Alternator or Rectifier/Regulator

I have had a battery failure, replaced the battery with a YUASA YTX20H LBS, now it really swings the motor over but....do I still have a problem, with the motor running the voltage goes to 13.85V never higher, the manual states at 2000rpm it should be 14.5V. If it’s less than there a series of test to do with the alternator wires to check if the cause is alternator or rectifier/regulator.

Who has had experience with this.

I decided to check the two plugs on the rectifier, they are all nice and tight. Maybe because the battery is fully charged that’s why I am getting the 13.9V readIng instead of the 14.5V.

Fully charged automotive batteries should measure at 12.6 volts or above. When the engine is running, this measurement should be 13.7 to 14.7 volts..

I would think that would be in range. You might try discharging the battery a bit with the headlights, then check it again.

Bediffetent, good suggestion, thanks.

You should get the highest voltage reading when the battery is fully charged and minimal accessories are on (engine running). Discharging the battery will result in a lower voltage reading until things charge up again.

Gregger said:

You should get the highest voltage reading when the battery is fully charged and minimal accessories are on (engine running). Discharging the battery will result in a lower voltage reading until things charge up again.

Click to expand...

I would say that is counter to how it should be. When you start a vehicle the highest charging rate is after you start the vehicle. As the regulator portion of the R/R senses the battery is at full charge the regulator drops the higher charging voltage back to maintain the charge and supply any accessories that are being use. You can see this type of activity by observing voltage gauge on your car instrument panel if you have more than idiot lights.

I think we are saying the same thing.... sort of.

Simply put, the alternator on a bike is a current generator which can push that current at very high voltages if not controlled. It pumps out amps to the battery/electrical system, especially when it senses a low resistance like the lights turned on and/or low battery (typically after starting the bike). When the battery voltage is low, the regulator senses this, allowing rectified current (amps) to flow into the battery which causes the voltage to go up as the battery charges, to a predefined level (Usually 14.5 volts). When this voltage is reached, the regulator causes the alternator to shut off the current flow preventing an overcharging condition. So the highest current flow (or charging rate) takes place when the battery voltage is low and the opposite happens when battery voltage is high. Talking current here.

Remember, current is flow and voltage is pressure.

Where are you testing?
If at a 12V plug somewhere, there's likely a voltage drop on the way.
Try testing at the battery with a meter.

@Gregger close but if I may clarify further; the stator is a current generator - however the regulator is a VOLTAGE controlled device - it attempts to 'regulate' the voltage to the system, based on the load demand and the condition of the battery; in order to achieve the desired voltage set-point, it does this by sending the excess current from the stator through the regulator's parallel shunts, rather than supplying it all to the load; this limits the system voltage; when it senses the voltage is rising too high, it sends more current through the shunt, in order that the voltage cannot rise beyond the setpoint. If you measured the voltage at the sense point inside the regulator it would indeed be at the pre-determined setpoint; it cannot 'push' current, the current demand is pulled - and based on the voltage applied to the load - the load of course varies, especially that of the battery itself, which will sink (rather than send) current if its voltage is lower than that of the regulation point; in essence the battery is always sinking current, rather than supplying it, when the engine is running - the amount of current depends on the voltage differential between battery and regulator sense point - if the battery is discharged then it will sink more current and also consequently cause the voltage of the system to be somewhat lower. As the battery reaches peak charge, the current into it will be minimal and therefor no voltage drop between the regulator and the battery terminals. The battery itself cannot sustain that voltage once the regulator is shut off. Once the battery reaches its peak charge, the generating system will still provide all the current consumed by the bike's other loads and you would expect none flowing OUT of the battery.

@motolink - a generating system producing 13.85V has nothing wrong with it.
A lot depends on what current load you have but I'd be surprised if a stock system was really at 14.5V regardless, especially on a system with twin headlights
And your battery size/condition will also play a significant part in that voltage - but be assured it cannot possibly produce 13.85V if either of the primary components (stator or regulator) is bad.

Are you using a good meter or a cheepie.
There are a lot of cheap metres for sale in car part shops there aren't very accurate.
Have you tried to measure the voltage on another bike or car with the same meter to see if that shows the same voltage as your bike.