I use a Battery Tender Plus. Haven't had the occasion to use it yet on my CDI, but other CDI owners assure me it will work fine.

On a related topic....hadn't worried before about completely disconnecting power from the car. On the W210 it's simply a matter of resetting steering angle sensor and resynching windows. But there's a lot more going on in a newer car. So Bergsma recommends using an adapter than enables you to keep 12v applied to all the electronics through the OBD port when disconnecting the battery cables. Assuming that such a procedure is perfectly safe, seems like the way to go.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife