Charging issues help required

Hi.
I am a new member with a bit of trouble with my series 3 swb petrol. Need some advice.
I am having trouble with the charging of the battery. When I drive the vehicle the battery runs flat. If I stop the engine I cannot re start due to a flat battery. I charge it overnight with a charger and all is fine again until I drive it again. The battery holds a charge when not in use and will stay charged for months. I changed the alternator thinking it was that but it is doing exactly the same. The battery is about a year old.
Any help much appreciated
N

You really need a multimeter or it's all guesswork. Does the charging lamp work correctly? Have you given all the charging circuit wiring a visual once over?

I do have a multi meter. If. Put t across the battery the voltage goes up when I start so I presume that’s the alternator reaching the battery
No charging light. Dash it a bit of a mess which needs sorting.

I don't know about series but some vehicles won't charge if the lamp isn't working, even if it'd as simple as the bulb blown. Do you know what voltage you were getting at the battery with the engine running?

About 13.7
I don’t know but what I don’t understand is that I can see the rise in voltage across the battery when the engine starts and the alternator is working but it doesn’t seem to charge the battery

I'd suggest a thorough check of grounds would be a good place to start, then alt wires for any suspect connections and corrosion where water has got into the insulation

I have checked the ground and it’s seems ok. I get the same reading from the Earth terminal direct to the positive as the positive terminal to the engine block.
All help and ideas are really appreciated
Thank you

Very odd. I'd have gone with a new alternator sorting it.

If you can read 13.7v then perhaps the alt isn't putting out sufficient current? I think that needs to be read in series rather than across the alternator.

As someone said above, make sure the charge light comes on with ignition, but if you're reading good voltage that can't be the problem, so scratch that.

With everything thing off, see if there is voltage drop across the battery. Perhaps there's a drain?

The charging light is the connection for the alternator exciter voltage for the armatrue windings so you need to find where that connection is going and get the charging light wired up right as a place to start.

I don’t think the light has ever worked. Why would the issue only start now. Do you know how the wiring is meant to be. I have two cables coming out of the alternator. The main one goes to the starter solenoid. Does the other go to the bulb.
Thanks

Mine was the same. Turned out to be a ****ted out battery.

Check the basic things, battery is quality, belts the right tension...

How's the water pump? Any grinding? Fine folks on here showed me that that was one of My issues that lead to poor charging. Too much resistance.

Post a pic of your alternator normally would expect three wires 1 voltage sense , 1 charge, 1 excitation.

If you have 13.7v at the battery terminals with the engine running, the alternator is ok.
From what you have said the battery seems ok.
Have a good basic check around the charging system (earth straps terminals etc).
Now with the engine off, and everything else off (interior lights etc).Remove the earth lead from the battery and put an ammeter in series between the battery earth terminal and the earth lead. This will show if you have an earth leakage fault. if you have a high leakage current you can start to pull fuses to locate it to a circuit.

Both this alternator and the old one have 3 connections but only two are being used. It looks like two are conennected together inside the alternator.

All terminals are good. It’s quite a new battery. Have tested voltages around and all the same so presume earth straps are good. There aren’t really any circuits to cause leakages.

Good alternator with running engine should give 14V , what happens to voltage when you load the system ie with engine running switch on all lights brake lights wipers indicators etc?

I will test this evening. What should happen

Fix your charging lamp.

The alternator needs this as a sort of kick start to get the alternator to begin charging.

Sometimes, the residual magnetism in the rotor is enough to start the alternator charging when the lamp fails, sometimes, it isn't.

Fixing your lamp should not cost much, and will rule out an obvious fault in your charging circuit.

Also, get a manual with the relevant wiring diagram. It's a nightmare chasing electrical faults without a diagram.

Col