Dual battery charging advice.....

OK, I am a little confused about setting up a dual battery charging system.

I currently have a cranking battery, and I want to install a deep cycle battery.

I'd like to just use a dual battery switch (1,2,all,off) so that I can start from the cranking and then switch to the deep cycle at night OR when the cranking is fully charged. My question is: how can I tell when the cranking battery is back up to 100% after I have started an engine, and it has been charging for a period of time. I heard that I could just test the voltage(ie when it's at 13.7v), but I thought that wouldn't work because doesn't the engine output more than this while its running??? (so I would be reading the voltage that the engine is producing, NOT the battery?)

You're over-analyzing the situation. Though BOTH batteries should be similar types, you need to start the motor on "position one", then switch to "all" for running.

A better setup is to have all deep cycle batteries, so you can keep the switch on "all" except while on the hook. (Then, you switch to "position two" so you'll have a fully charged starting battery at "position one" to fire the motor.) And, contrary to popular opinion, a properly charged deep cycle battery will start a motor with no problem at all.

A fully charged battery will "charge" one that isn't as charged until both finally receive a full charge, and without damage to either. People have been doing it that way for eons, so don't worry.

Jeff

PS: I have 5 batteries on my cruiser, with a battery paralleling switch that puts all 5 and both alternators on line simultaneously to recharge low batteries after an overnight. Works fine.

Hi sorry to butt in,any chance of a schematics for a two battery system Jeff in lay man's terms,so as even I can understand it.

If you don't mind the price, these are the way to go:
http://redarc.com.au/products-and-services/battery-isolators/smart-start-sbi
You can also use the trigger wire with a switch to use the deep cycle battery for emergency starts if need be.
They're a very good solenoid, indestructible and work very well.
They work by allowing the start battery to regain a set voltage, then allowing charge current to the second battery, as soon as start battery drops to 12.5v, charging is shut off to the second battery and start battery is charged.

Regards, Andrew.

On my (big boat) I run a three battery system.

A "starting" battery for my motor and a pair of deep cycle "house" batteries.

However, in my set-up it's a little more complicated than a simple selector switch.

In order to ensure that the "stuff" you have connected to your "house" battery doesn't drain your cranking battery (or vice versa), at minimum, you need to include a battery isolator into the mix.

An isolator will allow your motors stator/alternator to charge "all" the batteries, but will not let them (suck power) from each other.

My system goes one step further. Instead of a simple isolator I have an "automatic charging relay" which senses when one battery is topped up and shifts the charge power to the other batteries, plus keeps them all isolated when starting the motor.

So depending on how fancy (read how much you want to spend) your options and the solutions change.

Two similar batteries with a selector switch (which your really shouldn't change the switch while the motor is running - can cause electrical nightmares) is the "basic" setup.

It simply allows you to charge both batteries from the motor and start the motor with one, the other, or both.

A selector with an isolator - same as above with the added protection of not allowing the needs of one battery to steal from the other.

Or, the selector with a charge relay - that does intelligent charging plus isolation....

Jeez I think I'll buy an extra and charge after every outing, my poor brain is frazzeled trying to work that out

Don't blame you! My setup is ultra simple.

Jeff

You are onto something there.

Unless you have a relatively new motor with a high amp charge system (my 140 horse only has a 9 amp charge stator as an example), the easiest way really is to just charge when you get back on dry land.

Unless you are running a power sucking electric trolling motor the average group 24 deep cycle battery will supply all the juice you need for assessories during a days outing.

If you have a trolling motor, unless you run your big motor at full throttle for many hours (even a modern 90 horse with a 50 amp alternator could take 2-3 hours to replentish what gets sucked out) you still won't charge a drained battery back up.

So depending on your "style" of boating, the isolated charge system could be very much a waste of time and money. If you run your motor less than 2 hours during a particular outing (power out to fish for a few hours and then back, or pull the kids on a tube for a dozen laps around the lake) you likely won't even "top up" a fully charged battery - which means you will still have to throw it on charge when you get back to the dock...

hiiiiiii
this is a nice post and its intereting. given knowledge about yacht batteries
thanks.
luxury motor yacht

It's been a while since I've dropped by and a tad late to answer haventaclue but wanted to post this pic that may help others who has the same question.
BTW on some switches, it's recommended not to switch batteries while motor is running.