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yamaha xj600 mysterious battery draining

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Member - Posts: 1
Member spacespace
Joined: January 20, 2015
Location: Germany
Posted: January 20, 2015 at 6:31 AM / IP Logged  
Excuse my ignorance, but i have a problem with my Yamaha xj600 1986. Battery is new and holding correct voltage at 12.5 and maintaining while running. When running, the voltage meter shows an increase to about 14.5 or so while revving . Bike stays running without the headlights, but on full charge the bike will run with headlights for about 30 minutes until the charge is drained to the point where the battery can no longer supply a spark to the plugs and therefore the bike goes out. I am confused to know why it shows that the charging unit works, but independently the battery is drained due to the headlights. (Tailights are also independently draining the battery, just at a slower of course, due to its power consumption). I have also cleaned and steel wool brushed the stator. Also, before this problem occured, i had one of the prongs connecting to the regulator/rectefier which appeared burnt out (which originally didnt allow the battery to charge). I cut and rewired the burnt connection and made a direct connection, after this the battery began to show charge (as previosly explained. Thoughts/Comments ? Thanks for your reading and consideration!
Greetings, Ian Stephan
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 20, 2015 at 9:47 AM / IP Logged  
The charging unit might work but IMO it appears to have insufficient output. Maybe its brushes (if required) are worn, or there is some other fault.
One of the faults can be a faulty battery which is absorbing all available alternator capacity (until it explodes or is removed), but a fully charged or even 12.5V battery suggests it's more likely an alternator problem.
When charging, the battery voltage should generally be 14.2 to 14.4V.
Whether an alternator supplies enough power at (a low) RPM depends...
My 1980-1990 vintage alternators of ~80A rating in a 1965 ute held their rated 14.2 or 14.4V at idle after cranking with ~500W of lighting.
My GPz900s or older Yammy RDs only flattened after leaving parkers on for too long. Even my 1972 RD 350 handled a 100W spotlight without problem.
My 1972 Ducati GT750 and its whopping yamaha xj600 mysterious battery draining -- posted image. 120W permanent magnet 2-phase alternator only handled headlights when above 3kRPM which meant a normal roadspeed above our maximum speed limit. The engine had to run at 2kRPM to exceed battery voltage so any idling at lights with brakes and indicators drained the battery much quicker than ignition alone.
[ All above had electronically regulated voltage systems. I have ignored older electro-mag voltage regulators - as probably should everyone else - not to mention externally regulated (or fanned!) alternators. ]
But post-1980 Jap roadbikes not having decent charging systems - I guess there are some...
Simple checks:
Headlights or lights brightening with increased engine RPM indicates the alternator is charging. (Modern lighting may differ - their brightness can be independent of its varying +12V supply voltage.)
... But whether that "brightening" means sufficient charge...
Modernish alternators should be capable of supplying typical high current loads (wipers; lights) even at idle speed.
A fully charged battery should be ~12.67V @25C. That's after its surface charge has dissipated - allow up to 24 hours, or use headlights for whatever time is required (eg 1-15 minutes).
Decreasing alternator output due to worn brushes hastens the death of the battery.
By the time the battery is replaced, the alternator's output will have dropped further. The new battery may last a few months, or a few weeks.
Tho battery installers should check battery & alternator voltages first, voltage checking after installation is mandatory (... IMO; and even if the alternator was deemed ok with the old battery).
Never use anyone that replaces or installs your battery (or alternator) without checking voltages - that can cost you inconvenience even if they do subsequent repairs & replacements free of charge (no pun).
In automotive and similar charging systems, the battery and alternator are interconnected and hence have the same voltage except for fuse and other voltage drops.
If the voltage is above (say) 12.8V, then the alternator is supplying the load {ok, as is the surface charge). Below ~12.5V means some combination of alternator & battery supplied power.
After starting, the alternator recharges the battery to recoup cranking & last night's parking lights, and the overnight 0.1% or 1% self-discharge.
It's the alternator that then supplies all vehicle power until it's disabled, and during times of excess loading.
Paraphrased - it's a fixed voltage system whose voltage is matched to its battery's requirements - eg maintenance, charging & float voltages.
The fixed voltage charges the battery and it separately supplies current to other electrical loads.
The desired fixed or target voltage is maintained by the alternator whose regulator increases the rotor current - and hence the alternator's output current or voltage - to match an increase in loading or a drop in RPM, and vice versa.
Check your alternator's brushes - ie, they have good spring tension; are not near their wear limit, and have good clean slip rings.
It could also be a power diode failure (IMO brushes are more likely) but that needs specific testing. However with test method and tools that might be a valid first check if brush access is difficult.   (It's merely rearranging test order to take advantage of prevailing conditions.)

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