14 replies to this topic

[Andrew.B]

My battery has been failing to charge properly for a while, firstly I replaced and re-adjusted the fanbelt, seemed to work for a while (month or two) then the problem recurred. Yesterday I replaced the alternator with an uprated version from MiniSpares, seemed to be going fine until I tried it this morning, nada.

The only thing I can think of that might be causing it is a duff battery, I put a new battery in in late December so it seems odd that it would go bad so quickly, all the same, what should I look for to check if the battery is okay?

Thanks.

[Andrew.B]

Just a few more things, after I parked up last night after driving from the garage I thought I'd try and start it up again. It started fine. This morning -> flat.

When I turn the key the starter motor does turn but it sounds like it just isn't getting enough juice from the battery. After 1-3 turns it gives up.

[mini_dave]

yeah definatly sounds like your battery, your alternator can charge it, but you battery wont hold the charge, this is why it wont start in the morning..

[Silicon Skum]

You could have a "parasitic" load on the battery that is draining it overnight. If you have a multimeter, set it to CURRENT (A-- / AMP setting), disconect one terminal on the battery and use the multi to connect between the battery terminal and the battery clamp. make sure you have not got the key in the ignition (key off) and that ALL lights (interior light too), radio etc are turned OFF etc. Then check the reading on the multi. If you see anything higher than about 100mA (100 mili-amps / 0.100A) this could be the cause of the battery draining. Anything over 500mA / 0.500A is excessive!

The battery could also be dead from being incompletly charged due to the problems with the charging circuit. The battery *might* be able to be "recovered", but chances are it has suffered sulfation of the plates. Have the battery tested at a local motor factor shop.

SS

[Andrew.B]

You could have a "parasitic" load on the battery that is draining it overnight. If you have a multimeter, set it to CURRENT (A-- / AMP setting), disconect one terminal on the battery and use the multi to connect between the battery terminal and the battery clamp. make sure you have not got the key in the ignition (key off) and that ALL lights (interior light too), radio etc are turned OFF etc. Then check the reading on the multi. If you see anything higher than about 100mA (100 mili-amps / 0.100A) this could be the cause of the battery draining. Anything over 500mA / 0.500A is excessive!

The battery could also be dead from being incompletly charged due to the problems with the charging circuit. The battery *might* be able to be "recovered", but chances are it has suffered sulfation of the plates. Have the battery tested at a local motor factor shop.

SS

I can't smell the usual sulphurous smell from the battery that would suggest it was dead, but I haven't opened it to have a look at the plates yet.

Ordered a multimeter so I'll check for unwanted loads when it arrives.

Thanks for your help everyone

[Andrew.B]

Just got a multimeter.

It wouldn't start initially but I tested the battery and it had a voltage of 11.9v across the terminals.

Got it jump started and when I gave the engine a bit of gas the voltage went up to 12.9-13v but as soon as I turned the lights/fan/radio on the voltage would drop back down to 11.6-11.9v.

So, I'm now guessing (although I haven't yet tested for 'parasitic' loads) that the fault is to do with the battery not getting sufficient voltage from the charging circuit. Like I said, I've recently installed a new alternator and fanbelt so I'm pretty sure it's not that. What else could be responsible?

[mini_dave]

Test the output direct from the alternator, this should be around 14 volts, if not then your alternator may be at fault.

[Andrew.B]

Test the output direct from the alternator, this should be around 14 volts, if not then your alternator may be at fault.

Where would I connect to in order to test the alternator?

Thanks for the quick response.

[mini_dave]

Where your alternator is, there is a 3 pin connector coming out of it, just test it on the 2 larger wires with the engine started and see what that is putting out.

[Jimmyarm]

It wouldn't start initially but I tested the battery and it had a voltage of 11.9v across the terminals

Can I just clarify, it wouldnt start but the battery had a charge ?

