Twin engine dual battery question

SerenityPC · Posted: Sat Apr 08, 2023 9:43 am Post subject: Twin engine dual battery question

I have a 22 foot cruiser with twin 50 hp Hondas and dual batteries. For my port engine I use a marine starting battery. My question is for starboard engine, that cranks the engine, pulls electronics and overnight anchor lighting cabin lights and sometimes 12 volt overnight fan, should I use deep cycle battery with suffient cranking amps or a starting battery with high amp hours.

Robert H. Wilkinson · Posted: Sat Apr 08, 2023 10:07 am Post subject:

It would be better to use a deep cycle with a high enough cranking amp capacity or one of the dual purpose battery. Starting batteries should not be used for deep cycles. I would not use a deep cycle for a large V8 inboard or high compression engine. They can damage the starter from voltage drop if their cranking amp capacity is not high enough.

I do not recommend the 1000 amp group 24 batteries for marine use. Amp capacity is related to the number of plates in a battery. To reach the 1000 amp rating in a group 24 case means the plates are very thin and closely spaced. This does not seem to lend itself to the robust construction necessary for the marine environment.

Rob

-Waypoint- · Posted: Sat Apr 08, 2023 10:58 am Post subject:

This is what I am using on my Stbd. engine. My boat is wired the same as yours. I just bought the Engle 27 freezer/ fridge, am testing this battery out with more draw. New engines charges put out 17 amps no problems so far.

Interstate 27DC Marine / RV Battery
(this battery has been in the boat for two years now)

Specifications:
Group Size: 27DC
Part Number: 8-27MBP
Cold Cranking Amps(CCA): 600
Cranking Amps: 750 MCA
Reserve Capacity(RC): 160
Length: 12 3/4"
Width: 6 3/4"
Height: 9 1/2"
Weight: 50lbs

thataway · Posted: Sat Apr 08, 2023 12:17 pm Post subject:

I favor the Sam's Club Duracell group 31, FLA battery. ($117) Cap 100 amp hours, MCA 800. The reason, is I don't want that battery to run down, and not be able to use the Wallas Stove, or start that engine in the morning after all night draws. I have used this on both 25's Tom Cat 255 and both 22's. You always want a "combine" or better yet, a way to switch batteries to start the "house/engine start" to the other engine's starting battery. Get that other engine started before you start the "house" engine. I also carry "jump pack", which has many other functions, including charging my computer, any phones or electronics, plus the one I has will start a Cummins 8C (350 hp RV diesel), A GM 6-71, and any of the cars, and any outboard which a C Dory can use. I also use this compact battery to power the 12 V inflator for the dinghy (air floor). By using this I can blow up the inflatable anywhere on the boat, or even on the dock or land with the jump pac, and 12V inflator.

C-Wolfe · Posted: Sat Apr 08, 2023 1:26 pm Post subject:

Robert H. Wilkinson · Posted: Sat Apr 08, 2023 3:06 pm Post subject:

I agree with Bob - start your port engine then temporarily combine batteries to raise the voltage of the house battery before starting the starboard engine. Then switch again to separate batteries. If the house battery is drawn down more overnight than anticipated - over time it can damage starter motor.

Rob

-Waypoint- · Posted: Sat Apr 08, 2023 6:02 pm Post subject:

Caution! There is a reason that C-Dory wired the batteries separate on the twin engines boats. You can not have two running outboards on one battery bank! This will fry both outboard's charging system. Only one out board per battery bank. Carry a set of jumper cables only in an emergency jump start. This is only for a dead battery not good for continual charging all the time. Each out board will reverse feed each other when voltages peak.

Hunkydory · Posted: Sat Apr 08, 2023 7:07 pm Post subject:

To be more specific on Waypoint’s (Mark) comment. It can or maybe eventually will burn out the charging coil. When I didn’t know better during the first four years of running our CD22, which included three extended SE Alaska cruises, I would run with the batteries combined. During this 4 year period, I burned out the charge coil on one motor twice, but never both & this was with many hundreds of operational hours. Also, both times a coil burned out, I was making continual rpm changes with the motor controls where rpm was not held equal with each, due to sea conditions. When I repowered from the twin 40’s to 60’s, I was told this was no longer a concern by Sportscraft Marine. Still not trusting that information completely, I now normally don’t combine the separate batteries, one for each motor, unless using the windless or sometimes when operating lengthily periods on just one motor. I have forgotton several times when having the batteries combined to separate them when back to running both motors with thus far no problem with charging coils.

On extended cruises, I carry a Honda 2000 & as Bob does a powerful jump pack capable of Jump starting our trucks duramax diesel, so plenty of power for boat motor starting or auxiliary stuff.

Jay