A simple, inexpensive, yet powerful solar storage system

[ncbill]

I recently bought two of these used LFP industrial batteries. They are 24V 6p8s assemblies of Headway 38120 cells. There's no BMS.

The 6p8s configuration means 6 cells are parallel together in a group, then 8 groups are connected in series. One cell is capable of 200A continuous, so the assembly can do 1200A. At 24V, that's 28.8 kW (but for only 2.4 minutes). Crazy power!

The total capacity is only 1.23 kWh though, so this design was meant for short duration/high power applications.

I bought one, tore it apart to inspect and measure each of the 48 cells individually, then reassembled the battery. The 1st one I bought was nearly spanking new, so why was it made surplus? In opening it up, I thought I found the reason. One of the balancing wires had a loose internal connection, so perhaps that's why the battery was taken out of service

I liked it so much, I bought a 2nd one. This one came a bit dusty, so apparently saw some use.

These Headway cells have the spec of 8Ah each. With the 1st battery, I measured from 8.20Ah to 8.76Ah, with the average being 8.51Ah. This battery was nearly unused, as I thought.

With the 2nd battery, I measured the cells from 7.70Ah to 8.16Ah, and the average was 7.99Ah. Still not bad.

I had an application for these batteries in my home solar system along with more inverters and panels I already have, but still have to work out the details of installation.

Price?

~$10k will get you over 30 kWh of 48VDC LFP batteries in a server rack:

Or even cheaper, but with more assembly required:

 

[NW-Bound]

I paid about $350 a piece for the 2 LFP assemblies. At 1.23 kWh each, the cost is not really low, but fair for a self-contained, ruggedized assembly.

I was intrigued by the tremendous current capability of the Headway cells, and thought that if they don't work out for a 2nd auxiliary home solar setup, I can take apart the individual cells and build batteries for my dirt bikes. Or I can use them to run the AC of my motorhome.

About the LFP cells that are shown in Prowse's videos, I have been following the prices of this type. When I need to add to my existing 34 kWh set up, will buy this type.

An importer called Docan Power in Texas is selling brand-name CATL cells for $130/kWh, shipping cost unknown. CATL makes the cells used by Tesla in Shanghai.

 

[zinger1457]

Currently have my workshop in my garage, looking to build a separate workshop on my property. Problem I have is that my electrical panel is max'd out and since my utilities are underground the cost to upgrade the panel is well in excess of $5K. Sound like the solar system described (2400 watts output) would meet my needs, just want to confirm, this would be completely off grid. All my current power tools work no problem on a single 120V/15A circuit in my garage. The biggest tool I have is a table saw that's rated at 15A, only use one major tool at a time, have never tripped the 15A breaker in ~25 years. Would also need to power lights with this setup. Any thoughts, am I missing anything?

 

[NW-Bound]

Back 4 years ago when I first put together my DIY solar+battery system, high-current BMS were not widely available, and I built my own. This allowed me to watch the characteristics of the LFP cells, and I was astonished by what I observed.

Lithium cells require accurate charge termination. When they are full, any additional charge current will cause the voltage to shoot way up, causing danger of fire or at least shortening the life of the cell. LFP cells are even more prone to overvoltage when full.

My home LFP set up is built up from groups of cells in series, each group having the total capacity of 880 Ah. When the entire battery is near full, the high-voltage group may already reach the voltage of 3.6V, while the lower group may be at 3.5V.

Now, I set the voltage limit to 3.5V/cell in trying to have a longer cell life, and besides there's very little energy in the last 0.2V. So, when one group reaches 3.6V, it will need to be bled down to 3.5 V to match the rest. How much charge do you think I need to bleed off?

How about a mere 0.5Ah? Yes, 0.5Ah out of 880Ah total capacity, or a bit more than 0.05%.

Imagine filling a gallon jug of milk. When you get to the top of the jug, the neck narrows down and just a few additional tablespoons will cause the jug to overflow. Lithium cells are like that, and LFP has the most pronounced effect.

And that's why people who unknowingly play with lithium cells get them to blow up.

 

[NW-Bound]

Currently have my workshop in my garage, looking to build a separate workshop on my property. Problem I have is that my electrical panel is max'd out and since my utilities are underground the cost to upgrade the panel is well in excess of $5K. Sound like the solar system described (2400 watts output) would meet my needs, just want to confirm, this would be completely off grid. All my current power tools work no problem on a single 120V/15A circuit in my garage. The biggest tool I have is a table saw that's rated at 15A, only use one major tool at a time, have never tripped the 15A breaker in ~25 years. Would also need to power lights with this setup. Any thoughts, am I missing anything?

