Can you mix batteries?
QI have just bought a replacement starter battery and have found a spot for it in a separate battery box so that I can upgrade to four leisure batteries. I replaced the leisure batteries in June 2014 (Numax 100Ah) when the originals failed. I know you shouldn’t mix old and new batteries, but does ‘old’ just mean ‘dead’?
I have treated the new batteries well so would it be wrong or cause a problem to add one more (same brand/type) 18 months later? Or must I wait another three or four years until they need to be replaced and then buy four?
AIf your existing leisure batteries are in a good serviceable condition, then adding another should be okay. Fit the new one and then run the engine to recharge the whole bank. Any more discharged batteries will take a greater proportion of the available charge, so over a day or so, they should all be charged to the same degree.
Good practice says batteries should always be changed in whole sets and most professionals will be very reluctant to do otherwise. This is because when (not if) an older battery develops an internal short circuit it will flatten the other batteries in the bank. Then the newer batteries will sulphate, lose capacity and will possibly be ruined.
It is definitely best not to mix batteries with different maximum charging voltages because the charging would have to be set to the lowest value and that would increase the time taken to charge the whole bank. Without model numbers, I expect the Numax 100Ah batteries are either AGMs or wet open cell batteries so if you stick with the same type (not make) it should be fine. batteries with handles, so if your engine battery is in working order, forget about it. Although it seems counter-intuitive, the starter battery has a very easy life and rarely gets anything but a light discharge unless the engine is faulty. They will normally be all-but fully recharged less than an hour after starting.
An online manual gives no data for the starter battery size, but as long as it’s roughly the same size as your leisure batteries with a terminal layout that allows you to connect the leads, it will be fine. I would expect it to have a CCA (Cold Cranking Amps) rating of around 900 CCA and/or a minimum Ah (Amp hour) capacity of between 80 and 90 Amps hours. This will probably be overkill but it will fit next to the domestic bank and not require the additional packing to comply with the Boat Safety Inspection as a smaller battery would. An on-line outlet lists one at £75.