Originally Posted by oldtool2
Autocraft starting marine battery M24-3
cold cranking 800 amps
reserve 135 amps
marine cranking amps @ 32 degrees 1000 amps
To determine the AH rating of a Marine battery that gives capacity in RC (Reserve Capacity in minutes - NOT amps as you stated) you multiply the RC in minutes by 0.417.
So your single M24-3 battery has an AH rating of 135*0.417 or 56 AH at a 25 amp discharge rate.
25 amps at 12 volts = 300 watts Continuous
The attached chart shows a 100 AH battery as it gets destroyed as the current demand goes up. You can use it "as a percentage" for any storage battery (yours is not as it is a starting battery) so assume it will do MUCH worse than this example.
Enter the graph at 25 amps and you will see that it just about correlates with the 100 AH battery capacity line at 20 hours.
At a 5 amp continuous draw (60 watts), the battery will deliver 100 percent of its rated capacity for 20 hours.
At 25 amps (300 watts) it will deliver 56% of that 20 hours or 11 hours and 15 minutes.
Now for a typical electric coffee pot (1500 watts) @ 12 volts you would need to draw 1500/12 or 125 AMPS from the battery. As you can see it is "off the charts" and the battery won't last more than a few minutes before it is dead.
This is why you will need MANY batteries to feed a 3000 watt inverter. You will need to SHARE the draw across several 12 volt "stacks" to use that much power without destroying your batteries.
The ideal is to configure your battery stack such that no individual battery is loaded up with more than 25 amps EVER.
If you need 100 amps total (1200 watts) you will need FOUR 12 volt batteries so no battery sees more than 1/4 of the amp load (25 amps each).
Also remember that discharging a battery below 50% of its rated capacity before recharging will cause permanent plate damage and permanent loss of capacity.
Until you are ready to put in some heavy duty battery upgrades, I suggest a good quality pure sine 300 Watt inverter
for you and make your coffee on the stove.