Niagra,
I'm sure the zener could fool your automatic charger, since the charger is most likely working off voltage and maybe even current vs. time if it's smart. The zener could be draining current that the charger thinks is going to the battery. Probably best to charge batteries with the fuze out, especially with smart chargers.

I only use supplies with adjustable voltage and out of the circuit unless I'm charging Ni Cad or Lithium batteries in that case I use smart chargers. Maybe I'm wrong, but sometimes I think I'm smarter that the chargers, but I'm not sure about that, just ask our Terrier, he knows more than I do. I do know that my slow charger for the NiCad battery works better than the fast charger which does all kinds of smart things including measuring the battery temperature while it's charging.

More than you want to know.

Dave
69S