Ditto...a float charger or battery maintainer will keep your new battery from going into states of deep discharge...which will shorten the life of the battery. You want a device that will stop charging the battery when it has reached charging capacity. A regular charger will continue on to "overcharge" the battery...which is not good for it either. A float charger is the most inexpensive way to go...with the battery maintainer/tender costing a bit more. But a float charger only keeps a charged battery charged...it doesn't have the capacity to recharge a depleted battery. The slightly more expensive battery maintainer/tender can recharge a depleted battery then turn off when its charged. And you can use the jump start terminals in the engine bay to use either of these devices...if they have alligator clips. If they have a 12 volt cigarette lighter plug...then you know how to handle that.
If you do look into getting a float charger or battery maintainer...make sure that it works with an
AGM battery if that's what you'll be installing in the vehicle.
Also, if you're going to use OBC TEST 9 to check the battery's voltage...make sure that you wait at least 2-3 hours after running the engine so that all surface charge has been removed.
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My e38 doesn't get driven that much in the winter months...so I purchased a simple (and inexpensive) float charger ($3.99 on sale) back sometime around 2004...and it has worked perfectly. I got it when I replaced the battery (2004)...and it kept that battery going until Jan/2010 when I replaced it. I still use it on the new battery and that battery is still going strong 4 yrs later.
(here's a pic of my $3.99 float charger hooked up to my e38's engine bay jump start terminals)
