I drive my car maybe once a week. it's notorious for "consumer battery offline" messages. I'd have to run a 100ft extension cable to keep it on a trickle charger. I try my best to start it up during the week to get the juices flowing

But recently as soon as I get in the car (after being off for a week etc) a RED error message displays on the dash telling me that I have low brake power or something like that and that I need to turn on the car.

Is this related to the "battery offline" message- I noticed a lot of my systems are dead when the battery in the trunk is "dead"/"offline"

 

Good possibility, these cars don’t like weak or dead batteries and jumpstarts are electrical component killers. I would consider replacing the batteries first, and keeping a quality trickle charger on the consumer battery, or finding snd repairing the amp draw.

 

It is related, but one step worse than 'consumers offline'. At moderately low voltage, the 'consumers' are turned off, and the starter battery is connected to the consumer battery so that critical systems are powered - ignition, brakes, ABC, air bags, etc. If the voltage is still below the threshold needed for the brakes, you get the red brake warning. If the alternator is working, this warning will usually be brief as the voltage will quickly come back up to an acceptable level.

 

Thanks. you guys are the best. I'll run an extension cord to the trickle charger and keep the battery charged when the car is not in use. The front and rear battery were both replaced 2 years ago- genuine mercedes

 

Constant charge discharge and heat are battery killers. I wouldn’t be surprised if the rear battery is compromised at this point.