No isolated starting battery

My boat came with an 8D & 2 pairs of golf cart batteries. They are in 2 seperate locations, but essentially just function as a single bank. Obviously originally the idea was 8D for starting, and the golf cart batteries for the house bank.

I will be putting in new batteries, and for my uses, it makes sense to continue with the banks unified. One caveat is the space where the 8D is, does not have the extra inch of height to accommodate the taller trojans.

Two questions:
1. Is it a bad idea to mix a dual purpose 8D with trojan 105's for a house bank? Will the 8D have a comparatively shorter lifespan (than the trojans) when hooked up in this arrangement? (And therefore will be something I will need to monitor independently periodically?)
2. Would 4 pairs of trojan 105's be fine for starting? I am aware that different batteries are made for different specialties. Just checking if there's any reason I specifically need a purpose made "starting battery"?

What engines are you starting? Genset?

Cummins 6BT. Westebeke 7.6kw

Four pairs (8 batteries) of GCs will definitely be enough to start a 6BT. If you insist on keeping them wired together, this is probably the best solution because the batteries are all the same, although I would much prefer an isolated and combined/ACR starting battery.

David

The popular wisdom is for a given battery bank have the same batteries since otherwise they will charge / discharge at different rates affecting performance and probably useful life. Also, if you have start and house batteries in one circuit, you are vulnerable to not being able to start your engine or genset if you use too much energy for house purposes. It is usual practice to isolate start and house batteries with a switch, either a manual one or an Automatic Charging Relay or similar devise. In my case, both engines charge the house bank, 4 pairs of GC's. The charging is switched by an ACR to charge the start batteries, 2 group 31's, after the GC's reach 13.5 volts and at that point, house and start circuits are connected with the ACR closed. When the house bank drops to below 12.7 volts, the ACR opens and the two circuits are disconnected. My group 31's are in parallel and have plenty of power to crank my 6bt's. I agree that 4 pairs of GC's will have plenty of power to start your Cummins and genset.

I never worried since I had a generator with it sown start battery torecharge thebig batteries if needed. It doesn't take much charge to start that 6B

Mixing batteries in a bank is bad. Using an ACR with a mixed bank is also bad. People do it all the time and for the most part get away with it. When combining a mixed use bank for charging one should use a DC to DC charger, otherwise known as a SmartACR.

ERTF said:

I will be putting in new batteries, and for my uses, it makes sense to continue with the banks unified. One caveat is the space where the 8D is, does not have the extra inch of height to accommodate the taller trojans.

Click to expand...

I suspect that height limitation would make watering difficult, too?

Lifeline (and maybe others) makes an AGM 8D that might be useful if you stay with the mixed architecture...

Or... they also make 6V AGM GC2-type batteries... (which may or may not be as tall as T-105s?)... but which could also maybe be mounted on their sides...

We used two pairs of their 6V GPL-4CTs to start one of our Cummins 450s, worked fine.

-Chris