http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=44768
These are often on sale for $9.99. Watch for them on sale if you are an occasional "Harbor Freight Abuser" on a bender
, and throw one in as a filler on an order. Strictly a "maintainer", NOT a charger.
I've used one with no complaints, but I rewired my cigar outlet for an "always on" configuration so it would work while parked for a day or two.
As a charger, I use an old Sears with a ammeter and it works for glowplug testing also.