Need help with battery Ah and starter motor calculation

Discussion in 'General Electronics Chat' started by mrexp21, Nov 9, 2013.

  1. mrexp21

    Thread Starter New Member

    Nov 9, 2013
    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 ?

    I can see 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.
  2. MaxHeadRoom


    Jul 18, 2013
    "The current will increase if the voltage drops"
    Just the opposite, the voltage drops, that available current will drop and so will cranking torque..

    The type of motor used in cranking is a series wound DC motor, therefore the field current and hence torque drops with a drop in voltage.
    There are many variables to consider as to the amount of cranks can be obtained from a battery.
    Ambient and battery Temperature, battery charge, motor size and temperature etc.
  3. JohnInTX


    Jun 26, 2012
    True enough but a better way to look at it is that as the battery discharges with cranking, its internal resistance rises so, at the high currents pulled by the starter, the net battery voltage available at the terminals drops. Eventually, it will drop to a point where the starter is unusable but it will still have lots of amp/hours left, just not at a current/voltage profile that the starter requires.

    The amp/hour capacity is tricky. You can crank the starter until it won't go any more and the battery will still have some capacity. The problem is that with the higher internal resistance, it can't deliver the current that the starter requires at the voltage it requires - too much voltage is dropped across the internal resistance. You can demonstrate this with a 'dead' battery that won't crank the starter but WILL light the courtesy lights and play the radio (much lower current draws mean lower voltage drop across the internal resistance of the battery). Turn the radio on and hit the starter. IF the voltage left is enough to pull in the solenoid, the starter will attempt to run and pull the voltage down - that's why when you crank with a very low battery, the lights flicker as the starter comes up against a compression stroke and needs more current.

    What it means to you is that you need to look at the battery curves that show the current available at various 'degrees of discharge' as its called in the charger business. The available current at the necessary voltage will be way different than the rated amp/hours.

    And as Max says, all of this gets worse with age, temperature etc. so lots of cushion is necessary.

    EDIT: Check out Battery University. It has lots of good stuff. If you can get a copy of the Gates Battery Handbook, it has lots of useful stuff but I think its some $$$ these days.

    Ultimately , look at the battery as a little chemistry set that throws off electrons rather than some ideal source of electricity. As the battery discharges, its chemistry changes and its not as good as when its fully charged. NOt only does its internal resistance rise but its capability recover from high draws drops as well. And then temperature, age, current draw, duty cycle and lots more affect the amp/hours that you can get out of it. And don't forget that recharging is not a perfect process. If you pull 100A/H out of a battery, you have to put back more than that to fully charge it. As the battery ages, the charging process becomes less efficient and the final charge becomes less and less until the battery is kaput.

    What you are doing is akin to 'opportunity charging' (google it) i.e. charge while it can and run things on battery when it can't. Your profile is different but some of the same things apply. While stopped at a light, run off battery and compute the A/H taken from the battery while stopped. When charging again, monitor the AH back in plus some factor to compensate for charging losses. Keep a budget of charge out vs in. Know when the charge budget is below some point and keep the motor on at the light to maintain the battery.

    Its fun stuff. Have at it.
    Last edited: Nov 10, 2013