The smallest "size" starting battery
which will reliably start your diesel engine
is one SAE rated with a CCA (cold-cranking-Amps) value equal to or exceeding the stall current
rating of the starter motor
. I've not seen this concept
promulgated within the marine
industry so here it is. If you know the power rating of your starter motor
and not the stall current
rating divide the power, in Watts, by 5 (for nominal 12V systems) or 10 (for nominal 24V systems). A 5kW rated starter motor is 1500 Watts, thus the stall current will be close to 1000A and needs a battery
with a CCA rating of 1000 or higher.
If possible choose the battery having the lowest internal resistance value and, in no case, choose one having an internal resistance higher than 10 milli-Ohms (10mOhm). Neglect ALL other ratings like reserve capacity. Because we are attempting to minimize the size and expenditure never consider using a deep-discharge flooded electrolyte battery because of the higher internal resistance size for size compared to a start-only-start battery. If possible use the technically advanced cylindrical construction batteries
such as Optima or Hawker due to the low internal resistance and higher specific gravity (which directly translates to higher voltage under load) than flooded batteries
. In the case of these SLA (sealed-lead-acid) batteries they may be rated to serve as deep-discharge use as well yet because of their intrinsically low internal resistance they work
better than the "wet" ones, pound-for-pound.
All starter motors exhibit an output power versus current curve shaped like a parabola or inverted chain catenary. What this means to you is that when operating at any power out, other than exact maximum, there are two values of starter current which will deliver the SAME output of the motor. The high values on the right side of the power curve will, in general, be due to operating at sagging battery voltages (high internal battery resistance and cable and connector resistances. Using inferior batteries (like flooded deep-discharge ones) that are small WILL likely cause operation in this region creating unnecessary heating
and wear and tear on the starter motor even though the engine
may start. If the voltage to the motor sags enough then the starter motor will stall and you will be operating at the extreme right point of the curve with no engine rotation. However, if the battery fails to deliver this stall current you might have NO chance to get the engine rotating with cold oil
and freezing ambient temperatures.
In short, it is only motor CURRENT which causes the starting torque. It is starter terminal VOLTAGE which allows a peak power to be developed with that current which translates ultimatly into rotation speed. The faster the starter motor rotates the higher the compression
of the engine and the faster the engine will start.
I had the occasion to measure the effects of using a 2000 A-hr rated Rolls battery (great for operating lights for long periods of time) which would only slowly grind the engine barely starting it (sagging voltage caused by high internal resistance). Switching to a much smaller 1400CCA start-only-start battery (no good for running lights without ruining the battery) caused the engine to start almost instantly.
A note about reliability
: Flooded-cell constructed start-only-start batteries should never be trusted as being reliable if they have been left standing too long without a float voltage applied or if ever deeply discharged. Shallow discharging (on the order of a 3% capacity discharge) is also usually deliterious in the long term unless specifically designed to do so.
Hope this clears up some of the mis-information floating amoungst the marine