Two dual purpose vs. one deep cycle and one starting battery

Hi all,

I have a 95 GB 42. Currently each engine has a dual purpose battery that doubles as house batteries that need replacing. I have a switch for battery 1,2 or both. It's been recommended to me to go with one deep cycle battery for the house and a starting battery dedicated to starting. If I do this, and the starting battery is dead, will I be able to use the house battery to start the engines in a pinch? I assume I would put the battery switch to the starting battery to start and then switch back to the house battery the rest of the time? Any experience or expertise here?
Thanks
Jon

You would be better off with a deep cycle battery for the house load. There are lots of ways to do it using an ACR or DC to DC chargers. If the deep cycle is big enough it should start the engine in a pinch but if you are using the start batteries only for starting it should never go flat. In my boat I have 4 start batteries, 2 for each engine in parallel. The only thing they are used for is starting. The house battery does everything else.

You didn’t mention what size your batteries are.

tiltrider1 said:

You didn’t mention what size your batteries are.

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Nor the displacement of the engines.

But in any case I lean towards a dedicated start battery and a dedicated house battery, each sized and spaced for their intended job.

You may be better off with AGM batteries. These generally work well for both starting and deep cycle use. So, if it is sized right, the house could start the engine in a pinch.

David

I’m a fan of a Battery Bank to start and run whatever is powered that enables the engine to run. One Battery Bank for each engine, constructed from batteries of start battery design.

Then one Bank for house loads including inverter and Nav. equipment loads, constructed from deep cycle batteries.

With 2 start/run Battery Banks and a pair of jumper cables, your chances of being unable to start both engines due to not enough dc power is very slim.

JonMenig said:

I have a 95 GB 42. Currently each engine has a dual purpose battery that doubles as house batteries that need replacing. I have a switch for battery 1,2 or both.

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You might need to clarify this. We might assume your house battery function is separate from your start battery function... but the switch might not actually do that.

For instance, I could imagine your engines might each be hardwired to a specific battery and your switch might only select which (or both) would service house loads.

Also, might not apply to a GB, but many U.S. manufacturers use a combined start/house battery architecture, where a given battery bank services both functions... and start is not separable from house.

In these cases, dual-purpose batteries can be an OK solution. You'd want to meet minimum criteria for cranking amps (starting), and you'd want to aim for as much capacity as is practical (house). You could also increase capacity for house loads by increasing bank sizes, if space permits.

Flooded wet batteries can be good value, if you can do the service. AGMs can be useful if service is difficult, and they can bring the added advantages of less off-gassing (reduced corrosion in battery spaces) and minimum self-discharge rates.

-Chris

In response to your specific question regarding starting with a dead battery, you would most likely need to put the switch to “Both.” On my 1995 GB42 with Cat 3208TA’s, the “1,2, Both” switch is wired in such a way that it performs two functions. The most obvious is that it selects the source or sources of power for “house” loads. The less obvious function is that is where each engine’s start circuitry connects directly to its respective battery. If you have a dead battery on either side, you must go to the both position to power the dead battery side’s start circuit. The start circuitry energizes the start button, instruments for the respective engine, and ultimately the starter.

Sax

On my GB42-C, we set up a bank of 8 six volt GC batteries in the laz, then had a port motor set with an alternator specifically designed to charge those deep cycle batteries. The starboard motor had an alternator that took care of two 8D batteries which were used for starting batteries. Genset took care of it's dedicated start battery. Simple system that worked well for us for 15 years.

Ideally, you'd have dedicated start batteries, using batteries that offer the highest number of CCA for the foot print, and one for each engine (most engine manufacturers prohibit sharing start batteries, especially for those with electronic control), and a separate house bank made up of deep cycle batteries. You would also have a parallel switch between the two start batteries as well as one between one start battery and the house battery, you would be able to parallel all batteries with this arrangement. You also need to have isolation switches between all batteries and loads, so three of these. With a little creativity, you could use 1-2-BOTH-OFF switches for both isolation and paralleling. Personally, for simplicity, I prefer doing this with 5 ON-OFF switches, one each for isolation for each engine and house, and two for paralleling.

I would use 2 big deep cycle batts for the house, house 1 and house 2

Then a dedicated starting battery for each engine. 4 batteries.

Actually if it were me, I would go LiFePO4 for the house and lead for the starts -

Forty two feet sounds a little big to only have 2 batteries for both starts and house.