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Need help with battery Ah and starter calculation
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mrexp21
Novice
Nov 9, 2013, 12:17 AM
Post #1 of 8
(2329 views)
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Need help with battery Ah and starter calculation
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I wanted to calculate total allowed number of cranks until the starter not enough current to crank or battery dead. My battery capacity is 35Ah. Assume the power of the starter is 800W.
Assume fully charged battery : 12.6V
Calculation : P = VI
800W = (12.6V)(I)
I = 63.5A
so,
35Ah/63.5A = 0.55h
0.55h x 60 = 33minutes
33minutes x 60 = 1984seconds
If assume 5seconds for each crank, 1984seconds/5seconds = 396 times
According to calculation, the starter can be cranked for 396 times until battery is dead, nearly impossible. As I've heard, the starter can only be cranked 15-20 times maximum before the starter not enough current to crank or battery dead.
Is there anything wrong with my calculation ? Please help me. TQ.
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Hammer Time
Ultimate Carjunky
/ Moderator
Nov 9, 2013, 3:39 AM
Post #2 of 8
(2308 views)
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Re: Need help with battery Ah and starter calculation
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I would expect that all your numbers would change as the state of the battery becomes discharged. As the reserve capacity of the battery drops the starter will consume more current to accomplish engine rotation. The temperature of the starter would also effect current draw. Initial engagement would usually require a surge of far more current than continued operation.
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 9, 2013, 8:30 AM
Post #5 of 8
(2285 views)
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Re: Need help with battery Ah and starter calculation
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Engineers and mechanics don't get along most the time.. What happens to your equation if you have a gear reduction starter? I can tell you right now you are not going to have 12.6 volts at the battery when the starter is consuming current.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 9, 2013, 8:52 AM)
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Discretesignals
Ultimate Carjunky
/ Moderator
Nov 10, 2013, 7:33 AM
Post #8 of 8
(2254 views)
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Re: Need help with battery Ah and starter calculation
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I don't have the answer to your calculation, but someone else might on here. I'm not really good with mathematical word problems. The start stop system has been around since the 70's. You may need to find an engineering forum that can help you with your calculations.
I can tell you that the next step in start stop system is using internal combustion for restart. Mazda has already developed a system, you should check it out:
http://www.mazda.com/mazdaspirit/env/i-stop/
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Nov 10, 2013, 7:37 AM)
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