Can I charge 2 batteries w/ 1-bank charger using a battery switch?

sh00ter · Apr 28, 2021

And No, I can't afford a new 24v system right now, but having a second battery would be nice anyway w/ just the 12v system.

Current setup

12v TM w/ 1 battery that is connected to 1 bank on a 2-bank charger, so we have to treat this like a 1-bank charger for my question since the other charge bank is being used by the starting battery.

Desired setup

Second TM battery added to be able to switch over to if Batt #1 gets too low. I plan to buy a battery switch so I can easily switch power between Batt #1 & Batt #2 when necessary.

The unknown

How can I charge 2 batteries off a single bank of a charger? The second bank is already in-use for the starting battery and I do not want to buy a 3-bank charger right now since this 2-bank is basically brand new.

The plan?

Wire the single TM bank from the onboard charger to the new battery switch itself (instead of directly to a single battery), so when I switch the battery switch, not only will that control which battery powers the TM, it will also determine which battery will receive a charge at any given time from the TM bank on the battery charger.

IS THERE A WAY TO DO THIS?

THE JOKER · Apr 28, 2021

If you have a 12 volt trolling motor you can wire the batteries in parallel and one charger would work. I would put a load on it.
Positive to positive and negative to negative.
There really isn’t any advantage to switching between two separate batteries anyway

Oklahomabassin · Apr 28, 2021

THE JOKER said:
If you have a 12 volt trolling motor you can wire the batteries in parallel and one charger would work. I would put a load on it.
Positive to positive and negative to negative.
There really isn’t any advantage to running two separate batteries anyway

This would work. It would be best if batteries were at least similar in size and age. Everything I have been told its best to match batteries for this.

THE JOKER · Apr 28, 2021

You could just get a $20 maintenance charger for the cranking battery

Oklahomabassin · Apr 28, 2021

I don't put a charger on starting batteries except during the cold months or if I am parking it for a longer time. The outboard motor charges the batteries when it is running.

Oklahomabassin · Apr 28, 2021

I have separate battery bank for TM. I have a 3 bank battery charger on those batteries. After a trip, the main gets disconnected and the charger cord is plugged in. The starter batteries are 2, hooked together in parallel to have more battery bank for electronics between outboard runs.

sh00ter · Apr 29, 2021

THE JOKER said:
If you have a 12 volt trolling motor you can wire the batteries in parallel and one charger would work. I would put a load on it.
Positive to positive and negative to negative.
There really isn’t any advantage to switching between two separate batteries anyway

Yeah I was aware of that, but the batteries must be the same and in the same condition. I've read that it is common for batteries wired this way to fail prematurely unless you buy them the same time and are same model, etc. I sort of like the idea of running one down and then starting fresh anyway, but if the parallel thing would give longer run time, how would that compare to running each battery out separately?

THE JOKER said:
You could just get a $20 maintenance charger for the cranking battery

Yeah I have a few of those tying around too but the boat is kept outside and I like just being able to use the onboard. Also, the onboard is more sophisticated and temp adjusted, etc. than a cheap battery maintainer. I'd probably just add another single bank onboard marine charger if I had to.

Oklahomabassin said:
I have separate battery bank for TM. I have a 3 bank battery charger on those batteries. After a trip, the main gets disconnected and the charger cord is plugged in. The starter batteries are 2, hooked together in parallel to have more battery bank for electronics between outboard runs.

So how do you charge the starter batteries?

Oklahomabassin · Apr 29, 2021

sh00ter said:
Yeah I was aware of that, but the batteries must be the same and in the same condition. I've read that it is common for batteries wired this way to fail prematurely unless you buy them the same time and are same model, etc. I sort of like the idea of running one down and then starting fresh anyway, but if the parallel thing would give longer run time, how would that compare to running each battery out separately?

So how do you charge the starter batteries?

I don't know the run time difference by using 2 batteries wired in parallel or individually. If they aren't deep cycle batteries, discharging too low and charging shortens the battery life as well.

Starter batteries charge when outboard motor is running. When I do put a charger on the starter batteries I hook an auto charger up to the batteries and plug it in an outlet. It will read voltage and trickle charge as needed. The charger can be used as a 2 amp trickle charger, 30 amp fast charger, or 90 amp start boost. (IIRC the amps)

cowadle · Apr 29, 2021

what you need are deep cycle batteries. wire them in parallel +to+ and -to- with 12 volt the cables need to be larger than 24 volt. with deep cycle batteries you can safely discharge to 50 percent and not shorten their life. the batteries need to be exactly the same and condition or you are asking for trouble. you measure a deep cycle batt in amp hours and not cold cranking amps. you can even go with two 6 volt golf cart deep cycle wires in series P to N and N to P and you will even have more amp hours or run time and probably cheaper to boot.

sh00ter · Apr 29, 2021

so two 6v golf cart batteries in series will run a 12v trolling motor? I guess in series must make it 12v.

Can I charge 2 batteries w/ 1-bank charger using a battery switch?

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