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Matt, Mechanical Engineer
Category: Ford
Satisfied Customers: 22932
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Battery keeps going flat

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Battery keeps going flat

I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine / gearbox.


Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable. A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.


Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

Also che

ck that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.

if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement

And one last thing is – how old is the battery? – As even good quality ones need replacing every 5 years and faulty battery just won’t hold a charge

Customer: replied 3 years ago.
Put battery in car connect up nothing switched on 12.8 volts....start engine reading across battery 11.8 volts
Hello peter

Ok that means the alternator isn't charging

check the earth strap and alternator wiring as I mention above

and if thats OK then its time to get a new or repaired alternator
Customer: replied 3 years ago.
Hi Matt
I have checked the earth strap good contact
This is a new alternator that I have fitted,the voltage at the red thick cable is 18 volts is this normal
Helo Peter

ok 18v at the alternator is fine so its trying to charge, for whatever reason that current isn't getting to the battery so I'd check over the main wiring loom following the alterantor thick wire back to the battery / fuse box
Customer: replied 3 years ago.
Hi Matt what is the red alternator cable connect to as it disappears round the back of the engine block ,where would I fine it after that ?
Hello Peter

normally the thick red alternator cable will run direclty back to the battery positive postor into the high power section of the under bonnet fuse box
Customer: replied 3 years ago.
What section of the fuse box does the red alternator cable connects to ?
I can then isolate this to get a continuance reading to find out where the breakage is .

Hello Peter

there should be a high power ( 40A) section and its wired into here
Customer: replied 3 years ago.
Found breakage in wire ,where it connect up to starter motor.....,terminal disintegrated repaired this now got 13.5 volts at battery ...thanks for your time
Glad to have helped Peter
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