Most sailors don't stop to figure the actual amperage draw of their start batteries
. A little math is an eye-opener:
Typical Energy Usage on a 40-foot Sailboat per Da
: somewhere between 100 and 200 amp hours
(assuming 2 engine starts/day): total of 2 amp hours
Figure: 250 amps starting current
for 15 seconds per start (most engines take far less than that). But, for illustration, figure 30 seconds total starter time to start the engine twice a day.
250 amps x 1 hour would be 250 AH
250 amps x 1 minute would be 250AH/60 minutes = 4.2 AH
250 amps x 30 seconds would be half that, or 2.1 AH
Nothing, really, in the larger scheme of things.
These two AH are replaced in mere seconds after the engine starts.
THAT's why it's good practice to connect all onboard charging sources to the house batteries, and use a device like an EchoCharge or a DuoCharge to bleed off a bit of the charge current
to top off the starting battery.