So, i recently acquired a 1993 Mazda MX-3 with the 1.8L V6. The motor actually only had 40,xxx miles, and it runs pretty well. So far, the only real problem we have so far is the battery dying. I have to unhook the negative cable each time i'm done with the car to avoid this.

There is an external alarm that the previous owner had wired up, could this have anything to do with it? I imagine it's just something that's trying to use the battery while the engine is off.

Thanks in advance.

 

You don't say how long you can leave the car before you find the battery dead; neither do you say how old the battery is.
Has the car been in an accident such that cable harnesses might have been damaged?

But yes, you're right, ALL custom/ aftermarket stuff is suspect. If you know where it's connected, disconnect all of this stuff before doing anything else.
However, leaky rectifiers in the alternator can do the same thing; and corroded exterior sidelight sockets, brake lights, reversing lights, license plate lights, etc. etc. can also do strange things.

You should first check your car's general electrical health.
At fast idle, the alternator should push the volts at the battery terminals to about 14 volts or so.
With the battery full charged and with light or no load, the engine off battery voltage should be above 13 volts. Put the low beams on for 10 minutes, then turn them off. You should still see at least 12.3 volts on the battery.
If your electrical system passes the above tests, you can proceed as follows.

You will need a multimeter with a fuse-protected DC 10Amp. range.
First, with everything off, and engine stopped, disconnect the battery negative terminal.
Next, take a battery booster cable, connect the black clamp to the negative battery terminal post, and with the free black clamp, swipe a chassis ground connection bolt head lightly to see what sort of sparking you get. If it's a noisy splat with an orange color, you have a heavy leakage current, probably due to defective rectifier(s) in the alternator, and you should disconnect the alternator main output cable and repeat the spark test. If you now get a fairly meek spark, you have an alternator problem.
If you didn't get a heavy spark when you swiped the grounding bolt, you can proceed using the multimeter.
With the multimeter on the DC 10A range, connect the negative lead to the negative battery post, and quickly swipe the grounding bolt head with the multimeter positive lead just to check that the meter doesn't go offscale. If it does, you have a heavy leak load somewhere and you should proceed by pulling fuses, first in the engine compartment fusebox, then in the interior fuse panel until you find the cause. If the ammeter stays on scale with the current, you can safely leave it permanently in circuit, switch it to a suitable range, then go through the fuse pulling routine until you've nailed the circuit the "thief" is connected to. From here on it's a case of locating all the stuff on the leaky circuit and pulling the connectors until the ammeter readings drops to a reasonably low level.
(under 100mA. is good.)

 

^thanks for the input, we just bought a dmm and i will read through that all and try it now.

to answer your questions, we had the battery tested at auto zone and both the battery and alternator are good, and i believe the battery is less than a year old.

as far as how long it stays, according to the previous owner it takes a few days, so it's definitely a slow "leak".

Any further input is appreciated.

 

OK. That data was useful. By my calculations, the "thief" current is only in the order of 1 Amp. or 2, so you can omit the "splat test" with the booster cable, and safely hook your ammeter into the circuit without risk of frying it.
There is still a question hanging over the alternator though. They can test fine but still contain leaky rectifiers. You can either disconnect the alternator terminal, or you can risk assuming it's OK and proceed systematically with isolating the circuits by fuse removal. I advise locating all the add-on accessory circuits (if you can) first. Once you've done this, start with the engine compartment and work through it, then go to the driver's side fusebox down beside the clutch. Keep us posted on how you're doing.