Im posting here cos u guys are sure smarter than me- I have a yamaha vmax i recently acquired and its running good but im diagnosing the chargin system as the original piec eo junk reg was putting out .5 DC. So i put in an oem used SH775 mosfet reg.rect (popular upgrade) - Battery is 4 months old AGM, load tested good and around 12.5 now. Bike starts at the touch of starter. At idle im getting low DC charge.. 11.5 - when I rev is past 1600 its over 13v. With the used Reg/Rect connected I get 11.5v AC on all stator wires. Disconnect reg/rect and i get 23v AC at idle on all 3 pairs. Could a bad battery cell cause this? Or could a stator put out good AC volts but still be "weak"? Or is the reg/rect likely at fault?
First step is to look up that exact model online and search for charging issues. There may or may not be and issue as it might be acting like a one wire alternator on a street rod in that the engine has to be revved up before it kicks in and starts charging and if the rig start at idle and sits there and idles it never gets excited and starts charging.
Check your grounding out of the 'regulator'; make absolutely sure ALL connections are clean, and you have zero resistance between the regulator ground wire or base to the battery negative terminal. If there's only four wires connected to the regulator (three AC in, one DC out), I've ran a wire from the regulator base/mount to the negative battery cable attachment point on the frame to clear the issue. Most motorcycle alternators aren't 'regulated' in the same sense as an automotive unit. Where car units use a rotor with varying voltage/current applied to regulate output, most motorcycles use permanent magnets instead. In effect, they put out 100% output all of the time (with some variation due to RPM). The 'regulator' simply shunts excess voltage/current to ground to control output to the battery. A poor regulator ground will reduce output, but ANY poor connection will limit output. Here's a good step-by-step troubleshooting guide... https://www.vmaxforum.net/threads/how-to-fix-your-vmaxs-low-voltage-or-charging-problems.19108/
The Jalopy Journal
