BMW R1200GS Battery Charger


Last updated: 4-May-2005

The R1200GS is the first BMW motorcycle to use an AGM (absorbed gas mat) battery. Older BMW models all used flooded (wet), or more recently, gel batteries. AGM betteries have all the benefits of gel batteries but can take more abuse. They also have several advantages over both gel and flooded batteries, at about the same cost as a gel battery (2-3x the cost of a flooded battery). Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float. Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods.

The R1200GS has a "feature" where the on-board computer shuts off the power to the aux power outlet unless the battery needs charging. Once the battery is charged, it disconnects the aux power outlet and won't reactivate it unless the battery charger "signals" the computer to turn the outlet back on. Apparently, the BMW charger will do this, and other brands won't.


I'm not an expert in this area, so I took the easy way out and bought the BWM charger. The photo above shows what it looks like. It is a 4-stage charger about which BMW says: "in Stage 1 the battery is charged with maximum charging current until the charge end voltage is reached. In Stage 2 the voltage is preserved with falling current to bring the battery to maximum as quickly as possible. In Stage 3 the voltage is reduced to the maintenance charge. Reduction of the battery voltage in Stage 4 causes a reset -- and the cycle starts again from the very beginning."

The BMW charger has a series of 6 LEDs (4 along the top edge) to show you which stage it is currently in. It works just as advertised, and on my bike which needed a boost, it took several hours to run though all 4 stages.

The LEDs indicate the following:

Device Status (LEDs at bottom edge of charger)
LED Power (on) The device is connected to power
LED Error (on) Short circuit at the charging terminals, battery connected to the wrong poles.
LED Error (flashing) Battery error, charge interrupted. Wrong battery (e.g. 6V or 24V), battery is sulphated and revival failed, or a cell is short-circuited.
Charger/Battery Status (LEDs at top edge of charger)
LED 0 (on) The battery is connected, and the charger is in the main charging mode.
LED 0 (flashing) The charger is in de-sulphating mode with de-sulphating loops. If this step succeeds, the main charge mode continues. If it fails, the charge is interrupted and "ERROR" appears.
LED 1/3 (on) Main charging mode -- charging current has fallen to 1/3 of the initial value.
LED 2/3 (on) Main charging mode -- charging current has fallen to 2/3 of the initial value.
LED 3/3 (on) Battery fully charged -- charger is now in "maintenance" mode and can remain connected indefinitely without being overcharged.

My friend Steve says that technically, BMW's description is probably somewhat inaccurate. His view is that:

Which means that for some intial value of charge, current will flow to charge the battery until it is completely charged - at which time the current stops flowing. This is consistent with theory: a discharged battery has a large current and an almost charged battery has a small current. [for charging]

So in our case which of 1/3 or 2/3 equals a better battery condition? I'd bet it's 2/3 which then implies a lower charging current.

Battery Tender Plus (the competition)

In March of 2005, Scot Marburger did some investigating and wrote:

"Deltran makes the BMW charger for the Gel replacement batteries which BMW sells for the R11xxGS series. It's a relabeled 021-0156. Deltran does *not* make the $120 charger for the R1200GS. For the Yuasa AGM battery in the R1200GS, you need the BatteryTenderPlus model 021-0128. The difference between the two chargers is that the gel charger uses .2 less voltage during the absorbtion stage of the charging cycle to avoid cracking the gel matrix in the battery. It would probably be OK to use the gel charger on an AGM battery, but not the other way around. The correct charger for the 12GS is the model 021-0128.

The Deltran charger can't be used through the auxiliary power outlet like the BMW charger can. If you get the polarity wrong when you hook up the Deltran pigtail to your battery, you could take out one of the bike's computers that controls the electrical system. If that happens, it's likely your warranty won't cover the damage. The bottom line here is to be careful with what you connect to what if you go with the after market charger.

To wrap this up, use the BatteryTender Plus model 021-0128 to charge your 12GS battery. Or save yourself a big hassle and just spend twice as much for the BMW charger.

I've also noted that none of the four BatteryTender Plus chargers I have here now have any sort of part or model number on them. Only the packaging is marked."

Copyright © 2004-2005, by H. Marc Lewis & Scot Marburger. All rights reserved.