The battery is connected to 6 power resistors 3 Ohm each creating 0.5 Ohm. When verifying with the oscilloscope I could confirm the current and the voltage drop of my starter battery.

When my bike was actually running I measured the current when I ran the starter circuit (including the decompression solenoid) and got around 20A so this should work well as a dummy load to check if the starter battery is ok.

Wow!  I would have gotten as far as soldering the clamps onto the lead wire and then it starts getting over my head.

Smarty pants!  

@kamelryttarn, are you saying the S40 only draws 20A starting up? I find that hard to believe. Do not we look for around 200CCA when purchasing a battery?

Wouldn't it be easier to test the battery under normal starting load. I haven't tried it with the S40 because it's too annoying to get to the battery but on my GS500 I watch the voltage drop when I turn the lights on, then I use my phone to take a movie of the voltage drop as I crank it over. You need the movie because you only get a moment to see it, and it's easier to just play it back to check more accurately. The starter motor wants a couple of hundred Amps for a second. The battery might look good but still drop to 10V when starting which would indicate it's gone in spite of seeming to be good. I get worried if it drops to 10.5 while starting. I'm not an electrical engineer but it makes sense to me. Especially as I got stuck one day when what seemed like a perfectly good battery, suddenly when stone cold dead.

Edit: just checked the standing voltage and it was 12.3 under starting load for a few seconds it was able to hold 10.8V so I conclude the battery is fine and just needs a top up charge. Below 10V for a few seconds and I would begin to worry.