I have a Yale pallet jack model # MPB040EN24T2748 that will not charge. 3 of the 4 batteries were changed for new in February of 2015, the 4th was changed last week, when plugged in to charge it would go though the initial check and get to the point were the yellow light would flash showing that it is checking the batteries, after a few minutes I would get 3 flashing lights showing severe discharge. I checked all batteries and they were at 5.9 volts, I put all on a charger individually and now there charge is at 6.2. before I charged them I had no lift, after charging the lift is now working. but when I plug it in to charge I'm still getting the 3 flashing lights.