How long does it take for the alternator to recharge the battery? This question seems pretty straightforward - we have a 120A or 150A alternators fitted to the E31 - we have (around) 150 Amp-hours in total from our two 75 Amp-hour batteries - therefore it must take a little over an hour of driving to get a full charge. Well, fat chance of that for many reasons! • Lead Acid batteries cannot accept high currents without the terminal voltage exceeding 14.6V - around 30A is as good as it gets • The alternator only produces its rated current at 6000 RPM (around 2500 RPM crankshaft speed). Our alternators had an original specification of 14.6/80A/150A - this means a cut off voltage of 14.6V / 80A output at 1500 RPM / 150A output at 6000 RPM • The car is consuming a good proportion of the available current - the main consumers are • Heated seats, 10A per side • Heated rear screen 10A • Lights (main beams and fogs on in Europe) 38A (main beams and fogs on in the US) 29A • Auxiliary fan 10A at low speed, 22A at high speed • IHKA blower - up to 25A I have ignored a number of the electronic systems as the list above shows that it is quite possible to consume all of the available current from the alternator at idle - and most of it at cruising speed. It might seem an unusual situation to use all the consumers listed above at the same time - but in the UK this is the usual winter situation - heated seats on - blower and window defrosters on and the auxiliary fan running at half speed - lights all on and creeping along icy roads. So, in this situation, we are very lucky if the alternator is providing any excess current to charge the batteries. At the other end of the spectrum, a lovely summer day cruising along the motorway and the only large consumers being the auxiliary fan and the blower fan - in this situation we have lots of available current for the batteries - but the batteries can only use a small proportion for charging - around 30A for new batteries, and considerably less for batteries in poor condition. Unfortunately that is not the end of the story - as the battery voltage increases the less current is used for charging. The answer to the question: How long does it take for the alternator to recharge the battery? = somewhere between never and six hours When things start to go wrong Once you batteries have a low state-of-charge you will experience a number of problems. The low battery terminal voltage will play havoc with the 1980’s era electronics, the General Module can start to misbehave, fail to go into low-power mode and increase the discharge rate - in some circumstances it will corrupt its flash memory leading to even stranger problems. The same can happen to the EGS (gearbox computer) which may suddenly decide it has a shift-lock solenoid and bung up a TRAN FAIL-SAFE PROG - and the steering column module will reverse the motors so that UP is now DOWN due to the same corruption. .....and finally, the starter will just click - and that’s how you got here.... How to measure Current Drain (Closed-Circuit Current) Before you can perform an accurate measurement: • The meter must be capable of measuring 10A and have clamps large enough to get around a battery terminal • The batteries must be fully charged - the measurements are invalid with low battery voltage • The batteries must be in good condition - if in doubt buy two new ones, owning an E31 is never cheap • Make sure the emergency boot operation works just in case you get something wrong As the E31 has two batteries this simplifies the testing, we can leave one of the batteries connected while the meter is fitted to the other battery - this is important as it preserves the evidence that may be lost if the power is completely disconnected as this resets all of the electronics. When the meter is in place the second battery can be disconnected - here’s how to do it: Remove the carpeted trim from beside both batteries

The left-hand battery is more accessible so we will stick the meter on that side. Disconnect the negative terminal ONLY - thread the connector over the hold-down bolt to keep it out of the way. We now need to connect the meter between the negative battery terminal and the battery connector. If you need to buy a meter then most cheap meters will do nicely as long as it has a 10A/20A range -£20 ($25) is easily enough. You will usually find a common negative socket on the meter and a separate socket for measuring high currents. Here’s a generic meter showing the 10A socket on the left - and note that the range switch has a selection for 10A - this puts the left-hand socket in-circuit and effectively connects the Common and 10A sockets together via a low value resistor:

Although we are interested in measuring currents in the order of a few tens of milliamps it is imperative that the 10A/20A range is used - the lower ranges cannot pass the transient current demands and this will cause the E31 electronics to malfunction. Now connect the meter as shown below, correct sockets and correct range selected - do all of this BEFORE fiddling with the right-hand battery. We need this still in circuit to take the current as we prepare the car for closed-circuit current testing.

