brand new battery from BMW

My friend has a battery he gave me that's brand new. It came from a 2006 BMW X5 that died, before it died he had just replaced the battery.


If I keep this battery on a battery tender, can I use it to charge car batteries when they die? How long will the battery last for this purpose?


Thanks for your help.

Since those batteries run $200 and up. I'd sell it maybe 1/2 price then buy a decent battery charger.