R230 SL55 Auxiliary Battery Charging

[CheresX]

My sole ownership 1998 E300 has recently been accompanied by a 2002 SL55. Although maintaining the E300's battery charge is uncomplicated, I would appreciate guidance on how to keep the SL55's auxiliary battery charged. My current observation indicates that it seems to release in roughly three weeks.
Preferably, I wish to maintain the auxiliary battery connected and merely attach a charger to ensure it remains topped up or fully charged, thereby preventing error messages when I operate the vehicle.
Is this feasible? If that is the case, I would be grateful for information regarding chargers deemed appropriate.
Alternatively, should I contemplate detaching the auxiliary battery and subsequently linking it to my charger in AGM mode (I suspect the auxiliary battery is an AGM type) while supplying power to the vehicle through the OBD port?
I greatly appreciate it.

[CheresX]

Ultimately, I experienced hesitation about supplying power to the SL55 through the OBD port. I opted for a CTEK MXS 5.0 charger and utilised the provided connector to maintain the AGM auxiliary battery's charge on a 'as needed' basis, as the charger operates automatically. Detaching the charger prior to using the vehicle is uncomplicated, and thus far, I have encountered no warning indicators or error notifications related to a depleting auxiliary battery.
I trust this will assist those who sporadically utilise their vehicles.

[alpaco45]

I initially overlooked this discussion. You made the correct decision; supplying power to the vehicle through the OBDII port is feasible but perilous and may harm the car.

Did you utilise the eyelet connector for your CTEK? If not, they merit adjustment.

Moreover, utilising a Magcode Powerclip facilitates the most efficient and effortless battery charging process. Standard equipment on luxury vehicles.

[alpaco45]

https://www.mobilecentre.co.uk/magcode-powerclip-ctek-powerport-cover-pwr-lead-12v

[CheresX]

I greatly appreciate your reply. I utilised the eyelet connector provided with the CTEK, and with this configuration, I inferred that ideally, any power surges are mitigated by the battery's internal resistance; this may not hold true for power sourced through the OBD port.
I appreciate the information regarding the MagCode PowerClip.

[Blu_Ray_UK2007]

You ought to be capable of charging both batteries using a single charger if you understand how to connect a few wires in parallel. I intend to execute this on a Bentley I possess that contains two batteries in the trunk.

[dave783]

This is effective for you as you possess the requisite knowledge. I would not recommend this without additional direction and a comprehensive list of cautions.

What is the rationale for charging both batteries using a single charger? The starter battery (in an R230 or a vehicle with comparable batteries) experiences no external load (while stationary) and can endure an extended period without recharging. The consumer battery powers the operational components and may require upkeep if the vehicle is stationary for an extended period. Provided everything is satisfactory, a duration of three weeks should pose no issues, whereas an extended timeframe would.

[CheresX]

I appreciate your contribution. Although both the starter and auxiliary batteries in my vehicle are relatively new, as a retired mechanical engineer with ample time at my disposal, I would be grateful if you could share the specifics of your actions regarding your Bentley.
Although I value Diesel Benz's arguments, an idle battery ought to consistently receive the enhanced voltage 'stimulus' that contemporary intelligent chargers offer to maintain optimal condition.

[Blu_Ray_UK2007]

No issue, although I may need to withdraw that assertion. The Bentley is equipped with two batteries of identical dimensions, thus charging them simultaneously in parallel is acceptable. I am uncertain whether the same principle holds for batteries of varying sizes. I appreciate your revisiting the matter, as it allowed me to reflect further on it.