You can throw together a system that works for you for less than $5K. No problem.

The only thing is the surge starting current of the table saw may trip the current limit of the inverter. Electronic protection circuits are much faster than a mechanical breaker.

What you will need is an inverter that has a high headroom to tolerate the starting surge current which lasts less than 1 second. You either buy an inverter that is rated at higher than 2400W, or an inverter of the type called low-frequency design. More on this 2nd type later.

 

[zinger1457]

^^ Good info, thanks!

 

[NW-Bound]

If you are a handy type, I heartily recommend a solar+battery for your detached workshop. Not only the cost is lower than running grid power out there, you will have free electricity to keep a freezer out there, and in the case of power outage you will want to run an extension cord back into the home to get lighting. What's not to like?

Back on the inverter and surge currents, I have had no problem running a 2HP compressor off a MPP Solar 2400W inverter, but as the only load. Still, I have to put up a caveat that YMMV. My table saw, radial arm saw, and chop saw all work but they are a lighter load than the compressor, I believe.

I just saw that MPP Solar now has a 2700W inverter, 24VDC in / 120VAC out for $538 after shipping. That's less expensive than my old 2400W inverter.

 

[NW-Bound]

There are two designs of inverters: high-frequency and low-frequency.

Most inverters on the market are the high-frequency type. The design uses two stages. First, a DC/DC converter boosts the 12V or 24V or 48V up to about 200VDC for a 120VAC output. Then, an H-switch is used to steer the DC voltage to the load in an alternate manner to create an AC voltage. It's called an H-switch because the circuit diagram of the output transistor stage looks exactly like an H.

The 200V DC voltage is stored in a bank of capacitors to supply power to the H switch. To generate a sinewave, the H switch is turned on/off rapidly to modulate the duty cycle of the output. After passing through an LC low-pass filter circuit to average out the fast-switching waveform, the output looks like a beautiful sine wave.

It's called high-frequency because the first DC/DC converter runs at 100 KHz or higher. This is so that small transformers can be used at the high frequency. A small transformer of a few cubic inches can handle 1 kW easily. The design is light, and efficient.

A larger inverter has every component bigger, from the transistors of both stages to larger transformers, capacitors, and heatsinks for the transistors. To handle a large transient load, you can install beefier transistors, and keep everything else the same size, right? For example, the heatsinks will not overheat instantly with a 1-sec transient load.

However, the problem is that the capacitors that hold the high 200V DC voltage that sit between the two stages have to be upsized. This is the storage tank to provide that 1-sec surge current. Big high-voltage capacitors are not cheap.

The low-frequency design converts from DC to AC in a single stage. It uses an H switch to steer the DC input voltage across a transformer whose secondary winding provides the 120VAC. To generate a sinewave, the H switch is modulated to vary the duty cycle, just like the H switch of the high frequency design.

The H switch of the low frequency design cannot run at the same high frequency as the earlier type. It's because of the large iron core of the transformer which provides the 60Hz directly. High-frequency switching incurs larger losses in the iron core, compared to the ferrite material for the small high-frequency transformers.

The low-frequency design can handle a very large transient load which is a high multiple of its constant load. It's because the iron core transformer does not heat and burn up in a second. Its flux saturation characteristic is also a soft one, and not a brick-wall kind of limit. The main components that have to be able to handle the surge are the battery and the transistors.

The disadvantage of the low-frequency design is the weight and bulkiness of the large iron transformer, which is also expensive. However, it provides superior overload capabilities.

 

[Sunset]

Many portable inverters, meaning not meant for fixed installation, have built-in fuses. ... The inverter manual should mention this.

.... I also use breakers for the panel-to-controller and controller-to-battery paths.

If you use a fuse or breaker between the charge controller and the battery, be careful about breaking the contact to the battery while the panels are still connected to the controller. With a sudden loss of the load, the output voltage of the charge controller jumps up, and may destroy its output circuit.

.....

You should always have a fuse or circuit breaker as close to the battery terminal as possible. .........
The solar panel output is limited. ........ I put a cutoff switch on the panel input so if I disconnect the battery for some reason, the panel doesn't damage the controller, as NW alluded to.

I can see for the solar panel -> controller, a cut off switch or a breaker that can be tripped is much handier than disconnecting the wires and reconnecting each time the battery has to be disconnected, which will be each season for me.

For the actual cut off switch, can I use any type of switch , like a house light toggle switch ? or is there a difference between DC and AC that means the switches cannot be interchanged ?

