 2-HP).
2 HP = 1500 watts
At 12 volts cranking, 1500 watts means about 125 amps.
(from V=IR, circuit resistance is then about 1/10 ohm.)

Does the starter actually draw that much current ?

If the battery is drawn down to say 9 volts cranking, that would
indicate about a 90 amp draw, using the same circuit resistance.
(still assuming it takes 2 HP to crank the motor.)

Of course, different engines will need different power to crank.

Anyone measured this or looked into it closer ?

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Re: Cranking current draw battery/starter

Post  bruno on June 10th 2016, 3:33 pm

JBR-3 wrote:Trying to get a little better handle on this.

Suppose it takes 2-HP to crank an engine ( I don't know how accurate
this guesstimate is, it may be more or less than 2-HP).
2 HP = 1500 watts
At 12 volts cranking, 1500 watts means about 125 amps.
(from V=IR, circuit resistance is then about 1/10 ohm.)

Does the starter actually draw that much current ?

If the battery is drawn down to say 9 volts cranking, that would
indicate about a 90 amp draw, using the same circuit resistance.
(still assuming it takes 2 HP to crank the motor.)

Of course, different engines will need different power to crank.

Anyone measured this or looked into it closer ?


i would say most starters draw way more than that . On average 200-300 amps
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Re: Cranking current draw battery/starter

Post  BOSS 429 on June 10th 2016, 11:27 pm

mine with 1 red top, power master starter drops to 9.4 to 9.8 volts, this is with ing switch on,and cranking . starts great

volt meter test at firewall solenoid
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Re: Cranking current draw battery/starter

Post  whatbumper on June 11th 2016, 12:36 am

My starter pulls about 350 amps on 16volts while on. It is only about 100 amps when on the bench with the same battery. On a customers 900+ inch motor the starter we have on it pulls 500 on the hit. Anything else won't spin it over fast enough to fire it. We run 00 wire from battery to starter and it is hot after cranking. Twisted Evil

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Re: Cranking current draw battery/starter

Post  JBR-3 on June 13th 2016, 2:05 pm

Thanks guys, I thought my estimate seemed low.

Whatbumper, That is quite impressive, how are you measuring the current draw ?

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Re: Cranking current draw battery/starter

Post  whatbumper on June 13th 2016, 5:19 pm

JBR-3 wrote:Thanks guys, I thought my estimate seemed low.

Whatbumper, That is quite impressive, how are you measuring the current draw ?

clamp ammeter

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