Need help with battery Ah and starter motor calculation


Old 11-09-13, 08:26 PM
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Join Date: Nov 2013
Posts: 2
Need help with battery Ah and starter motor calculation

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
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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Old 11-09-13, 08:36 PM
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Join Date: Nov 2013
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I know that the voltage drops with every starter motor cranking. The current will increase if the voltage drops, I understand this concept. Using this concept, how I could calculate total number of times can be cranked ?

Kindly note : I'm doing a project on engine start/stop system during traffic light stop. Thats why I'm calculating the total number of times can be cranked. Please give your suggestions. TQ.
Old 11-09-13, 11:16 PM
Join Date: Oct 2013
Location: United States
Posts: 235
Starter Amps

Your calculation looks ok for AH's however a battery needs to have around a 75% charge to actually operate the starter correctly. You cannot figure the starter will operate until the battery is dead. There are two values to consider, AH is the current the battery can deliver for about 20 hours. You need to be working with the CCA (cold cranking amps) which for a typical starter may be as high as 750amps. The CCA is a rating for cranking that last about 10 seconds. Repeated cranking will run the battery down below the 75% in a short time. The repeated cranking will increase the IsqR losses because of the heating of the battery, cables and the starter. If your study is based on start stop engine operation at intersections I would wonder about the pollution from the unspent fuel passing through the engine during the cranking process each time the engine is started.
Old 11-09-13, 11:54 PM
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There are way too many variables to compute the actual amount of starting times.

Realistically based on real world experiences you may get only 25-45 starts. Battery condition, engine condition, starter condition, weather/temperature all affect the outcome.

A starter has a limited lifetime and continuous starting like that would wear the starter out rather quickly too.

One way to extend the number of starts would be to increase the battery size and the size of the wire from the battery to the starter.

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