The Simple Facts Of Battery Charging-101

I have seen more than one thread in the forums recently concerning the charging of batteries. One question, “can the vehicles alternator fully charge a battery”? Another concerning the best type of charger/converter for battery charging and concerning the cycling of batteries and the effect on battery life.

Most of the answers are relatively simple. To charge a battery one needs only to have the voltage from the device charging the battery higher than the internal voltage of the battery. But not too high that would cause the batteries to boil and overheat. Remember that voltage is pressure just like in a garden hose. When you open the nozzle at the end of the hose the pressure is then higher at the end of the hose connected to the house plumbing and lower at the open nozzle. The water flows out of the nozzle.

When a battery has been discharged the internal voltage, the pressure is lowered. Attaching a battery charger to the battery and then turning it on voltage will be built up in the charger to a level higher than the internal battery voltage. Current will flow to the battery.

The alternator of your vehicle will fully charge the battery but there are limitations. An alternator unlike the old DC generator that it replaced needs electricity to produce current. A DC generator that we used on cars and light trucks until the late 60’s and early 70’s had built in magnetism that would start to produce current as soon as the fan belt turned the armature. And it would produce full power right off the bat without regard to the state of charge of the battery.

The alternator that replaced the DC generator needed current fed to the field terminal to build a rotating magnetic field that would then produce electricity when the fan belt was turning. However the amount of current developed was in part controlled by the input voltage at the field terminal. If the battery was dead then the output of the alternator would be diminished until the battery charged up. So getting a dead battery to full charge took longer.

Most alternators have internal voltage regulators today that control the peak charging voltage to around 14 volts. And as I said before if the charging voltage is higher than the internal battery voltage then current will flow from the charging device to the battery. And the greater the difference the more current flow up to the limit of the charging device.

For instance I have a 150 amp output alternator on our old Foretravel. After 4 or 5 days of boondocking my battery bank is at about 60%, even with the solar charging. The house battery bank is comprised of six Trojan 6 volt batteries and one 8D battery totaling over 1000 amp hours of power. That is the equivalent of 4 batteries in parallel. Now when I fire up the old Detroit Diesel the alternator kicks in. I get near full alternator field because the field reads from the chassis battery, which has not become discharged.

150 amps rolls out of the alternator and runs to the isolator and to the batteries. Now that 150 amps is divided to the six different paths/banks of batteries with the amperage flow though each bank being determined by the internal voltage of each bank. If each bank takes the same amount of current that is then only a 30 amp charge rate per battery, well under the charge rate in your average automobile. However if the internal voltage in the chassis battery is higher, as it most likely is, then it will only accept a lower charge rate and the excess then will be distributed to the other batteries with lower internal voltage.

So, I can get a fully charged bank of batteries but it will take a longer time. I used to tell my students when I was teaching Automotive technology, that everything about the car is basic math, everything. In this case, amps in X time = charge

Next time we will look at some of the differences between battery chargers, inverters, and other charging devices. Till then keep camping.

Brad