Mixing Batteries

[Bristol14]

I've rewired my '23 to accommodate two batteries now that I have an outboard with an electric start and power tilt. With spring sales now being advertised, I will be buying another battery to go along with my fairly new deep cycle lead acid battery. I've read several conflicting posts about mixing AGM batteries with regular batteries. There are some pretty good deals on AGM's as well as long term advantages. Any thoughts on mixing battery types. I have installed a regulator and mixing switch.

Thx.

[slode]

If the new battery is for starting the motor only and you're good with the capacity of your existing deep cycle get a marine START battery and keep it separated from the house battery, just tied to the motor.  If you want the extra house capacity keep the same size and type as your deep cycle or if you get an AGM deep cycle put it on a switch so you can keep them separated.  No harm in starting the motor with a deep cycle, start batteries are just cheaper.  Probably a good idea to separate them anyway just in case one draws down too far you have the other to start the outboard.

Not a good idea to wire different battery types together in parallel.  AGM and wet cell batteries have different charge curves.  If you keep the start battery and house battery separated by the switch and charge them separately you will be fine with having different types.  But don't mix different types (or sizes, or significantly different ages) in a parallel installed bank.

IMHO I have not found AGM deep cycle batteries to be worth the extra cost when the batteries sit on a good 3 stage charger most of the time.  As long as you're willing to do the routine maintenance (check electrolyte levels a couple times a season and top them off), store them properly off season, and never draw them under 50%.  If I were off shore cruising more often I may consider.

[Bristol14]

I wired a Blue Sea Mini Add a Battery that includes a Dual Battery switch and a relay that's supposed to switch over when one of the batteries is charged as I was planning to wire them in parallel.

[wes]

The problem is not just that you don't want to put batteries of different types in parallel, but that the charging voltages are different. AGMs are expensive, and you will shorten their life by charging at the voltages that are typical for conventional flooded lead-acid batteries. So the battery relay isn't a complete solution.

Recommend RC Collins' excellent blog, www.marinehowto.com, for objective, clearly written (although sometimes very opinionated) info on batteries and charging.

My setup is two Trojan group 27 AGMs on a Sterling smart charger. I generally run them in combined mode. I chose the Trojans for their excellent (US-made) reputation and because they are physically on the small side for Group 27s, so I was able to squeeze two of them into my CP 27's battery compartment. Two Lifelines would not have fit. On the (initially) expensive side, but I expect to get years of maintenance-free service. So far so good.

Wes

[slode]

Bristol14,

The automatic charging relay that comes with your kit isn't designed to only charge one battery at a time.  It has a relay that senses voltage rise when a charge is present and opens to allow the charge current to flow to both batteries.  Once the charge level voltage disappears (engine turns off) the relay closes to keep the house and start batteries separated so one doesn't drain the other.  Charge will always go to both batteries unless you flip the manual switch.  That's a good piece of kit and with wet cell or high charge voltage rated AGMs you'll be fine combining and charging off the motor and/or charger.  The website Wes references actually has a article specifically about charging relays https://marinehowto.com/automatic-charging-relays/

[MacGyver]

I wouldnt mix as the first reply said, they do have different charge curves, and that will cause a issue, if not a few, not just limited to early death of the superior battery.

AGM I felt were always good for the Jeep guys, rock crawlers, etc. Off roading really needed a better option to keep the acid in, and not just dumping out all the time.

Not sure what the good points would be to have it on a boat necessarily, Looks like I have something to google later LOL

Mac