A major component of the van are the battery packs. Lithium technology brought the weight down considerably and increased cycle times and higher discharge levels make the technology quite attractive. There are a few downsides though. One has to be careful with the charging regime not to overcharge and also make sure that each cell (4 in a 12V battery) has the same voltage, meaning the cells are balanced. The battery needs a BMS (Battery Management System) which monitors the cells and will disconnect a charge line or a load line in case of over charge or over discharge.

It is also an interesting study subject to learn a bit more about the behaviour of the batteries as well as getting a good feel for the efficiency and capabilities of the whole electrical system. The batteries deliver power for all 12V devices as well as power for the inverters to create 240V. Each of the two banks has a healthy discharge current of 350Amps and can deliver more over a shorter period of time if required without being stressed too much. However there are possible situations which might diminish the battery capacity over time if certain conditions are not observed. I do not want to explain Lithium technology here, I leave that to others on the web.

The only aspect for me is to get good statistical information to observe my system and to optimise my charge and usage regime. This is what the battery monitor is for. One can buy pretty nifty monitors on the market which do most of the things am talking about here, but first of all they are not cheap and secondly where is the fun in that. All my electronics in the van will cost less than one battery monitor.

battery_bank1

Once the battery page is selected it will display an overview of information for battery bank 1. The same page exists for battery bank 2. It shows voltage and temperature for each cell of the bank and also has two amp meters. One shows charge current coming in and the other show load current going out. I allow up to 500 Amp discharge per bank, above that value the bank would be isolated from the inverter, which is the one drawing most of the power. The 12V supply connected to the battery will remain connected. The display is pretty straight forward and self explanatory.

battery_statistic

The next page is the more informative page. Also available for both battery banks, it shows a live chart (updated in real time) with 3 graphs. Charge in, battery temperature and load out in one diagram plotted over time. Each vertical line in the grid correlates to one hour elapsed. The chart will show a whole charge cycle if we assume a duration of 7 hours per charge cycle from solar energy. I wanted a quick overview of our daily input and output. All data is stored on a SSD dive and is available to analyse further. I also record all the other values for each bank with the same timestamps as the entries in this chart. This way I can correlate all info later for analysis. The column with 7-segment numbers gives a summary of values since the last reset (when fully charged). It shows, state of charge, KW in from solar, KW in from AC charger, KW in from DC charger (vehicle) and also the KW used by 12V only and by 240V inverter. These are accumulated figures of the individual sample values, which are used to plot the chart. At this stage I record a set of values for charge and load every 200 Milli seconds (5 per second) to get good precision for the summary values. Temperature and cell voltage are not recorded at that rate, because it does not change that rapidly anyway. I can increase the data rate if required and the precision for state of charge calculation might not be sufficient. I will determine this in a practical test.

At the bottom of the chart one can see summary values for the last solar charge cycle. The system will determine when charge current comes through the solar charger and will record the beginning of a cycle after a longer period of sleep (no sun) and will start accumulating the energy coming in. At the same time accumulation for the load current will begin, which delivers a total amount of energy taken out. This way I get a quick overview of my energy balance during the charging period (daylight).

I will also record the SOC of the bank at the beginning of the cycle and at the end of the charge cycle. This tells me if my energy balance is positive and if I have sufficient input to compensate for drawn energy over night and during charge.

With todays technology this is rather trivial it only requires a few shunts in the 12V supply lines and a few analogue inputs on a microprocessor. I use two independent data collectors, one for each battery. These are little Arduino boards with a CAN bus interface communicating with a data hub (also an Arduino), which relays the information to the central system via Ethernet.

battery_statistic_final

Above picture shows a snapshot of the moving chart. The values do not resemble real life. They are just ransom numbers in a testbed to verify the functionality of the software. Once the signals are connected it will look differently.