It looks like Yamaha uses thermal protection in their battery during the charge cycle. After my commute home yesterday, a hot (45C) day today with a headwind my Yamaha PW-SE battery was down to 25 percent so I brought it in the house and put in on the charger. Uh oh; all four battery level lights blinking. Thinking that the fault was temperature protection in the battery, I removed the battery from the charger and waited; did this every few minutes. After ten to fifteen minutes the battery finally started charging.