On Board Charger Question

[Boss Hawg]

Hope i ask this right :redface:
I have a 2 bank on board charger--
One lead to my starting battery-
One lead to my "accessary" battery-
I also have a 3rd battery (just in case) next to my accessary battery under my center console-
Question is------Would it be OK to just connect those 2 together to keep the 3rd "mantained? They are about the same condition & age
Right now i hook a regualar battery charger to the 3rd every couple of trips out just to make sure its "topped off" even though i dont use it -
I have my onbank charger on a timer to come on 6 hrs every night-
Thanks

 

[Silvertip]

Re: On Board Charger Question

If you hook the two of them together you create a parallel system so your accessories will now use both batteries, not just the one. Keep in mind also that because you have two batteries connected to one charger it will take longer to charge them both because they appear as one very large battery to the charger. Another consideration is that even though you say the batteries are the same age and condition, are they the same size as well. Those two batteries will not have the same identical capacity so one will always be stronger than the other. That means the weaker one will always drain the stronger one until the two are equal. Not a big deal if all things are prettry equal. It is a big deal if one of the batteries is considerably weaker than the other. You can avoid the situation by simply installing an ON-OFF swtich between the two batteries. Turn the switch off to separate the third battery from the second. Turn it ON when charging both or if you want to run all accessories from both batteries.

 

[EricJRW]

Re: On Board Charger Question

That switch idea is a good one Silvertip.

Boss Hawg, if you are comfortable with electronics, you could add a 110v "normally open" relay to the output of your timer. Then, when the timer comes on, the relay closes and the batteries are connected. Whenever the timer is not on (also covers the time you are on the water) the batteries are isolated from each other.

PS. You might want to still install that switch as Silvertip suggested, as this would let you tie the batteries together while on the water, allowing you to draw from both. Just be sure to size that switch and wiring for the amps you plan to draw.

 

[fish_on_the_deck]

Re: On Board Charger Question

That switch idea is a good one Silvertip.

I was thinking of a switch too when I read this. But maybe a off/1/2/both switch would be even better. You could switch between 1 and 2 every time you go out and let the alternator charge one of the batteries each time, but keep the weaker of the two from harming the stonger.

This would also allow easy parallelling for charging them both at once. By putting the switch on both when youre at the dock, and the automatic nightly charge would maintain them both.

Although I'm not sure how good of an idea that is... you mean you charge both batts 6 hours every night regardless thier charge status? hopefully your charger has a built in feature to prevent oversharging?

 

[EricJRW]

Re: On Board Charger Question

I have an on/off/on switch that I use as a "bank charger" for my two batteries (not connected to each other). I just take turns which one my trickle charger charges. Once I see it's topped up, I flip the switch.

 

[Silvertip]

Re: On Board Charger Question

Don't know how big your engine is or what it's alternator output capacity is but most outboards do not have the capacity to charge two batteries much less three. It is once again very simple math to show the reason why. A mid size outboard of 50 - 75 HP has an alternator that outputs about 15 amps at wide open throttle. If you drew 15 amps out of just one of those batteries you would therefore need to run the motor wide open for one hour to replace that current. If you have a high output alternator as many newer EFI/DFI outboards and I/Os have, then a dual battery switch or ACR makes sense.

 

[Boss Hawg]

Re: On Board Charger Question

All good info guys