OK gang, please help me out (and bear with the long description). I have a problem that is driving me crazy!

My 2003 R53 runs and drives great, but it has a frustrating problem that causes the battery to go dead on a regular basis. This problem first came to my attention almost a year ago, when the car wouldn't start - even though I had just driven 35 miles and had only been parked for five minutes. And what's more, it wouldn't start using my jump starter box or using jumper cables connected to a running car (even after 20 minutes of charging). The battery was stone dead - and couldn't be revived on my charger at home either.

Thinking that the five-year-old battery had just met it's natural end, I swapped the dead battery out with the good one from my R53 parts car. Predictably, the car ran well for a few days - then that battery went dead too. This battery was revivable on the charger, but it was no spring chicken either. I wondered whether that battery was also near the end of life, or if I had a charging problem.

As you might imagine, I began to suspect the alternator was bad. But to ensure that the old batteries weren't the culprit, I bought a new Optima Redtop battery on sale (for $140!). I installed it and the car started and ran fine. I measured the voltage both under the hood and at the battery at idle, and my trusty multi-meter showed 14.3 volts. When I turned on a bunch of accessories at once (lights, AC, full blower motor, radio), the multi-meter still showed 14.1 volts across the system at idle. That voltage should be more than enough to consistently charge the battery, and suggested the alternator was working fine.

So I drove the car for a week, until this new battery fully drained too. I took the battery to a manufacturer authorized shop, and they said it was severely drained; but they managed to charge it up. Once reinstalled, the car predictably started fine, and I drove it home - where it has been parked ever since.

Before I ask questions of the society of electrical Mini experts here, let me offer a few more helpful details:

First, and I think this is important, the car has never stalled or shown lack of electrical power while running. If I can get the car started, it seems to run just fine on the power supplied by the alternator (no matter how many accessories are running). But even if I start with a fully charged battery, the car may drive fine for a few days, but the battery will consistently go dead within a week. The battery might even refuse to start the car two minutes after parking, even if I just drove the car for an hour.

Second, for a while I imagined that there was some vampiric power drain in the electrical system, but the car has been sitting in my driveway for months, and the battery has not gone dead just sitting there. It has power to reliably lock and unlock the doors from the remote every time I try it; and the car has started the last five times I've tried it (over a period of a few months).

Third, a previous owner or mechanic had broken the metal part of the hot (red) battery terminal cable and had reattached the metal bits to each other by bolting them together. It's stupid and inelegant, but the connection is snug, and it seemed to work fine when I got the car - as I drove it reliably for two years with no power or electrical problems. (As such, I don't think this broken and bolted lead is the problem; but I figured it's worth mentioning because it's non-standard.)

So now I'm frustrated and confused. My best thinking suggests that the battery isn't charging. The alternator seems to be working fine, but I imagine it's possible that it's putting out erratic voltage under load (unlikely, but possible). Also possible is that the alternator is working fine, but that there's another reason the battery isn't charging - but I don't know what that is or even where to look.

Does anyone know of something else that could go wrong in the car that would keep the battery from charging? Do any of the electronics attached to the hot battery cable control this (and is it possible they've failed)?

From one Mini enthusiast to the NAM community, thanks in advance for all your help!

 

I would do a voltage drop test across that bolted connection. Your suspicion that the battery is not being charged may be a good one. If there is a big drop across that connection then replace that cable.

You can also do some current checks to see if the battery is accepting charge. (assuming you have a clip on current meter).

 

any decent parts store can check the alternator for output ..... usually for free