How To Match Battery Bank to Charging Panels

[DixieMtCR]

I have 6 100w 12v MONO panels wired 3s2p. Panels are tilt-able and can also easily pan to track the sun during the day (manually).

I have 4 new 100Ah Flooded Deep Cycle RV batteries for storage, all wired in parallel for 12v.

I have a 12/24v 40A MPPT Charge Controller to charge the battery bank.

I have a 3000w Pure Sine Wave Inverter connected to the battery bank to supply 115v AC. Using 4/0 AWG 36" cabling for the connection to the battery bank. Seriously considering switching to 24v to cut down on the current. But, would require a new inverter.

Currently, nothing is connected to the LOAD terminals of the MPPT CC.

For testing, I have been running my 14A skill saw, table saw, chopsaw, bandsaw, sawsall, and other tools intermittently during the day. Also, I have run a big freezer and a big refrigerator 24/7.

On a sunny day here in Oregon, the battery bank tops off in 2-4 hours. Then the system stops producing since the batteries are full. Wasted potential storage time.

The rest of the potential charging time for the system is wasted each day because the battery bank is filled early in the day. The system just trickle charges the rest of the day

Would it be safe to say that I could double the battery bank to make more storage? I would not be using more power, but there would be more stored for those days when there is less sun, and therefore, less potential for charging.

Purpose for the system is to provide emergency house lighting, medical equipment power, Microwave, and power refrigerators and freezers when power is out, which happens frequently where we live. We have a generator but there have been been several times where the outage has been long enough that we run out of stored gasoline, and the snow and ice is so bad that we cannot get out to the nearest town. We also do not run the generator at night while sleeping, so lites in the house would be needed.

Where is the balance or how would one calculate that? Any thoughts?

 

[grizzzman]

More battery bank will give you more time with low output from the solar. lower DOD the longer the battery life will be.

 

[time2roll]

Since you have the equipment.... best test would be to switch the main breaker off 48 to 72 hours and see how it really goes. Seems light on both solar and battery to me but with a generator you can always top up the battery into the evening if needed.

 

[Bud Martin]

So you have 600W of solar panels, the batteries are charged to full just about all the time. So do you run anything off the inverter so you will not waste the power produced by the solar when the batteries are full?
Do you have Kill-A-Watt to measure your power usage of each electrical device? You can measure KWh for a couple of days to get average KWh.

 

[DixieMtCR]

Since you have the equipment.... best test would be to switch the main breaker off 48 to 72 hours and see how it really goes. Seems light on both solar and battery to me but with a generator you can always top up the battery into the evening if needed.

It is actually, extremely lite to power the house. We have an all electric house with the normal 240v loads, and a 680ft deep water well. Without grid or generator, we are bare bones.

I have had the refrigerators connected for about 3 weeks and it holds fine. That is about all I can connect the solar to without shutting the entire house down.

I have made a "Generator Cable" that matches the one for the 12,500watt gas generator. The generator inlet on the side of the house then acts also as the solar inlet. There is a Main Transfer Lockout Slider that ensures the main in the load center is off before the generator/solar is connected to the load center. Once the main is off, and grid is disconnected, I turn off all breakers except the lighting circuits needed. Then, turn on the 50A generator inlet breaker to connect the power source to the load center. NO 240V circuits can be on for solar. And sorry, but the horse's trough heater is off too!

I have tested my chosen lighting circuits in the house, running them on solar power. Works great. I would need a transfer switch to run them continually from solar.

For new houses being built I would have the electrician run extra lines for 12v and have at least 1 12v light in each room. Much less efficiency to use the inverter to make ac for lights. Idle current on the inverter is 2.5A. Disgusting.

I have made up a spreadsheet with all of the items and wattage they need. The spreadsheet also computes the possible Wh generated throughout the seasons, at 100%, 75% and 50% solar system output. That gives me an idea of what I can make and use and make adjustments as needed.

 

[DixieMtCR]

So you have 600W of solar panels, the batteries are charged to full just about all the time. So do you run anything off the inverter so you will not waste the power produced by the solar when the batteries are full?
Do you have Kill-A-Watt to measure your power usage of each electrical device? You can measure KWh for a couple of days to get average KWh.

I run the refrigerators and freezers off the inverter. I do have a "Kill-a-Watt" meter and have watched the power on the appliances. I have changed all of the lights in the house to LED, to save power. Thing that have Amperage on the electrical place, i make note. Other things I use the Kill-A-Watt.

