Checking battery voltage from jump points under bonnet

[JimmySpeed]

As above really, is the voltage the same at the jump points as at the battery when the engine is off?

I would like to check my voltage a few times over the weekend but don't wan't to have to move my LPG tank each time

[X5Sport]

The jump point +ve is directly wired back to the battery with the same cabling that the starter motor uses. The -ve point is on the chassis and providing there are no bad connections between front and back, the voltage drop will be minimal (a couple of 1/10ths maybe) as your battery -ve terminal is also direct to the chassis. If in doubt, then disconnect the battery -ve which puts it into an 'open circuit' condition and will tell you what the volts are on the battery alone.

These measurements will only tell you what the volts are without the battery being under any stress. I've certainly used those points very successfully in the past.

This might help too...........

These are general voltage ranges for six-cell lead-acid batteries:

Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V (2.10-2.13V per cell)
Open-circuit at full discharge: 11.8 V to 12.0 V
Loaded at full discharge: 10.5 V.
Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5 V for AGM (absorbed glass mat) and 13.8 V for flooded cells. You may well have an AGM type as they are now pretty common.
Fully charged, engine running: 14.0 - 14.4V typical on the X5.

You do need to take the meter accuracy into account too, hence there may be a variation of +/- a few 1/10ths. There is a clear difference between good and bad though, as a dead cell will show a drop of at least 2V.


After full charge, terminal voltage drops quickly to 13.2 V and then slowly to 12.6 V. - this is why you don't check the voltages less than 4 hours after last charge so that any residual charge on the plates dissipates

[JimmySpeed]

So while running I should see 14-14.4 and after 4 hours of no use with battery still connected I should see 12.6?

Let's say I get a good reading when off but low when it's running, that does that point to alternator? And if it's good when running but low when off, then that points to bad battery?

[X5Sport]

Theory sounds OK, but what it should indicate is where the issue could lie. The only way to really check a battery is to disconnect and remove it from the vehicle as that ensures there are no parasitic draws and allows you to also carry our a 'stress test' on the battery where a whacking great resistor is connected between the battery terminals to test whether the battery is delivering what it should.

To help you though, the first check should be to fully charge the battery and then test it not less than 4 hours later. The 4 hour figure is the one BMW themselves use. That will tell you whether the battery is OK. You can then do the alternator check. Doing it the other way around could throw you a curve ball as you may have a bad cell which gives a wrong alternator output.

How old is your battery? If more than 5 years, or if it has ever gone very flat, then it may be time to change it. Car batteries are 'starter' batteries rather than 'deep-cycle' batteries and as such have their useful lives shortened if allowed to go completely flat.

If you think you have a parasitic draw somehere then testing gets a little more complicated. What is making you check the electrics?

[JimmySpeed]

The battery is not the original but unsure how old.

My battery will die if in ignition 2 to use tv or stereo for say 20 minutes without starting. It died about a month ago like that.

[X5Sport]

Sounds like a dying battery, or at least one with a problem.

One of the issues with the AGM batteries that are used these days is that they are very intolerant of being run flat. It shortens their life considerably. The more times it goes flat, the shorter its life gets.

[JimmySpeed]

I will begin investigations when I get a volt meter then. Thanks for the info!