Required Tools: DC voltmeter or battery tester

As a general rule, modern charging systems are efficient, reliable and maintenance free. However, there are exceptions to every rule and few problems will leave you stranded as quickly as a charging system that’s gone kaput. Fortunately, keeping tabs on the system is relatively easy, all it takes is a simple DC voltmeter or battery tester.

To start, let’s clear up a fundamental misunderstanding about batteries. Batteries do not create electricity. At least no more than a refrigerator creates food. In both instances the devices are merely storage containers. Both accumulate and return what you put into them. If you use up all the energy stored in a battery without replacing it, it’s as if you’ve eaten all the food in your refrigerator, so until you refill it, you’re going to be out of power or mighty hungry.

It’s the job of the charging system to keep the battery full, which it does first by converting the AC current provided by the alternator to DC (direct current). Batteries can only store and release DC, then send that rectified current to the battery at a slightly higher rate than the battery’s standing voltage. Normally a fully charged 12-volt battery in good condition has a standing value of right around 12.6 volts. This can be slightly higher or lower, maybe 12.75 or so, to 12.4 to 12.5 volts, depending on the battery’s efficiency and state of charge, but in round numbers 12.6 is about it. Once the battery charge drops to somewhere around 10 volts you’re done and the engine will simply shut down.

Batteries only accept a charge when the voltage flowing into them is greater than the voltage flowing out.  Unregulated, the average motorcycle-size charging system is capable of pumping out around 75 volts, which is far more than the battery, or the bike’s electrical system can safely absorb. This is where your regulator comes into play. The regulator, which is normally built into the rectifier (some older bikes have a standalone device), reduces the output to slightly more than the battery’s rated voltage. In most 12-volt systems this equates to a normal charging system output voltage of around 14-to 14.75 volts (however that’s a rule of thumb). The charge rate is dependent on a number of factors, among them the charging systems design, the quality of the components and of course the motorcycle’s electrical demands, so some variation is to be expected.

For example, a small dual-sport or commuter bike doesn’t require a massive charging system. As long as the input voltage exceeds the output you’ll be fine. You should expect to see a reading towards the low end of the scale, maybe 13.5 to 14 on a bike like that. By that same token a sportbike, cruiser or touring motorcycle needs a robust charging system, so if you are checking a fully loaded touring bike you want to see at least 14.4 to maybe 14.7 volts, especially when all of the accessories are on (you’ve got to love those heated grips and seats on chilly mornings!).

The bottom line here is that you need two things to make your electrical system work. The first is a fully-charged battery and the second is a charging system that keeps it that way. The question being how do you know which portion of the system is at fault when there’s a problem?

Here’s an example. If your battery doesn’t seem to hold a charge between rides it might be because the battery is on its last legs, or it may be because the charging system isn’t performing as well as it should. So, which is it?

To answer that you’ll need two things: a fully-charged battery (no charging system can be reliably tested when the battery’s discharged) and a voltmeter or dedicated battery tester, like the TecMate TestMate mini, which is capable of reading at least 15 volts DC.

Charging system voltage tests are always performed with the battery fully-charged and connected to the motorcycle, so the first order of business is to make sure the battery connections are clean and tight. Many charging system problems have been cured after checking the battery connections.

Set your voltmeter to the DC scale. If it has a calibration adjustment, set it to read above 12 volts and attach the clamps or probes. If you’ve never used a voltmeter before, here’s how this works. The meter’s positive or red clamp or probe attaches to the positive battery terminal, then the negative or black probe connects to the battery ground. A healthy battery should read right around 12.5 volts. If it’s much lower than that, say 12.0 or less, charge the battery before proceeding. If the battery won’t hold a charge, or shows signs of damage, you should replace it.

Assuming the battery voltage is good, start the bike and watch the meter. Depending on the type and efficiency of your charging system, the meter may show only a very small increase or even a decrease in voltage at idle. However, as you increase the engine speed you should see a rise in voltage. At any speed above a fast idle the voltage reading should rise above the standing voltage before leveling off. Preferably you’ll see something like 14 to 14.5 volts, although there are exceptions, so don’t be overly concerned if your readings are slightly higher or lower. If they’re considerably lower or higher you’ll need to take corrective action. For that you’ll need the motorcycle’s service manual and some understanding of how the charging system generates and regulates the current, which is outside the scope of this brief introduction.

On that note, here’s some final advice. If the charging system isn’t working properly, don’t ride the bike until it is. I know that sounds obvious, but I’m always surprised at how many guys will try nursing a bike along when the charging system is marginal at best. A dead battery creates all sorts of unpleasant scenarios and if one of them occurs at rush hour on a busy stretch of road you’re going to be in for an interesting experience pushing your bike through traffic, followed by a long wait for help.

The flip side, an overcharging battery is just as bad, perhaps worse, because under the right conditions the battery could literally explode beneath you. Admittedly that’s an extremely rare occurrence and the more likely outcome is that it will blow out your lights due to high voltage, but that’s only marginally better. The bottom line is that being able to determine where the problem is located will only be half the battle, but at least it’s a half you can fight and win in your own garage.