Please recommend a battery tender for a 2007 boxster

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Ctek is tried and true as a reliable tender for lead acid batteries. Just plug it in and forget it until spring.

Only crazy motorsport types like me use 5 lb lithium batteries that require special care and feeding. I don't recommend leaving them in your car at all for winterizing but rather remove and store separately. I have a NOCO battery charger maintainer designed to charge lead acid, AGM, and lithium, batteries with their specific needs. The trouble is, this is a very smart charger with microprocessor technology to sense your battery type and condition. In a power outage it may lose it's memory and software crash... no charging taking place at all. I like it but keep it in the garage where I keep a close eye on it.