Flat battery

tworodrod · 24th March 2020, 13:06

Hi all can any one please tell me when trying to start my car with a flat battery my headlights come on but not when the battery fully charged many thanks Rod and I do hope you all keep safe and well

macafee2 · 24th March 2020, 13:10

A flat battery messes with the electrics and anything can happen, best is not to try and start the car with a flat battery. Your key and car can lose their ability to "speak" to each other.

macafee2

raykay · 24th March 2020, 14:21

The Light Switch Module has an emergency backup system, when the battery voltage is low, it switches on the dipped headlights and tail lights and inhibits the other lights.
When you try to start the engine with a low battery, the starter solenoid will engage and connect the starter motor, which may not operate, but pull the voltage low enough for the LSM to go into its backup system, and the solenoid to disconnect. The battery voltage then rises enough for the LSM to go back to normal, and the headlights and tail lights switch off.
If the key is kept in the 'start' position, the battery voltage will keep rising and falling as the solenoid engages and then disengages, causing the headlight to 'flash' as it does so.

24th March 2020, 13:06	#1
tworodrod Newbie Rover 75 saloon Join Date: May 2019 Location: tyldesley Posts: 23 Thanks: 17 Thanked 0 Times in 0 Posts	Flat battery Hi all can any one please tell me when trying to start my car with a flat battery my headlights come on but not when the battery fully charged many thanks Rod and I do hope you all keep safe and well

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24th March 2020, 13:10	#2
macafee2 This is my second home Rover 75 Saloon & Tourer Join Date: Sep 2012 Location: Lincoln Posts: 14,908 Thanks: 1,630 Thanked 3,032 Times in 2,181 Posts	A flat battery messes with the electrics and anything can happen, best is not to try and start the car with a flat battery. Your key and car can lose their ability to "speak" to each other. macafee2

24th March 2020, 14:21	#3
raykay I really should get out more....... Now Other Manufacturer Join Date: Nov 2006 Location: Co. Kerry Posts: 2,786 Thanks: 16 Thanked 310 Times in 283 Posts	The Light Switch Module has an emergency backup system, when the battery voltage is low, it switches on the dipped headlights and tail lights and inhibits the other lights. When you try to start the engine with a low battery, the starter solenoid will engage and connect the starter motor, which may not operate, but pull the voltage low enough for the LSM to go into its backup system, and the solenoid to disconnect. The battery voltage then rises enough for the LSM to go back to normal, and the headlights and tail lights switch off. If the key is kept in the 'start' position, the battery voltage will keep rising and falling as the solenoid engages and then disengages, causing the headlight to 'flash' as it does so.