 

[DixieMtCR]

More battery bank will give you more time with low output from the solar. lower DOD the longer the battery life will be.

Good Word! Thanks

 

[time2roll]

I think you know your limitations. You asked about balance... not sure there is a magic ratio. I would normally say 400+ watts with 400 Ah so you are already in the zone. Either more solar or more battery would improve the system.

In time you might find more capacity and value in a self built LiFePO4 battery and a couple more panels.

 

[DixieMtCR]

Yeah, but I'm an old retired geezer and$1000ea/100Ah LiFePO4 is out of my reach. Advantage of the lithium batteries is that the voltage stays flat longer before rolling off. But they are still 100Ah. 5a for 20 hours. They are light weight too but my batteries are installed in the garage and I don't make it a habit of moving them around

 

[chrisski]

If you want to add something to your system, I’d recommend getting a bigger SCC, 50 amps, and perhaps 2 or 3 more panels.

Looks like with the kilowatt mater, you have an idea of the energy consumption you need, so I’d figure out if you think you can get through the night on 2.4 kWh, which is the 50% limit of your 4.8 kWh battery bank. Better yet, 1.2 kwh so you can go a day without sunshine. My recommendation is based off meeting your energy requirement.

With a lead acid battery bank, probably should not be charged at more than 50 amps, so you’re at a 40 amp controller, so there is room to up the SCC. Charge rate of a lead acid battery is usually about 1/8 of the total amp hours of your system.

I have found my panels can put out nearly their rated value when they are tilted and move with the sun. I have three banks of panels, and the portable panels are 400 watts and get 320 watts typically, and have put out 412 watts before. hat is by tilting them and facing them into the sun 3 times a day.

With the portable panels, and the other two banks of panels, I can have my batteries charged in a couple of hours, 100 AH, when I have used the propane heater and the energy sucking blower motor. I have a total of 1350 watts of panels.

I have managed to balance my panels so they can charge at the max rate, 60 amps, a couple hours after sunrise. When the batteries are fully charged, the excess production isn’t really stored, so that’s when I switch the propane fridge to electric, going from 6 watt consumption on propane, to 330 watt consumption on electric. I think of this as coming straight from the sun to the DCC to the inverter, directly to energy. I may run the microwave for a couple minutes also. When I turn the microwave on, the inverter can pull 155 amps, and I’ve seen my system produce as much as 1040 watts, or 74 amps from the SCC, and the rest from the battery.

Your system can produce about 41.6 amps after taking the panels and turning the output to charging voltage. 50 Amps is as much as the batteries can take. If you got 3 more panels (3S3P), you could get 61 amp. You could also do 8 panels total (2S4P) and get up to 54 charging amps to the battery. For me that’s a way for my microwave to pull as much directly from the SCC without draining the batteries. Sinc you’re using this, you can see how much you’re likely to get.

I do like to do the high amperage stuff in the day.

Another think I started doing after my system is watch energy production on cloudy days. The cloudy days I had were one of the reasons I added the other 300 watts of panels on my roof.

 

[DixieMtCR]

I have 6 100w panels, wired 3s2p. I need it that way in oregon because there are lots of cloudy days when I need to have the panels in series to get enough voltage to get the SCC to start working.. Watts are watts, but the SCC needs a higher voltage to get going.

Since I have designed and made a setup for tilting and turning all the panels at once to track the sun, It would be hard to add more panels to the rack. But I have considered another rack in a different location with its own SCC, feeding the same battery bank.

Old growth timber shades the property in various places during the day. Having two sets of panels in different major sun locations would really help.

 

[Shotgun]

I have the same question on a slightly different system setup.
I have 1500W of panels, feeding through a PowMr all-in-one system to a couple 12V/143Ah batteries in series. I feed the 3kW capable output of the all-in-one to a breaker panel, that I've tied 4 of my house circuits to. These circuits feed computers, TVs and lights, such that the draw is a fairly constant 300W to 500W.

The all-in-one protects from low DoD by switching over to power from the grid. So, the game I'm playing is how do I not drop any of the photon energy the panels collect, while not blowing it out buying batteries.

I've got the technical details I needed from PowMr so that I can start tracking the system with SolarShed. Just need to write a little python code.

But, until I have that running, what would be the advice on spending another $469 (plus shipping), to double the battery capacity? What would be the decision process?

 

[pollenface]

My decision process would include a few beers and a credit card.

You're not going to lose by doubling your capacity.