There are areas/channels/fields/groups that you can check within CAN-Gateway that
store the car's interpretation of the present "useful capacity", "charge throughput" and "internal resistance of the battery" - recording them maybe once a year must be a good idea as to how things are with the battery.
The trouble with just using a DVM across the battery is, that battery is on load even if the load is not huge. Due to having some issues with a late 2009
Ibiza, I treated my self to a CTEK battery tester/analyser, it is a Midtronics designed tool made in China for CTEK, it can be used across a battery while it is in circuit in a car and seems to give out reliable readings. I have read that with any new battery, prior to fitting it, if you check it with a battery tester, it should return a reading of at least +10% over the CCA rating quoted on the battery case, and for 2 new AGM Bosch batteries I have found that to be true - I've forgotten but maybe the value is meant to be even more than +10%, either way what I found tied in with that. Once in circuit and well charged and with all the car's systems asleep, the measured value should always be at least or near the quoted CCA rating. Maybe annoyingly, the factory fitted AGM battery in my February 2011 Audi S4 that does not get used much (still under 30K miles) but gets its battery maintained with a CTEK charger, was still handing out CCA values very near to its quoted CCA when I replaced it in April/May this year - just as I didn't want to need to buy a new battery when away from home, I considered that 11 years was enough for any battery, no doubt it will find other uses around the house/garage! The EFB in my wife's August 2015 VW Polo 1.2TSI 110PS, is an Exide one and it is dying gracefully - I have already bought a slightly larger capacity, but cheaper than if I bought the same capacity as is fitted battery, and AGM Bosch one, for that car, to get fitted hopefully before the original one fails, that car has run 40K miles, a new battery tray and insulator was also needed to do make that change.
The areas/channels that I record are:-
IDE01834 Battery voltage 12.050 V
IDE01836 Battery current -5.589 A
IDE01837 Temperature of battery sensor 24 °C
IDE01838 Battery temperature 9 °C
IDE01839 Battery charge level 75 %
IDE01841 Battery internal resistance 6.6 mOhm
IDE01842 Usable battery charge 24 Ah
IDE01843 Battery voltage at rest 12.5 V
IDE07616 Battery degradation for start/stop mode Phase 1
IDE07681-MAS06086 Battery total charging throughput-Total re-charge 204.5 Ah
IDE07681-MAS06087 Battery total charging throughput-Entire energy throughput 208.2 Ah
IDE08347 adapted battery capacity 64 Ah
The charging throughput values, for that 2015 Polo are skewed a bit because I changed the battery's serial number back in November last year to try to "improve" or extend its useful life - looking back, I should really have logged all these values BEFORE I changed the serial number! Some of these values might have no use at all in real life.