Charger question: 2 batteries w/ switch but no isolator

[NWVintage]

I tried to make the title descriptive...

I have a two group 27 dual-purpose wet-cell marine batteries that run through a selector switch. There is no isolator in the system, just the switch. ie. I can choose either or both but the whole electrical system runs off whatever I choose. This boat is fairly new to me and thus I replaced both batteries but now I need a charger so they don't get ruined. This will likely be used primarily as a maintenance charger but there will be overnights and such when the batteries will become deeply discharged, so I would like to have a charger that is reasonably capable for that as well.

Thoughts? Suggestions? Wiring options?

 

[Bondo]

Re: Charger question: 2 batteries w/ switch but no isolator

I tried to make the title descriptive...

I have a two group 27 dual-purpose wet-cell marine batteries that run through a selector switch. There is no isolator in the system, just the switch. ie. I can choose either or both but the whole electrical system runs off whatever I choose. This boat is fairly new to me and thus I replaced both batteries but now I need a charger so they don't get ruined. This will likely be used primarily as a maintenance charger but there will be overnights and such when the batteries will become deeply discharged, so I would like to have a charger that is reasonably capable for that as well.

Thoughts? Suggestions? Wiring options?

Ayuh,.... Ya don't need an isolator in a 12v system,....

Just hook up yer charger to a point in the electrical system,...

Then use the selector switch to charge whichever, or both batteries,...

 

[bruceb58]

Re: Charger question: 2 batteries w/ switch but no isolator

Or buy a dual bank charger.

 

[achris]

Re: Charger question: 2 batteries w/ switch but no isolator

Yep... I just hook a trickle charger up to one battery, and leave the switch on BOTH....

 

[UncleWillie]

Re: Charger question: 2 batteries w/ switch but no isolator

...there will be overnights and such when the batteries will become deeply discharged, so I would like to have a charger that is reasonably capable for that as well...

You did not mention what size boat and what charging system the engine has but....

An overnight discharge should drain A Battery, and Not THE Batteries.
When stopping for the night be sure to select just one of the batteries, or the option to start the engine in the morning will be gone.
One the engine is running, select BOTH Batteries to recharge the depleted one.

Adding an ACR to the existing system and separating the house and engine loads to separate batteries would make your life much easier.

 

[NWVintage]

Re: Charger question: 2 batteries w/ switch but no isolator

You did not mention what size boat and what charging system the engine has but....

An overnight discharge should drain A Battery, and Not THE Batteries.
When stopping for the night be sure to select just one of the batteries, or the option to start the engine in the morning will be gone.
One the engine is running, select BOTH Batteries to recharge the depleted one.

Adding an ACR to the existing system and separating the house and engine loads to separate batteries would make your life much easier.

1978 19.5' Glasply Cuddy Hardtop (saltwater fishing style). Merc 470. Original stator-style charging system with standalone water cooled voltage regulator that produces around 14 volt peak and still drops to a little over 12 after the boat has been run a while (it still works!).

I understand how to use the battery switch when using it for overnights. I think that an isolator is simply money that I'd like not to spend on this boat.

I suppose my biggest concern is overcharging the one that's still charged while the system is charging the deleted one. Could this occur? I suppose I could charge one, then the other, then switch to both for storage.

Really, I'm looking for the cheapest option (that is still a quality product) in an on-board charger which is designed to be able to recharge a depleted group 27 battery and will switch to a storage charging program once the battery is charged.

So maybe a single bank charger around 10amp? Brands?

 

[UncleWillie]

Re: Charger question: 2 batteries w/ switch but no isolator

...I suppose my biggest concern is overcharging the one that's still charged while the system is charging the deleted one. Could this occur? I suppose I could charge one, then the other, then switch to both for storage.

The battery is actually in control of the amount of charge it takes.
The charger is a simple power supply at its heart. It supplies a voltage.
The battery creates it own voltage that is countering the charger. POS to POS - NEG to NEG.
As the battery's voltage increases it counters the chargers voltage and the charge current eventually reduces to Nil.

If two batteries with drastically different capacities and charge levels are paralleled to a singe charger,
Each battery will independently control the amount of charge current it will accept.
The full charged battery will have minimal charge current and the depleted battery will draw the majority of the current.

A smart charger will monitor the two batteries as if they were one. When it sees the current has reduced to a minimal level; i.e. both batteries have reached full charge, it will stop charging and wait for the joint battery voltage to fall into the lower 13 volt range before it resumes the maintenance cycle.

You can safely charge two or more batteries in parallel without destroying the fully charged battery in the group.
It is really no different than using a full charged battery for 3 seconds to start the Boat/Car and then have the alternator supply it with ~14+ volts, charging it for the next half dozen hours. No Damage, No Foul.

Just be sure to check the Water levels in the cells occasionally.
Once the plates become exposed, Bad things can happen!

 

[NWVintage]

Re: Charger question: 2 batteries w/ switch but no isolator

Thanks Willie! That's precisely the info I was seeking.