thank you for the help.
The story is like this:
Originally, I can charge the battery pack about 1000mah. it will run very good in 10 minutes because the Kubelwagen is not heavy and ESC is strong enough to push it.
Just recently, I charge the battery either at 1A or 0.1A, it will only take about 200mah charge, then stoped. It took 1100mah charge last night.
but the battery look just does not have enough juice to push the car. I measured the Voltage, it is 8.9V. what I do not understand is, why the voltage is high, but not strong enough to push a little car. the esc is Futaba 230 with reverse.