BMW Airhead Charging System & AGM batteries

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

I like the Deka ETX20L battery, but the stock system does not fully charge them resulting in an accelerated loss of capacity and reduced battery life – A daft situation given the cost of these batteries and their otherwise excellent life if looked after. So I spent some time trying to improve things.

The Deka’s will fully charge at a maximum regulator setpoint of 14.6-14.8V (always measured at the battery). 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) which is less than optimal for AGM’s and becomes hopeless with the headlight switched on, which is all the time right?

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 adjust 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. 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 14.5V 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’ and AGM in the same setting are clearly not adequately catering for AGM! There are quite a few of them.

For a more generic charger with an  AGM setting, my research suggests 14.6V as a good maximum charge voltage.

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