# Is there a way to reliably test voltage of a 12V battery when it's not disconnected (i.e., partly loaded)?

I'm having an issue where a fully charged battery doesn't necessarily report the proper voltage when checked from the socket within the car, because in order to have the socket activated, the car has to be ON, which, in turn, puts non-trivial load that drops the voltage below what should be the proper voltage without the load (as the 2.1V+ per cell rating, 12.6V+ total, is the full-charge at-rest rating of the battery without any recent charging or load).

In fact, I often find that it's even a problem when trying to measure the voltage of the battery directly from the battery after opening the hood, because the car continues to consume some extra above-idle power until a certain timeout of some number of seconds and/or minutes.

Is there any reliable calculation, formula or a rule of thumb to quickly assess the charge of the battery in such conditions, without fully disconnecting the battery, nor resorting to very expensive specialised measuring tools?

• I'm thinking there must be some known estimates how much a given car is expected to consume when the key is ON, parking break is activated (i.e., all lights are off), radio and inside lights off etc.

• Likewise, there should be some tables on how much the voltage would drop in the situation as above.

• What exactly are you trying to measure? Just the battery voltage? I think your idea that when the car is on, or at a minimum is in the accessory position, the load is quite trivial. Reading the voltage directly from the battery with everything off is going to give you a very accurate reading of the battery. Unless there is a major draw (something which shouldn't be happening), you should get the voltage without issue. – Pᴀᴜʟsᴛᴇʀ2 Jan 27 at 0:08
• If this is a "big" problem, your battery is on the point of failing through high internal resistance anyway. Professional battery testers (that cost \$100 or more) intentionally put a high load on the battery, to measure what it does in a realistic working situation. – alephzero Jan 27 at 1:16