testing 6v house batteries

[Mikeopsycho]

I've always thought I'd install an external alternator if the existing quit working. It does seem odd that the set up I have now worked so well in the past. We used to get 5 days out of the batteries without starting the engines. If I install an external alternator what amperage should I look at? Maybe changing both engines to external would be best? Money is an object for us, so I'll have to make upgrades over time.

 

[bruceb58]

It does seem odd that the set up I have now worked so well in the past. We used to get 5 days out of the batteries without starting the engines.

Over time, the capacity of the batteries get less and less, especially if you are deeply discharging them. Pretty normal!

Add the alternator to just one engine.

I take it you have the house battery shared with a start battery on one engine and the other engine just has the one battery on it?

 

[Mikeopsycho]

"Obviously boats aren't aircraft, where weight is a prime consideration, but I'm still not tracking with what he said, or what your experience was. Were the 12v type 27's of the same capacity, chemistry, and make, as this 6v series bank you've been using? Were they used and cared for the same (charged and discharged?)."

Spoilsofwar, no, the two type 27"s were not of the same capacity as the two 6V. They were used the same. I don't feel this is a inquisition at all, and I thank you for participating in this thread. I'm OK with the direction this post has veered, any battery discussion is relevant and interesting to me at this point.

I have two type 27 house batteries in my small motorhome that I installed about three years ago, and I've been happy with them so far, but they don't have to power the fridge like in my boat.

My next question was going to be "can you get one large 12v that has the same capacity as two 6v", but Bruce answered that above.

 

[Mikeopsycho]

I take it you have the house battery shared with a start battery on one engine and the other engine just has the one battery on it?

Huh, I can't figure out how to do the quote thing. Anyway, it doesn't matter which engine is running, or if just one is running, whichever battery I have the switch set on charges, as far as I know.

 

[spoilsofwar]

Problem is, that is not possible. You actually have more cabling for the parallel set of 12V vs 2 6V in Series.

Right, but that's why I said, assuming you could do the job with ONE 12v battery as opposed to taking two 6v batts and wiring in series. again, it's my aircraft (electric, unmanned) background coming out. The fewer solder joints, the better.

 

[bruceb58]

Huh, I can't figure out how to do the quote thing. Anyway, it doesn't matter which engine is running, or if just one is running, whichever battery I have the switch set on charges, as far as I know.

So...do you have just one start battery that starts both engines?

 

[Mikeopsycho]

So...do you have just one start battery that starts both engines?

Yes, that's correct. Only one 12v starter battery.

 

[bruceb58]

Ok...so that probably means both alternators are hooked together. That definitely helps your charging output situation.

 

[Mikeopsycho]

Oh, I see. So I am likely getting up to 80 amps with both engines running at higher rpm's. Is that considered adequate?

 

[bruceb58]

It helps. Not sure you are outputting even half of that at 1800 RPMs though. I Mercruiser manual may spec what the output is at various RPMs

 

[Mikeopsycho]

I'll try to find some info on that bruceb58, and thanks for all the help. Thank you to everyone for giving me lots of great info to mull over.