Varley battery

[andygo]

How do I tell if my Varley battery is working ok? Seems to be ok, but a friends Varley would start his Formula Ford 1600 ok, show good voltages, keep its charge etc, but apparently caused the car to lose performance at the top end

He changed the battery and bingo, speed was regained.

My battery is a 3 years old and just wondered how to properly test it.