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Hi. Does anyone know the reason why my auxiliary 12volt Battery reads 12.67 volts in the boot, but only 11.4 at the cigarette lighter at the front of the car.

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Are you measuring under the same conditions?

If you have to power up any systems to get the lighter socket to energise then that will pull the voltage on the Battery down. If you measure from positive terminal on the lighter socket to positive on the Battery then you will get a truer reading of any possible volt drop that may be happening.

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It all depends on how the circuits are made up and what controls and connections it uses you will also find the dash and aux socket will be voltage regulated

 

Hybrid electrics are very different compared to a conventional system

 

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18 hours ago, Bob110023 said:

Hi. Does anyone know the reason why my auxiliary 12volt battery reads 12.67 volts in the boot, but only 11.4 at the cigarette lighter at the front of the car.

Silly question (not your's)!

Are you measuring both using the same meter/device?

As Jeff and Bob point out, there is a longish length of cable from the Battery to the socket.  This along with connectors, fuse etc. will cause resistance losses along the way. Even the Battery terminals, if not clean, add further resistance. You can check if there's any voltage drop there by testing on the Battery posts and comparing with the terminal clamps.

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I have a couple of plug-in cigarette-lighter-socket voltage meters, of slightly different designs, bought about 2 years apart from eBay.  Both were less than £2 each, so a bit of a bargain.  

I recently discovered that they both display a slightly incorrect voltage....sometimes.  And that it is as much as 0.15 volt adrift, so not huge.

After realising this, I connected them onto an adjustable bench PSU - there actually seems to be some hysteresis in their readings when the  voltage is slowly adjusted up and down around 12 - 14 volts, and they both do this.  I've not seen this in a volt meter before. 

Not a game-changer!  But if you are using one of these plug-in voltmeters it is worth keeping in mind.  (I do have other 'normal' volt meters which I was using to check the PSU at the time btw.)

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There are multi meters and multimeters i have used and tested some cheap Chinese ones over the years the difference in the leads and even the way you're holding them can make a reading off

 

I had my fluke recalibrated last year as it was drifting a bit

 

Also, take into consideration the Aux circuit is a switch live not a permanent live, you may also find the Aux voltage stabilizes with a load, most in car chargers/adapter outputs are 5v or even 3V as such they have a wide input voltage 8-15v

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