Sounds like high Ri cells or one of the cells is bad. You need to load test the cells so you can see what they are doing during use. Could be several other things although the fact it is working for 5-10 minutes points towards poor quality cells or they are going/gone bad.
What pack is it you have so i can get an idea of what the cells are supposedley capable of. Should be able to come up with a way you can load test the cells as long as you have a fairly accurate way of monitoring the voltage.
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