Battery keeps going flat .. lights dont go brighter when revd

the battery keeps going flat especially if the head lights are used
the engine becomes sluggish and starts to play up
hard to start ( slow crank speed)
a simple test is to ( at night) run the engine and pull the head lights on
if the alternator is not performing ,the lights will be dull and will not get brighter as you rev the engine up
If the lights do get brighter , then the problem may be in the battery
have it load tested
take the vehicle to an accredited auto electrician and have it tested properly.as it may be the regulator that is faulty.

Normal.

When the ignition is first turned on (but before the engine is started), the light is running just off the battery, which may not be fully charged.

When you engage the starter, this is the biggest power draw there is from the battery, causing the light to dim even more.

Once engine is running (if everything is working right), the alternator powers the light, as well as everything else.
In fact, if may/may not get brighter as you rev the engine.

What year and model is your BMW. How old is your battery? More than 3-4 years, may be past best by date. Once running does it keep running? Lights work? Does the battery go dead if the bike sits for a week or more? Was it kept charged when not in use? If yes to the above, battery is probably no good. If the alternator isn't working, your bike would run for a while after charging your battery, then die. The lights should become brighter as the rev's are increased from idle.

most newer acrs have a charging relay if this is goosed it will still show that there is a fault and will not charge the car easy way to check if its working wait untill its dark park up to a wall or garage door dont turn on engine but turn on lights start car see if the lights get brighter after a couple of seconds or see if the lights get brighter when you rev it if they done the relay has gone hope this helps ivan

hi , If the battery is flat please follow these instructions: 1 - borrow a charger , charge the battery and start the bike 2- while the bike is running , turn the light on and while the bike revs up notice if the light becomes brighter . 3- if it does , the recharging process is good but your battery is faulty 4- if it does not become brighter the charging magneto may be faulty or the regular may be faulty as well . Depending how old is the battery , the best thing to do is to change it every two years . Let me know how you go .

it is the alternator. 100% Once the alternator goes out it is not uncommon for the lights on the dash to act strange. Go to your local parts store and have them do an alternator output test. Don't go driving too far because when that battery goes dead the car will die also because the alternator is not keeping a steady charge.

could be alternator have it checked out at a parts store AKA napa ect..

If the light is flickering and not just becoming brighter or dimmer with engine rpm, I'd suspect an electrical short, possibly between the battery and alternator or inside the alternator itself. have the alternator output checked before going further with diagnosis and before it fails completely and becomes more difficult to diagnose.

Lights going brighter on reving generally means the battery is getting weak. Check the water in the battery. There should be enough to cover the top of the plates. When the engine revs the flywheel generates more electricity. The battery buffers the power. A weak battery does not buffer the extra power very well.

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After you start the bike put the headlight on high beam. Rev the engine and the light should get a bit brighter. If it stays the same or dims the battery is discharging.

If you have a meter, take off the seat and start the bike. Place the meter in DC Voltage mode and place the red probe on the + side of the battery and the - probe on the negative side. It should read 12v to 14v. Start the bike. The voltage should rise to over 13V or at least higher than it read when the bike is not running. If you rev the engine the voltage should also rise a 1/2 volt or so.

If the voltage remains the same or decreases you have a charging problem. Here are the two most likely fixes.

Tighten the battery connections.
The rectifier is not working. It is located on the right rear of the bike under the seat fairing.The rectifier converts the AC voltage coming out of the alternator to DC voltage which is what the battery needs to stay charged.