Your battery will not maintain a surface charge with a load connected to it as the 'floating' electrons are soon bled off.
What happens if you disconnect the battery, charge it at a good 14.5 volts or so with your external charger and then measure the open circuit battery voltage ? I used to run a bank of 4 Optimas in the Oka as a house battery and they would often hold a surface charge of 13+ volts overnight.
Regarding your battery tester, what battery internal resistance does it show ?, I find this to be a more accurate measure of battery condition than a derived CCA figure. I'd expect less than 4 milliOhms for your battery.
Deano
66 SIIA SWB .......73 SIII LWB diesel wgn
86 RR 'classic'......99 Range Rover P38a
94 Defender 110..95 Defender 130 Ute
96 D1 300TDi.......99 D2 TD5 (current)
04 D2a Td5..........02 Disco 2 V8
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