Diagnosing low charging voltage

While you are ridding your motorcycle, the charging system provides electrical power to run electrical components and accessories. It also recharges the battery. Most of the electrical components will operate satisfactorily with a voltage between 12 and 15.8 volts DC. The battery is a little pickier about its charging voltage. Below 13.8 the battery will not fully charge. If the voltage is to high the battery will charge too quickly and may over charge. Either way high or low charging voltage can lead to battery problems. In order to get maximum battery life proper charging voltage must be maintained.

Last month I talked about what to do if your battery is not staying charged. One of the tests was to check the charging voltage at the battery. This article will cover how to find the problem if your voltage is low.

 There are two main problems that can result in a low voltage condition at the battery. The first is a voltage drop between the regulator and battery. The second is if the electrical system is drawing more amperage than the charging system can put out.

In the April 2010 issue of Iron Works I wrote about voltage drops. In that article I made an analogy about water flowing through a pipe that has restriction. As the water current flows through the point of restriction the presser drops. Lets take this water analogy one-step further. If you have a pressurized water system and water is being taken out faster than it is being put in the presser will drop. Voltage is the force or presser that makes electrons flow. Amperage is the actual flow of electrons.  When the electrical system is drawing more amperage than the charging system can produce, system voltage will drop down to battery voltage. At this point amperage will be drawn out of the battery to make up the difference.

In order to diagnose why the charging voltage is low, I would start by testing for voltage drops between the regulator and the battery. (See voltage drops in the April 2010 IW) Next I would do a charging system out put test. This will tell you if the charging system is operating properly. You will also find out exactly how much amperage you have to work with. Than I would do a system usage test to see how much amperage the bike is actually using. By comparing charging system out put to amperage usage you can see if the charging system can keep up. There needs to be enough extra amperage available to recharge the battery after starting.

 If your motorcycles electrical system is not keeping up with demand you have two choices. You can either reduce amperage usage or increase charging system output. To reduce usage you can eliminate accessories or reduce the wattage of light bulbs.

To increase the output of the charging system you will need to upgrade to a system with more output. Cycle Electric Inc offers many kits to do this. You can find them on our web site www.cycleelectricinc.com.  We offer 40 different alternator kits with amperage output ranging from 22 to 50 amps.

Determining Usable DC Amperage (System Output Test)

The object is to add electrical load until the voltage drops below the normal operating voltage. At that time the charging system should be producing maximum amperage to try to keep the voltage up. The amount of extra load you will need depends on the output of your changing system compared to the usage of you motorcycle.

 At Cycle Electric Inc. we define usable amperage as the amount of DC amps the charging system can deliver while maintaining 13.8 volts at the battery. If voltage drops below 13.8 the battery will not fully charge. By the time voltage drops to 12.7, you are drawing amperage out of the battery, so any increase in amperage output after the voltage drops to 13.8 will be considered unusable.

Note: On Cycle Electric Inc. low volt models normal operating voltage is 13.8. It will be necessary to drag the system voltage down below 13.8 to perform this test.

 To test charging system amperage output, you will need a voltmeter, an ammeter that is rated to measure higher amperage then the charging systems can put out and extra electrical load to add to the system.

 To add electrical load you can use power resistors, a battery tester or light bulbs. Light bulbs can make an economical load dump. One H4 bulb with a 100-watt high beam and an 80-watt low beam will draw 15 amps with both filaments lit.

Note: If your system voltage is already low it will not be necessary to add extra load.

 Connect the voltmeter across the battery. Connect the ammeter in series with regulator output (measure amperage in the wire from the regulator to the battery). Start the motor and allow it to warm-up enough to obtain a steady idle. Connect load dump across battery. (Add light bulbs, resistors or whatever you are using for a load dump) Continue to add load until the voltmeter reads between 13.8 and 13.5. Read the ammeter. This is usable low speed amperage. Now increase RPM to normal cruising speed (usually between 2500 & 3200 RPM depending on your model and riding style) add load to drag the voltage down to 13.8. Read ammeter. This is usable high-speed amperage. On some systems maximum amperage will come at 4500 RPM or higher. Unless you cruise at 95+ DO NOT consider this as usable. USABLE DC AMPS SHOULD BE AT LEAST 1OR 2 AMPS HIGHER THEN SYSTEM LOAD.

Determining Amperage Usage

The object of this test is to determine the actual amperage usage of your motorcycles electrical system.

1. Disconnect your charging system. Put a battery charger on the battery to keep the system voltage up.

2. Use a clamp on inductive amp meter on battery negative cable.

3. Start the engine and turn on all normal electrical accessories. Read amp meter. Add 2 amps for battery draw. This is normal usage.

4. Turn on all accessories. Read meter. Add 2 amps for battery draw. This is maximum usage.

Note: Amperage usage should be at least 2 amps lower then charging system output.

 If the charging system is not capable of producing close to full rated out put it has a problem. I will cover diagnosing charging systems in the next couple of month. That subject needs to be broken into two articles. One for generators and another for alternators.

Until then I hope your motor starts crisp and your lights stay bright.

Let the road you choose to ride bring you happiness.

Karl S Fahringer.


Cycle Electric Inc