I use a xantrex true charge battery charger, if you want to do multiple batteries at once get a true charge unit tha is capable of more than one bank. Most of the units in the link are meant to be mounted and used as an on-board charger but all you have to do is make a set of cables to.use as a portable charger.
The ones I use are the true charge 10, I happen to own four of them as I got a deal I couldn't pass up. Take a look at them you won't be dissapointed.
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