Need help with battery Ah and starter 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

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.

Yep, the full capacity of the battery is not available over the entire discharge curve. Also, surge current capability of the battery will come in to play and vary over the discharge curve. You will need to find the datasheet for the particular battery you are using and look at what it shows for capacity across the discharge curve. Keep in mind that as you load a battery, the voltage will drop. The power required by the load however will remain relatively constant. So the battery may show 12.6V with no load, but as soon as you load it with 60A, the voltage may drop to 11.5V. As the voltage drops, the current draw will increase to maintain the power.

-Rod

Yep, the full capacity of the battery is not available over the entire discharge curve. Also, surge current capability of the battery will come in to play and vary over the discharge curve. You will need to find the datasheet for the particular battery you are using and look at what it shows for capacity across the discharge curve. Keep in mind that as you load a battery, the voltage will drop. The power required by the load however will remain relatively constant. So the battery may show 12.6V with no load, but as soon as you load it with 60A, the voltage may drop to 11.5V. As the voltage drops, the current draw will increase to maintain the power.

-Rod

Hi,

Thanks for the response. How I can calculate voltage drop for battery and what is the minimum voltage acquired by starter usually ?

Hi,

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.

Again, you need to get the datasheet for the specific battery you are considering and see what the battery is capable of sustaining. If you do not have a specific battery in mind, only a capacity, then pick a specific battery and list that as an assumption in your report. Then once you study the discharge curve, you'll be able to see at what point the battery will no longer have the capacity to support the starter motor. From that point, you'll want to de-rate the battery for a margin of safety. You may want to de-rate as much as 50% to account for sulfate buildup over time.

-Rod