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Installing IP22 12/15 (3). I plan to connect 2 x 110Ah leisure batteries and 1 x 110Ah starter battery, all lead acid, in parallel. If the leisure batteries are on their way out and only charge to say 12v maximum, will that mean that the starter battery will not charge beyond 12v, because the charger wants to keep the voltage the same across the 3 batteries?
The charge controller doesn't know you have three batteries joined together in parallel. It only sees them as one single battery. If two of them are no longer able to charge fully yes it will effect the good one.
Personally I would not use the starter battery as part of the whole just to gain some amp hour capacity. Starter batteries are not designed internally for that kind of use and vice versa for the leisure batteries which might explain why they are on there way out.
If I've misunderstood your question please provide us with more details of your setup including the type batteries used for leisure use and what kind of starter battery you have. The more information we have the better.
Thanks for yiur reply. Not on boat atm so I don't know the make of battery sorry, only that all 3 are 110Ah. Are you saying as a general rule that you would not advise using one charger to charge the 2 leisure batteries and the starter battery at the same time? In other words, I should buy another charger for the starter battery and connect that up separately?
My rule of thumb, through some experience with RVs, is let the starter battery do one job and one job alone and that is to start the engine and run the electrical load of the vehicle, boat, RV, whatever. Everything NOT associated with motive duty can be powered by the leisure batteries. If you've been docked somewhere and solar input has been low and you need to move on short notice a fully charged starter battery should be able to get you moving quickly barring an unknown key-off parasitic draw. If that applies to you it should be looked into and repaired. Think of the starter battery like the one you have in your vehicle.
You could carry a second charger. They are always handy to have whether away or docked at home. Myself I always carry a rechargeable booster pack like this: https://no.co/gb40. They can hold a charge for a long period of time thus good for emergency use. You'll have to decide for yourself what best suits your needs best.
As for the batteries, starter batteries are designed to put out a high current load (Amps) in a short period of time to turn over and start the engine. They are not designed for extended low current duty. Leisure batteries however are ideal for lower sustained current draw for longer periods of time to power less demanding items like overhead lights, water pumps, etc.. Starter batteries are usually rated in Cold Cranking Amps (CCA) whereas leisure batteries are rated in Amp Hours (AH).
Hope that helps.
