The red battery light came on the dash on my 2004 Land Rover Discovery 2. The truck has about 68k miles on it and was running all good before and after the battery light came on, didn't notice much difference after the battery light showed on the dash. I measured the voltage using multimeter. It's at 12.78V when the car if off, and 13.89V when the engine is idling. Same voltages at the battery terminals and the alternator connection pole. I kept driving the truct for about an hour with the battery light on, the truck worked just fine. And the voltages are about the same as I measured before.

Any idea what triggered this battery light on?

Thanks,

 

Overall, whenever something inexplicably weird happens in the electrical context, it's almost always remedied by charging the battery. FWIW, I have to charge the battery every couple months just due to how often it's driven, and it almost always needs about 50-60%. Try that before anything else?

 

I’d also check the state of both the alternator and battery grounds. The voltage from the alternator seems just a scoshe low, but probably sufficient.

Charging the battery for a bit couldn’t hurt- oh yeah, check the wiring and connections at the battery terminals as well.

 

When this happened with my truck about four years ago, it was simply because my then 23-year old son hadn't tightened the nuts for the connectors for the two wires on the alternator enough. Perhaps try removing and cleaning both of them, after first disconnecting the battery.

Sometimes connectors may get a little oxidation or something that's not really visible that prevents them from carrying current as designed. I had that problem with my first car ~45 years ago (a '65 Mustang). It wouldn't start, but I solved the problem by simply disconnecting the primary lead from the starter, scratched up the surfaces a little bit, and reconnecting it. You could get fancy and use an emery board or a little piece of sandpaper, but even just scratching around with a screwdriver or a nail may do the trick.

Try that out and let us know what happens.

BTW, the smaller wire on the alternator runs to the battery light in the instrument panel. According to the RAVE Electrical Library description, "A warning light in the instrument pack illuminates if the charging system is not generating a voltage equal to or more than the battery voltage." I'm not sure by just what sorcery that works; perhaps someone here who knows more about electrical systems than I can explain.

 

The way this works is that normally an LED is connected positive (+12V) at one side (the anode) and ground (0 V) at the other (the cathode). The voltage difference causes current to flow from high (12V) to low (0 V) and light up. But only works 1 way, an LED is a diode which is a one way valve for current, so if the "wrong" side is high it won't light up because it blocks the current. With the charging LED warning light both sides are connected to positive, one side from the battery and the other side from the alternator. In normal operation the alternator has a higher voltage but it's connected to the low side (the cathode) of the LED, so it doesn't illuminate because the LED blocks the flow of current the "wrong" way. BUT! If the alternator fails and the battery (which is connected to the high side, the anode) has more voltage it can push current back through the LED to the alternator and the LED lights up. This works even if the alternator is producing a voltage because voltage is technically a measurement in the difference so if the battery is +13V and the alternator is at +10V the 3V difference is enough to push sufficient current "backwards to the alternator" and illuminate the LED. There is probably a little more wizardry going on to reduce the difference needed to light up the LED because the typical voltage drop required for a red LED is 1.7V, so it wouldn't illuminate until a 1.7V problem occurred (and who knows- maybe it doesn't!).

But yeah, check your terminals- I swapped mine out with ones that actually fit (the stock ones are terrible) and noticed an immediate improvement.

 

Thanks so much to greisnb for this explanation!

We are used to seeing electrical circuits terminating in a ground (earth). It’s odd to see a circuit with power supplied to both terminals of a light (LED).

Here’s my attempt at an emoji representation of how this works.

Battery 13.89v > 🤜➿💡➿🤛 < 13.89v Alternator

I depicted equal voltage on both sides because the battery and the alternator are connected.

Most of the time, there is equilibrium in the system. From where the LED sits in the system, the voltage it’s seeing from the battery = the voltage seen from the alternator. This produces a standoff, and the LED (represented here by an Edison incandescent) does not illuminate.

But, let the alternator voltage drop and current then begins to pass through the LED from the battery, thus illuminating the LED. In other words, the alternator is not producing enough voltage to battle the battery voltage to a draw (pun intended).

This may not mean the alternator is not producing any voltage at all; it means that it’s not producing as much voltage as the battery is supplying which will eventually discharge the system.

Thanks again, greisnb !!!