After the ride when bike became cold, I couldn't start it again outside (exhausted one litium and one lead battery when trying). The spark was from the level of one spark in the beginning to none. After that I took bike inside and started tearing it open: I checked most of the connectors for dust or moisture, wires and wire harness visually, used new plug without change, put in new regulator as I happened to have one, swopped places of relays and no change for better. Then I plugged the charger on the battery (because it was starting to get weak) attached to the bike and after a while, the spark came back and was perfect time to time. Was the reason the charging of the battery only or how much did effect the fact that bike got warmer in the garage within those 6 hours?
Next day the bike started straight away again in the garage and I did ride about few hours, during which the starting became worse and worse. It was -11C outside. So, now I'm still confused what is core reason for this behavior. Have those batteries again gone bad? They seem to have normal voltage. Last time I measured the one on the bike, it was 13,3V. I think the latest one is third battery for this bike and fourth is now ordered. Can it be so that bike does not charge battery well enough during the ride? But for the last ride I put in a new regulator, so it shouldn't be the reason.
Can I check all wires by with multimeter so that I put one lead to the pin in the main connector and another lead to the other end of that wire (for example wire coming to relay)? Or I need to tear open the whole wire harness and look for cuts, bruises etc. on the wires?