Here we go then, time to do some measuring - but wait, before disconnecting the right-hand battery we need to do some stuff to simulate normal operation. This what you do next: • If the bonnet (hood) is open - close it • Get in the car, close all windows, doors and sunroof • Key in the ignition - switch to position II (dash lights on) but don’t start the car • Leave in position II for 30 seconds • Switch OFF the ignition and remove the key • Double check everything is switched OFF and get out of the car • Shut the door behind you At last it is time to disconnect the right hand NEGATIVE battery terminal (don’t fiddle with either positive terminals) With the right-hand negative battery terminal disconnected  the meter is now in circuit and will be measuring the current demand - move the meter out of the boot (trunk) and put some cardboard over the trailing leads - it is extremely unlikely that the leads would be cut, but it is safe just to err on the side of caution - nothing would explode even if they were cut - you would just need to use the emergency unlocking procedure. • Shut the boot (trunk) • If you have remote locking lock the car from the key fob • if you don’t, lock the car with the key in the drivers door

START THE MEASUREMENT BY LOCKING THE CAR The video below shows you what to expect when you lock the car: • an initial > 10A demand as the locks activate • Current settles to 500mA or so • An additional 125mA will be seen in double bursts every 6 minutes • After close to 16 minutes the current should drop to around 50mA If the current reduces to around 50mA then your work here is done - that is the correct closed-circuit current demand and a decent pair of fully charged 75Ah batteries will keep the car ready to go for 100 days leaving a few Amp-hours in hand.

To summarise: • Don’t bother measuring closed-current demand unless your batteries are in good condition • The most likely cause of dead batteries are batteries that have not been correctly maintained • If you don’t drive the car that often (a couple of times a week) get a battery conditioner • Disconnect all after-market electronics if the closed-current exceeds ~50mA What to do next if the above fails (without just guessing) So you have good batteries, you have disconnected all the after-market stuff, you’ve waited 16 minutes and the current is way above 50mA - what next? We know a number of usual culprits, the General Module is a contender and you could whip it out, replace the capacitors and stick it back in again - always worth a go. But there is a more scientific way of tracing the errant current demand - and that is to measure the current in individual circuits. We need to get to all of the fuses for this, fortunately the car still goes into closed-current mode even if the boot (trunk) and bonnet (hood) are open - so we open the boot and bonnet, lock, unlock and wait 16 minutes at which point the current should have reduced to 50mA, the only difference is that the current will be higher initially as the boot lights should be on - this can be useful as if the boot lights stay on after 16 minutes then the fault will be with the General Module or the Consumer Cut-off relay. The boot lights can be extinguished by disconnecting the boot switch as shown below . As long as you have BMW fuses fitted there is a simple and accurate way to measure the current being passed by each fuse without removing it (as this could reset and lose evidence). Here’s the method: Measure current demand without removing fuses Using the above method and armed with the function of each of the fuses as detailed below: BMW E31 Fuse Allocations ...tracing the excess current demand is a bit long-winded but reasonably simple and will find the cause. The link above contains hyperlinks to the relevant sections of the circuit diagrams which will pinpoint the circuit that is demanding excess current. Above I am poking a fuse in the Auxiliary Fuse Box - there’s no current across that one! That’s all on this for now - we have only skimmed the surface but if you are worried you have battery drain - measure it! Time for a cup of tea! Equipment used in this article:

