What Size Battery Do I Need To Start A 3.9L(236ci) Perkins Diesel?

How far from the Starter motor do you intend to mount the battery?  You should also consider the size of wire you intend to use.  Are using the unit in a cold environment?  Do you know how the engine behaved on starting?

Considering these questions,  I would recommend a group 30 battery and 1 gauge battery cables if you are within about three feet of the starter motor with the battery.  A group 30 is typically 650 CCA.  

If you are requiring battery cables longer than three feet to reach the starter, or if the engine is a tad lazy, or if you are in a colder climate, or if you just want a little better system.... Go for a group 31 which will be 950 CCA or better.  

Also consider gauge 2/0 battery cables if they need to be much longer than, say, four feet.  They are harder to get in pre-made sizes, but a tractor or truck dealer should be able to make them to your spec.

When buying your battery, pay attention to the rating you are being quoted.  CCA is Cold Cranking Amps and is a rating of the power available @ 0°F.  CA is "cranking amps", and is a rating at 0°C (32°F).  As a result of the temperature difference, a vendor may be able to impress you with numbers on a battery that is no better than a competitive battery.  If I told you I could offer you a group 30 with 1000 CA, it may, or may not have 650 CCA so don't pay a premium price for it with out more information.

Good Luck

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