Is it okay to link two batteries of unequal size?

Hi,

It is not best practise.
This is what is balanced and best for twin twelve volt batteries.



As it often doesn't cost a dime more to do this, I think it is worth the trouble.

If you wish to understand the "why" surf here:

correctly interconnecting multiple twelve volt batteries

Others may say it doesn't matter--but unless there is a compelling reason to not optimize charging and discharging why not do it the best possible way?

I would think that with different type batteries the charge rate by the onboard charger may be unequal...I suppose its possible for the smaller battery to receive an overcharge, but I haven't a need to try this...

As BFL said, yes you can safely parallel those batteries. Not the best solution but for the price of one extra battery than must be 12 volts not a bad idea. Generically a Group24 Deep Cycle is 80 AH and the Group27 is 100 AH giving you 180 AH battery bank with 90 AH being useful (try real hard not to take those batteries below 50%). You should get a couple years of use out of them

When they eventually go replace them with dual 6volts. I just picked up 2 new Trojan T105s for $90/each at a golf cart outlet. This would give you 225 AH which again shouldn't routinely go below 50% but with these batteries you can safely go down to 20% SOC occasional without significantly reducing life. Much more useful, robust and forgiving for boon docking purposes.

Well, the dealer threw the battery in at no charge, so I didn't get "cheated", but I don't believe it is the optimum type for an RV. However, I have noticed that some brands, such as Sears DieHard do not carry the DC, they only carry the MS, which is a "hybrid". Yes MS is a Marine Starting battery, which has a higher CCA rating than a true Deep Cycle, but is also rated for deep cycling. Anyway, I appreciate the response as I wanted to go with the 27Dc, and already have the box for it.

Isn't the MS a marine "starting" battery? You don't want to use that for your Rving, which calls for a "deep cycle" battery, but many have the hybrid deep cycle/starting type at least (such as that 27DC)

If that is a starting battery, then your dealer cheated you on the battery.

Anyway, going back to the question, yes you can parallel the same kind of batteries even if they have different capacities, such as a 24DC with a 27DC, as long as they are nearly the same age, and usage.