Is my car battery health bad and should I replace it?

Question

I bought a 2010 Golf in march and I do not know how old the battery is (no dates on the battery) and with winter coming in I feel a bit nervous.

Usage
I do not use the car very often but I try to give it a 30min ride once a week.
The car is parked outside on the street, no garage or access to power socket to charge the battery.

Measurements
Before driving, the battery will typically show 12-12.15V and the car starts even at -5°C. After driving 40min the battery shows 12.7-12.75V. Measures were made with the battery connected and car locked to minimize load. A few days after driving the battery will be back to ~12.1V.

Is my car battery health OK or is it close to end-of-life?

Answer 1

I think your battery is okay. You are getting 12.7 volts after driving, which means that it is fully charged. It's the charge after sitting a few days that concerns me, 12.2 volts is generally considered 50% charge, 12.0 is 25% charge, so after a few days your battery is dropping below 50% charge. That's actually fairly normal as modern cars have battery drains from alarms and other devices that stay on even when the car is off.

One thing to check is the battery capacity, and make sure it's sufficient for your car. Sometimes people skimp on this and get a battery below the recommended capacity or on the low end. A lower capacity battery will drop more when sitting under typical standing loads. The capacity should be written on the battery somewhere. If it is low for your car then I would replace it with the highest capacity battery you can fit in the car, this will have a higher charge after sitting a few days.

I would also do a battery drain check on your car using the ammeter function of your multimeter. If you are getting more than 100 milliamps drain (as measured 30 minutes after you turn the car off) then you have a high drain and need to get it looked at. If you are getting close to 100 then see what you can do to reduce that.