Battery - Cranking Voltage?

[PSL184]

Trying to troubleshoot a starting problem.... Battery volts at rest 12.8v, battery voltage running 14.2v. Turn on all electrics (lights, heater etc) charging voltage 13.9v.
Nine times out of ten engine wil start fine (although does turn slowly at first), but once in a while it will just be dead. Jump starts fine then runs without problems.... The only wierd thing I've noticed is if I check the battery voltage when cranking the starter they drop to 9v momentarily then jump back up as the engine fires....?
Is this relevant or indicative of a battery problem?

[Alec]

Hello PSL,

battery voltage does drop when cranking. A check of the battery terminal connectors, solenoid and earth strap would be the first check.

Alec

[bmcecosse]

I agree - sounds like bad connections somewhere. When it's 'dead' - does that mean no lights, nothing - or does it just mean the starter doesn't run? The volts all sound fine - and yes 9volts cranking is fine, and obviously as it fires the load comes off the starter and the volts jump up. Could be something draining down the battery from time to time - nothing left on? Could the alternator be draining back - although i would expect that to happen all the time - not just now and again...

[PSL184]

I agree - sounds like bad connections somewhere. When it's 'dead' - does that mean no lights, nothing - or does it just mean the starter doesn't run?

That's the bit I am not sure about as it has only happened twice when my good lady has been out in the car herself. Each time she mugged a passing motorist to jump start her so by the time I get to check it over everything is fine again
I've checked connections and earths etc but all seem good......

[bmcecosse]

Well - if jump leads get it going -it's possibly a drained battery - any chance lights could be left on? Consider a 'lights on' warning buzzer!

[PSL184]

She assures me nothing was left on either time......

[Jefftav]

Last winter I had the same problem and assumed a dodgy battery etc but eventually tracked it to a faulty starter motor. It ran fine off the car but on the car would work sometimes but not others.

[beero]

Could be poor connection on the engine/gearbox earth strap .
If someone put the black jump lead to the engine it wouldn't show as a fault.

[PSL184]

I'm definitely thinking poor connection but all look good, so I may try another earth strap to eliminate that. Good point about the starter to Jeff. Problem is that it is so intermittent - It's only happened twice in 12 months.....

[lambrettalad]

my mrs sometimes "forgets" she has left things on

[PSL184]

I couldn't possibly comment any further