Two important features which we feel necessary to include in the node are both an ammeter for the solar panel output and state-of-charge indicator for the battery. The ammeter has been going well and at the moment we only have some small calibration issues. Measuring the state-of-charge however proves to be much more difficult.
The state-of-charge (SOC) of a battery is referring to the amount of potential energy it currently contains which is also the same as saying the number of amp-hours left. Chemically, this refers to how much of the discharging chemical reaction is still able to take place. There are a number of things which one can measure to estimate the state of charge but many of them were inappropriate for our situation. (such as using a hydrometer to analyse the electrolyte) The website here was pretty good at explaining all the different ways to measure SOC.
The easiest method for us will be measuring the voltage across the terminals of the battery. The problem with this is that if the battery has been charging or discharging for a while there is a significant variation in the terminal voltage and this variation will not be removed for a solid 2 hours. This was confirmed when measuring the voltage of the battery after a long discharge time in the lab. It is quite likely that the battery is frequently being charged and discharged and so getting an immediate and accurate SOC reading would be impossible.
We also looked into the idea of using a GMR (giant magneto resistance) which, through the phenomenon of quantum tunneling, varies its resistance due to the magnetic field it experiences. The way to use such a device would be to place it on one side of the battery while a bar magnet sits on the other side. The permeability of the magnetic field is different for plates of lead and lead-oxide (the product in a discharged battery) and so this change could be detected. There are a lot of questions to be answered with this method such as how sensitive does the sensor need to be, in what range of operation and how how strong does the bar magnet need to be. It appears that no one has really used this method before except in very recent prototypes. It was deemed that this method was too new to try and implement in this project.
So the solution that Lydia came up with reduced flexibility but really gives the same impact. We wanted the user to have the ability to know the SOC of the battery so that they could better judge the prioritization of charging the battery or powering a load. What we will implement is a SOC meter which measures the open circuit voltage of the battery terminals but the condition of using it accurately will entail only being able to use this device after a long period of rest. This period will be night time and so immediately before the day's use of the node, the user will record the SOC of the battery and at that moment make a judgement about whether or not to prioritize charging or loads for that day.
What we are desperately looking for now are the voltage values which correspond to the state of charge for our particular battery. We will call Interstate Batteries and hope that they have this information.
The state-of-charge (SOC) of a battery is referring to the amount of potential energy it currently contains which is also the same as saying the number of amp-hours left. Chemically, this refers to how much of the discharging chemical reaction is still able to take place. There are a number of things which one can measure to estimate the state of charge but many of them were inappropriate for our situation. (such as using a hydrometer to analyse the electrolyte) The website here was pretty good at explaining all the different ways to measure SOC.
The easiest method for us will be measuring the voltage across the terminals of the battery. The problem with this is that if the battery has been charging or discharging for a while there is a significant variation in the terminal voltage and this variation will not be removed for a solid 2 hours. This was confirmed when measuring the voltage of the battery after a long discharge time in the lab. It is quite likely that the battery is frequently being charged and discharged and so getting an immediate and accurate SOC reading would be impossible.
We also looked into the idea of using a GMR (giant magneto resistance) which, through the phenomenon of quantum tunneling, varies its resistance due to the magnetic field it experiences. The way to use such a device would be to place it on one side of the battery while a bar magnet sits on the other side. The permeability of the magnetic field is different for plates of lead and lead-oxide (the product in a discharged battery) and so this change could be detected. There are a lot of questions to be answered with this method such as how sensitive does the sensor need to be, in what range of operation and how how strong does the bar magnet need to be. It appears that no one has really used this method before except in very recent prototypes. It was deemed that this method was too new to try and implement in this project.
So the solution that Lydia came up with reduced flexibility but really gives the same impact. We wanted the user to have the ability to know the SOC of the battery so that they could better judge the prioritization of charging the battery or powering a load. What we will implement is a SOC meter which measures the open circuit voltage of the battery terminals but the condition of using it accurately will entail only being able to use this device after a long period of rest. This period will be night time and so immediately before the day's use of the node, the user will record the SOC of the battery and at that moment make a judgement about whether or not to prioritize charging or loads for that day.
What we are desperately looking for now are the voltage values which correspond to the state of charge for our particular battery. We will call Interstate Batteries and hope that they have this information.
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