Pre-Trip Solar Testing

Up until a week ago I had been working on a smart relay controller to manage the charging and power sourcing from two batteries. Well, that ended in smoke a week ago - literally.

I had completed the prototype including all electronics, wiring and housing. Unfortunately, there were some design limitations (such as MUST have two batteries connected) and exclusions (such as over voltage and digital input protection) that I was hoping would not get in the way of a great blog, but as destiny would have it I saw smoke after a few hours into the programming.

 

I won't go into the details in this blog but I think the long leads to the car batteries were a contributing factor due to increased inductance (and the fact that I only had one battery connected to the second battery connection) which caused the circuit to toggle on/off at a rapid rate - a known limitation of my circuit that I failed to think about before I connected the battery). I have some new ideas in mind that will greatly simplify the design and prevent the same disaster in version 2. Stay tuned for more on that.

With a pending camping trip due to commence in a few days I decided to spend this weekend testing my panels and batteries just to make sure everything was in good condition. This blog is to report on the test setup, conditions and final results.

Our setup is made up of the following:

• Two 100AH deep cycle batteries wired in parallel (positive to positive and negative to negative)
• A Ctek 250S Dual DC-DC converter / charger capable of giving out 20A.
• 3 solar panels providing up to 240W. These panels are documented to output a maximum of 18V and 13.33A (it was originally supplied with a 15A regulator). I've removed the supplied regulator and feed the panels directly into the Ctek charger. As you'll see in my notes below I have recorded close to 22V and 16A peaks (not at the same time). I've even noted a peak power (voltage x current) of 266.7W !!!
• All cables to the charger and battery are 4mm rated to approx. 30A. These are all soldered onto either Anderson plugs or terminals that are screwed onto the battery. No cables are crimped.
• The output of the charger is connected to the battery such that the positive lead connects to battery 1 and the negative lead connects to battery 2.
• The output cable to be fed to devices (such as our Engel fridge/freezer) is connected in the opposite fashion (positive lead to battery 2 and negative lead to battery 1).

All my reading on how to connect batteries indicates that when a battery is being drained by a source the electrons flow from negative to positive and when a battery is being charged the electrons flow in the opposite direction. The wiring configuration indicated above helps to balance the electron flow. With my limited knowledge this sounds like it would equate to more even charging / discharging and therefore helps to extend the battery life.

The Experiment

The aim of this testing was to start with a battery at around 50% discharge and then connect everything for approximately 6 hours and see just how well the battery would charge over this time period.

Preparation began yesterday by defrosting the Engel refrigerator and disconnecting the AC based charger I use to maintain the batteries. The refrigerator was then connected to the batteries with its thermostat set to its maximum setting (5 out of 5) and left running overnight (opening the door from time to time to help kick the compressor into life). My aim was to get the battery as close to 50% as I could (12.3v).

The testing began at 08:30am this morning. Local conditions are quite cool (approx. 20C degrees), there's a lot of cloud cover and random periods of light rain. The battery is sitting nicely at 12.3v (open circuit) - perfect. I've reset the thermostat to 3 (typically keeps the freezer at around -10C when conditions are ideal).

There's a few points to note that will influence the interpretation of results (your conditions will more than likely be less than ideal as was the case in this test scenario).

• The temperature is nice and low (helps reduce the number of times the refrigerator's compressor needs to kick in)
• The lower temperature also helps improve the solar panel efficiency
• The cloud cover reduces the ability to capture sun (which is great for this test)
• The refrigerator door was not opened during testing (I forgot to do this)
• The open circuit battery readings taken during the course of the day may be artificially elevated due to being connected to the charger. A true gauge of open circuit battery status can only really be gained after 8 hours of rest (I've read 2 hours is the absolute minimum)
• The Engel refrigerator draws around 2.5A / hr - when it kicks in it draws around 45-50W

And here's the data:

08:30am

• Ctek input: averaging around 1.05A, 19.2V and 25W
• Battery: open-circuit at 12.3V, closed circuit (under load) at 12.17V

08:40am

• A small break in the clouds providing some sun
• Ctek input: averaging around 4.00A, 19.76V and 79W

09:30am

• Overcast conditions with light rain
• Ctek input: averaging around 2.22A, 19.48V and 43W

10:30am

• Ctek input: hovering around 3-5A, 17-19V and 50-60W
• Battery: open-circuit at 12.95V, closed circuit (under load) at 12.88V

11:30am

• Filtered sunlight
• Ctek input: hovering around 3-4A, 19V and 85W
• Battery: open-circuit at 13.10V, closed circuit (under load) at 12.95V

12:20pm

• I moved the panels 90 degrees (the sun had moved substantially by this time)
• Ctek input: averaging around 4.7A, 19.2V and 91W
• Battery: open-circuit at 13.20V

1:30pm

• Ctek input: averaging around 4.5A, 20.5V and 90W
• Battery: open-circuit at 13.29V

2:30pm

• Sun has broken through the clouds
• Ctek input: moving around 5-10A, 19V and 100-150W
• Battery: open-circuit at 13.4 - 13.6V

2:35pm

• More sun (5 mins since previous reading)
• Ctek input: averaging around 15.7A, 21.6V and approx. 228W
• Battery: open-circuit at 13.4 - 13.6V

At this point I noticed the current being drawn from the panels (by the Ctek) was cycling from 0A up to approx. 15A. At this point I suspect it has changed it's method of delivering power (it's a 7 stage charger)...at this point the closed circuit battery reading is 13.05V.

Conclusion

While I can confidently state the battery was around 50% capacity when I started (because it wasn't connected to the charger during the evening) I cannot (with certainty) claim to what level the battery has been restored after 6 hours of charging.

The capacity of a car battery is meant to be read open-circuit without any charging for at least 2 hours; 8 hours seemingly the preferred period. Depending on the reference material you read, a battery at 100% is typically sitting around 12.7V. I think it would be safe to assume that my final reading of 13.6V wouldn't drop much lower than this after 8 hours of rest.

I'd like to think this has been a success even though the testing has been biased by the fact camping conditions have not been accurately reproduced. I was only after a rough guideline for performance.