Boat battery question

Santiago - 8-8-2020 at 03:53 PM

My Baja boat (now in NorCal) has two group 24 marine starting batteries, going thru a Perko selector switch, standard OFF-1-BOTH-2. Either June 2017 or June 2018 I replaced #2 with a Cabelas AGM. #1 is a standard flooded cell lead acid marine starting.
I have no clue how the boat batteries are wired but I do know the following:

#1 typical battery cables and a few smaller wires to both terminals.
#2 only has large battery cables.
The boat will not start unless there are two batteries, even if one of them is dead and the switch is on the the good one.

During the last three years, the Perko switch has been on #1, never moved to the AGM. I pulled both batteries today and #1 had typical corrosion on the terminals, #2 was clean, looked brand new. I hooked the AGM up to my "Smart" charger and after going thru its analysis startup, started charging at 7 amps. Normally a deep cell battery will start at 25 amps and slowly drop as the battery becomes charged, sometimes taking 6 to 8 hours to finally shut off. Am I to assume that at 7 amps the AGM was still in a mostly-charged condition?
Further, was it being charged by the motor charging system even though the Perko was set to the other battery all this time?
Lastly, how can I test an AGM, obviously can't use specific gravity, right?

And let's dispense with comments about turning in my man card and the like, I will stipulate to all that.

JZ - 8-8-2020 at 05:49 PM

55steve - 8-8-2020 at 06:34 PM

Engine charging system only charges the selected battery if everything is wired correctly. I usually have the selector to both when making a long run and #1 when reaching destination. #1 battery is wired to run pretty much everything - #2 is there if I need it - which I seldom have during my 45 years of boating.

SFandH - 8-8-2020 at 06:59 PM

Quote: Originally posted by Santiago  

The boat will not start unless there are two batteries, even if one of them is dead and the switch is on the the good one.

That makes me think the switch is wired incorrectly. Find the wiring schematic at the manufacturers website and check it out.

larryC - 8-9-2020 at 12:11 PM

It does sound like the agm battery was nearly charged so that is why only 7 amps at charge start. If your battery sw is always on #1 then that is the only battery that would charge while motor is running.

State of Charge on a AGM battery in your situation is probably done by batt voltage. 12.7 to 12.9 being fully charged.
Battery switches are pretty simple, and yours sounds like something is wired wrong since you have to have to have 2 batteries installed for it to start. To check the wiring on your sw just make sure that the positive cable from batt 1 goes to terminal 1 on the back of the switch. Then connect the positive of batt 2 to the #2 terminal on the back of the sw. Now connect your motors main wire harness positive battery cable to the common terminal on the back of the sw. Next you can eliminate the small wires coming from the #1 battery by running a wire large enough to carry all your secondary loads from the common terminal to a fuse block and then from the fuse block to your radios, bilge pumps, etc. Battery negative wires connect to your motors main wire harnesses negative lead. Wired that way when you turn the battery sw off everything turns off which is nice if you have an electrical short and need to shut everything down.
Good luck with it.

Santiago - 8-9-2020 at 12:47 PM

Turns out I was wrong, again. Everything runs fine with one battery installed and switch to that. Also, the smaller leads on #1 is only two grounds, looks like #10 or 12. I will follow those and see where they go, The positive terminal only has one battery cable going to the switch, The common on the back of the switch does have a lead to the fuse block so that is normal.
I find it kind of amazing that the AGM held a charge for 2-3 years just sitting.
This boat is starting to get used more, in 2017 thru 2019 I doubt there was ever more than 20 hours each year.
However, the salt rust on the trailers cross beams look pretty bad, I will need to replace those.