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On most 12v cars,the #1 cause is a light left on somewhere in the vehicle.#2 cause is a bad diode in the alternator and #3 is a bad battery which self discharges and is usually over 5 years old.
An easy first step is to disconnect the charged battery over night.If it still has a full charge the next morning,after you reconnect it and if the car starts,you can rule out the battery as the culprit.Now,if the battery passed the first test,now we go onto #2 test and disconnect the alternator output wire from the alternator overnight.If the battery still has a full charge the next day we will now know the alternator has a bad diode and will need to be repaired/replaced.If the battery was found to be low on charge the next morning with the alternator disconnected we will now have to go on to test #3 to find the drain in the vehicle. Disconnect the positive battery cable and place a 12v regular bulb type test light between the positive cable and the battery positive post.You should have a bright light on the test light if you have a drain in the vehicle from the battery.(don't worry about a dim light as electronics need a little memory voltage) We are looking only for a bright light.With the key off (as it would be overnight) you are now going to remove one fuse at a time while watching the brightness level of the test light.Take one fuse out, look at the light and if no change in brightness put the fuse back in and on to the next fuse,one by one until you see the light go out or dim.The fuse that dims or removes the bright test light will be the circuit that is draining your battery.Now you have your "battery drain" circuit identified and can inspect/repair the item/items on that circuit.
REMEMBER an open door,trunk or hood may cause a FALSE bright light on your test light because of courtesy light switches and alarm switches so manually close those switches if you need to keep one of the doors or hood/trunk open to gain access to the multiple fuse boxes in many vehicles.
DO NOT TURN ON THE KEY during this test, as all power to the vehicle is now going through the test light and things won't be happy if you do.
Yes this can be a normal condition caused by load being applied to the headlights and that lowers the voltage, the lites dim, then the load is removed and the lights brighten. Typical loads that cause this are AC climate control related. Have the Alternator load tested to confirm there is no problem in the charging system. Also have the battery tested, a battery that is starting to fail can cause this problem as well, a failing battery takes a huge amount of charging system power to keep it up, so that makes the lighting see higher spikes in voltage
Either the battery is not fully charged or it is time to replace it. Try charging the battery. If that doesn't work you will need to dissemble the light to obtain the replacement battery numbers. It should take (2) Model CA645 Batteries. They are 6 volt 4.5 amp hr.
ok , you have checked the brightness settings, check battery power settings as well. you may have to do a hard reset so back up your info. If doing a hard reset, pull the battery and charge it on its own if you have a cradle. Hope this works for you