[Andrew.B]

It wouldn't start initially but I tested the battery and it had a voltage of 11.9v across the terminals

Can I just clarify, it wouldnt start but the battery had a charge ?

That's correct. The starter would turn a few times without enough 'umph' to actually start the engine. It sounds and feels very much like it just isn't getting enough power from the battery. The Haynes manual says that a fully charged battery should have a voltage across the terminals of 12.4v, anything below 12.2v is essentialy a flat battery (just because it has a voltage does not mean to say it has any more than a fraction of an Ah left in the battery).

[Andrew.B]

Where your alternator is, there is a 3 pin connector coming out of it, just test it on the 2 larger wires with the engine started and see what that is putting out.

So start the engine up with the alternator disconnected and then test across the terminals? I'll give this a try this afternoon.

[Jimmyarm]

Do the test SS mentioned, if there isnt any significant draw with everything switched off dig out the battery receipt and take it back

[Dan]

Do not run the engine with the alternator fitted but disconnected, it is not good. Remove the back of the alternator connector but leave the terminals hooked up. For one thing if you run it with the small terminal disconnected it won't produce a reliable voltage anyhow.

[Silicon Skum]

The Haynes manual says that a fully charged battery should have a voltage across the terminals of 12.4v, anything below 12.2v is essentialy a flat battery (just because it has a voltage does not mean to say it has any more than a fraction of an Ah left in the battery).

Actually the HBOL has got it wrong again! A fully charged battery should have a voltage of about 14.2 - 14.4V! However this will drop to about 13.4V within 24 hours. If your battery is being charged when tested or has been charged within a 24 hour period, a "surface charge" effect will play a role indicating a higher voltage or state of charge than is actually available. The reason for this is the way lead acid batteries work.
During charge the battery will rise to 14.4 v, at this voltage the battery will be drawing MUCH less current and will be in the gassing stage (where the battery is bubbling not just "fizzing"). This is known as "topping" charge or if the current is greatly reduced to the battery and susstained for a long time - is refered to as a "float" charge. This is the voltage that automatic chargers cut off the charge.

ALL car battery chargers output about 14.4V, and once the battery cell chemistry has settled, the battery voltage will then drop to the final "charged" voltage of 13.4V.

11.9V indicates aprox 40% charge state. A car battery is considered to be "discharged" at around 10.5V.

12.9 -13V at the battery terminal with a discharged battery and with the engine at IDLE sounds about right! The altenator does not make anywhere near full power below about 1,500RPM (about 30 - 40MPH). Also charging by the alternator will take about an hour of motorway driving to reach the "charged" voltage for the battery, because the alternator reduces the charge current according to the battery voltage (so it does not overcharge the battery). So the closer it gets to a charged state, the longer it takes to reach it.

The lower voltage at the battery terminals COULD also indicate that the connections might need cleaning. So clean the battery terminals with a bit of sand paper, clean the batter terminal clamps, then move to the front of the car. Clean and check the state of the battery cable connection and the condition of the alternaor output (brown wires), then clean the connection for the engine earth strap.


If the battery has been TOTALLY flat (0 Volts) a number of times, or has been left in a discharged state for any great length of time (like a couple of weeks), then the battery may be the problem. Car batteries REALLY don't like being discharged.
As I said, try taking the battery to the nearest motor factors (I think even hallfrauds can test your battery for you). You CAN test it yourself using a multimeter though.
You will need to FULLY charge the battery overnight and leave it disconnected for about 12 hours, then connect it up to the car. This will help reduce the surface charge and give you an acurate reading of the voltage. Note the voltage of the battery (should be around 13.5V), then (with the engine off and keys out of the ignition) flash your FULL beams and hold them on for 30 seconds and note the battery voltage (should drop to about 12.5V). THEN turn off the lights and check the battery voltage again. If the voltage is LESS than 12.9V, the battery is faulty.

SS

Edited by Silicon Skum, 18 April 2007 - 05:13 PM.