Voltage/Current reading overnight

I left the arduino logging current and voltage readings to the SD card last night. I switched on the fridge in the van and a pair of halogen lights. It looks like the fridge attempted to run right at the start but then gave up. I'm guessing as it was running from just a single battery, the low voltage cutout kicked in straight away. This is not a good sign for the health of my batteries :(

Voltage and Current readings of single 115Ah battery overnight

As you can see, the voltage drops down immediately as a load is applied. It was sitting at 12.80 volts at rest. As soon at the load of 3.5A was applied the voltage dropped down to around 12.45v and then kept falling overnight. What is interesting is the shape of the voltage line over time. With a constant current it takes a distinct change in slope at around 1 hour in.

When I went outside to switch the load off at 7:30am you see the current immediately drop to near zero and the voltage immediately jumps up. It then takes about an hour to stabilise. Then settles at around 12.3V.

I'm not sure exactly what this is telling me, apart from something does not look right. The total Ah discharged over the 9 hours is 29Ah. Which is about 25% of the capacity of a 115Ah battery. The battery was at around 10 degrees C last night, so would have had a bit smaller apparent capacity than the stated 115Ah. That said the 115Ah is the C/20 rate (ie. when drawing 5.75A) so we were drawing less and so that probably would have cancelled out the effect of the cold.

A voltage reading of 12.3V indicates a state of charge of approx 60%. I'd be expecting, based on the Ah drawn to be around 75% SoC at the end, and so a voltage of approx 12.40V.

I'm about to take my daughter out to pay for the weekend, and will leave the monitor connected to the battery and hopefully we will see the effect of the Sterling B2B charger kicking in.