4Ah battery (instead of 12Ah) in KZ400?

Hi KZriders ,
a question: has anybody an experience in running a KZ400 (77') with a 4 Ah battery (instead of 12Ah)? I´m running my bike since a few days in that configuration. It worked only a short while... After 20 kilometers the battery (new) was empty and the ignition failed. Now I´m wondering
- anything destroyed in my charging system? I checked with a meter the Volts during idling (12,5 V) and during 4000 RPM (14,7 V); that´s why I think that the charging system works...
- anything destroyed in my rectifier? I checked it in both directions: from - (minus) to all three diodes: working (one way flow, other way no flow); from + (pus) to all three diodes: (one way flow through two of the three diodes, other way now flow through all three diodes): IS THIS NORMAL?
- is my battery shit? It was a very cheap model. I charged it again. The output after fully charging was 12,3 V.
Has anybody a idea what could be wrong here?
Unfortunately I can´t change back to the 12AH battery due to limited space. A gel battery would be possible (with more than 4 Ah), but I think the original KZ charging system is not compatible with a modern (sealed) gel battery.
Thanks for sharing your experiences with me...

The battery serves an essential function of "filtering" the high voltage AC ripple coming off the rectifiers. If the battery flakes out severely, high voltage pulses can hit the electrical system.

When that happened on mine, it blew out about half the light bulbs on the bike and also took out my electronic ignition.

Running a severely undersized battery is kind of risky.

anything destroyed in my charging system? I checked with a meter the Volts during idling (12,5 V) and during 4000 RPM (14,7 V); that´s why I think that the charging system works...
- anything destroyed in my rectifier?

That part sounds encouraging. maybe it was only the battery that cooked? If you are getting 14.7V at 4k then I think the alternator and rectifiers are working.

Sounds like your test of the three diodes in the rectifier shows they are good. They are like a oneway door, power only flows in one direction.

What made you chose such a Tiny battery? :pinch:
That sure didn't last long.

:S limited space under my seat was the reason to choose such a tiny battery. If I could use an AGM or a gel-battery, I could change the position (because no fluid is running out) and a large one would have enough space. But I´m afraid that such a "modern" battery-type won´t work together with my "old" charging system. I´ve somewhere heard that more than 14V are able to cook those gel types...