If a person has a fuse or breaker between the Controller -> battery, won't this mean if it gets tripped/melted that the controller will then probably be destroyed ?
I'm thinking what if I put in too small a fuse, it will trip and I've destroyed my controller on day one

 

[NW-Bound]

I can see for the solar panel -> controller, a cut off switch or a breaker that can be tripped is much handier than disconnecting the wires and reconnecting each time the battery has to be disconnected, which will be each season for me.

For the actual cut off switch, can I use any type of switch , like a house light toggle switch ? or is there a difference between DC and AC that means the switches cannot be interchanged ?

If a person has a fuse or breaker between the Controller -> battery, won't this mean if it gets tripped/melted that the controller will then probably be destroyed ?
I'm thinking what if I put in too small a fuse, it will trip and I've destroyed my controller on day one

Because of the arcing danger of DC voltages, DC breakers are of a different design than AC breakers. However, this is only for DC voltages of higher than 40-50V or so.

For your low voltage panels with Voc of 24V, a regular light switch works fine, particularly as the current is also only 10A.

About the connection from the controller to the battery, you've got a good precaution there. Maybe hardwiring is the thing to do, particularly as many controllers already have a built-in fuse.

 

[NW-Bound]

By the way, be careful with observing polarity when connecting components together. Use a DC voltmeter to be sure which lead is + and -.

I once wired up a panel backward to a charge controller. The current from the battery flowed backwards through the controller, and into the solar panel. By the time I saw that the long wires from the controller to the panel were hot and started to smoke, the high current already destroyed the bypass diodes inside the panel.

Thank goodness, the 80A controller was not damaged. Its connection to the big battery bus was via a 100A circuit breaker which did not trip. However, the unknow current was too much for the 10AWG wire, and the 30A diodes of the panel.

I was able to replace the panel diodes, and that was all the damage. It was a good thing that was a temporary wiring setup so I could check out the panel while it was laying on the pool deck, hence the wiring was loose 10AWG wire, which I could feel the heat.

How can the battery current flow back through the panel via the controller? I can show this easily via a circuit diagram of a typical charge controller. It's possible to provide diodes for protection, but this controller does not have them.

PS. For the above temporary wiring to testing, I did not have a breaker between the panel and controller.

 

[youbet]

I know this is an old thread, but now I'm going to start out with a small solar system for my remote no electricity cabin.

Initially I want to be able to charge up cell phones, so we can reduce trips to go charge the phones

Just charge phones in the car. No need to go someplace or even run the engine while you're doing it.

and hopefully power up a 900W coffee maker for fast morning coffee.

Heating water with electricity generated with solar sounds unnecessarily complicated. Why not solve that issue with a simple propane burner ($29.99) and old fashioned percolator?
A 20# propane cylinder will last a long, long time just used for that.

I'm not suggesting that getting involved with solar is a bad idea. Just saying that the reasons you mention are really easy and convenient to solve using your car and propane.

When we're dry camping (I'm assuming this would be similar to your remote, off-grid cabin) we don't bother with solar despite many of our friends going to elaborate systems. We run the fridge, furnace and stove on propane. We power the the lights, furnace blower, vent fans and computer/phone charging off the house battery. When the house battery runs low, we charge it from the truck via jumper cables. If we aren't using the furnace, once a week does it. The furnace blower does have a significant draw, so in cold weather we might need to charge the house battery every other day.

I'm likely going to get some solar going before we leave for Florida next January just for the experience of learning the ins and outs. But we don't expect to be able to do anything with it we don't do without it now, at least as far as dry camping goes.

Great information in this thread!



[/QUOTE]

 

[travelover]

..........

For the actual cut off switch, can I use any type of switch , like a house light toggle switch ? or is there a difference between DC and AC that means the switches cannot be interchanged ?

For a small system I'd just use a toggle switch.

If a person has a fuse or breaker between the Controller -> battery, won't this mean if it gets tripped/melted that the controller will then probably be destroyed ?

If the fuse trips on the battery, you've got bigger problems to worry about. I just use Renogy PWM controllers that are 20 or 30 bucks on my RV, so not a big deal. And unless the panels are in bright sun when it happens, it won't necessarily damage the controller.

I'm thinking what if I put in too small a fuse, it will trip and I've destroyed my controller on day one

Size the fuse to the smallest wire than could melt due to a short.

 

[Sunset]

Just charge phones in the car. No need to go someplace or even run the engine while you're doing it.

The cabin is on an island, so we would have to cross the lake to get to the car, pretty inconvenient when it's windy due to rough waters or when raining as we would get wet.

If we weren't on and island, we could charge using the car, which is what we currently do, but sometimes have to make special trips just to charge or can't make the trip due to the climate conditions and have devices run down.

