Technical Battery warning light

[tobywood13]

Hi all, my ‘93 Panda is displaying the battery warning light. It appears a few seconds after start up, and remains visible while driving at various speeds and RPMs. Interestingly, yesterday I took it out for a drive and the headlights were very dim, but when I did a hard pull they suddenly got quite bright and then remained bright for the rest of the drive, as if the alternator suddenly generated a burst of charge. I’ve replaced the alternator belt, but that doesn’t seem to have made a difference.

When I bought the car I drove it two hours home with this light on the whole time, and it survived the trip. When running the battery measures about 11.5 volts, so it’s just barely below remaining neutral but far from the approx 14v it should be at. I’ve done around 200 miles since I got the car, and it has had the light on the entire time but the battery has not died yet. It starts up easily every time.

What might be the cause of this low charging? Is it a bad alternator, or a parasitic drain somewhere perhaps? Basically, what things can I check before I go and buy a replacement alternator. Thanks!

 

[the green vanper]

Hello,

Nominal voltage of a lead-acid battery in good health is 12,6 V. The alternator charging voltage should be around 14,4 V.
10,8 V is the critical low, something around 15,4 V is critical high. Both can damage the battery.

I'd check earth cables, ground points (both of the alternator and battery). Those can be a problem in a 30 years old car. Cables tend to rot in the inside. I'm using good quality copper welding cables to replace the ones I'm no longer sure about.

Alternator being another possible cause, but also check the voltage regulator (should be on the alternator, at least it was in other older FIATs.

 

[tobywood13]

I tested it again this evening, it was reading 12.5v with the car turned off, then 14v with the car running. It stayed at 14v with all electrical things turned on as well.

The battery light was still showing, even though the voltage off the battery seems perfect. Is there any reason why it might be showing the battery light despite generating 14v?

 

[the green vanper]

Having the electric diagram would help someone who can read them. I can't

If possible, I'd check if the warning light is still on conected just outside the alternator, to eliminate a possible short somewhere on the loom.

 

[tobywood13]

Voltage is ok, but light is still on. There must be a problem with something other than charge voltage.

What other things can I test to determine where something is going wrong? I am planning to:
- Check earths on the battery and alternator.
- Check voltage at the rear of the alternator, compare to voltage at battery.
- Check voltage going to the battery warning light.
- Clean all connnectors on battery and alternator.
- Check negative cable from battery using a jumper lead.
- Try a different battery.

Can I use the car in the meantime despite this issue, or is that likely to do more damage until it fails completely?

 

[tobywood13]

Tonight’s testing revealed a 1v difference between power at the alternator and power at the battery. Alternator is generating 14.7v, battery is receiving 13.7v when idling. What does that indicate? My thinking here is that the battery positive cable is corroded and losing a volt somewhere between the alternator and the battery. I'm not sure if that would affect the light on the dashboard, however, as that runs from its own separate wire, I believe?

I switched batteries with my other car, exact same battery model, known to be good and working with regular usage. Still no change.

There was also a loose nut on the rear of the alternator where the wires are attached. I tightened that up, still no change with the battery light.

I replaced the negative battery cable with a jump lead grounded to the engine mount, that also did not change the battery. That rules out the negative cable being the cause of the fault.

Where is the alternator ground strap? That’s my next thing to check.

 

[the green vanper]

The alternator is grounded by the engine (or gearbox) ground. In the older cars usually it was a braided strap of this kind (lower one):

This one is from my girlfriend's 18 years non-FIAT car. And it doesn't look bad, the one on my 3 years older car of the same make looked much, much worse. It had so much oxidation that it almost doubled the diameter. I've replaced both with the one above on the photo, an insulated copper welding wire adding some copper terminals and heatshrink insulation: