Internal Resistance of Battery

I might be in the wrong place but I want to try and get a decently credible answer.

I have just bought a brand new 12v AGM car battery. The voltage drop in the battery when a load is applied appears to be too large. It's voltage when disconnected is 12.8V. I know my car draws very close to 1amp when a battery is first connected and my clamp meter confirmed it did again this time, but the voltage of the battery dropped to 12.7V at the posts. R=V/I, thus the internal resistance is apparently 0.1 ohms. Is this not an appropriate way to measure internal resistance, as I believe 0.1 ohms is way too high?

• Suspect measurement issues first, e.g. 12.76 rounds up to 12.8, 12.74 rounds down...
– user16324
Jul 16, 2021 at 0:41
• I would suggest using a meter with at least 4.5 digits, that gives you a decade above your ref voltage. Connect your load and let it operate for at least 10 minutes on a fully charged battery. At that point the voltage readings should be stable, then you can calculate the internal resistance and be close.
– Gil
Jul 16, 2021 at 2:16
• In principal what you are doing is right . You might need a bigger load to do this measurement with a reasonable precision, maybe a set of DC bulbs acting as load. Note that the internal resistance of a battery changes with it's state of charge and cell life. Jul 16, 2021 at 5:25
• The meter has 4 digits.It read 12.80 and then went down to 12.70. It then slowly creeps back up to 12.8V 1/100 of a volt at a time... Jul 16, 2021 at 6:32