Simplest way is to buy a multi-meter (very useful to have anyway) set it to 20 volt dc range and test across the battery terminals. You should have above 12 volts. To check for load, have someone inside the car pull the headlamp flasher for a while then test again (whilst lights are pulled) you should not get below 10 volts.
To check if the battery is being charged by the alternator, start the engine and test across the terminals. You should read about 14.5 volts.
Hope this helps.

 

Thinking about my reply, I’m not sure with led lights that it would give enough load ?
Maybe something needing more power plugged into the 12 volt adaptor.
Also, keep those battery terminals clean.

 

@SweepieW176 the OP may be asking about the HV battery pack in his hybrid car, as opposed to the 12V starter battery

In any case - sorry to disagree with you - but using a voltmeter will only give an indication of the SOC of the battery (and maybe if a cell is down) - not the SOH of a battery.

if you really have nothing better to do and the alternative is sticking matchsticks in your eyes - have a read of the thread below, it will explain why a simple “voltmeter test” does not apply to “modern” cars with EU6 “smart alternator” technology ……

https://www.aclassclub.co.uk/threads/eu6-smart-alternator-charging.23533/

(TLDR: Many owners are getting “partially charged” messages, despite their batteries reading a healthy >12.5V, but due to deteriorating SOH, the capacity of the battery is below optimum).

12V Battery Warnings

Getting the alert on Mercedes Me that my battery is low, probably because I haven’t been out on a long trip for a couple of weeks Did a short one and noticed stop/start was working Guess it is a case of too much information

www.aclassclub.co.uk

 

Hi spot on, the Hybrid battery pack, how do i check it's health?

@SweepieW176 the OP may be asking about the HV battery pack in his hybrid car, as opposed to the 12V starter battery

In any case - sorry to disagree with you - but using a voltmeter will only give an indication of the SOC of the battery (and maybe if a cell is down) - not the SOH of a battery.

if you really have nothing better to do and the alternative is sticking matchsticks in your eyes - have a read of the thread below, it will explain why a simple “voltmeter test” does not apply to “modern” cars with EU6 “smart alternator” technology ……

https://www.aclassclub.co.uk/threads/eu6-smart-alternator-charging.23533/