Monitoring battery electrolyte levels

[chrisjx]

I am putting together a project to monitor the electrolyte in about 20 batteries at an off the grid site.  The owners of the house live most of the time in another part of the country but want to know when any of the electrolyte is low.  If they know, then they can send someone locally to check and fill.  I suppose a future project would be to also automate the fill up, but one step at a time...

I've been looking at the IOBridge setup for a while and I think this might be a good fit, especially if I can get the IO expander that it described here: http://www.iobridge.net/forum/index.php/topic,570.0.html

I am awaiting a device known as a Blinky (http://www.phlsci.com/support/html/blinky_electrolyte_monitor.html),
that is inserted into a cell of a battery and indicates the appropriate level by an LED indicator.  I plan to take the signal going to light up the LED and send it to the IOBridge, to the web service, etc.

The Blinky goes for about $20, the Smart Blinky goes for about $50 (kind of expensive for 20 batteries), so I am going to see what they're made of and see if it's reasonable to build the rest.

Any suggestions, better ideas for this project?

TIA,
Chris.

[nick]

The blinky is US Patent #5936382.  If you google it you can get a circuit diagram of how it works.  It is a very simple circuit.  A wire is attached to one terminal of the battery, a probe is put in the electrolyte next to the other, and an LED and a resistor go between them. The probe is made of lead so as not to corrode in the electrolyte.  It is made long enough so that it touches the electrolyte when the level is good and doesn't when it is low.  Making a probe like that should cost pennies.

I don't believe the probe is covered by the patent, just the LED circuit. 

If it were me, I'd use a Teensyduino as outlined in this post:
http://www.iobridge.net/forum/index.php/topic,567.msg2354.html#msg2354

Attach each battery to one digital input of the teensy and the teensy to a serial smart board.  Write a very simple sketch that sends status to your website every 10 minutes or so.  On your website have a page that sounds the alarm if the status changes.  Have a job that sounds the alarm if you don't get a status in a set period.

Some improvements: The blinky only monitors one cell of the battery. A more thorough approach would be to put one probe into each cell, although you may find that when one cell is low the rest tend to be also.  Use an AND gate --powered by the battery-- so there is one signal per battery that is high when all of the cells are good and low if one or more are low.

I would imaging the batteries are wired together in an array, so the probes will have to be  electrically isolated from each other.  Use optoisolators, powered by the battery.  Even a "dead" battery will have enough juice to power an IC like that.

The biggest challenge I would see is wiring the whole thing up robustly so that there are no false alarms, that's a lot of connections.

[nick]

The rock bottom cost way to do this would be use AND gates and AND all of the sensors together and attach to a digital in of the IOBridge.  You could be notified when a sensor changes state, but you wouldn't know which one.  Cost would be an optoislator (about 15 cents) for each battery and six quad AND gate for 20 batteries (about 50 cents).  The wire and connectors would be far more expensive than the logic chips.

However I think it would be worth your while to spend about $50 more and get the serial smart board and teensyduino and have some ability to track what's happening.

Nick

[nick]

Use an AND gate --powered by the battery-- so there is one signal per battery that is high when all of the cells are good and low if one or more are low.

After some thought I realize that won't work. Each battery has a different ground level, but the signals need to be tied together. Tying them together would short out the batteries. So you need to have the logic all powered by a separate power supply with a common ground.

After additional thought I think that if you are going to go to the trouble of running wires and connectors to all the batteries you should measure the voltage of each battery.  During both charging and discharging if there are significant differences in voltage in your batteries you run the risk of damaging the pack, monitoring the voltage of each individual battery would give you excellent protection capabilities.

You could still monitor the voltage using the same probe, which would give you a two in one, since voltage would drop to zero if the electrolyte dropped below the level of the probe. This would mean an all analog system instead of digital.  Substitute a linear optocoupler for optoisloator and an analog multiplexer for the AND gates in the discussion above.

Nick

[electroman]

Chris,

You might be able to use a mini float switch for this application.  Easy to interface and fairly cheap.  Seen them for US$6 on ebay.  Ok for acid too.   For example:

http://www.maplin.co.uk/module.aspx?TabID=1&criteria=usb&ModuleNo=221982&C=SO&U=Strat15