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Battery charging / Alternators

Kevin Wood

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Kevin Wood

Why is my battery not charging?

How do I tell if it's charging / the alternator is working?

How do I charge my battery?

How do I keep my battery topped up during winter?

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Kevin Wood

To make the measurements here you'll need a multimeter capable of measuring down to 0.1 volts or so accurately. You'll also need to connect it exactly as described as voltage drops can occur around the wiring loom. Dashboard battery voltage gauges are not 100% trustworthy for this reason and should be treated as a guide.

Battery charge level.

Although rated at 12 volts most car batteries, when fully charged and disconnected from any charger, have a voltage of around 12.5 Volts. If the voltage is less than this the battery is in poor condition or not fully charged. If the voltage is down to 12 volts the battery is actually pretty flat. Under 12 volts and it's seriously flat. Measure the voltage with no load and no charging (engine stopped) and measure directly at the battery terminals if possible.

Battery condition

If the battery is showing a low terminal voltage, charge it fully as described below. Let it settle for a couple of hours and measure the voltage again. If it's less than about 12.5 volts the chances are it's seen better days.

Measure the drop when you switch on the headlights. If it drops more than say 0.3 v or continues to drop further as the lights are on for a minute or two it's probably seen better days. If it'll crank the engine, measure the voltage and look under the cell caps while cranking. If one or two cells gas while cranking or the voltage drops to less than 10 volts it's probably seen better days.


Batteries are charged by placing a larger voltage than their rated voltage across the terminals. For trickle charging, a voltage of 13.8 volts will bring a flat battery up to 100% charge in 24 hours or so and will do no damage if left charging permanently. Increasing the voltage to 14.4 volts will charge the battery much quicker (a couple of hours) but will do harm if left connected indefinitely. Less than 13.8 volts is probably never going to bring the battery to a state of full charge. Less than 12.5 Volts and the battery is being discharged! Substantially more than 14.4 could be dangerous to the battery, and if much more (15.5 - 16 volts or more) could be dangerous to anyone in the vicinity!

Whether a battery is being charged by your alternator or a battery charger, measuring the voltage directly across the terminals and comparing with the above will tell you what's going on.

An alternator really should be able to manage 14 volts or more at engine speeds used when cruising (perhaps not at idle). This is the only way that the huge amount of energy taken from the battery during starting will be replaced during a journey. If the alternator can't make this voltage, or can't maintain a reasonable voltage with a few electrical accessories switched on, the battery is likely to be running flat. The alternator should not charge at more than 15 volts or so under any conditions or the battery will be overcharged.

If the charge from an alternator looks low, measure the voltage between the alternator B+ terminal (the big one connected to the battery) and the alternator case. If this looks OK according to the above, don't condemn the alternator. It could be a dodgy earth or poor wiring between alternator and battery.

The above voltages also apply to battery chargers, some of which are not very well controlled. When charging a battery, check the voltage periodically while the charger is connected and use that to decide how much longer it is safe to charge the battery. To summarise 13.8v or less - indefinately - down to a few hours or maybe overnight at 14.4 volts. If the voltage exceeds 14.4 volts under charge the battery is probably either fully charged or at the end of its' life.

Conventional liquid electrolyte batterys can tolerate a bit of abuse when being charged. Gell batteries such as light weight racing batteries are much more easy to destroy so pay attention to the voltage. There is no harm in charging these on a conventional car charger to get you going, despite what those who market more expensive chargers might say, if the battery is flat. If the battery doesn't need the charge, it could be damaged. As a rule, if the voltage climbs to 14.4 volts while charging, it's probably time to turn the charger off and try to start the car. Go for a blat and allow the alternator to finish the job.

Leaving batteries on charge for long periods (months), even at a low level of 13.8 volts or so, is not a good thing to do. This is why many conditioners cycle the charging on and off and even periodically discarge the battery. This type of charger may be left on the battery for long periods but a cheaper and more fun way to keep the battery in good condition over the winter is to either drive the car occasionally or use a cheap trickle charger for maybe a couple of days at a time once a month or so.

Despite the fact that most car battery chargers have ammeters showing the current that they are supplying, it is the voltage across the terminals of the battery which is the key to knowing what's going on.

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  • 7 years later...
Andy Banks

Lifted from a response from peterg to a question about charging and alternators:


Battery voltages:


12.7 fully charged.
12.4 75% charged.
12.2 50 % charged
12.0 25% charged.
11.9 DEAD.


When charging the alternator will show around 14.4v to the battery


The voltage coming out of the alternator depends on two variables: the amount of current flowing through the field coil (i.e. the strength of the magnetic field) and the speed at which the alternator’s field is rotating. The alternator has a regulator that tries to keep the voltage across the battery at a steady 14.4V (the optimal voltage to recharge 12V car batteries). It does this by regulating the amount of current flowing to the field coil. Once the alternator is self-sustaining, the only current flowing to the field originates from the alternator itself. If the output voltage is too high, the regulator lowers the current flowing to the field coil. If the output voltage is too low, the regulator increases the current flowing to the field coil. Simply put, as long as the alternator can maintain at least 14.4V across the battery, making the pulley spin faster or slower will have absolutely no effect on the power output.


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Mine stopped working this weekend at Curborough. I was going to buy another one until I notice that my battery pre charge indicator bulb looks like its blown. 

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