2005 S211 - New auxiliary battery installed - won't start

[SLMILLEMIGLIA]

2005 S211 E350.

Has run perfectly for three years. Replaced main battery in boot, continued to run without missing a beat for a year. Car was parked up over winter, left with interior light on. Attached AGM battery charger to main battery in boot, clicked green in a couple of hours. Attached charger to auxiliary charger points in engine bay, correct settings 12v standard, wouldn't take a charge. Replaced the original battery as per MB Service Dept instructions removing negative lead from main battery in boot first, leave 5 mins etc. Reconnected negative lead to main battery, noticed no small spark on reconnection. Ignition on, dashboard usual lights on at stage 2, paused for a minute, twist to start ... a few clicks from the starter, wouldn't start. Red warning, 'reduced braking power'. The interior lights are now off, does this suggest the main battery fuse is blown?

 

[OneForTheRoad]

2005 S211 E350.

Has run perfectly for three years. Replaced main battery in boot, continued to run without missing a beat for a year. Car was parked up over winter, left with interior light on. Attached AGM battery charger to main battery in boot, clicked green in a couple of hours. Attached charger to auxiliary charger points in engine bay, correct settings 12v standard, wouldn't take a charge. Replaced the original battery as per MB Service Dept instructions removing negative lead from main battery in boot first, leave 5 mins etc. Reconnected negative lead to main battery, noticed no small spark on reconnection. Ignition on, dashboard usual lights on at stage 2, paused for a minute, twist to start ... a few clicks from the starter, wouldn't start. Red warning, 'reduced braking power'. The interior lights are now off, does this suggest the main battery fuse is blown?

Check battery connections in the boot , my w211 did this . I loosened them and turned them , then tightened again.

 

[SLMILLEMIGLIA]

Will try that on Friday when back at the storage, thank you very much for this, I knew it would be something simple but oh my that is a beginner level error by me! Fingers crossed. Any other suggestions welcomed in the meantime!

 

[OneForTheRoad]

Will try that on Friday when back at the storage, thank you very much for this, I knew it would be something simple but oh my that is a beginner level error by me! Fingers crossed. Any other suggestions welcomed in the meantime!

One other thing springs to mind , never charge the battery via the battery posts . The battery monitoring system won't recognize the charge, you can use the live but use an earth from the chassis or something before the earth on the battery. . Hook up the battery charger earth to the BMS before the battery earth .