Battery Voltage vs Percent Charged

The charge state of a lead acid battery (% charged) can be estimated from the voltage measured at the terminals of the battery.  The chart below shows the relationship.

First, it is important to understand the terminology.  “C” is the capacity of the battery in amp-hours.  For example, this battery has a rating of 215AH when discharged over 20 hours.  Note:  The Duracell GC2 battery is a 6V golf cart battery.  So, the chart is applicable to two of these batteries connected in series.

For easy reference, here are calculations of the currents shown on the chart:

Curve Current Draw/Charge Approximate Wattage Draw/Charge
C/3
71.7A
860W
C/5
43A
516W
C/10
21.5A
258W
C/20
10.75A
129W
C/40
5.4A
65W
C/100
2.15A
26W

Here are several examples of interpreting voltage:

Scenario 1

It is night and no solar energy is available to charge the battery.  Several small pieces of electronic equipment are connected to the battery.  They consume about 25W or so.  The battery voltage is 12.5V.  This means that the battery is about 60% charged.

If the battery voltage were 12.0V, then the battery would only be 10% charged.  Likewise, if the voltage were about 12.7V, the battery would be pretty much fully charged.

Scenario 2

It is night and no solar energy is available to charge the battery.  An inverter, which draws about 2A (~25W) of current is powering a small air conditioner and 6 high intensity recessed LED lights.  The air conditioner consumes 500W of power and the lights consume 75W.  So, the total power drawn from the battery is about 600W, which is between the C/3 and the C/5 curve – but closer to the C5 curve.

If the battery voltage measures 12.0V, then the battery is almost certainly fully charged.  If it measures 11.5V, the battery is about 60% charged.  And if it measures 10.5V, it is somewhat less than 10% charged.

Note that discharging a battery at the C/3 or even the C/5 rate decreases the life of the battery.  C/10 or C/20 is much safer.  In addition, battery life is shorted by discharging a battery below 50% of its capacity.  600W represents 50A of current.  So, using a Duracell G2 battery pair like this for more than two hours decreases life because of the discharge rate as well as discharging too deeply.  For usage such as this, it is advisable to use at least 4, if not 6 of these batteries, in series/parallel combination.  6 batteries would reduce the discharge rate to C/16.7 and would increase the safe usage time to about 6 hours.

In the chart below, the area in green represents the safest discharge rates and amounts for best battery life.  Note that battery terminal voltages under 12.0V are always suboptimal.

Scenario 3

It is a sunny day, nothing is using power from the battery, and a solar panel system is producing about 10A of current.  The voltage at the battery terminals measures 14.0V.  This means that the battery is approximately 90% charged.  On the other hand, if the voltage at the battery terminals measures 13.0V, the battery is approximately 50% charged.

Vertamax 3000W Inverter Review

I have used the Vertamax 3000W inverter on and off over the past two months with various types and configurations of solar panels and batteries. It performs well under a load of up to 1200W and probably performs well beyond that, although I have not tried. It is well built.

One thing that I have noticed is that, as stated in the description, it does shut off under low voltage. However, the shutoff voltage is around 11.8V or 11.9V – not the 10.5V (+/- 3V) shown in the instruction manual. Of course, this is good for a lead acid battery under load because the inverter will shut down before the battery completely runs down. Running the battery completely down will dramatically shorten its life.

The description/manual also states that “When the input voltage rises to approximately 11.4 – 11.9V DC, the inverter restores to normal operation and the red FAULT indicator will turn off.” This is not correct. Even after a battery is fully charged to 12.9V, the fault like still blinks and the inverter does not come back on. I called WindyNation and spoke with a knowledgeable gentleman who verified that the only way to get the inverter to resume normal operation is to manually toggle the ON/OFF switch. This means that, at a remote unmanned site, a low voltage condition will cause the power to be off until someone can visit and reset it.

WindyNation told me a good rule of thumb to minimize the likelihood of batteries being run down below 50% – which is about as low as it is safe to go without damaging lead acid batteries. The rule of thumb is that choose a bank of deep cycle batteries that have as many amp hours as your solar panels have wattage. For example, six 100 watt solar panels should have a battery system rated at 600AH. This is general guidance and the exact ratio depends on the amount of sunshine in your location and the season. But, my experience over the past few months in North Texas is pretty much in line with this recommendation.

Renogy Rover Monitoring with the Raspberry Pi

The information, below, was posted on the Renogy Forum by a user with the screenname lindsey.  The forum recently moved and the documentation was temporarily lost.  The information was reposted; but I wanted to put it here for easy reference in case it gets lost again.

This information is about connecting the Renogy Rover to the Raspberry Pi for monitoring.

First, here is a general link discussing connection of the Raspberry Pi to a solar battery charger.

https://www.rototron.info/raspberry-pi-solar-serial-rest-api-tutorial/

Here is the wiring diagram that the Renogy Forum post provided:

Here is a sample output on an Android from the Python scripts.

Here is a sample database query.

DatabaseQuery

The diagram, below, is a diagram of how the Rover’s RJ-12 port splits out into RS-232 signals.  Note that only TX, RX, and ground are used.

The link below was put together by lindsey.  It describes the needed hardware (in addition to the Pi) as well as the general functionality of the Python code.

Raspberry PI Writeup

Finally, here is a zip file with the Python code.  Unfortunately, I do not have a way to contact lindsey.  The code comments say that her name is Lindsey Crawford.  If anyone knows how to contact her, please let me know.

SolarMonitor

Solar Panel Performance

Here is a link to a website put together by the California Energy Commission to provide evaluations of solar panel performance:

http://www.gosolarcalifornia.ca.gov/equipment/pv_modules.php

I find it interesting that there does not seem to be much of a difference between monocrystalline and polycrystalline panels.   Both seem to produce about 90% of their rate values under standard test conditions of:

  • 20C air temperature
  • 1 meter/sec wind speed (2.2mph)
  • 10 meters (33 ft) above the ground
  • Air mass of 1.5
  • ASTM G173-03 standard spectrum
  • 1000 watts/sq meter solar irradiance

Ok.  So, how is that practical?  Well, here is a calculator, based on historical data, of how much irradiance is expected in a given location.

http://solarelectricityhandbook.com/solar-irradiance.html

For Dallas in September, expected solar irradiance of a South facing panel is 4.96kWh/sq meter/day.  So, it appears that a real world 100W panel should be expected to produce 9% of this amount – or 446Wh.

To put this in perspective… 446Wh would allow a person to use 18.6 watts of electricity constantly over a 24 hour period, assuming that the system has a battery to store energy for use at night and cloudy days.

Note that solar irradiance is about half that amount in the winter.

Bottom line:  An ideal 100 watt solar electric system in Dallas allows a person to constantly use about 18 watts of power during the month of September; about 9 watts in the winter.  Real world results are almost certainly less.