A question of Battery power ..

This has me stumped so I hope someone can advise ..

I have a battery box containing 3 X 7 aH UPS batteries which I use in my camper for internal lighting ..

I also have a 26 aH UPS battery which I use at home for testing polarity etc etc ..

I tried to run my Waeco CF50 on the 26 aH and it kept going into red flashing error mode BUT the 3 X 7 aH powered it no problem at all ..

All of these batteries hold charge and appear to be OK but I can't figure out why the bigger one fails at running the fridge ... It has a 7.8 Amp rating stamped on the side so that should be OK for a 4 Amp fridge ...

And before you all jump to the wrong conclusion - I run the fridge from a 100 aH in the back of the Jeep under normal use..

This was just a test I was doing ....

Any ideas ? I assume the 26 aH is unwell ?

Cheers

Steve