Monday, February 13, 2012

Flakey CellLog8s alarm

Today's episode:
Still charging up the cells one at a time...

When no.4 got to be full, I decided to play with the CellLog8s and its alarm output.  I'd noticed that when it gets to (and beyond) the set point of the over Voltage alarm, it would flash "over" on the display but not always beep.  It goes though random phases of beeping every 4 seconds (like it should) and then not beeping for a while.

So I set it up with the provided alarm output cable (with a tiny plug and tiny thin wires...) to a 12V LED.  This  has the current limiting built in for use as a panel lamp in cars. You can see the alarm port on the device and the external LED circled in green in the photo.
The plan is that the CellLog8s will be my low Voltage monitor, both for the pack as a whole and individual cells (as any cell that goes below 2.0V will be permanently damaged).

The 3kW inverter is my only load and has a LVD cut-off built in, but there are two problems with this.  The first is that it has a fixed cut-off Voltage of 21.0V, which is too low.  It's 2.62V per cell.  For high drain applications (0.5C / 200A discharge) 2.8V is the recommended cut-off.  For lower currents, the cut-off Voltage is actually higher.  The cell can be considered "empty" when it gets to 3.0V.  This means that for the many hours of a day where the inverter is drawing a mere 3-4A while doing not a lot, the worst case applies and I need to shut the thing down when it gets to 24.0V pack Voltage.

Then it's also possible for the pack to be out of balance and one cell get below 3.0V before the others as the pack nears "empty".  If I accurately bottom balance the cells, this shouldn't happen but I want to catch it if it does.

The CellLog8s does both of these things.  It monitors each cell Voltage, and it monitors the pack Voltage.  And the alarm output can be triggered at any programmable level.

Now, back to the test...  Because I'm charging the cells, I set the over Voltage alarm to 3.65V so that as the cell gets near to the end, it would alarm so that I could watch it finish and shut the charger down.  Just to see the alarm work.

Ideally, the alarm output would trigger and latch.  That way, if the threshold is crossed, the load will be disabled and then require manual reset before it could be enabled again.

Anyway, for the test, I had the alarm set to "Normally Closed" output.  This means that the LED comes ON when there is NO alarm condition.  This would mean the CellLog8 has power (to drive the output transistor) and the wiring is working.  This would provide a fail-safe "inhibit" signal to the inverter remote port.  When there is no alarm, it's safe for the load to run.

When the alarm is triggered (by low Voltage), the LED should turn OFF.  This would signal to the inverter that there was either a low Voltage alarm or that there was a fault in the CellLog8 (open circuit wiring or no power to the device).  In either event, the inverter should be disabled or shut down.

Well, as you can see in the video, it sort of worked.  The alarm was triggered at the appointed Voltage and the LCD display started flashing "Over" to tell me what kind of alarm it was.  The CellLog8 started beeping (as it should) and the LED turned off.  But... It then came back on and randomly turned on and off.

At first I thought it was a hysteresis problem (with the alarm threshold being crossed multiple times as the Voltage crept up) but with the cell well over the limit, the LED continued to randomly turn on and off.  No good.

Time to post a bug report on the RC Groups forum where the manufacturer hangs out.  They've been quite good at listening and producing bug fixes and new features for the device firmware but this is a pretty basic problem that should have been ironed out by now.

Failing that, I could work around the problem with an externally latching switch / relay, but if the software on the logger worked properly in the first place, it would reduce the interface complexity and so the number of points of failure.


  1. Hello!

    I seem to have the same problem as you have.

    In my case I am using four cellLog 8m to monitor the voltage of four 12 volt batteries in series. I have them hocked up with one relay each and they switch a lightbulb on if the voltage goes below 10,5V or over 14,9V.

    For the low voltage it just works like a indicator but for the higher voltage it also works like a balancer when charging. A small bulb at 5W seems to hold down the voltage enough to let all batteries hit fully charged when charging in series.

    However the alarm switch on and of even if the voltage is in the alarm range. Just like it does in your videos. I hope that this can confirm to you that the problem appears even if the device is not powered from usb.

    Unfortunately the 8m model doesn't have a usb-port, so firmware update is not possible. :(

    I hope that Junsi comes up with a solution for the 8s model. (I have some of those too)

    Best regards Mattias from Sweden

    1. Thanks for your feedback, Mattias.

      I'm glad in one way that it's not just me having the problem. But I'm also now worried that it's a real software problem with the CellLog, or worse, a fatal hardware design error and it will never work properly.

      If the CellLogs don't work out for you, you could always try out the 12V monobloc balancers that I'm using on my 24V bank. You can use two of them across the two pairs of your 48V bank and it will keep all of them in balance. Each one can shunt 1A, so a bit more powerful than your 5W lamps.

      See here:

  2. Thanks for the links you've posted. I have been looking on things like that before. However I do not want to spend more money on the lead acid bank. I'm going to build a very simple balancing circuit like this:
    I'm still going to use cell-log but their job will be to light up a big red lamp in my eyes when it's time to charge. The blinking would still tell me what it is about, even if the light should be constant.

    Maybe I should tell that my battery bank is powering an electric scooter with a peak draw at 4kW. or ≈80A
    It's built up with four 60Ah batteries (Greensaver) which have a low peukert constant (for being leads)

    At the 2H dischargerate they should deliver 43,2Ah.
    Under the winter I have measured them up and it's only one battery that performs like it should according to the spec. The rest gives from 25 to 30Ah.

    Lithium would be fantastic, but I can't really says that I must buy it now. The scooters range is still enough for me.

    I'm following your blog and the thread on RCG and it will be interesting to see how things works out.

  3. Sounds interesting about your scooter.

    I'm making some progress with the CellLog8s. Junsi has been a star and turned around a beta firmware for me in just one day!

    Testing it now and it seems to have partially fixed the problem. Stay tuned for another main blog post on the details...