The battery in the automobile with combustion engine is older than the generator. It already supplied the ignition with electric energy, before it could be recharged through driving. This had to be done at home before almost every trip. It took over the role as energy supplier for the starter only much later, when the starter edged out the hand crank.
If one compares the battery with most of the other vehicle components, one will note that it has basically, undergone the least amount of development. Even though it is now (2010) modern, maintainance-free and drier, it remains a heavy, lead-acid battery able to store only a relatively small amount of energy. The demands made of the battery however, have increased drastically.
More than 1000, 2000 and even 3000 watts of output performance were previously, at most, only conceivable for luxury busses. The very quick change over from current supplying to current consumption also poses a problem today. As a result, it's quite possible, that even new cars , after having stood still for some time, can have a defective battery while still at the dealer.
Actually, there should be no possible working conditions whereby the battery has to give out more energy than it receives. This situation can occur when idling, where the generator only achieves a good one third of its performance. Of course, a negative charging balance can also occur when apparently everything is switched off, however, e,g., the interior anti-theft is active and is waiting for a remote control radio signal.
Chronologically seen, the demands made of the battery are even higher. Since exactly then, when it has produced the performance necessary for starting the engine and is only being sub-normally charged, is the consumption paticularly high. This begins in the Diesel engine with the after-glow equipment, it concerns, e.g., the secondary air-pump for the catalytic converter heating, which can also be done by electricity, and further, the windscreen heating, the lighting and a number of other things as well.
The battery is charged by the generator, because this delivers a higher voltage than the momentary battery voltage. This succeeds best of all from the mid-range RPMs upwards at moderate temperatures, if the temperature is too high, this can cause a too high internal resistance of the generator. The power requirements in automobiles can be found here.
The charging voltage of the battery has been increased over the years from approx. 13,8 V to 14,2 V and up to 14,7 V. In this case, the (admittedly, not very often) installing of an older battery in a modern automobile can pose quite a problem.
Nowadays, one should look for the battery, not only in the engine compartment, if you find one, don't stop your search. Not only large cars and campers can have two batteries. One is there, just waiting for the next engine start, while the other can be discharged to distinctly less than half its power.
Excessively high temperatures in the engine compartment or too low temperatures in the luggage compartment can also strain the battery. Sometimes there is also too little space beside the engine. Indeed, because of the current to be transferred, at least the starter battery should not be too far removed from the starter. The weight distribution of the vehicle can also play a part here.
The regulation of the charging voltage has also undergone a great deal of development. After the introduction of the alternator, it seemed that a relatively simple regulator would be sufficient. Only with the advent of pulse width modulation and the partial take-over by the engine- or electrical systems control, have the possibilities been clearly extended.
During the start-up procedure, the generator charges only sparingly, to prevent an additional strain on the battery. Then, depending on the requirements, more or less pulse width is generated, always analog to a desired charging voltage which can, by the way, be higher with a cold battery and lower with a warm one. All this is done more effectively the fewer the (also the high frequency) fluctuations in the voltage are.
At this point, we should discuss the Electric Energy Management and the battery sensor. Since somebody must measure the temperature at the battery and, e.g., charge or discharge two batteries at the right time. Particularly important in this context is the emergency function, i.e. the prolonged negative energy balance.
The battery must urgently be brought down from its top position in the breakdown statistics. If necessary, its breakdown must be prevented by the consequent shutting down of larger consumers. The sensor directly on the battery terminal enables through several, virtually concurrent measurements, an exact impression of the state of the battery. It also enables (through the LIN-Bus) a complete error analysis. 11/10