2 parallel deep cycle batteries to start engine

kloudie1 · Jun 10, 2011

Depends a lot on how you use them, but I have 2, 110 amp hour deep cycle batteries that are 1 and 2 on the switch.. I use them alternately for all the electrics, I usually charge the one I've just used and use the other for the rest of the day .. I am on shore power most of the time, so charging is typically not a problem.. I don't use them together.. I am not a high power user..
so .. yes you can but I prefer to not use them together.. One battery will start the engine.. if the engine fires quickly, it does not use a lot of ampere hours..

Don S/V ILLusion · Jun 10, 2011

Engine starting does not create an excessive burden on any battery so you certainly could do what you propose, however
I assume you know the argument about having a separate dedicated start battery for obvious reasons.
If you haven't already done so, my suggestion is to start from the basics - that is, figure out your total anticipated energy needs in amps on a typical cruise or between recharge, multiply by two (so as to not discharge more than 50%) and see what you calculate in terms of total capacity need. The unasked question then becomes whether your existing charging system fits that picture

Maine Sail · Jun 10, 2011

njl said:
Would it be unreasonable to use 2 deep cycle 12V batteries in parallel to start a Volvo Penta MD2B and provide "house" electricity?

I have an AGM and Gel both on their way out and needing replacements. Originally the AGM was for "house" and the Gel was for engine starting; the Gel barely functions, so currently the AGM is supplying all electrical needs (which are quite meager).

Anyone here use a single bank of Deep Cycle batteries for all electric?

This is absolutely fine and in most all cases will provide significantly more cranking amps to the motor than a single "starting" battery will. When you parallel batteries you combine amps and this includes cranking amps.

Our house bank of three deep cycle 12V batteries provides over 3200 cranking amps at 70F. Our single "starting battery" provides about 670 cranking amps. You do not need a dedicated "starting" battery to start a small aux diesel engine and a parallel multi-battery house bank will give you more oomph.

Roger Long · Jun 10, 2011

Maine Sail said:
This is absolutely fine...

And I'm one that does it. I always kept my battery switch on "Both". When I put in the battery monitor, I tied the batteries together and replaced the three position switch with a simple ON/OFF switch.

My engine 32 year old, 20 hp, diesel always starts like a new car. Push the button, boom, it's running. My "starting" battery is one of those portable car jump starter units the I keep charged up and stored in a locker. With a little care I should never have to used it. The battery monitor, with it's alarm, should make that even less likely.

Roger Long · Jun 11, 2011

njl said:
battery monitor are you using.

Look for the several threads over in Maine Sail's section on Monitors. For less than the price of a one night tie up with dinner at a marina, there is no excuse not to have a Victron.

Winkfish · Jun 13, 2011

Those jump starters are a great thing to have. I always carry one on my fishing boat. The one time I had to use it was when another boat's battery died. I pulled up, passed it over to the other boat, he started his motor, and passed it back to me. No messing with jumper cables over water, etc.

Tropic Cat · Jun 15, 2011

My first post here.

Small diesels don't need all those CCA that our cars do, especially in warm weather. I too carry 2 deep cycle batteries and no starting battery. What's more, they were the cheap group 27's, sold at Walmart 3 years ago and they work just great. I just put them on a load tester last weekend because I was curious and they still test good.