Arbtalk.co.uk | Discussion Forum for Arborists

I had some trouble with poor starting in my car.It has two batteries and now the Battery furthest from the Alernator is running down.

I have Replaced the Alernator,Both Batteries,cleaned up the earth points on the Negitive Cables.Both positive leads are clamped together each side of the Supply Cable from the Alternator.I guess that the only thing it can be is that the Cables forming the Circuit from one Battery to the other are deteriorated internally and the Current is not reaching the second Battery?

Or the Earth cable on the second battery is internally knackered?

Auto Electrics are a bit of a mystery to me.

You have got the jist Mike..

Charge the battery up which isn't charging, fit it back on the vehicle then disconnect the good batteries earth so in effect you are only using the bad battery... crank the motor over on that battery only.

It will, if there's a fault in the circuit either not crank at all or crank over slowly, it may if even heat up where the fault is and reveal where in the circuit it is.

If it cranks over fine, then your guess is as good as mine mate

May sound obvious but check the weak battery hasn't got a dead cell. It wont show up on a meter except under load. Take all the caps off the cells (if there are any) and have some one crank it over, if you see bubbles in one of the cells its fecked.

get someone to drop test meter on the battery to check for deterioration under load, also try a continuity test on the leads between the batteries

Thanks for the Replies.

I will reinstall the Battery thats on charge now and try it with the first Battery's negitive lead disconnected.

Both Batteries were receiving the correct amps when I checked them after replacing the Batteries.So the only un replaced part of the circuit are the Cables.....

I will have a look at the Cells,but I am pretty sure that they are sealed units.

If thats the case Mike do the same cranking test with a meter on the battery, the voltage should drop but not right down. If 1 battery is dead there should be an obvious difference between them under load even if they both read the same not loaded.

If the batts are getting the correct amps then why suspect the cables?
For the cables to be at fault the volts & amps to the bats would be low.

Test the bats.
try swapping them over & seeing if the fault stays in the same position or mover with the bat.

My money is on the bat.

Does it have a split charge system / controller or are they just linked together?

Not neccessarily... the charging amps are far lower than the cranking amps, you can get charging amps through a partial connection.

Bad connections may allow a small current but not a stonking big one, especailly if its an intermittant fault.

I think he has replaced the batteries for new as well which dis-counts the batts

is it always the battery in the second position thats faulty (if you swap the batteries around, will the battery which was in position 2 become good and battery which was position 1 now bad)

if so something which can be an intermitent fault is the voltage regulator

Also try putting a jump lead from the earth of the second battery,to the engine block then crank it over. If it spins faster you need to look at the earth on the second battery.

Martin