Starter motor and Battery Sizing

I have a 12 volt starter motor on a diesel engine that is rated at 2000 W. Typically a fully charged lead acid battery is 12.7 volts. Therefore the current draw would be 157.48 amps but for only a few seconds. What is the correct way to size a battery to this starter? Do I look at amphours (battery capacity) or CA/CCA?

• CCA, though the battery voltage is about 7.2V and for no more than 30 seconds as in SAE spec. See here for an example: yuasa.co.uk/info/technical/understanding-the-specifications Mar 16, 2018 at 18:00
• What you really need to worry about is the stall current when the motor is first running. This might be useful: mechanics.stackexchange.com/questions/47237/… Mar 16, 2018 at 18:02
• From the link you provided it sounds like most dc motors pull 2-3 times more current on startup than what they are actually rated for. I'm assuming the starter is fine for the diesel engine as it came from the manufacturer but I'm just not sure the proper way to size a battery based off the 2000W starter Mar 16, 2018 at 18:07
• Assuming 12.7V and 2000W, you get about 160 amps. Multiply that by 3 and get about 480 amps, so 500 CCA would probably be sufficient. 400 is the low end of available batterries and they go up to around 1400. You also need to take into account that your battery will age and you still want it to start when it is not fully charged. So, perhaps something in the 800 CCA range? Mar 16, 2018 at 18:14
• Ok thanks. So is the battery's capacity (Ah) even useful to consider when looking at just starting the engine? I take it that the battery capacity rating is instead just a benchmark to go off of if your alternator breaks or something Mar 16, 2018 at 18:17

1 Answer

It depends on the battery climate temp. If sub 0’C use CCA , CA rating is sometimes 30% higher.

Since diesel has 22:1 compression ratio the start torque is much higher than a car at 8:1.

The proper way to choose a starting battery depends on the Battery/Motor ESR ratio under any condition as any drop in battery voltage is due to battery ESR and load current when starting to crank the pistons is due to the sum of both Battery ESR and starter ESR.

Battery ESR rises with Depth of Discharge rapidly above 80% and motor ESR rises due to the Reverse EMF of the motor rising with RPM. The motor rated current will stall at same voltage at about 8~10x rated current or ~ 1000A in your case.

simulate this circuit – Schematic created using CircuitLab

Considering many users replace a battery if CCA test <50% (or less) of rating you need to consider coldest temperature of battery cost and turning speed and motor oil viscosity and starting reliability /risks you want for the life expectancy in your application.

In Winterpeg, I had lots of CCA battery experience with cars and slow turning engines at -40'C and sometimes relied on battery and/or oil heaters or inline water heaters. Usually everyone had jump cables in the trunk for helping others that underestimated their battery needs and would willingly stop and help anyone. Occasionally to help themselves... For these climate conditions you always choose more CCA margin than you need in the summer.

• "Winterpeg" :D Nice. Toronto to Calgary and now extreme Northern BC for me. Mar 16, 2018 at 20:50