I saw this kind of product been advertised through lelong and seems like very convenient as plug & read instead of using a multimeter to test it.
So i am wondering how the accuracy of reading it has?

Not accurate at alll. but it gives you rough idea about your battery condition. My ciggie lighter voltmeter readings were 12.8v, when plug in charger to charge phone it drop till 11.5v, sometimes drop till 10.Xv then i know its time to add battery. and at least you know your alternator is working.

My obd2 reader, 13.1v to 13.5v all the time. so plug the voltmeter direct to battery is still the best option.

l think u blame the wrong guy.. in your case, is not the volt meter that is not accurate, it is your car wires that are weak ,which happen in older car. Check/Replace the rusty ciggi socket wire/connector n it will reduce the voltage drop n your phone will be charge better /faster.

to double confirm my statement, u could try connecting the meter direct to battery, u should get similar reading as your obd2.

that car just 2 years + old oh.

Agree it is not the product that is not accurate and i never blame the product oh, but it is the way it is connected that make the readings inaccurate. there is too many things before it reaches the ciggie port, then ciggie port got more resistance. then ur handphone charger, gps, ionizer , shaver, all dragging power, lowering the voltage in that ciggie port(this i dont understand why only the ciggie port voltage drop till 10v and others still maintain 13v).

thats why i suggest that connect voltmeter direct to battery is still the best option.

But ciggie lighter voltmeter is still useful for rough idea of the battery condition. I myself is still using ciggie lighter voltmeter because i don't want to temper my wiring and lazy to open up my hood to check the battery voltage.

13v - 10v drop is quite high. some connectors are not plug in properly. cheers..
on a free day, check the cigar fuse reading to find the culprit. happy diy..

okie, will check see any problem or not. thanks.

but usually when drop till 10v. it is time to add battery water. after add then it goes back to 11v when i charge HP.

dont waste money on those equipment, just request your mechanic to check your battery during services.