Looking for advice. I tried to charge my 500W battery after leaving it for 1 week almost empty. After 5 hours still only 1 bar illuminated and no other leds flashing with charger connected and on either on the bike or removed. Held battery on/off button for 15 seconds, it flashed once then started flashing indicating charging until 2nd led illuminated after which charging again stopped with 2 leds on. Have tried holding down power button for 15 secs again but it is not charging.