BM2 Battery Monitor

A couple of months ago a friend showed me his BM2 Battery Monitor installed on his 420SEL.    It is a bluetooth enabled battery monitor that connects to a phone app.   The app lets you monitor the battery in real time, run some tests and download history from the battery monitor device.

I already have various battery testers which are useful for standalone battery testing.    However, I didn’t have a good way of checking the overall charging system of the cars.    In particular, I have seen some strange behavior with the radios in my 250SE and Citroen DS that may be related to over or under charging.

I ordered the BM2 Battery Monitor devices a while back, and they finally arrived yesterday.   I had the E-Type at my house this week, so thought I would try out one of the devices.

There are two apps you can choose from.   The free app, that lets you connect to one Battery Monitor at a time, and a paid one that lets you connect to four.   I started with the free app.     You don’t connect your phone directly to the BM2 battery monitor bluetooth, you start the app and give it permission to use the bluetooth device.

When I first connected the phone, it showed the current status of the battery in the car.   As expected the battery was fairly low.   The last drive was quite short, in the rain and dark.

When I started the car, I was able to use the cranking test which passed (as expected).   Next, I tried the charging test.   This is where the unit somewhat fails on older cars.   According to the BM2 Battery monitor, the car wasn’t running, so I couldn’t start the test at idle.   Its not really surprising as modern cars have very large alternators that provide plenty of amps even at idle.    The E-Type has a 43 amp alternator.   It doesn’t produce a whole lot at idle.    I was only able to start the test holding the revs above 2000. I imagine it would have even less luck on an older model equipped with a generator.

In any case it showed that once the revs were up, the alternator created plenty of current.   This was even more apparent during normal driving where the Alternator was putting out plenty of power.  I was seeing a regular 14.5-14.6 volts while driving.

I did get an alarm at idle when the cooling fan was running and I switched on the headlights.   Once I took off again there was plenty of power.   The E-Type has pretty low power needs.   During normal driving only the fuel pump, the ignition system and sometimes the cooling fan are drawing power.

I think this will work well to test the charging system of the 250SE and Citroen DS.   I may even get a couple more to keep them on the older cars.     It would be good if the app could be put in a mode where the screen does not turn off, as it could function as a temporary voltage gauge.    The range is quite good as I was able to connect to the device from outside the garage.