Battery keeps dying...
 

So this may sound weird but I am going on my second battery and they won't hold a charge. I went to the auto parts store and they tested my alternator and it was fine. However I did notice, when the car is off, there is a ringing noise coming from the engine and it doesn't stop. Could that be why the battery is constantly driving? If I charge it and let is sit for more than 12+ hours, the battery is guaranteed dead. I know for a fact nothing is staying on and draining it (well, visually).

It's a 1996 Land Rover Discovery

Need to find what is making the noise that is a bad parasitic draw

any idea on what could be causing the noise? its a very faint ringing noise...its a noise that you wouldn't think a car would make. it sounds like it could be more electrical based. however I know nothing about cars........

Start with fuse box under hood. Unplug the fuses one at a time. Perhaps ABS pump. Maybe rear door sub woofer amp.

Disconnect the ground cable from the battery.
Place a meter reading amps (or a test light) between the negative post and cable lug. See what your drain is.
Unhook the meter and disconnect the charge charge cable from the alternator (make sure the end isn't touching metal).
Reconnect the meter, or test light. Is there a drop in the reading? If so, you probably have bad diodes in the alternator.
If little to no change;
Remove fuses one at a time to see when you get a significant amp drop. See what that circuit supplies.

Don't forget to reconnect the alternator cable before reconnecting the ground cable.

What Antichrist says doesn't really show on alt test at store

Before removing the ground cable from the battery it should be checked if there is a specific procedure related to the alarm system (especially if the alarm sounder with integrated battery back-up is installed, it has a square body).

Normally without any device switched on the current should be very low. Don't switch anything on while an amperemeter is in the loop as it could easily blow the measurement instrument's fuse. For higher currents a DC clamp amperemeter is the most convenient instrument but reliable ones aren't cheap (make sure that it can measure DC as many can only measure AC).

For low currents (i.e. below say 2 or 3 A or so but it depends on the instrument) clamp meters are often not enough precise and especially not to measure small leakage currents unless using very expensive special instruments.

Some faint buzzing-like sounds we believe to be of electrical origin can also come from vacuum, pneumatic or other fluid circuits during when pressure change (also due to temperature changes).

Beside defective devices, insulations problems, for example due to water+dirt+salt can easile cause leakage currents. Also corroded wires don't necessarily create a short circuit if their insulation is damaged.


Edited:
Forgot to mention to first check the charging voltage when the engine is running. Should be around 14 volts. Also check the battery voltage though that is not a stricly reliable indication if a battery is good.

Unless it's some installed after market system it's not an issue on a '96. I don't think it is on any D1.

I suppose it depends a lot on the country. There are not only NAS (North American Specification) vs. rest of the world differences but the theft alarm system configuration depends also on local (i.e. country-specific) regulations. For example, even between different European countries details can vary, especially about audio alarm and flashing lights. Country-specific parameters are programmed in the theft alarm unit.
Where applicable the battery disconnection procedure is explained both in the owner's manual and the RAVE documentations.

It also seems that some ECUs don't like to be disconnected from the battery but I don't remember details, here I try to avoid battery isolation as much as possible (one futher reason being that electronic circuits, and especially aged ones, don't like inrush currents when being reconnected so reconnections should be kept to a minimum).