How to do dual battery

ccna4481

Recruit
Joined
Jan 28, 2016
Messages
4
Hello all,

First post here...always been a pwc and quad guy, but just bought a 2002 Marada 18ft runabout...i wanted to put a second battery in to run accessories etc off of....what all do i need to do that and make sure the alternator would charge both batteries?

Thanks for your help
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
:welcome: ccn4481 to iboats...

First of all, the engine alternator will never maintain the batteries over time. You will still need to use a separate battery charger. And the reason is you don't run the boat long enough to bring batteries back up to full if basically drained. Maybe that will work for the starting battery, but certainly not the accessory battery. But to be on the safe side, I would still use a separate battery charger myself. And leave the accessory battery alone from the engine alternator and charge that battery after the outing. So either charge each battery separately, or only charge the cranking battery with the engine alternator making sure it stays up.

I am sure others will offer their opinions and there are lots of ideas concerning this subject as well. JMHO!
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
Howdy

Need a battery switch and manually move it to both after motor is running
OR
A battery switch and an automatic charge relay ACR
OR
A battery switch and an isolator
OR
Just and ACR or isolator

ACR_alternative_to_multiple.jpg


Two Bat one switch.jpg

Two Bat Isolater no switch.jpg
 

poconojoe

Lieutenant Commander
Joined
Sep 10, 2010
Messages
1,966
Alldodge posted some nice diagrams.
I have the ACR setup. Blue Sea makes the ACR and it will automatically charge, with your alternator, whichever battery that needs it or both if need be. It gives you peace of mind. Yes, it wouldn't work well if you completely discharged a battery under a large load such as a trolling motor, but normal accessories such as a stereo it will be fine. You can add a shore powered battery charger to the circuit as the diagram shows. The ACR will control which batteries the shore charger will charge, just as it does with your alternator.
Like I said...peace of mind that you always have a good battery to start your engine.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
Alldodge posted some nice diagrams.
I have the ACR setup. Blue Sea makes the ACR and it will automatically charge, with your alternator, whichever battery that needs it or both if need be. It gives you peace of mind.
That's not how they work. They sense the voltage on each side of the ACR. If either side gets above a setpoint, the internal logic will activate a couple of relays inside and join the two connections. It also has a failsafe so that if either voltage is below a low setpoint, it will not allow a connect. This way, it will not try to charge a completely dead or failed battery.
https://www.bluesea.com/support/arti...harging_Relays
https://www.bluesea.com/products/765...Kit_-_120A/FAQ

If you ran your trolling motor long enough to completely discharge the battery, the ACR will not connect.

I use the BlueSea ACRs as well. They have some limitations but they do work very well and I prefer them to diode isolators.
 
Last edited:

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Great advice from the guys as usual...

The ACR will (help) keep your house battery charged and the battery switch will keep you from getting stranded if your engine battery goes dead (and the house battery isn't depleted). I would do both the ACR and battery switch.

If you are partying and blasting the stereo at the beach, then running quickly back to the dock, the ACR won't do squat to keep the house battery charged. If you throw in a couple hours of skiing and have the engine running for a long time, the ACR may keep the house battery topped off.

Personally, I have several trolling motor batteries I can charge back up while trolling off the main engine, but it takes hours of trolling off the main engine to charge the trolling batteries. Even trolling for 1/2 -> 1 hour will throw enough charge back into the troll batteries to extend my day.

I also have an onboard charger, as mentioned above, to keep the batteries charged at the house. The worst thing you can do to a battery is leave it in a discharged state for an extended period of time, or leave it sitting all winter long with it slowly discharging. Best to keep it charged up every month or so.
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Also, tinned marine battery cable, lugs and heatshrink tubing can be purchased inexpensively from genuinedealz.com. They can also crimp and finish off the cables for a nominal charge of something like $1/end, you just have to be sure of the cable lengths you need.
 
Top