BMW Airhead Charging System & AGM batteries

The stock Airhead charging system was designed for flooded cell lead acid batteries but doesn’t cut the mustard for many Absorbed Glass Matt (AGM) lead acid batteries like Deka and Odyssey that require higher maximum charge voltages.

The oft used Deka ETX20L and Odyssey PC680 AGM batteries will charge most efficiently at a maximum regulator setpoint of 14.6-14.8V (always measured at the battery and around 20C ambient temp if possible). However, with the stock loom and charging system in good nick, you might see a maximum charge voltage of around 14V at best but usually lower  (even with the 14.2V setpoint of both the Bosch RE57 and RE55 regulators). This is less than optimal for AGM’s and becomes hopeless with the headlight switched on and/or in stop-start city traffic.

The issue is this: The last 10% of the charging process is the most challenging. In an ideal situation, it is achieved during a relatively long ‘constant voltage absorption phase’ after the charge voltage has reached the setpoint of the regulator. The lower the setpoint voltage compared to that recommended for your particular battery, the longer this critical phase takes. Moreover, unless you are zipping along at highway speeds for an extended period, the time spent at the setpoint voltage is anything but constant.

At trip’s end, many Airheads will arrive in the garage with a less than fully charged battery. This is when the damage is done: Any lead acid battery left standing in a partially charged state will begin to sulphate resulting in accelerated loss of capacity and reduced battery life. A daft situation given the cost of these batteries and their otherwise excellent life if treated well.

This highlights the importance of hooking up a good battery tender after each trip to ensure the battery has a chance to return to 100% charge.

The Diode Hack

I first tried the oft cited ‘diode hack’: Placing a diode in series with the D+ line to a Bosch RE55 regulator. The diode’s forward voltage drop (~0.6V)  tricks the regulator into thinking the battery is flatter than it is and pumps up the charge voltage (theoretically 14.2 + 0.6V = 14.8V max charge voltage).

Charge regulator diode hack

With headlight on, I started to see healthier charge voltages though not sure I’d recommend this hack. I did it on two bikes, both fitted with Deka’s. On the bike with a considerably older Deka, I saw voltages creep beyond 15V and believe this is due to the fact that a diode’s forward voltage drop is not necessarily a constant as often assumed, but increases with increased current which is directly related to the state of the battery.

A diode with higher voltage/current rating or a Schottky diode with its lower voltage drop might help but I decided to sack this idea and try something else.

Adjustable Voltage Regulator

These regulators are available through several suppliers. They have a trimpot that allows adjustment of the maximum voltage setpoint and come pre-adjusted to a setpoint of 14.3V. Not quite enough for my precious Dekas.

You can increase the setpoint on the bike if you are sure the battery is nearing full charge but it’s a nuisance to continually remove the tank to gain access to the trimpot between tests. I adjusted the setpoint to 14.8V on the bench with a variable power supply and a couple DMM’s which removes some guesswork when adjusting on the bike.

Testing in the bike with a well charged battery, the charging voltaged hovered around 14.5-14.6V with the head light on and nudged 14.8V with it off. That’ll do.

Adjusting setpoint on BMW Airhead charge regulator

BMW Airhead with adjustable voltage regulator

‘Smart’ Battery Chargers?

Here’s another opportunity to mistreat your AGM battery. There is nothing smart and everything suspicious about a lack of specification. In chosing a battery charger/tender, I avoided ones that have an AGM setting but don’t tell you the corresponding setpoint. Different AGM battery manufactureres recommend different maximum charging voltages ( I also don’t buy batteries that lack this sort of specification) so if you really want to optimise your battery’s charging regime – and life – then the charger should ideally match.

Chargers that lump ‘Pb’ (flooded cell lead acid) and AGM in the same setting are clearly not adequately catering for AGM! There are quite a few of them.

The ‘Smart’ in Smart Charger refers to a charger’s ability to optimise the charging cycle by automatically switching between certain charging modes that best suit the stage of the charging cycle.

Minimum ‘Smarts’ is a three step charging regime:

1) Bulk Charge at constant current: Charge is accepted at an elevated rate and peters off as the voltage steadily rises until the regulator set point is reached.
2) Absorption phase at constant voltage:  Voltage is held around the regulator set point voltage while the critical last 10% of charge is more slowly accepted. This is the phase that is not adequately addressed by the stock regulator.
3) Float phase: A lower voltage (~13.6V) maintenance phase after battery is fully charged.

Some smart chargers have several more steps such as desulphation and further maintenance modes.

For Odyssey owners, buying a Odyssey charger specifically designed for your battery is clearly a smart move! And will be good for the Deka’s too.

Alternatively, I use, and can recommend the CTEK MXS 5.0. Ideal for both Deka’s and Odysseys, it’s a multistage charger that has both Motorcycle and Car settings depending on the size of the battery. It has a stage beyond the Float step Called ‘Pulse’ that monitors battery voltage and starts up the charge cycle when required. Making the charger a truely set and forget proposition that can remain connected to the battery indefinitely.  Importantly, it has separate Pb and AGM settings with an AGM setpoint of 14.7V. Perfect!

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