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I bought two from Harbor Freight over 4 years ago for our cars and they both are still working. One has been on the vette almost constantly. They were about $10-12 if I remember correctly. You can pay up to $80 for one, but I could never figure out why anyone would pay that much for one.
Wal Mart has a nice Black & Decker unit. It is small, compact and has the quick connector that can be left installed on the car's connectors and not the battery. The big "no-no" with battery tenders is disconnecting the tender from the battery with it is still plugged in. Most mfgs say disconnecting from load without unplugging from the wall first will fry the battery tender! Hence, the alligator clip-on connectors would be more-than-likely to become disconnected and possibly fry the battery tender.