By Wesley Pestana | Updated Nov 26, 2018
Why won’t my Motorcycle Battery Charge?The alternator is not working correctly. In other words, if your battery doesn’t receive a top-up charge during the normal running of your motorcycle, the fun is going to end pretty damn quickly.
Should you be the proud owner of an early 1930’s Scott Flying Squirrel, you may want to skip this article and enjoy a cup of tea. Why? Because, quite simply you and the majority of smug looking vintage bike owners, happen to have magneto ignition, dispensing with the need for a battery.
For us lesser mortals though, a fully charged battery in a state of high excitement throughout the duration of a ride is a must.
In other words, your bike is running fine, so why, when the revs drop right down, does it start to splutter? Also, after a short journey, why isn’t the battery left with sufficient juice to crank the engine?
Look no further than our previous article; Why Motorcycle Batteries Die. However, for anyone who didn’t get to read it, read on. It’s because, although the alternator provides all of your electrical requirements when riding, it’s the battery that takes over this duty when the engine revs drop to around tick-over speed.
During the regular course of events, the alternator produces well in excess of your motorcycle’s needs in terms of electrical supply. The excess is then fed into the battery to keep it fully charged.
When a problem occurs somewhere along the supply chain, and your motorcycle battery won't charge, any stored power is going to be drained by the electrical components on your bike.
As we’ve covered the dead battery scenario before, for the sake of this piece, we’re going to concentrate on testing the alternator. So here is the run down.
The stator (outer casing and windings) and rotor, (inner component) fed from the crankshaft, form the alternator. The rotor revolves at crankshaft speed while the stator remains in a fixed position. Together they generate AC current. A rectifier then converts the AC into direct current or DC.
When considering this question keep in mind that it’s slightly different from the highly common scenario of, my battery keeps dying. In the latter, a number of factors can be at fault from a cracked battery to a dodgy wiring harness.
To kick things off; to check the alternator, you need a fully charged battery. Using the meter, check that the battery reads between 12.6- 12.8. If it doesn’t (and the chances are it won’t), put in on a trickle charge.
By the way, if it doesn’t take the trickle charge, you know your battery is toast, but let’s assume for now it’s okay. Test the battery with the meter again, with the engine off and take note of the reading.
Start the engine and see if the meter reading increases, if it doesn't, you have your answer. The lack of an increase in scale means there’s no output from the alternator, so the next step is to check its various components.
The first thing to do is locate and remove your alternator cover. You may need to check out the owner’s manual. For example, Honda’s tend to be on the right side crankcase, while Suzuki’s are on the left.
With the cover off, first check the rotor. Namely, the cylindrical part that stays attached to the engine and spins, while the stator is the collar shaped component that remains fixed in the outer casing.
Although these components and the covers they hide behind may look different from bike to bike, they all do the same basic job and the test to check for continuity is the same.
To test the rotor, it's back to the trusty multimeter. Set it to 200 ohms and make sure the leads are in the correct sockets, ie. Black lead in the common port and red in the socket marked with the Greek letter Omega (Omega looks like a line drawing of tomato dropped on the floor).
You are testing for resistance, and an ohm is a measurement of resistance. A common or garden Japanese rotor is made-up of slip rings. When rotating at engine speed these slip rings contact with brushes fixed in the stator housing.
Test the resistance by placing a lead on the outer and inner ring. You should have a reading of between 4.2-5. If it kicks in significantly less, then it’s safe to assume the rotor is goosed and needs replacing.
Next, it's the stator, and with this, we're checking to see if there's a fault in the wiring. Follow the wiring from the stator to where it connects to the harness and unplug.
Usually, there’s a block with a 2-3 formation of bayonet slots; the two will feed off the brushes and the three connect to the stator windings.
Put the red lead in the first slot of the connector and the black in number two slot then number three. Any reading will tell you there’s continuity, meaning that the wiring is okay.
You can carry out a secondary check on the stator housing by connecting the black lead on the outer casing and going from slot to slot with the red lead. In this instance, you don’t want to see any reading at all, meaning nothing is shorting out.
To check the brushes, one lead goes on the housing, maybe on a fixing screw on the brush backing plate and the other on the brush. Check both brushes, and here, we want to see a reading, so if it's a big fat zero, the stator is junk.
That my friends, is pretty much that. Depending on if we require a reading or not, determines the functionality of the components and the need to replace them. Sometimes the fault is in the alternator, other times the battery needs to be replaced.
This article has been quite specific in its content purely because, in our other feature, we’ve already been through the process of looking for faults in the rest of the line.
So next time your motorcycle battery won't charge, you'll have the knowledge to shine the spotlight directly on the alternator. It may even save you a buck or two. In the past, I have to admit to junking the lot - when in reality I could have isolated the problem in one component. Hindsight is a wonderful thing!