Further data I found on your problem http://versioni.addr.com/Tech/battery.html
What do you Mean the Battery is dead? I just charged it!...
This shamelessly stolen from
Pre 85 E30s battery charging problems
by Chris Diersen
If you own a pre-1985 318i, you may fall victim to the same electrical system flaw that stranded me three or four times and cost me a new battery and alternator (replaced unnecessarily) before I found the true problem! That is: BMW designed (in their infinite wisdom) the idiot light for the charging system (the little red light with a battery on it) in SERIES with the charging system, as opposed to PARALLEL, the logical choice... This occurred only in the pre 1985 318i's. As a result, if the bulb burned out, or the contacts on the instrument cluster board went bad (as in my car) the charging system would cease to work. Read: alternator no longer charging the battery. In my case it was double bad, as the bulb made intermittent contact, so it only failed sometimes! I replaced the battery and alternator (ouch!) before reading of this problem in the Haynes (thank you Haynes!) manual. I diagnosed the circuit board failure by banging the instrument cluster with the ignition on (but the engine not running). The battery light would flicker on and off. BMW's solution to this problem was to (on later models) wire a resistor in series with the bulb. That way, if the bulb or its contacts went bad, the charging circuit would not be broken, and the charging system would still work. If you experience a failure similar to mine (where the contacts on the circuit board for the instrument cluster shot craps) you can replace it with the resistor-equipped circuit board (dealer part only: $176.00!) or you can do what I did: Buy a brand new bulb that is identical to the bulb for the charging system idiot light ($1.20 at AutoZone). Locate the wires that run to the charging system idiot light by tracing the connections on the instrument cluster circuit board from the charging system idiot light to the multi-pin connector that connects it to the wiring harness. There are two wires that run to this light. You may want to make sure you have the right two wires by checking the continuity with a Multimeter. Once you locate them, splice a wire into each of these, *soldering* the splice to be sure it takes. After completing the splice, connect the two new wires to the new bulb that you bought, affix the bulb under the dash somewhere (I hot-glued it to the center console), and re-assemble the instrument cluster. That's it! When you turn the ignition on, the new bulb should light up until you start the car, at which time it should turn off (sometimes slowly). Essentially, you are adding a "resistor" (the bulb) to the circuit, just like BMW did later, only you can check your "resistor" by looking at it. Not a bad idea if (like me) you're not an electrician and bungle thing quite frequently. You may want to do this to your pre-1985 318i BEFORE it fails, although the dealer mechanic says he *never* heard of this problem actually happening to anyone...(Sure; that's why it's one of the first electrical system notes in the Haynes manual...) You *do* have to remove the instrument cluster to make this repair (unless you're a contortionist)! To do this, simply remove the steering wheel (pop off the roundel and use the BMW-supplied wheel lug wrench (it fits!) to unscrew the wheel...my little trick! Be sure to mark the wheel's position on the stem with a marker before removing it, or be prepared to suffer eternal "mis-aligned steering wheel spoke annoyance!"), unscrew all the trim around the instrument cluster, remove the four bolts that hold the cluster in place, and tipping it *forward* (back may break the top two "tangs") carefully lift the cluster out. I won't tell you what color wires to splice into, because they're probably different on your car. Good luck!
This tip from All BMW PARTS
Ever wonder why BMWs don't come equipped with maintenance-free batteries?
BMW's come with lead-acid batteries, but most maintenance-free batteries are lead-calcium batteries. Interstate's new Pinnacle battery line uses 'silver calcium' chemistry. These battery types differ in their characteristics and operation. Maintenance-free batteries require a higher charging rate than lead-acid batteries, but BMW voltage regulators are calibrated at a lower charging rate than the maintenance-free batteries require. Another difference between the two battery types is that lead-acid batteries have a different internal resistance than maintenance-free batteries. This internal electrical resistance difference causes the voltage regulator to set the charge rate even lower than it would if a lead-acid battery was installed. In effect, it "fools" the voltage regular into thinking that the battery is in a higher state of charge than it really is. What occurs is that the maintenance-free's need for a higher voltage is not met, but the battery's different resistance causing the charging voltage rate to be lower resulting in a perpetually undercharged battery condition. Recharging a lead-acid battery requires around 14 volts. Many maintenance-free batteries need closer to 15 volts to fully recharge (a Delco Dura-Power, for example). A typical BMW charging system ranges between 13.8 to 14.5 volts. I have personally seen up to a .5 voltage drop caused by a maintenance-free battery installed in a BMW. Dropping one-half volt of charging rate causes the voltage to fall out of the acceptable range if that particular car is marginal to begin with. One case in particular stands out in my mind. A BMW owner was mystified that his battery would discharge after sitting in the garage for only a few days. The combination of a maintenance-free battery and a typically low charging rate setting for that model never allowed the battery to attain a full charge level. In normal driving the battery would slowly discharge. By the time the car was back in the garage, the battery had been partially discharged. The normal radio memory, accessory draw and computer memory finished the rest off. When he went to start the car, the battery was stone-cold dead. Testing for a parasitic drain would always come up negative and a low charge rate didn't raise any flags. No conclusive fault could be found with the voltage regulator or the alternator. Installing a new lead-acid battery cured the problem entirely - despite the chain stores' recommendation for maintenance-free batteries in BMW applications. Be SURE to specify the correct lead-acid battery for your BMW.