Adding a second battery to be charged while engine is running. - Question

[kevinstan]

I know there are multiple threads already on this - but the reason I created a new one is for some of the additional questions I will be adding once I do figure out how to do this properly. It will prevent me from having to bring back an older thread just for my specific needs.

So here's my question: I have two batteries on my pontoon boat, I have one under the center console for the livewell and one in the back used for starting the engine. Can I run these in parallel by simply running an additional pos and neg cable to connect the two? And will this allow both batteries to be charged while the engine is running?

I do not use the livewell on the boat, instead I am going to be installing a stereo and want to use the extra battery for that so I am not draining my engine starting battery.

What is the best way to have both batteries charge from running the motor - but only have one for the radio, and one for engine start?

 

[dingbat]

You can install a 1-2-both switch and worry about remembering to turn the switch to the right position or install one of these and leave the worries behind.

http://www.iboats.com/Mini-Battery-S...iew_id.1085020

 

[kevinstan]

Is there a way for me to do it without even using a switch? Or do I at minimum need a switch to charge and use both batteries?

 

[dingbat]

At the bare minimum, you need switch to prevent equalization of the two batteries when connected to a common source of charge

When batteries are tied together they act as one. The battery with the higher charge will discharge to the lesser charge battery until the charges is even.

You need a means (switch) to keep them separated while in use yet combined to charge both with the motor.

 

[ondarvr]

You asked about the "best way", there are completely automatic systems that monitor every aspect of each battery independently and charge them with no input from you. These systems do cost more and may need someone that's familiar with them to install it (that's if you don't feel comfortable with it).

​There are some shortcommings with these systems, which result in some people not liking them, but most people that have them like them.


But, and there's always a but, how well it charges and meets your needs depends on the motor and how you use it.

​If the motor has a low AMP output you would need to run it for a full day at high RPMs to do much charging, and even if it has a high output system you still need to run it long enough (hours) for any meaningful charging to take place.

So look at how you plan to use it and if investing in a better charging system is going to pay off.

 

[Silvertip]

You mentioned adding a stereo system. Therein lies the reason you need to isolate the two batteries. If the two batteries are simply connected together in parallel, the stereo cannot determine which battery it gets its juice from. It sucks it from both batteries since they are ties together. So yes -- you need a battery switch or an ACR (automatic charge relay) to keep the batteries separated but automatically charge which ever one needs help.