This is the current situation, looking for some expert advice.

Total amp draws with night time running lights + GPS + other accessories I am at about 70 amps. My alternator puts out max 50 AMPS

Currently, I have 2 batteries hooked up. Two high quality but different Marine batteries. I haven't run into any power issues yet as I don't always run at night so my lights aren't always on.

My question is this.

To be safe I am planning on hooking up two additional batteries to the system so I will have enough stored power to make sure that if I have to boat at night 2 days in a row I would have enough juice. I know I would be in a losing battle but with 4 batteries I would think I have a fighting chance.

I was looking and found the Optima Bluetop D31M. With 2 of them, I will have a huge power bank.

I am also worried about my alternator charging the batteries as they are not all the same.

What should I be installing with my 2 new batteries to not ruin them?

  • 70 A for running lights, GPS and a few others? That sounds like a lot: 1 kW of continuous draw. Are you using high-intensity spotlights or something like that?
    – Hobbes
    Commented Nov 22, 2018 at 12:38

2 Answers 2


I can't comment yet so I'll just put this here instead. As you stated using the amount of power you're using is a losing battle as the alternator cannot keep up at maximum output. Alternators, however, aren't designed to run at full tilt for extensive periods of time and by running it over a period of several hours you run the risk of damaging or causing the alternator to fail permaturely. It's always recommended with alternators to have a slight margin of extra power the alternator could generator at full draw simply to extend the life of the unit and prevent premature wear.

  • My alternator copes with running at full tilt when all the lights, heating etc etc is on when thrashing up the motorway in winter at night basically 8 hours driving... and so does the engine...
    – Solar Mike
    Commented Nov 22, 2018 at 8:53

You have to look at what is taken out of the batteries in x time compared to what goes in in the same time...

If the alternator can’t keep up then the system ends up discharged.

If the energy supplied in 24h exceeds the demand in 24h or 5h whatever time period you use and you need to consider several periods to establish what will happen.

Two points, one fitting a more powerful alternator is one possibility..

Two, using a split-charge system can separate the starting batteries from the load batteries - split-charge relays are easily found...

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