Tweet
Hi A couple of years ago I installed 2 solar panels – UniSolar PV 68 Watt on the roof of my camper. My battery bank was a pair of 12 volts – Interstate Deep Cycle and Costco Marine Deep Cycle. Controller was a SunForce PWM 30 AMP . Overall, I was reasonably happy with the system. Now my batteries are 5 years old and a couple of times I ran the voltage down to 10 or so volts, so I thought I'd replace them.
That started what now appears to be a major upgrade. I now have a pair of Interstate 6 volts golf cart batteries from Costco. Costco is light on the specs but said they were 208 AH - similar to GC2-RD-UT directly from Interstate. But the Costco price was about 1/2. My inverted is a PowerBite 1100 W that I rarely use and when I do, it's less than 200 watts of load. The inverter is fused with an 80 Amp fuse, so I'll never use the full1100 watts. The inverter/battery connection loop, including positive and negative connections is 5 feet of #6 gauge, so I figure for voltage drop I've got 2.5 feet of one way cable. The positive cable is about 2 feet, the negative cable 3 feet to get to the other side of the batteries as well as hook into the ammeter shunt. That gives me a voltage drop of 1.36% at the full 80 amps. But at a more typical 20 amps usage, the voltage drop is .34%. So I think I'm okay for the battery/inverter wiring.
I decided to add some more panels, so now my system will have 4 UniSolar PVL 68 watt panels. My camper is a popup with a thin skin aluminum roof over a foam layer. Light and easy to raise but not much to attach solar panels to. That's why I like the Unisloar. They are roll on self stick and they really stick. I'm sure other panels could give me better wattage at a better price but the stick on is key. These are the panel specs. For wire sizes I used STC (Standard Test Conditions) as this would be the worse cause scenario.
STC
(Standard Test Conditions)
( 1000 W/m2, AM 1.5, 25 °C Cell Temperature)
Maximum Power (Pmax): 68 W
Voltage at Pmax (Vmpp): 16.5 V
Current at Pmax (lmpp): 4.13 A
Short-circuit Current (Isc): 5.1 A
Open-circuit Voltage (Voc): 23.1 V
Maximum Series Fuse Rating: 10 A (UL), 8 A (IEC)
NOCT
(Nominal Operating Cell Temperature)
(800 W/m2, AM 1.5, 1 m/sec. wind)
Maximum Power (Pmax): 53 W
Voltage at Pmax (Vmpp): 15.4 V
Current at Pmax (lmpp): 3.42 A
Short-circuit Current (Isc): 4.1 A
Open-circuit Voltage (Voc): 21.1 V
NOCT: 46 °C
There are 11 solar cells per panel with bypass diodes across every solar cell. From the panels to the controller is approximately 7 feet. With the initial 2 panels, I ran 2 #10 gauge wires to the controller. Because the roof goes up and down, I needed wire that was flexible. I thought 2 #10's would be more flexible than a single #8. That was for a 2 panel parallel setup If the 4 panels are now in series, a single #10 is way oversized. The wire size calculator at http://www.freesunpower.com/wire_calc.php says I could get by with #24 gauge. Seems too small, but another calculator at http://www.powerstream.com/Wire_Size.htm confirms this. Am I doing something wrong?
From the controller to the batteries, there is also a 7 foot run. Here I've got #8 gauge. The voltage drop is 1.24%, well below the target of 3%.
So now, finally the questions.
That started what now appears to be a major upgrade. I now have a pair of Interstate 6 volts golf cart batteries from Costco. Costco is light on the specs but said they were 208 AH - similar to GC2-RD-UT directly from Interstate. But the Costco price was about 1/2. My inverted is a PowerBite 1100 W that I rarely use and when I do, it's less than 200 watts of load. The inverter is fused with an 80 Amp fuse, so I'll never use the full1100 watts. The inverter/battery connection loop, including positive and negative connections is 5 feet of #6 gauge, so I figure for voltage drop I've got 2.5 feet of one way cable. The positive cable is about 2 feet, the negative cable 3 feet to get to the other side of the batteries as well as hook into the ammeter shunt. That gives me a voltage drop of 1.36% at the full 80 amps. But at a more typical 20 amps usage, the voltage drop is .34%. So I think I'm okay for the battery/inverter wiring.
I decided to add some more panels, so now my system will have 4 UniSolar PVL 68 watt panels. My camper is a popup with a thin skin aluminum roof over a foam layer. Light and easy to raise but not much to attach solar panels to. That's why I like the Unisloar. They are roll on self stick and they really stick. I'm sure other panels could give me better wattage at a better price but the stick on is key. These are the panel specs. For wire sizes I used STC (Standard Test Conditions) as this would be the worse cause scenario.
STC
(Standard Test Conditions)
( 1000 W/m2, AM 1.5, 25 °C Cell Temperature)
Maximum Power (Pmax): 68 W
Voltage at Pmax (Vmpp): 16.5 V
Current at Pmax (lmpp): 4.13 A
Short-circuit Current (Isc): 5.1 A
Open-circuit Voltage (Voc): 23.1 V
Maximum Series Fuse Rating: 10 A (UL), 8 A (IEC)
NOCT
(Nominal Operating Cell Temperature)
(800 W/m2, AM 1.5, 1 m/sec. wind)
Maximum Power (Pmax): 53 W
Voltage at Pmax (Vmpp): 15.4 V
Current at Pmax (lmpp): 3.42 A
Short-circuit Current (Isc): 4.1 A
Open-circuit Voltage (Voc): 21.1 V
NOCT: 46 °C
There are 11 solar cells per panel with bypass diodes across every solar cell. From the panels to the controller is approximately 7 feet. With the initial 2 panels, I ran 2 #10 gauge wires to the controller. Because the roof goes up and down, I needed wire that was flexible. I thought 2 #10's would be more flexible than a single #8. That was for a 2 panel parallel setup If the 4 panels are now in series, a single #10 is way oversized. The wire size calculator at http://www.freesunpower.com/wire_calc.php says I could get by with #24 gauge. Seems too small, but another calculator at http://www.powerstream.com/Wire_Size.htm confirms this. Am I doing something wrong?
From the controller to the batteries, there is also a 7 foot run. Here I've got #8 gauge. The voltage drop is 1.24%, well below the target of 3%.
So now, finally the questions.
- Does what I've written above make sense?
- How will shading impact the performance with multiple panels. I'm thinking that panels in series with bypass diodes are the best configuration. I'm most concerned with shading by trees that will not be uniform over the camper roof rather than cloud shading which should be more uniform.
- I'll need a new MPPT controller. Something from MidnightSun is a bit too much for me. I'd liked get something in the $250 range or below as long as I'm getting reasonable quality. I'm assuming that I will need something that will handle the 12 volt output amps, for me 16.52.
- I think I could have a single fuse on the roof for the input into the controller. The specs for the panel list a maximum series fuse rating of 10 Amps. Is this what I use? For the output side, where does the fuse go, near the battery or near the controller. Or do I need 2 fuses since the are 2 power sources – the controller and the battery? If the wire were to short, there could be voltage at either end. At the controller end, there would be maximum amperage of 20 amps because that's all the panels would supply.
Thanks for any help, comments.
Comment