voltage of "still" charged car battery

[Thunderchild]

what will the voltage of a car battery be when full but juts sitting ie: not under charge of load ?

 

[bychon]

Somewhere around 12 volts if it's an American car. (I only know about American cars.)

 

[HarveyH42]

Lead-acid batteries don't like to just sit around, usually self-discharge, lead sulfate crystals form on the plates, and the battery refuses to take a full charge. Eventually, the battery becomes useless. The voltage would depend on how long the battery has been sitting.

 

[AlainB]

"Around 12 volts" is describing any state of charge of a car battery, except maybe "completely dead"

From autobatteries.com:

**broken link removed**

 

[Gary B]

I have been attempting to load a battery discharge curve, but I can’t get it to work.

Basically, a fully charged led acid battery will have a voltage of 2.25 volts per cell so a six cell car battery will have a post to post voltage of 13.5 volts. A fully discharged one will have 1.70 volts per cell or 10.2 volts post to post. To discharge below that value is to risk permanent damage to the battery. As stated in the previous answer, it will only show that voltage right after removing the charger. It will drop to 2.20 volts per cell quite quickly but, the self discharge rate drops off with time and appears to very close to the same curve of a discharging capacitor through a very large resister.

 

[AlainB]

As often stated in batteries litterature:

The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and resided at a steady room temperature.

( Re: **broken link removed** )

The voltage should then be according to the chart posted.

Not very convenient however!

 

[Hero999]

It's true that an unloaded lead acid battery with a terminal voltage of just 11.9V is completely discharged.

However when a load is connected to it, the voltage can be lower, even if it's fully charged which is why you should measure the voltage with the ignition switched off.

 

[Thunderchild]

I'm just asking what the voltage is when the car is stopped so no charge going in and no drain on it.

 

[ericgibbs]

I'm just asking what the voltage is when the car is stopped so no charge going in and no drain on it.

hi TC,
Look at this link for a useful guide.

LEAD ACID BATTERY CHARGING INFORMATION, BATTERY TYPES AND OPERATION - SOLAR NAVIGATOR WORLD ELECTRIC NAVIGATION CHALLENGE, NELSON KRUSCHANDL, BLUEBIRD ELECTRIC LAND SPEED RECORD CARS

 

[Thunderchild]

hanks Eric, basically what i needed to find out is that it's 12.7 V tops unless under charge

 

[AlainB]

what will the voltage of a car battery be when full but juts sitting ie: not under charge of load ?

Well, as I understand it, if the battery is good and unplugged for at least 3 hours and tested with a good multimeter with an accuracy of .05% or better the voltage should be pretty near of 12.6 volts. If it is tested while plugged the result could be meaningless.


EDIT: I was posting after #9 but you ended up posting before me.

 

[jrz126]

I left a SLA battery (about the size of a car batt.) on the charger over night once, it was still reading 13.2V a day or so later. I'd assume a regular vented batt. would have just boiled the water out.