Diagnostic voltage vs direct battery voltage

[dannymax]

Firs, I apologize if this has already been covered, If it has I can't find it.

My '05 was turning over slow and a couple times needed to be charged before it would start, bad battery symptoms it seemed. So I replaced the battery, charged it overnight and everything appears to be ok, motor whips over quickly and starts right up.

Today I checked my diagnostic voltage reading, 11.8V....this was the same as the old battery I just removed.

Then I checked voltage at the battery through a fused accessory link connected directly to the pos/neg terminals, 12.72V.

I started it and checked again with the bike running, 14.41V.

Why is the Diag. voltage reading different from the meter reading at the battery and does the charging voltage sound normal?

Thanks.

 

[Welldunn]

I don't know anything about the diag. voltage. Your real battery voltage and your running voltage are in the good/normal range.

 

[yamafitter]

Why is the Diag. voltage reading different from the meter reading at the battery and does the charging voltage sound normal?

Thanks.

It could be as simple as a calibration error between the diagnostic voltage reading and your meter or the diagnostic reading does not read true battery voltage. I'm not sure.

As far as the old battery and new battery reading the same voltage at rest this is not uncommon. The only true way to test battery condition is through a proper load test to determine if the battery is capable of delivering the required current (cranking amps) to spin the starter while maintaining an acceptable voltage.

When the bike is running the charging system will maintain a slighty higher voltage to allow for charging of the battery that was discharged during starting. Your reading of 14.4V is OK.

If you would like to know more here is a link to an article from the Yuasa Battery Website:

Understanding Batteries

 

[ionbeam]

When you use the diAG to measure voltage here is what is happening:

• The voltage being read and displayed on the diAG screen is being measured by the ECU from the main harness that powers the fuel injectors.
• For diAG to work the key must be on but the engine isn't running so battery voltage is all that is available.
• There is a voltage drop between the battery and the ignition switch, and a voltage drop between the ignition switch and the injectors.
• With the key ON, the battery is supplying power to the meter assembly, nine bulbs in the Meter Assembly, the running lights and ECU.
• This causes battery voltage to drop to ~12.2 volts, then drop to ~11.8 volts after the ignition switch (key).
• The ECU monitors voltage at the injectors and factors this into the Fuel Injection volume calculation.

As the others have said, the voltages you measured are correct. The FJR has some small quiescent current draw from the electronics that will very slowly drain the battery. Add in some cold weather, let the bike sit for a week or more and you will get slow cranking at first start-up, after that it will be fine until let sit again.

If you have a suspected weak battery, start the engine then quickly use the red RUN/STOP button on the right handlebar to kill the engine but leave the key ON. This keeps the headlights on creating a 100W load on the battery. In roughly 1 minute, turn the key off. Measure the battery voltage, it should be ~11.5 volts or higher at the battery terminals. Record the reading. After doing this, let the bike sit for 1 hour with the key off. Now measure the battery voltage again with the key off, DO NOT turn the key on at any time. A good battery should have recovered at least 0.2 to 0.5 volts. This is a quick field test, don't sweat it if your readings are off my 0.1 - 0.3 volts; if you have a problem your voltages will be significantly different.

 

[dannymax]

I wondered if the diAG mode placed a load on the battery and, in effect, it does. Also the fuel pump runs adding further load.

Thanks for the information, it clears this up very well....also lets me know I need to get a decent automatic battery charger like the Yuasa Smart Shot to maintain my bike batterys properly.

Thanks again, you guys are great! :yahoo: