Today was a bad day to test a relay that opens based on solar power. Worse sun angle of the year, shortest daylight of the year, and it’s a cloudy day. But it’s all hooked up and ready to go.
Big Red
How to charge all lead acid batteries; how to charge SLA lead acid batteries,a tutorial for engineers about lead acid chargers and charging.
When you charging a starting battery or other battery that cycles a fair amount and has a limited amount of time to charge, you want to push the voltage to the point where it's close to gassing voltage, but not above it. Too much voltage and you'll start turning the water in the battery into hydrogen. Too little voltage and battery won't charge properly in the cold.
That's why a modern car alternator might be put out 15.2 volts are freezing, and 15.4 volts around 10 below -- but only 14.1 volts in hot weather. So if you see your alternator is putting out a higher voltage in cold weather, this is totally normal.
Voltage switch
It seems like that voltage switch will work well for my purposes…
While I’m using it to connect the solar and accessory batteries to the starting battery, I am using it in the voltage discharge monitoring (out) mode rather than the charge mode, because that mode makes the most sense for my purposes.
The low current internal relay in the device is switched on the positive side which means that I can easily excite the big relay that is between the starting and accessory batteries. I wasn’t sure until I tested it – a lot of transistors are NPN sink current so they operate on the negative side.
Settings I’m planning on using in Out Mode:
- Upper Voltage – 14.0 (maximum is 60 volts)
- Lower voltage – 13.2 (minimum is 6 volts)
- FON (First On, aka ignore initial voltage sag due to inductive start up load) – 10 seconds (maximum is 10 seconds)
- PON (Pause On, aka delay before re-closing relay to avoid burning contacts) – 300 seconds or 5 minutes (maximum is 999 seconds or 16.6 minutes)
How it works in the out mode I’m using …
The solar panel in the sun produces a rising voltages as the accessory batteries charge. The voltage switch monitors the voltage on the accessory system until the upper voltage is reached (which I set at 14.0 volts).
At the point the upper voltage is reached, internal relay closes, which sends a positive voltage to the positive output terminal. That via an exciter wire and a diode will active the main relay in the truck connecting the starting battery to the accessory system and the solar panel.
The FON (First ON) countdown begins, displayed on the screen and ongoing for the ten seconds I set it to. The voltage switch will temporarily ignore any temporarily voltage drop during the FON interval to ensure its not accidentally shut off.
Once FON countdown is done the relay will remain closed and solar power will flow from the panel to the starting battery until the input voltage drops to the set lower voltage, which I set to 13.2 volts. Once the lower voltage threshold is reached the relay will open. The exciter line will be off and the starting battery will be disconnected from the accessory batteries to avoid discharge from accessory loads when the panel doesn’t have excess current to spare.
Once the low voltage threshold is reached, the switch enters the PON (Pause ON) mode for the 300 seconds or five minutes I set it to – and is displayed as a countdown on the screen. During this period the voltage switch will not reclose even if the voltage does once again go above 14.0 volts to protect the relay from excessive switching on and off due to variable lighting conditions or other factors that could cause the voltage to fluctuate.
After the 300 second PON delay I set is over, the voltage switch will monitor the voltage and once it’s over 14.0 volts the process starts all over again.
Voltage low, disconnect relay
Dumping Excess Power Into Starting Battery
Often when I am camping or when I leave my truck parked for a long period of time at home, I think it would be it would be nice to have the solar panel dump excess, unused power into the starting battery.
This means I would always have a very happy starting battery, ready to give maximum spin to the engine even in cold winter weather. The way to do this would be to have a voltage monitoring switch, like this $7 dollar XY-CD60 voltage-monitoring switch I got from AliExpress. For about twice as much, you can get also get it on Amazon. It came in the mail, and it looks just like below — I haven’t had a chance to test it, but it looks like it will work perfectly for my situation.
My plan is to use this “voltage switch” to connect the relay between the starting battery and other batteries charged by the solar to dump the excess charge into the starting battery. I will probably set the unit into out mode (discharge protection mode), with an activation voltage at around 14.0 volts, with a delay of maybe 120 seconds, so that the relay isn’t activated until I know I have a sufficient charge to keep the voltage from dipping back below the threshold when the starting battery connects. It will take a while to figure out what settings works well for me — I don’t want to connect the starting battery too soon and have the voltage drop have it shut off the relay prematurely.
The product didn’t come with any documentation but the AliExpress website has some decent documentation in somewhat broken English, but I think I’ll be able to get it working fine. I’ll let you know how it works out.
Logic of an Andy
Last summer: “I have so much battery capacity, I’ll go ahead and remove the under voltage reset switch. On the rare chance the voltage dips and I need a reset I’ll just start the truck.”
This autumn: “Darn, in the cold, pulling heavy electrical loads on these long nights the voltage sometimes dips below 12.1 temporarily. Not because the charge is dangerously low but the load is high and the battery is cold. Sure would nice if I could just manually reset the voltage disconnect. So I reinstalled the switch.”
Holding off on a MPPT controller
I’ve decided to hold off on getting an MPPT solar controller for my truck for now – even when the benefits are obvious like on a partly cloudy day like this morning when I shut off the inverter, changing the resistance and actually boosting the output of the panel.
For the most part I have sufficient power with the two accessory batteries and don’t need the benefit compared to the PWM charger I currently have. The system preformed well enough on my October road trip even if I know on many days I’m not necessarily getting the best possible performance out of the solar panel. I say replace it when the PWM charger dies but the truth is without capacitors or non solid state components that’s unlikely to happen any time soon.
Plus if I were to get the larger Renology Rover MPPT charger I’d have to relocate the inverter and several other components which would require lengthening and cutting several wires to make things fit which is quite a bit of work. I do like all the diagnostic features of that charger though. One of the off gridders I follow online is a big fan of the Victron MPPT controllers which are pretty much the size of the PWM controller currently have.
Honestly the system works fairly good. Why put more money into something that works fine now?
