In house DC bus

[DThames]

This will make someone's eyes roll.

I have some extra panels and have decided to make some standby emergency power. Because I will not be charging my batteries daily, I am looking for a way to use some of the available power. It was easy to bring in the DC wires to my hot water closet, then from there to the garage. When I am not charging batteries I can preheat water or add heat to the house. So I have been playing around with that.

I have my little heater running on medium setting and it is nice and warm, loading the 3 series panels to 108v right now at 11:30 AM. The MPPT charger did a little topping off of the battery early today. When we have full sun there is plenty of power that the 10A charger can get its power and the voltage on the heater sags only slightly. The PWM charger is not connected but could be in an emergency. Also, PV4 is not connected to PV3 either, yet.

I am brought six 10ga wires into the closet and then took four 10ga wires to the garage. I can rearrange the wiring at the terminal board to deliver several power options to the garage, if need be. I have 4 more panels in storage, so I might beef this up later.

 

[gnubie]

Nice, voltage to the heater is close to its normal operating voltage and current available from PV is near to what it will draw at that voltage so a good amount of heat can be produced (approx 900w). If you didn't know what you were doing and tried to run it at 60v you'd only get 480 odd watts out of it.

<insert usual warnings about switching DC here>

 

[DThames]

Nice, voltage to the heater is close to its normal operating voltage and current available from PV is near to what it will draw at that voltage so a good amount of heat can be produced (approx 900w). If you didn't know what you were doing and tried to run it at 60v you'd only get 480 odd watts out of it.

<insert usual warnings about switching DC here>

I was trying to teach my wife how to MPPT with the 3 power settings but yes when you step down on the power switch it fries a bit. Thermostat set to full so it will not switch off. I think I will just leave it set on medium setting to save the switch. I plan to connect to my preheat hot water tank when I get time. I will most likely get a temp sensor and a solidstate relay.

 

[tictag]

Never seen such a circuit design before but my 'spider sense' is tingling a bit because PWM controllers are just switches with a programmable mark/space ratio, meaning that on the 'marks' you'll be directly connecting your battery to a very low resistance heating element, which is probably going to blow something.

Not sure how an MPPT would respond because it would be connected to a potential divider between the panels and the heater.

 

[DThames]

Never seen such a circuit design before but my 'spider sense' is tingling a bit because PWM controllers are just switches with a programmable mark/space ratio, meaning that on the 'marks' you'll be directly connecting your battery to a very low resistance heating element, which is probably going to blow something.

Not sure how an MPPT would respond because it would be connected to a potential divider between the panels and the heater.

The heater is to use the power from the panels when there is no need to charge the batteries. If the solar charger pulls the voltage down and the heater is not heating, that is not a problem. If the heater pulls the voltage down (it when the sun power is not high) I can turn the heater off if I want the battery to take what solar power there is. The MPPT charger works fine while the heater is on. I have not tested the PWM. It is there for some additional options if needed....charging a second battery or charging with more power.

 

[gnubie]

Never seen such a circuit design before but my 'spider sense' is tingling a bit because PWM controllers are just switches with a programmable mark/space ratio, meaning that on the 'marks' you'll be directly connecting your battery to a very low resistance heating element, which is probably going to blow something.

Not sure how an MPPT would respond because it would be connected to a potential divider between the panels and the heater.

Using Ohm's law we can calculate the oil heater's element resistance to be approx 9.6 ohms (V x V / W, 120x120/1500). PWM battery, I'm guessing, is 24v based on panel voltage. 24v into 9.6 ohms = 2.5 amps ( I = V/R ), 60 watts ( VxV/R), so not a lot of current, not a lot of power, nothing should blow up or catch fire if that element was connected across a 24v battery, even if the PWM controller permitted current to flow in that manner.

 

[tictag]

Using Ohm's law we can calculate the oil heater's element resistance...

A very fair comment.

 

[DThames]

Never seen such a circuit design before but my 'spider sense' is tingling a bit because PWM controllers are just switches with a programmable mark/space ratio, meaning that on the 'marks' you'll be directly connecting your battery to a very low resistance heating element, which is probably going to blow something.

Not sure how an MPPT would respond because it would be connected to a potential divider between the panels and the heater.

Actually if the PWM charger could do that (no direction flow control), the battery would be in parallel with PV3, PV4, and apply 24v to the solar panels, which normally provide 36ish volts to the heater. So the affect of switching the 24v battery in place of the 36v solar panel for 1/3 of the heater series circuit would be pretty small.