DeWalt lithium ion 18v issues

Hoping someone can help me out here I have 4 x 18v li ion batteries 2 of which are 5 months old & 2 which are 8 months old. When at work the other day one of the batteries ran out so I put it on charge to discover my other 3 were also flat, after charging all 4 batteries until the charge complete light came ON I went to use them to find that they all went flat after about 5 seconds of pressing the trigger on the drill so I then tried these batteries in other cordless tools such as my impact driver with no luck.

 I tested each battery with a multimeter and each battery had between 10-11 volts.

 I find it hard to believe all 4 batteries would die on the same day so I stuck my multimeter in the charger and although the 'charging' light would not flash the charger was out putting pulses of between .5 - 3.5v DC, is this normal?

 Does anyone know a method of pinpointing the problem? Eg battery cell test or charger test?