Three six volters

[captain nodge]

Having just picked up 3 yuasa endurance 100amp 6volt batteries from the recycling yard where i work,(the forth one was knacked) and rigged 2 up to my 12volt system by connecting in series, i was wondering wether i could connect the third one in parralel to one of the two in series without making the total voltage up to 18volt, main reason for this is that i havent got a 6volt charger, and trying to avoid changing over one of em. i googled these  and found out that they are quite expensive and are of a good quality, hopefully i might get a couple of years use out of them

[Flux]

Trying mixed capacity in series strings always ends up in disaster unless you can spend a lot of time babysitting the things and keeping them in step.

The only real clue you have to state of charge is specific gravity. I suspect these are sealed and you can't use this criteria.

Best I can suggest is that you build a little switching step down regulator to maintain the odd battery fully charged as standby or for small loads at 6v ( LED lights ?)

If the other battery was knackered it probably means that the others have a limited life. It seems a good idea to maintain the spare one for use when one of the others die but I would just keep it charged and not try to use it as part of the bank.

Not having a 6v charger makes this difficult and the regulator I suggested is really nothing more than a 6v charger. Unless you can build it it's probably useless advice.

Flux

[captain nodge]

cheers for speedey reply, have taken your advice on board,guess il just change em over periodically, the last yuasa batt i rescued from yard 5years ago is still going fairly strong that one is a 150amphr 12v and has had continious use