Sorry I've not written much in a while. Truth is I've had hardly any time to work on the battery monitor. A few months back I managed to damage a trace on the Logshield board with my clumsy soldering and broke the 5v supply to the 3.3V regulator. That meant that the INA219 board (the voltage and current sensor) and the SD Card both had no power.
|New magnifying glass and workbench|
Since then we've managed to buy the house that we have been renting for the past 2 years (Yay!). As part of this I've been sorting out our garage and building better storage and a proper workbench in there. This now means I have a permanent place to work on the Arduino. I can now do soldering without worrying about melting the kitchen tablecloth! I also don't have to set up and tear down before every meal… so I can do some more ad-hoc work.
I also bought a cheap little magnifying glass off eBay. It is a great little unit with a weighted base, two croc clip claws on arms, a magnifying glass with LED lights and a soldering iron stand. Using this I was able to spot where I'd damaged the trace on the Logshield board. I was able to scrape away a bit of the protective top insulating layer on the board to expose the trace and solder down a strand of copper wire to repair it. It is not the prettiest repair, but hopefully should hold.
Last night I started putting everything back together again and put the Arduino in the van and connected it up to the 12v supply for power and to the shunt to measure current. The battery monitor now continuously transmits voltage, current and temperature readings via a MiRF chip. I have a second Arduino with another MiRF chip plugged into a laptop to read the readings and display them.
I'm pretty impressed with the range of the MiRF chips, with only an onboard PCB trace antenna I was still able to read the voltage from within my house, with the van parked outside. So that is going through the metal body of the van, and the wall of the house. And about 20 meters of air. As I'm only really wanting the RF transmission so that I can transmit data from the battery monitor to other Arduino units in the van (e.g. one with a 3G card so I can get data remotely, or to one in the van cab to display the readings) it has more than enough range.
The battery monitor is logging the values to the SD card on it, so I will be able to retrieve historical data, plot it, and try and work out a State of Charge (SoC) algorithm. The battery was fully charged when I left it for the dat today. I left the compressor fridge running, so I should get a day's worth of data as to how the current, voltage and state of charge relate. The sun is starting to come out and as I left the solar panel was putting in about 0.2A to the battery.
At the moment the current readings I've been seeing seem to vary quite a bit from what my Stecca solar regulator is reporting. I'm not sure which is accurate though. I did attempt to put my multimeter in series with the battery last night to see what it was reading, but for some reason it caused the load to cut in and out. I fear I may have shorted out my multimeter a while back and its current shunt is a bit damaged. If the current readings are out then it could point to the shunt not being the exact 15 milliohms that it reports to be. I don't have a meter accurate enough to be able to test that. I might see if the local Bristol Hackspace has a more accurate meter I can use.
This weekend we are away camping for the bank holiday weekend. We will be arriving on site on Friday evening and leaving Monday morning, so will have two and a half days running off the batteries so will see what data we can collect in that time.