Bill
I suspect that by a '12v car-type charger' you mean one that has a plug which fits into the 12v lighter socket of your car and NOT one designed to charge a (full-size) car battery. It's not a good idea to use this type of fast charger for the first few charges of a new battery.
The "mains" (slow) charger is a better bet but you've not said what its output current(s) is e.g. 100mA. It should be marked clearly on the charger. You need to calculate how long a full charge should take with this beastie by dividing the battery capacity (1600maH) by the charger's output (e.g. 100mA) and then multiplying the answer by a factor of about 1.4. So if your charger
does put out 100mA then the equation is (1600/100) x 1.4 = 22.4 hours. This will fully charge a pack from a value of 1.0v per cell, which can be considered as the minimum voltage to which the pack should be allowed to discharge. Any lower voltage runs the risk of damaging one or more of the cells in the pack.
NiMH packs will take three or four full charges before they reach maximum capacity. That said, Time Bandit may well be correct in that the motor is taking so much current from the battery that it's flat in just a few minutes. 1600maH is a very small-capacity battery by these days' standards; 5000maH is quite commonplace. You would need a special power-monitoring meter to measure just what sort of current the motor is drawing under load e.g.
http://www.4-max.co.uk/wattmeter-budget.htmThe other thing to consider is that the battery packs may have been sitting around so long in stock that they have become fully discharged (by virtue of their own internal resistance) and are now beyond rescue. For that reason alone it's advisable to buy batteries from a supplier who sells a lot of them and so is constantly replacing his stock. These guys are recognised as one of the best suppliers in the UK
http://www.componentshop.co.uk/DM