WHAT TO DO IF THE CURRENT DOES NOT DROP TO 50mA Don’t panic if you are only a few milliamps over 50mA, this value is for the standard E31 electronic systems only. My car initially showed a demand of 66mA; but 15mA was tracked to the..er...tracker! Yes, a tracker was hidden away and consuming a small amount of current. The final 1mA or so was traced to the built-in Ctek charger that has a small standby current to keep the electronics alive. If you have aftermarket stuff fitted such as a different head-unit, telephone, bluetooth, charger, amplifier or alarm fitted these will demand some current if not correctly installed. Of the above, an aftermarket telephone system can often be a battery killer - so strip all of this before worrying about our 80’s era electronics. Of all the E31 electronic systems, the General Module (A1 - ZKE) is usually the trouble maker. This module is central to controlling a lot of the cars systems and governs the 16-minute time-out that puts the car into closed-current mode. In particular, the ancient electrolytic capacitors can dry out, start passing DC current and stop the timer operating correctly. Fortunately these can be replaced quite easily and the parts are readily available. There are a couple of useful threads on Bimmerforums that cover this nicely. The General Module shares a problem that affects other modules such as the seat/mirror module, motorised steering wheel adjustment module and the gearbox ECU (EGS) - they can suffer from corruption of the flash memory which is used to hold the variant information (such as RHD/LHD, USA/Euro) - this can reverse the action of seats/steering wheel action - add a non-existent shift-lock solenoid to the EGS - and more relevantly for this article - it stops the General Module entering closed-circuit mode after 16 minutes - it is all explained here: Bimmerforums thread discussing the General Module FULL ROUTINE VIDEO

Timm's BMW E31 - How To Measure Battery Drain It’s much more likely you have killed your batteries The E31 Forums are stuffed with posts regarding ‘battery drain’ - usually along the lines of: The car was working flawlessly, I didn’t drive it for a week and then it wouldn’t start because the batteries were discharged, hardly a click, I think I have battery drain, how do I fix it? Well, you might have battery drain, but in 99% of cases you haven’t, you just haven’t looked after your batteries, or they have come to the end of their useful life. We might as well start with the reasons your battery is dead to start with: The BMW Battery Most battery 'drain problems' are due to the battery state of charge. Where a car is used for short journeys, especially in cold weather where many electrical consumers are in use (heated rear-window, heated seats, blower on full-speed, auxiliary fan, headlights etc), it is possible that the alternator cannot sufficiently charge the battery. In these situations a fully-charged battery's state of charge will slowly reduce to a point where there is insufficient energy to operate the starter motor. Where a battery is operated with a low state of charge over any period of time it will be permanently damaged due to sulphation. This effectively reduces the battery's capacity (the amount of energy that it can store), and its ESR (effective series resistance) which reduces the amount of current the battery can supply during cranking. Sulphation can occur in a matter of months if the state of charge is maintained at 20%. I have often heard It can't be the battery, it's only 6 months old. Unfortunately, yes it can, if you do not maintain a good state of charge the battery is not going to last long, even if it is the OEM Bosche unit. More information on the BMW can be read here. Maintaining a good state of charge Each time you start the car a large amount of energy is consumed. This energy has to be replenished by the alternator, the amount of time it takes to replace this energy is entirely dependant on what consumers are operating and the RPM of the engine. With many consumers operating (especially the blower, auxiliary fan, lights and heating elements) the alternator will not replenish this energy unless the RPM is greater than 1500RPM. Any lower than this and the battery is actually being discharged. If very few consumers are operating the battery will charge even down to 600RPM but nowhere near the rated 100A for the alternator, much closer to a 10A charge. These situations are quite normal, especially in the winter. If the journey is quite short then the alternator will not replenish the energy consumed when starting the engine. Over a period of time the state of charge will reduce, sulphation will increase and the battery will slowly lose its efficiency. If your use of the car is similar to this, then you will have to maintain the battery using a battery charger. Choose a battery charger that is automatic, these types fast charge the battery and then change mode to maintain the battery without over-charging. My 840i has a BMW charger permanently situated behind the battery, in winter or where the car is less used, I run a mains lead under the boot lid and leave it on charge. Below is a table so that you can check your current state-of- charge: