This circuit looks much better. For anyone who wants to try it, here are a few things to check.
Measure the battery charging current using an analog, dc amp meter at maximum rpm with a low battery and make sure you do not exceed the maximum allowed by the manufacturer. Too high a charge rate will at least overheat and damage the batteries. They may actually explode. You should be using a battery pack capable of supporting all of the lights anyway, which is a significant amount of current and should survive the available charging current.
Also, check the charging current with a fully charged battery and make sure it is low enough to not upset the battery. Probably less than 50 ma is ok. This can be adjusted using different value zener diodes.
The zener and rectifier diodes must be single units. Semiconductors vary slightly in specifications from one to another and if you parallel them, they will burn out serialy. You should check the temperature of these diodes in operation, if you can not hold them with your finger tips, mount them on a heat sink. If your diodes are stud type, held in place by a nut, then the current rating assumes mounting on a heat sink.
Here is a piece of random info I read in a friends ham radio magazine a few weeks ago that was news to me. Apparently this article was written after consultation with major ni-cad manufacturers and satelite manufacturers who use ni-cads exclusively for powering satelites during dark periods of orbit.
With the possible exception of single cells, never discharge ni-cads completely. Apparently, if you discharge a ni-cad battery pack to 0 volts, because the cells do not have exactly the same capacity, some will discharge completely while others are still providing power. The continued current flow through the discharged cells, charges them in reverse which is very detrimental. This also causes high initial charge rates when first recharging because the reversed cells are adding to the charging current.
Electronic equipment and "smart" charger/conditioners for ni-cads, sense when the terminal voltage of the battery pack drops to a certain voltage and cease discharging.
Now I know why my lap top's battery pack died precipitously after I hooked up a brake light bulb and discharged it til it was 0 volts!
I hope this helps. I think if I were going to try this, I would at least use a 6 volt gel-cell, which is a lot less sensitive to over charging and may not need a zener at all in this circuit. The charging current should still be tested as described above and a correct zener selected if necessary.
Now you know why I never answered this originally. I knew there were lots of variables in a circuit like this and I know I have not even covered them all here. Remember that without carefull testing of a circuit like this, it may seem to work ok for a while, but then begin to fail from over stressed components. At least with this circuit, you can switch back to the alternator and mo-ped back to the drawing board.