yet another battery question

[Jack]

Our usual practice is #1 battery for the house and #2 battery for the start. They are the same grp 27, deep cycle that the dealer installed. There is a refrigerator, but we keep a careful eye on the power consumption. After an over night with these same practices I switched over to battery #2 started the engine. All went as usual until i moved the battery selector to 1&2 so they would charge both batteries as we motored along a bit. There was a noticeable draw on the engine that lasted about 1.5 seconds. She then went back to idle speed and sounded normal. This is the first time this has happened. I assume that the house battery had more of a discharge than the cute volt meter indicated but it's never been wrong before.

So here are my questions:

1. Did the house battery have an excessive discharge that may have caused the engine to draw down when it was connected? and why does this happen?
2. Because they are both deep cycle should I be swapping the house load from 1 to 2 and back again to keep them more balanced?
3. Is there a better way to do this?

thanks again for any advise.
jack & robin

 

[Don S/V ILLusion]

There are differing views on how to manage batteries, most of which are simply different ways of doing it right- it seems everyone has their own personal favorite.

To your questions, a typical Gr27 battery has a usable capacity of +/- 90 amp. Your refrig likely draws 5 amp-hrs with a 50% duty cycle so in 12 hrs, you just ate up to at least a third of your capacity. Unless the battery was at an initial full charge, more. Bottom line - a Gr27 battery isn't enough for a typical refir plus whatever add'l demands the night brings. It should be noted your battery voltmeter is almost useless at measuring capacity.

This is likely why your engine (alternator) didn't like starting up. When the alt kicked in, it got a surprise as it sees the lowest battery as the demand.

There are lots of better ways to design a battery bank, one of the most common being to get a couple of 6V golf cart deep cycles as a house bank and keep your start battery for the engine.

This is a common discussion (about battery management ) with lots of stuff in the archives if you want to browse.

 

[Ron20324]

Just wondering if you tried to start the engine on the house battery before switching to #2? That MAY have worked, and then charged just the #1, the drained one.

 

[Jalepeno]

You should consider rewiring the two existing G-27 batteries in parallel to become the main house bank on B-1 and buy a cheap G-24 starting battery, battery box and cables to become the reseve B-2 battery. This way you double your main battery capacty at minimal cost and still have a reserve. Use B-1 for everything, including normal motor starting. Don't use the BOTH (B-1 + B-2) switch position but remember to switch to B-2 once in a while to recharge it. Do not switch through OFF if the motor is running.

 

[Warren Milberg]

Another thing to consider here is whether your main switch is a "make before break" type of switch or not. Most newer boats will have "make befor break" switches that allow switching between batteries when the engine is running. Here is an easy way to tell:
With the engine off, put the main switch on "Battery 1" and turn your cabin lights on. Then switch to "Battery 2." If the lights don't flicker, you have a "make before break" main switch. If they do flicker, you don't -- and shouldn't be changing battery selections while the engine is running or you could fry the diodes in your alternator.

 

[Jack]

thanks all

"Just wondering if you tried to start the engine on the house battery before switching to #2? That MAY have worked, and then charged just the #1, the drained one..."

didn't try to start on B-1, started on B-2 then switched to Both 1&2. When i switched was when i got the draw on the engine.

Thanks for all the additional advice...
decisions to be made...
jack