volt drop testing, handy for slow cranking engines

http://media2.turbosport.co.uk/2011/...p pic jpeg.jpg

ok, for all those of you that have slow cranking starters, first you need a decent preferably digital voltmeter, you also need a charged battery, if the battery doesnt show at least 12.2 volts then charge the battery, a full charge is 12.8 volts.

starters often crank slowly because there are bad connections between the battery and starter or engine earth, checking is very simple.

like the pic take your voltmeter put one lead on the battery terminal itself, the other onto the stud on the starter where the main battery cable goes, then get an assistant to crank the engine, although both leads of the volt meter are on the positive you will see a voltage reading, that reading is the different between the battery and voltage reaching the starter, anything more than 0.5 volt indicates a bad connection, or maybe cables which are too thin.

you can do the same with the earth side of the circuit, hold one lead directly on the battery terminal and the other onto the starter body, again crank the engine, you should see no more than 0.5 volt, as above if you have more than 0.5 volt you have bad connections or cables that are too thin

infact you can volt drop between any two points of a positive or earth circuit to narrow down where the fault actually lays

90% of cars which crank slowly but have a healthy battery are suffering from bad connections that could be found with this simple tests hopefully this will help some of you

why is volt drop so important? well picture this, a 3/4 charged battery will be about 12.5 volts, when you crank the heavy current load a starter demands will drop the voltage down to around say 11 volt, if the volt drop in the positive side of the circuit is say 1 volt and you have a 3 volt drop in the earth side of the circuit then the starter only sees, 11-1-3 or in other words 7 volts, nobody would ever expect a battery with only 7 volts to spin an engine over

Well said and well described 👍

Seems loads of people suffer the same problems which can be easy to check with even a cheap digital meter.

Cheers for that Graham, great post! I'll get it checked out when I get the car back from having its holes fixed!

I did this a few months ago a found the problem was due to a bad earth :thumb:

Nicely done Graham, as always, very well explained :thumb: :clap:

SHH, volt drop is a breakdown company secret lol, This can be applied to almost any circuit also, just make sure you know the values you are looking for and that the circuit is under load at the time of test.

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Originally Posted by Scott_B

SHH, volt drop is a breakdown company secret lol, This can be applied to almost any circuit also, just make sure you know the values you are looking for and that the circuit is under load at the time of test.

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absolutely scott ;)

the 0.5volt drop would also apply to an alternator + and earth connection, on all other circuits the volt drop should be more like 0.03v or less

good post mate. i'm sure it will help many people who haven't got your mechanical knowledge..:thumb:

Good post Graham :thumb:

Just stumbled on this.
Good call Graham. I had starting troubles with my engine, it just wouldnt fire.

Turned out after doing a volt-drop test the wire from the battery to the starter were too thin.
The ECU was only getting 7.5V !!

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Originally Posted by Graham

why is volt drop so important? well picture this, a 3/4 charged battery will be about 12.5 volts, when you crank the heavy current load a starter demands will drop the voltage down to around say 11 volt, if the volt drop in the positive side of the circuit is say 1 volt and you have a 3 volt drop in the earth side of the circuit then the starter only sees, 11-1-3 or in other words 7 volts, nobody would ever expect a battery with only 7 volts to spin an engine over

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Eh? - you appear to have double dropped the starter draw? A fully charged battery should show 13.2v (1.2v per cell) so allowing for average Battery at 12.5v and 3v drop across the starter you're at 9v and change. Its the Ignition that suffers if using a 12v coil and with the other draws the coil probably will only see 8v - nearly a 1/3rd down!

firstly a fully charged battery will show 12.8 volts, any more is a surface charge, secondly there are two halves to a circuit, you will get volt drop on both halves, in an ideal world it should be negliable, but a big volt drop in either side of the circuit will cause problems

I think I can just about crasp this, I think!!! :confused::thinking: I spose I knew someone else would come a long an question it though.

When you say any more is a surface charge Graham what do you mean by that please?

Thanks
Rob

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Surface charge is the uneven mixture of sulfuric acid and water along the surface of the plates as a result of charging or discharging. It will make a weak battery appear good or a good battery appears bad. You need to eliminate the surface charge by one of the following methods

Allow the battery to sit for between four to twelve hours to allow for the surface charge to dissipate
Turn the headlights on high beam for five minutes, shut them off, and wait five to ten minutes
With a battery load tester, apply a load at one-half the battery’s Cold Cranking Amps rating for 15 seconds and then wait five to ten minutes.

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think of it like the froth on top of a pint of beer it makes the glass look like its fuller than it really is

Ok Graham, thanks.
I think I understand what your getting at. :thinking:

I know this is and old post graham but hope you can help

Is there anyway I can do this if the battery is in the boot

If I use a piece of wire from battery to the meter will this interfere with the reading?

as long as you have a decent connection your bit of wire should not interfere with anything