3 Battery system question

Greetings All. I'd like some opinions. I recently purchased a new Lund fishing boat. It came with a two battery system. One battery is isolated for the engine, the other a battery exclusively for the accessories, trolling motor, radio, etc.

I'm wanting to add a 3rd battery to the system to be a 2nd battery exclusively for the accessories. The reason I want to do this is that I actually troll with my trolling motor for long periods meaning that the trolling motor runs continually for very long periods. With 1 battery I usually get about 5-6 hours of continuous trolling for 3/4 knot troll. I want more troll time.

My question is, what is the best way to set that up? Should I just drop in that other battery and hook it up in parallel with the other accessory battery or is it better to get a battery switch? Also when I add the 2nd accessory battery is it best to go to a 2 bank onboard charger or can I stick with a 1 bank since the batteries would be hooked together anyway?

The battery for the engine is already isolated with it's own charging system and I'm not wanting to mess with that.

Thanks in advance for your thoughts.

Re: 3 Battery system question

The best way to go is with a battery switch. You need to look at the info on your charger. My 1 bank is hook thur a secound battery switch. So when one battery is done charging I select the secound battery & charge it. I'm not sure if all chargers can be hooked up this way.

Re: 3 Battery system question

See I was wanting to avoid charging 1 battery at a time. My thought was to put in the switch but to also directly wire a 2 bank charger to the batteries and leave the battery switch in the off position. That would allow me to charge them both at the same time.

Why is the switch best though? Does it give you better battery life or something?

Re: 3 Battery system question

The 2 bank would work out fine for that. Using a swith will let use both batteries at the same time or just one battery at a time. So you have a choice on how you want to use them.

Re: 3 Battery system question

If you have a low/marginal battery, and your batteries are connected in parallel, the good battery will stabilize (drain it) to the lower voltage. This is why I would have the batteries on a battery switch, so you can run them separately. Also, if one of the batteries goes bad, it can ruin the other battery,,, if connected in parallel.

What type of trolling motor are you using? The newer digital TM's will run far longer at slower speeds than the old style (cheap) TM's. They are more expensive, but in the long run probably cheaper since you don't have to buy additional batteries, and additional batteries in the future. I've had both a MinnKota Traxxis and a MotorGuide Varimax, the Varimax was cheaper, literally! The Traxxis was worth the extra money. I've trolled a full day (8 hours+) on a 55# Traxxis with 1 group 29 battery.

Re: 3 Battery system question

I have a system just like you're thinking of. Mine hapens to have a ACR/Switch but if your's is already isolated and you want to keep it that way, then you could add the second battery in parallel. A single 20amp onboard charger would be all you need then. The general condition of both batteries should test good with a hydrometer before you hook them together. If one won't hold a charge it will drag the other one down with it.

I can go for about 3 days on a single charge with my group 29's. It's amazing how much more I enjoy getting up early when all I have to is walk down to the slip, jump in and go. It takes a good 10 hours to charge them back up to full from 40% with a 20amp charger.

Re: 3 Battery system question

If you just bought a new Lund boat I seriously doubt the second battery is dedicated soley to the accesasories. Better check your wiring. You can do this by disconnecting one cable from the "second" battery and then see if the accessories you mention work. If they do, those accessories are wired to the starting battery - not the house battery. Another way to check that is to look at the STARTING battery. There should be the two LARGE battery cables connected to that battery and there should also be another red/black pair of #8 or #10 wires conencted to that battery as well. THAT pair of wires feeds the "BOAT" electrical system (or the accessories as you put it). If someone moved those two wires to the HOUSE battery, that is NOT the way the system was designed to run. The second battery is intended for the troller.

As for adding a third battery there are as many ways to do that as their are opinions on politics. One thing is certain, connecting two batteries in parallel does NOT cause the weaker battery to pull the stronger battery down to its level. It results in the two batteries "equalizing" meaning the stronger battery will tend to "charge if you will" the weaker battery until the two voltages are equal. This is very different that was stated earlier. Adding a switch or ACR only adds complexity and potential points of failure in what is a very simple system. Why not add a simple "A" / "B" switch capable of handling the troller motor current so you can run the troller from battery "A" until it is discharged to about 10.5 volts and switch to battery "B". This prevents equalization, you can use a two or three bank charger connected directly to the batteries and have a very simple system. Use a dual battery swtich if the cost of an A/
B switch is greater than Guest or similar switch.