In November 2012, I bought a trio of the Deltran Battery Tender Junior models off of Amazon.com. They were on sale, and I thought they would be useful for my motorcycle, and E500, and other vehicles as needed to maintain a battery charge particularly in the colder winter months here in south-central Texas (i.e. Dec-Jan, 30-50 degrees F).
These are the small, plug-in units that go to the wall and extend to the vehicle with a charging cable. Amazon has them for around $27 each.
A couple of years ago, one of the Battery Tender Juniors (which I'd used to keep my motorcycle battery charged) failed somewhat unexpectedly, giving a constant blinking red light and refusing to charge. I threw it out after confirming that it was bad.
Last week, the second Tender Junior failed, which had been used MUCH less than the motorcycle unit. I have used it constantly since the fall to keep the battery in my E500 charged, however.
This is an unacceptable failure rate for this particular product, and I would highly recommend that folks NOT buy it, and instead spend a bit more money for a more robust Battery Tender unit. They make several units in their product line for cars, and all of the other products are more robust.
I settled on purchasing two of the Deltran Battery Tender units. These are much larger and more robust, and properly vented with a metal housing. They cost around $49 each, but the fact they use larger components, and are better vented, should make them last much longer.
I wanted to make folks aware of this, because these are important for those of us who don't daily or weekly drive our E500Es.
I'm sure there are many other brands of these battery maintainers out there, many of which are quite good. I know that Griot's Garage has a good battery maintainer product, although it is overpriced IMHO.
Hope this info is of use to someone.