OX66 charging second battery

[Chinookie]

As has always been the recommendation for parallel battery banks, it is important that the same type, model and age batteries be paired together.
Not only is the voltage of the batteries important but more so is the internal resistance/impedance of the batteries. That parameter can differ for different battery technologies.
I am not certain but the article may have been referring to paralleling different voltage batteries like a 12v and a 6 volt together as opposed to a 12v at 12.5 volts and another 12 v at 11.2 volts

I have forever cemented myself in Grady White forum lore as the person who questioned 40 or more years of common practice. Sigh, You all are too kind not to beat me up for reposting a Reddit question. But at risk of getting another dunce cap for today could someone please explain what Skunkboat means when he says switching between 1 and 2 is stupid? Does that mean he moves his battery selector from off to 1 (starter) but never to 2 (house) or both, because the aux charge cable is taking care of the charge on the house? Or does he move from off to both and never to 1 or 2?

 

[Chinookie]

If you have a severely discharged or damaged battery and you parallel it with a good charged one, it will "try to charge" the bad battery. In extremes, it can move a lot of current and not be pretty.
It will definitely discharge the good battery, possibly leaving you screwed.

For that reason, BOTH position is supposed to be a last resort.
I know many people use it as a "charging" position. Many people doing the wrong thing doesn't make it right. Some dealers even tell people to do that.
Using BOTH defeats the point of having two batteries and a switch. Many people forget to take it off BOTH.

You have a one motor, one battery switch, 2 battery setup. Lets call them 1 & 2.
Neither is technically the "primary" battery. Whichever position the switch is on connects that battery to start and charge ...AND the House 12V to the dash.
The point of that setup is to always have one battery that is fully charged in reserve, in case the one you are using won't start your motor.
If 1 doesn't start, switch to 2 (not BOTH). If neither will start you are already screwed so try BOTH.

Often the auto-bilge pump, and possibly stereo memory, is directly connected to the "1" battery. A second autobilge may be on the "2" battery.

Ideally, you would use the batteries equally. Switching the one that you use each trip. That is a pain. Nobody remembers. Its stupid.
Use the Aux charging cable...

I think I may be starting to understand what I had backwards in my head, thanks to your reply, and it has implications for what types of batteries go where. Please tell me if I have this straight. If I move from Off to position 1 when I use the boat, then battery 1 should not be a starting battery, it should be a dual purpose battery, because it is going to start the engine AND run the house. The backup battery number 2 will never be drawn from normally, it is there like a shadow, and is always kept topped off because it has an aux charge line on it. Number 2 battery can be a dual purpose battery as well, but in most cases is probably just a starting battery because its ONLY purpose is to start the boat if #1 is unable to. Do I have it right?

 

[SkunkBoat]

...when he says switching between 1 and 2 is stupid? Does that mean he moves his battery selector from off to 1 (starter) but never to 2 (house) or both, because the aux charge cable is taking care of the charge on the house? Or does he move from off to both and never to 1 or 2?

in a typical setup from grady, one motor, one switch, two batteries, "2" is not "the House". The house is connected to the FEED (or COM) of the switch along with the "starter" cable of the motor.
The house battery is whichever one the switch is on. The same one that starts the motor and gets charge from the motor.

You can separate the House from Start. IMO, it makes more sense than the OEM setup. If you do, you need to charge the house somehow. Either an AUx cable or an ACR.

 

[SkunkBoat]

I will preface this by saying that I ran a 20 footer for 18 years with a single Group 27 dual purpose and no battery switch. Every 5 years I replaced the battery.
As long as there was nothing left "on" on my dash, I didn't need a battery switch. I had a VHF, gps/ff and lights. No stereo.

I now have a 26ft and twins which is different than your case, so I won't talk about that.

You COULD separate the 12V house(which we will label (2) from the Start (1).
In that case you can have a deep cycle House battery (you can still use a Dual Purpose thats same as your start battery)

Move the House feed from the FEED(COM) post of the switch to directly to the "House(2)" battery pos terminal. Doing this disconnects House 12V from the MOTOR.
Connect Aux charge cable to House(2) battery pos terminal so that it will charge from the motor

Bat 1 is connected to switch position 1. In normal operation, you put switch on 1. Bat 1 starts the motor and charges thru the starter cable.
Bat 2 is connected to switch position 2. You can use switch position 2 in an emergency if your bat1 will not start the motor.
Bat 2 is always connected to the House. (you should put in a switchable breaker to turn off House). bat 2 charges from the Aux cable whenever the motor is running, regardless of switch position.
I would connect autobilge to Bat2


You could replace your 1/both/2/off switch with a blue seas dual circuit plus switch which will turn ON/OFF the start and house and allow emergency start using a "combine" feature which is like a "both"

Blue Seas has a "add a battery" kit that does all of this using a switch and an ACR https://www.bluesea.com/products/7649/Mini_Add-A-Battery_Kit_-_65A



What you gain from separing the House is that your House loads will not be draining your START battery, ever. Your start battery starts your motor and raises and lowers your motor and charges while its running.
You turn the switch to 1 and go boating. You turn the switch off when you are leaving the boat. You never use 2 or Both unless there is a problem
Your Bat2 charges regardless of switch position.
Your Bat1 will not be cycled low over and over again from running a stereo at the sand bar all afternoon.



quick drawings...

What you have


to separate house and add Aux charging

 

[DennisG01]

No worries, Chinookie! Everyone starts somewhere with their "knowledge base"!

As you're starting to figure out, it's a little confusing because there are multiple theories on the best way to use two batteries. And it really boils down to how they are set up for charging (one or two charging leads and where they connect, ACR, etc) and what makes the most sense for YOU.