as far as "checking the battery", first, pull the battery from the machine, and in compliance with local regulations, get all the battery corrosion from the truck, and replace ANY power cables that have been resting in that corrosion, or that have ANY damage to their insulation. Fully charge the battery, than test -each- cell with a volt meter and record each cell's voltage. check each cell's specific gravity with a good temparature compensated hydrometer. if there is more than a 10% difference between any 2 cells voltage or specific gravity readings, then you have a battery problem. next you would "load test" the battery using a volt meter and the hydraulic lifting motor stalled at the top of it's stroke (but never stall more than 3 seconds), a voltage drop of 10% or more is also a sign of a bad battery.