Phaska

I am struggling with figuring out the pattern of this problem and I don't think my mechanic knows either.

Here's the situation: I have a 2000 XJ Sport, and earlier this year the battery kept dying without any lights or anything being left on. When I jumped it, the battery would inconsistently hold a charge, sometimes refusing to start up again even after i drove it, turned it off, and tried to restart. Mechanic said the battery was old and replaced it. Then, the exact same symptoms happened again a couple months later, and the mechanic looked again and thought it was due to the alternator and battery, and replaced them both. All worked for a few weeks but then sure enough, same problem is happening again now.

I plan to borrow a multimeter from a friend and check for invisible power draws but I'm not sure where to begin or if that is even the probable cause. Any ideas of what is going on? Things I can test for?

Saudade

First, I'd verify your system is charging. Battery voltage should be 12.5-ish with the key off, doors closed, noting else on. If less than 12v, you should charge it until it's full. Then start the car and let it idle. Voltage at the battery should be above 13v (like 13.5-ish). If you rev the engine to about 1500 rpm, voltage should still rise. Let it idle and reading should drop back to 13.5-ish. Give it another rev and volts should increase. If this all checks out, then you most likely have some kind of parasitic draw.

If not, check your battery connections at both ends. Make sure the ground connections to the engine and chassis are good and clean. Carefully check your battery cables themselves and look for signs of corrosion where the copper cables exit the insulation. Visually inspect as many of the harnesses as you can, looking for any burns, chafes, or other visible damage. Pay attention to the ones near the steering shaft, exhaust and drivetrain.

There are lot's of vids on checking for parasitic draw. Basically, you measure the amp draw off the battery with everything off. If you get more than about 50 milliamps, you start pulling fuses one at a time, until the draw drops. The fuse you pulled indicates the circuit with the draw. Bear in mind, you may have more than one issue.

Have you done any other electrical work to it recently (like before it started doing this)? Install a new radio/speakers? Upgrade lights? Etc? Do you live in a rural area where rodents have have set up housekeeping?

lawsoncl

On the parasitic draw, you may have to wait a few minutes after connecting for things to settle down. Radio might wake up and try ejecting a CD, etc. Newer cars take several minutes for the various computers to go back to sleep. Some cars like Subaru, have "intelligent charging" where it alternately lets the battery run down and then charges it back up, which can trick you into thinking the alternator is bad. It's really hard on batteries, but somehow gets them 0.05 extra mpg. I'm pretty sure replacing the expensive AGM battery every 2-3 years costs more than the fuel saved.

If you're looking for an excuse to buy a new tool, this is my favorite for doing parasitic and current measurements. It's a clamp-on current meter that works on both AC and DC, so you don't have to remove the battery cable. You can also clamp it on after starting to see how many amps the battery is charging or discharging. It also has voltage, capacitance, frequency, diode test, resistance, and non-contact voltage testing. Lowest range reads milliamps, and the highest range goes to 100-amps. I picked it up on sale for $33 last year.

Phaska

Thanks both. This advice is super helpful. I'll implement tomorrow and hopefully figure this out.

Roler

Some good and comprehensive advice there.
Let us know what you find.
When you test the alternator with the engine running, measure the voltage over the battery terminals with accessories turned on (radio, whipers, AC blower on high etc, all except the window defroster). Raise the rpms to about 1500-2000 and check the batt voltage. It should be a minimum of 0.5 Volts (and preferably at least 1 Volt) above what the battery showed as battery voltage without the engine or anything else on.

Phaska

An interim update:
I tested the battery and found that its health was toast, so I had to replace a battery for the third time in about 6 months. I now have a fully functional new battery that shows voltages in the correct ranges when the car is off, and on, and revving. I began checking the connections, harnesses, and fuses but have not found any issues or parasitic draws yet, but will continue to explore. I am hoping that withthe solid baseline of a new healthy battery I can finally figure out what is going on and not waste another battery.

Also, to answer your earlier questions Saudade, I have not done any electrical work. However, I bought the car used last year and a few powered parts are missing or dont work (i.e., the sunvisors and their lights are not installed, the rear windshield wiper doesn't work, the work light under the hood doesn't work, and there is a thick audio cable for a subwoofer leading to nowhere).

I also do not live in a rural area, and though there are rats and squirrels around here, I see no signs of chewing or nesting. I also spray the wheel wells and engine with a strong anti-rodent mint spray.

RockyMtn96XJ

If any of those items that dont work have loose and/or exposed wires, they could possibly be touching something (metal) they shouldnt and that is completing the circuit, hence drawing current 100% of the time when it shouldn't be. Even with the key off.