Marine battery reserve amps? Sounds too good to be true.

viper · May 2014

Interesting comment from the battery store.

Marine deep cycle and starting battery has a "reserve" amperage. This particular battery was about 800 amps with a reserve of 140 amps.

The salesman was trying to explain to me that if I ran the stereo for so long that the voltage dropped to a level where my stereo quit functioning there is still 140 amps to start the motor left. He further was trying to explain to me that the starter only requires a certain amount of amps to turn over the motor (i forget but he was saying it was about 40-60 amps required).

So communicate to him what I understand is that I can run my radio off my battery till it quits, then attempt to the start the battery for an hour before the battery is completely dead, then I'm stranded? Answer "Yes"

*assuming this is a brand new battery

212rowboat · May 2014

I'm sure it can be done with some whiz bang controls, but I'd like to see it south of the price of running two separate cells with a simple switch.. :-)

Michael T · May 2014

The basic answer is that the salesman's comments are absolute nonsense unless you have a pcb instaled that will cease power to the battery when it reaches a certain voltage (like many cars/trucks have). I doubt very much that has been installed on your boat or would be practical for your application.. If you run your stereo until it shuts-off your battery will be essentially "dead" it wouldn't light-up an led party latern yet alone your motor - particularly if it is an EFI and for sure if it is a later model EFI or MFI that requires a lot of start up power to initialize the engine's electronics. If you want to blast your tunes - I suggest you get a second battery and a battery switch. Simple, relatively inexpensive and safe. Reserve minutes is a calculation of how long your battery can sustain a load of 25 amps before it drops to 10.5 volts (DEAD) Sooo, a battery rated at 150 minutes could run a 25 amp stereo for 2.5 hours then the battery becomes a cheap anchor as in DEAD - at which point it couldn't start an argument yet alone your engine. If your battery voltage drops to 11 or 10.5 volts you don't have any "reserve" amps - they've left the house! That's my understanding of battery dynamics for what it may be worth. :-) MT I should add that there is another calculation that refers to straight "amp hours" that means the number of hours a battery can run a constant and sustained load for 20 hours. Therefore if you had a 140 amp hour battery it could power 10 amps for 14 hours - then it would be dead. BTW 10 amps is not much power and long before the battery went dead at 14 hours it would have ceased being able to start your engine. Drew said it in two sentences - I've explained the dynamics ( I hope) same answer - get a second battery and switch and have fun with total certainty that you get to go home without a sea tow! MT

Dream_Inn · May 2014

Well, let's just say I wouldn't be buying any batteries from that salesman. As mentioned, your best option is to have a second battery on a switch. MT mentioned some different voltages. If that battery gets down to just 12V, yes 12V, you have done some damage to that battery.