Well, unless you have recently installed a new battery, the drop you are seeing is due to the internal resistance of the battery, which increases as the battery gets older or is not fully charged.
Charging the battery from the alternator does not necessarily mean that the battery gets a full charge. The battery can only absorb so much, the rest of the current just heats up the battery.
Suggest that you use a trickle charger (no more than 2 amps) overnight. If the trickle charger is not regulated, the you will have to keep an eye on the battery voltage to ensure that it doesn't go above 15 Volts. Fill the cells with distilled water before charging.
You didn't mention if the headlight was on when you tested the batery with the key on. If the headlight was on, then the voltage may drop as low as 10 volts with anything other than a new, fully charged battery.
A new, fully charged battery should read 12.6 volts at room temperature, even after sitting overnight.
Last Edit: Feb 14, 2009 16:42:22 GMT -5 by pamcopete
hi Pete cheers for the reply the headlight was not on during any of these tests. i have a trickle charger & will charge up the battery overnight. having read several posts about the voltage drop at the coils & how this can cause problems, i have a fouling right plug, should the voltage at the coil be tested with the engine running? And what sort of voltage should I expect with a fully charged battery and the ignition on? I was sure Id found a problem with such a low voltage at the coils, Ive checked the resistance of the plug caps and they are both fairly new and give the right readings. once again cheers for your expertise on this Pete, you are the electrical guru
Well, if you were to measure the voltage at the coil, with a fully charged battery, you could expect about 11.5 volts. Even a fully charged battery has internal resistance, so allow about .5 volt drop on the battery itself and another .5 volts dropped across the fuse, ignition switch and kill switch and the various connections.
Last Edit: Feb 14, 2009 20:10:46 GMT -5 by pamcopete
Cheers Pete you were spot on, I charged the battery up overnight voltage across battery with ignition off = 12.9v with ignition on (lights off) = 12.3v voltage at coil= 12.1v checked valve clearances exhaust .007" inlet .003" checked float heights, tuned & balanced carbs took bike for 7 mile test & so far all is ok Ill keep an eye on the battery to make sure its not on its way out but charging seems ok with 14.5v with engine running ;D
Those are very good numbers, especially the 12.1 at the coil, which means that you are only dropping .2 across the key switch and kill switch and .6 volts in the battery internal resistance. If the battery still reads 12.6 in the morning, then it is probably ok as well. 14.5 with the engine running is right on and would inducate that your charging system is OK as well.
It also means that you probably do not need the relay mod.
Last Edit: Feb 15, 2009 20:17:10 GMT -5 by pamcopete
r80rt: :)This is a splendid write up, THANK YOU!!!!
Dec 26, 2015 16:54:05 GMT -5
mashermoto: What the what is this shoutbox for? Or should I shout, WHAT THE WHAT IS THIS SHOUTBOX FOR!
May 12, 2016 22:22:21 GMT -5
motormike: Good Day ...just a FYI.. Wild Cat is this Labor Day weekend. Still go'n on. A few 650's still make the scene. Various rides at different skill levels each day.. awards dinner sunday eve. BS in the park'n lot after dinner.
Aug 23, 2016 7:57:01 GMT -5
lsettle: First post in over 6 years!
Aug 28, 2016 19:45:17 GMT -5
kardon: Wow quiet here, where did they all go.
Oct 7, 2016 2:18:13 GMT -5
gggGary: XS650.com LOL
Feb 16, 2017 20:49:02 GMT -5
buell88: Anybody have any idea who built the xs650 tracker used in the Draggin Jeans Ad on Pipeburn?
May 16, 2017 16:24:09 GMT -5
joshua: The regulator/rectifier unit I got from Mikes PMA has closer hole spacing than original. Are we supposed to mill the holes to fit?
Jun 7, 2017 19:25:58 GMT -5
joshua: My bad. I found the bracket they supplied to mount the rectifier/regulator... Uh oh. How is this meant to work? Which way is it meant to mount? Looks like I have to use some spacers or something. Good instructions would be nice.
Jun 7, 2017 22:09:52 GMT -5
craig8johnson: Not sure if this is the right spot or not but oh well...
Jul 11, 2017 18:04:15 GMT -5
craig8johnson: I'm checking the charging system. I'm afraid I might have fried the last battery. With the solid state regulator (after market) what should it be charging at. The book says, I think, 15 volts. I'm hitting 17 when revved. Is that too high?
Jul 11, 2017 18:06:19 GMT -5
gggGary: Yes 17 volts is too high, about 14 volts is max, sounds like you don't have the right regulator or it's wired wrong.
Jul 13, 2017 6:54:33 GMT -5
dean: Just saying Hello, new member who is going to fix up a 79 XS650 that has seen its better days. I will definitely have lots of questions coming up.
Jun 23, 2018 16:45:32 GMT -5
tt650: Trying to breathe life into a 1980 that's been stored (indoors) for 25 yrs. Air filters are dry rotted. Have cleaned carbs but she only runs briefly on choke then stops. Need jet setting info and other ideas that might help her idle and rev.
Jul 3, 2018 19:15:19 GMT -5
trix13: Hey everyone! New to the forum and am fighting those gremlins!! New charging system, ignition switch, coil and fuse box. Resistance is higher than normal on the ignition pick up so I'm trying to find one. No such luck yet.
Aug 16, 2018 8:50:48 GMT -5
trix13: Also, even though all of my connections are clean, I cannot seem to find out why , when at higher rpms, I hit the signals and the rpms pulse with the signals...hahah. same with the brake light. I appreciate all input!!
Aug 16, 2018 8:55:08 GMT -5
bob99: how do I post a question. I am new to the forum
Jun 6, 2019 10:52:10 GMT -5
adlpc: Hello all! Been enjoying your edifying posts and discussions - and as a fan of the XS650 since my early teenage years happy that I now own a 1980 Special which I plan to turn into a vintage half faired racer. Thank you all for being here!
Oct 1, 2019 0:58:56 GMT -5