Hi, Scratchbuilder has the questions that need answers.
I have a lot of batteries and run my yachts on 4 cell AA battery packs. Lately 2500mAh or so, mine are relatively new and last for at least half a day.
If you have old batteries the capacity slowly reduces and the cells fail. One cell will fail first and it either goes open circuit or short circuit.
With the former there will be no charge current possible but if short circuit it will seem to charge but not last long running the receiver.
Are your batteries single cells in a battery holder or a 4 cell welded pack? Good quality cells last longer.
One of the frequent causes of failure is the black (negative), lead it changes colour from the copper metal colour to a black brittle wire. More often seen in transmitters. The wire needs replacing if that is something you can do.
But 4 cell packs are not that dear and it may not be worth the effort. I go for an average mAh capacity as they are usually cheaper.
This is on the basis that I know how long the battery needs to last, no point in paying for the extra capacity if you never use it! You will find if you buy say 4 packs then the postage per pack is less.
I favour the ones that retain their charge. I only buy from UK stockists that are well known for supplying batteries.
A year or so back I bought 8 cells from a well known supermarket as I used to get a discount. They were total rubbish and Which magazine had them as a 'don't buy.
It is worth having a cheap multimeter that can measure up to 10 amps, most checks can be done with this.