Guide & Tips: How to know if your BMW's battery is any good!

[genphreak]

As our cars get older or are used less frequently, they get more electrical issues, and a lot of members fail to suspect their battery's performance or measure its state of charge properly. So I thought I'd share the key steps necessary to avoid such trouble.

Using these steps you will be able to verify / know for sure if the battery is good, or if the car is not charging it. After all, we all know how important a good battery is to keeping a BMW running at its reliable best, right?

Car Batteries
12 volts (V) is the standard automotive battery in most petrol vehicles. A car battery has 6 cells, each with a voltage of around 2.1V, so a fully charged car battery has 12.6V-12.7V.

There is a very big difference when a battery drops even a small amount of voltage. When a car's battery voltage drops, for instance, from 12.6 to 12.0V, there will be 75% difference in available capacity- i.e. its power drops from 100% to 25%. At 12.4V, a car battery is 75% charged, while at 12.2V its 50% charged.

Tip: Consider your car battery charged at 12.4V or higher and discharged at 12.39V or less.

How to Check Battery Voltage and State of Charge
It is important that you first check the battery. To check most simply, turn on the headlights- if they come on with normal brightness it is possible a no start condition/problem is not the battery. The problem could be one of the following:

• bad starter relay or solenoid, connection to them, etc.
• Engine immobiliser, car alarm or safety switches (on some cars these are installed on the brake, the gearshift- some racing cars even require a special start sequence)
• a poor wiring connection between the relay and starter, imobilisation system.

If the headlights are dim, or there are no headlights at all, check on the battery voltage and charging output.

To check it more accurately, you will need a voltmeter. On the meter, select a suitable range/scale for 12V. Most have a 12V or 20V DC scale. Now connect the voltmeter's red positive test lead to the battery positive terminal (+), and the black negative test lead to the battery negative terminal (-).

Make sure the car is off and its lights and all electrical accessories are off. Modern cars use a small amount of power until the car's systems shut down, often this can take up to 20mins. During this time the battery voltage will read slightly lower than it really is, depending on how much current the car is drawing. Also, if you still have accessories on, dome lights with delays, (E.g. the radio, antenna and amplifiers on some cars still work until the key is taken out of the ignition) the voltage will appear lower.

If the car is not using power, the battery has a reading of 12.4V, it means it is low (discharged) and should be recharged. A battery fully charged has a reading of about 12.6 volts. It should still start the car but it won't sound very keen about it!

Hint: A car may use up to 40mA at rest with nothing on. This is because some things ARE actually still on, such as remote key locking system, etc. This is the reasons why a car battery should be disconnected whenever you leave a car for a long period. And why if you don't, the battery will be flat after 2-5weeks. Any more than 50mA draw and something is drawing power that probably should not be. This is called a parasitic drain/loss and will cause the battery to go flat quicker than it should, and over time, will cause the battery to wear out quicker than it should. Some BMWs have several states of standby/sleep, and take hours to step down from say 120mA to their lowest power state (20mA).

Check Car Battery Charging Voltage
After measuring the battery's standing voltage you should measure its charging voltage. This can checked after jump-starting the car. 13.5 to 14.5V at idle should be produced by a charging system operating normally (some may even measure higher than this).

If less than 13.5V. the alternator is probably not putting out enough current to keep the battery charged. You should then test the alternator (can be tested at an auto-electrician or an auto parts store). Sometimes the regulator is worn out or faulty- and can be checked and replaced easily, however if not the regulator, consider replacing the alternator if its rated output current is not up to specification.

After recharging the battery, test it to find out if it is capable of holding the charge. Do this by use of a hand-held electronic battery tester or a conventional load tester. If the battery is good or bad, be sure that the tester will tell you. It is important to note that for accurate test results load testers require the battery to be fully charged. Reliable test results are given by most electronic testers even if the battery is not fully charged. Replace a battery if it fails a load test.

12V car battery voltage chart

Battery voltage	Volts per cell	State of charge
12.70	2.12	100%
12.50	2.08	90%
12.42	2.07	80%
12.32	2.05	70%
12.20	2.03	60%
12.06	2.01	50%
11.90	1.98	40%
11.79	1.96	30%
10.50	1.75	0%

Tip: Most BMW's Engine Management Systems (EMSs/ECUs) will not work when its voltage drops below 12.0V. If your battery is in a very low state of charge, when you crank the engine over the car's voltage may drop below 12.0V- in which case the ECU will simply fail to start the engine. It may push-start if you have a manual car and the battery is standing above 12.0V with ignition on and no cranking, but depending on how quickly or fast you do it, it may not!!!

The average life of a car battery is 4 to 5 years (some can last up to 7). This reduces to around 3 years in hot climates such as Arizona and Florida, where a battery does where the heat causes the battery to deteriorate chemically, faster. If the battery is mounted in the engine bay, it will not last as long as batteries that are mounted elsewhere in the body such as in the trunk or under the rear seat.

If your battery is getting on and its voltage is dropping, it is likely that it has reached the end of its service life.

Should you continue to use an old battery it will eventually be unable to start the car, or worse, begin to lose charge when you are driving- even with a functioning charging system. This will become very apparent when you begin using electrical systems that draw large amounts of power such as window demisters, air conditioners/fan blowers, electric windows, windscreen wipers and headlights. The risk you take driving with an old battery is that the car's electrical system will shut-down when it gets gets dark, cold or hot outside, starts raining, and so on. This may mean the engine will stall and if so, it will not re-start. Also, an old battery will cause the charging system to work overtime, increasing wear on the alternator, belts, belt bearings, and increase fuel consumption.

Note: All these figures are general rules of thumb, if your experience varies, post it up and let us know how!