Hello, I recently installed a new stereo with subwoofer that's max 600W, when doing so I had the radio on for 30 min with low volume while I was putting interior panels back in place. I ended up discharging the battery enough for the starter motor not to turn. Funny enough it only took 2min with charger and the car would start.
Since then I have been avoiding having radio on with the engine off, not even for few minutes.
I can see it's possible to buy voltage meters for the cigarette lighter port also I can monitor the battery on a phone app, I have an ObdII reader.
Can that be used to make sure I have enough juice to start the car? What is the threshold for when the starter motor is disabled?

While I'm here... has anyone tried to put in a secondary battery in the engine bay? It looks like there is space on the other side of the engine.

what sized engine is it? if its a 1.2 petrol the battery capacity wont be much, add in the fact that [possibly] the battery might be a few years old then chances are it will run flat

might be time for a new battery

12.2 is around 50% charged
And your avarage car battery should not be discharged below 40% for any length of time as it starts damaging them I'd say 12.2 is a reasonable battery voltage

A battery starting a car is not just dependent on the voltage. The amount of current the battery can produce comes into play, because simply measuring a lead acid battery should typically give you about 12.3 volts due to the exact nature of how a lead acid battery works.

What happens when you start the car and ohms law comes into play.

The bigger the starter motor the more current is needed to spin the engine. If 100 amps is needed at 12.3v to start the engine, then this equals 1.2KW of power being needed.

If the starter needs 200 amps then the power needed doubles to 2.4KW

If however the starter needs 100 amps and the battery can only supply 800watts because of damage or a low state of charge of, then this has the effect of dragging the battery voltage down to 8volts to try to supply the needed 100 amp current. But only while the engine is trying to start. When you get back up and measure the voltage it may still very well say 12.3v.


This is why batteries are tested with a “drop tester” this puts a very low resistance across the battery terminals and then measures how low the voltage drops, as an indication of battery health.