Ok, while I'm waiting for parts to get my new car (off-road as it happens) on the track I decided to cycle all my batteries and take fresh accurate readings with my latest gadget a Watt's Up watt meter.
Anyway the batteries are:
2 x 1400 mah scr +
2 x GP 3700 (unmatched)
2 x Intellect 3600 (matched)
2 x GP 3700 (matched).
Charging source is a 12v battery from a light truck.
I charged in the order they are listed and the first 1400 pack gave a reading of 1484mah (the second 1400 was very similar) and on the 20amp discharger it used 1250mah.
So from there the rated mah of the batteries as they were charged was 3700/3700/3600/3600/3700 and 3700. Note that after the second 3700 the source was charged overnight before continuing with the last 4 batteries.
The first 3700 gave a reading of 3600mah on charge and the second gave a reading of 3382mah.
After charging the source overnight the first 3600 read 3334 and the second read 3137. The second to last 3700 read 2901. The last battery is on as I type this.
Progressively each battery is giving a worse reading than the one before it, but the really weird part is that even after an overnight charge of the source battery the readings are getting worse with each battery in turn.
Even more weird the meter said the charger was delivering the charge it was supposed to.
Edit: The last battery actually went up by 116mah over the previous one.