Starting Battery w/ Deep Cycle Battery

[Chesapeake Bob]

Maine Sail,

With an old, traditional battery selector switch on a 1984 sailboat with a Universal M18 engine, what would you think of simply putting in a new group 24 starting battery with a new group 27 deep cycle battery. (Lead acid batteries)

 

[Maine Sail]

Maine Sail,

With an old, traditional battery selector switch on a 1984 sailboat with a Universal M18 engine, what would you think of simply putting in a new group 24 starting battery with a new group 27 deep cycle battery. (Lead acid batteries)

Not an issue unless they are used/dischrged when in the BOTH position. BOTH is fine when charging but you'll want to isolate them when discharging.

 

[swallowtail]

what about the old dogma of keeping the batteries identical if possible ?

 

[Maine Sail]

what about the old dogma of keeping the batteries identical if possible ?

They are both flooded lead acid so for charging purposes they are as identical as is necessary.

For discharging, as a "bank", then yes you need to try and keep them identical and preferably the same age/date code and model #. Does not sound like these will be discharged as a "bank" and will be used independently but charged together.

 

[swallowtail]

thanks. the reason i asked is because Bob's set up sounds similar to my own . My house batt. needs replacing . I was thinking of replacing both batteries with new agm but reading here i'm not so sure thats a good idea. My batteries only parallel when there is amp from the alternator so I think Im understanding that I could have a 90 ah deep cycle lead acid house with a starting batt. I could maybe use the money saved to buy some solar for battery maintenance. Any thoughts on this?

 

[Chesapeake Bob]

Maine Sail, you are the best and I refer boaters to your website often.

What about the rare situation when the starting battery needs extra help to crank the Universal M18? Under my proposed battery set up, is it OK to switch the battery selector switch to "both" for this purpose? Once the engine starts, I am assuming that I would leave the switch in the "both" position, and after shutting down the engine and while under sail power only, switch the battery to my house battery. Do I have this correct. I just want to sure and I trust your advice.

 

[Maine Sail]

Maine Sail, you are the best and I refer boaters to your website often.

What about the rare situation when the starting battery needs extra help to crank the Universal M18? Under my proposed battery set up, is it OK to switch the battery selector switch to "both" for this purpose? Once the engine starts, I am assuming that I would leave the switch in the "both" position, and after shutting down the engine and while under sail power only, switch the battery to my house battery. Do I have this correct. I just want to sure and I trust your advice.

Yes you can combine in that instance but you would be better to use the "full" battery not combine a dead battery with a good one. If a starter battery is having trouble with an M-18 there is likely an underlying problem that should be addressed. These motors take very little to start.. The easiest thing to do is to use your house bank and not draw it below 50% SOC and it should start your motor with ease...

If you don't have an ACR or Echo type device then your battery switch is a "charge directing and use" device. So yes, leave it in BOTH when charging and isolate it when not charging.

 

[Chesapeake Bob]

Maine Sail, say the engine starts on just my starting battery. Should I then move the battery selector to "both" while the engine is running to charge both starting and house batteries?

 

[LakeOntario270]

My Catalina user manual specifically states do not move the battery selector switch while the engine is running.

 

[weinie]

Maine Sail, say the engine starts on just my starting battery. Should I then move the battery selector to "both" while the engine is running to charge both starting and house batteries?

Yeah, don't do that.

You will blow the diodes in your alternator if you disconnect the battery while the engine is on. Make sure you shut the engine before turning the switch. I'm surprised no one mentioned this earlier.

 

[Maine Sail]

Maine Sail, say the engine starts on just my starting battery. Should I then move the battery selector to "both" while the engine is running to charge both starting and house batteries?

If you have a "make-before-break" battery switch moving between 1, 2 & BOTH is fine, just do not go through OFF..

The big problem is you likely have the old red toggle selector Cape Dory installed. They often become "break-before-make" as they age. I replace an awful lot of those old red CD installed switches.... Break before make can be bad for your alternator.

To test it turn on the cabin lights and move the switch slowly between 1, 2 & BOTH. If the lights dim at all when switching then it is breaking before making. If it is then don't move the switch with the engine running.

 

[onecoolair]

thanks. the reason i asked is because Bob's set up sounds similar to my own . My house batt. needs replacing . I was thinking of replacing both batteries with new agm but reading here i'm not so sure thats a good idea. My batteries only parallel when there is amp from the alternator so I think Im understanding that I could have a 90 ah deep cycle lead acid house with a starting batt. I could maybe use the money saved to buy some solar for battery maintenance. Any thoughts on this?

If you get the optima batteries they charge best with charger set to wet cell. Read and study the manufactures instructions for charging and discharging plus study their heavy pure lead spiral bound design specs.. http://www.optimabatteries.com/us/en/technology/
http://www.optimabatteries.com/us/en/
I was instructed a long time ago to select the wet cell setting and not the gel cell setting on my pro-tech 4 multi-stage three battery bank charger. Asked why and was told that the optima's are not gel cells and because of the different design specs. Their charge rate is closer to a wet cell then the more charge sensitive gel flat plate design.
Ask me how I know this....That's because I have been using optima's in the real world for a good thirty years. My first bank of optima's lasted at least 18 years, before I had to replace them. The current set is over 8 and working just fine. And that's a fact jack !!!

 

[swallowtail]

It seems the main rap against agm batts is that they are more prone to damage if used in a situation where they are not fully recharged soon after discharge.Have I got that right?
Another thing which occurs to me is that if the optima charges on the same setting as a lead acid could I put one with a lead acid (I have a good LA starting batt)

 

[Maine Sail]

It seems the main rap against agm batts is that they are more prone to damage if used in a situation where they are not fully recharged soon after discharge.Have I got that right?
Another thing which occurs to me is that if the optima charges on the same setting as a lead acid could I put one with a lead acid (I have a good LA starting batt)

The internal resistance of a TPPL AGM batteries is much lower than that of an FLA battery. Thus if "combining" your charge source will usually flow to the battery with the lowest resistance. This can result in undercharging of the FLA battery as the TPPL can take much if not all the available current. When the TPPL/AGM is used as a "start" battery this is usually not an issue but it is generally advised to use an Echo type charger when mixing battery types such as AGM/FLA, GEL/FLA, GEL,AGM etc...

Combining batteries of the same type FLA/FLA, AGM/AGM or GEL/GEL is fine. In most instances if the AGM is a starter you'll be fine but it is not "optimal" to combine differing types of chemistry.