NiMH's are not the way to save money now sadly.
If you think one of the cells is duff check it with a voltmeter. If it is completely out of lign with the other cells in the pack or at 0V it's a goner.
We used to try bumping them back to life by giving them them high amps on the single cell but it only worked for a charge or two. If you carry on using it the pack will feel bad and the duff cell is more than likely going to blow up in your car or while charging.
Get yourself two decent lipos. Cheaper in the long run and fine to buy secondhand usually too.
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