Heating water with electricity generated with solar doesn't make much sense. Why not solve that issue with a simple propane burner ($29.99) and old fashioned percolator?
A 20# propane cylinder will last a long, long time just used for that.

Currently we do that. But in the morning I'd like my coffee faster as the percolator over-boils if the gas is on too high. So I have to watch it.
A cheap $10 drip coffee maker like we use at home, would be nicer.

.....

When we're dry camping, we don't bother with solar despite many of our friends going to elaborate systems. We run the fridge, furnace and stove on propane. We power the the lights, furnace blower, vent fans and computer/phone charging off the house battery. When the house battery runs low, we charge it from the truck via jumper cables. If we aren't using the furnace, once a week does it. The furnace blower does have a significant draw, so in cold weather we might need to charge the house battery every other day.

Partly I'm doing this to learn, and once I get better at it I'd like to add in a couple of lights , and ideally more generation and batteries to run a small fridge. Our propane fridge started leaking, and we could have burned up in a fire as the leak was impossible to find until we took apart the burner assembly.
That has made me pretty nervous about propane fridges in the cabin.

A new 15 cu ft propane fridge is ~$2K

I may never get to an electric fridge stage, due to the issue of isolated cabin invites theft over the fall-winter-spring and even solar panels on the roof could be attractive.

But some lights would be nice instead of flashlights

 

[NW-Bound]

I may never get to an electric fridge stage, due to the issue of isolated cabin invites theft over the fall-winter-spring and even solar panels on the roof could be attractive.

But some lights would be nice instead of flashlights

You don't need much of a solar set up to run LED lights.

And about the fridge, a portable 12V fridge can be run off a small solar system. And I mean a real fridge with a compressor pumping freon, and not an inexpensive thermoelectric cooler.

When I get back to RV'ing, will get a portable 12V fridge to augment my propane fridge. This keeps us from opening the propane fridge all the time, which takes a heck of a long time to cool back down. Also, I will get a countertop ice maker.

PS. Once you have power, a lot of things become possible. If you have spring water, can run a small 12V pump + RO filter to get safe and good potable water. Total cost: < $100.

I have been thinking about getting the pump+RO setup for emergency use, to get good drinking water out of the pool if the city water supply gets intermittent because of the drought. Perhaps I am just too paranoid.

 

[youbet]

The cabin is on an island

When you said remote, you really meant REMOTE! I was thinking you were driving into an "out past the power lines" situation.

But in the morning I'd like my coffee faster as the percolator over-boils if the gas is on too high.

OK, makes sense. We haven't been dry camping as much the last few years as previously. And then, an extra few mins brew time doesn't matter, at least to us.

Partly I'm doing this to learn,

That's why I'm looking at it.

and once I get better at it I'd like to add in a couple of lights , and ideally more generation and batteries to run a small fridge.

Lights are easy. And I've noticed many late model campers are coming equipped with compressor fridges (which draw 2 - 3 amps) instead of evaporative 3-ways like I have.

Our propane fridge started leaking, and we could have burned up in a fire as the leak was impossible to find until we took apart the burner assembly.
That has made me pretty nervous about propane fridges in the cabin.

A new 15 cu ft propane fridge is ~$2K

A leak would be scary. Our little Dometic 3-way has been getting the shake, rattle and roll treatment bouncing around in the camper for over 13 years and so far no problems. And I hope it stays that way.

I do notice that some newer campers are being delivered with 12V compressor fridges and solar panels. My buddy's newer Alto has 200W of solar on the roof and a lithium battery. He says he's always gotten enough enough from the solar panels to run the fridge 24x7 indefinitely. The fridge draws about 2.5A when the compressor is running. [/quote]

I may never get to an electric fridge stage, due to the issue of isolated cabin invites theft over the fall-winter-spring and even solar panels on the roof could be attractive.

What general area is your cabin in? Sounds great.

Have fun!

But some lights would be nice instead of flashlights [/QUOTE]

 

[youbet]

The cabin is on an island

When you said remote, you really meant REMOTE! I was thinking you were in a drive-in but off grid situation.

But in the morning I'd like my coffee faster as the percolator over-boils if the gas is on too high.

OK, makes sense. We haven't been dry camping as much the last few years as previously. And then, an extra few mins brew time doesn't matter, at least to us.

Partly I'm doing this to learn,

That's why I'm looking at it.

and once I get better at it I'd like to add in a couple of lights , and ideally more generation and batteries to run a small fridge.

Lights are easy. And I've noticed many late model campers are coming equipped with compressor fridges (which draw 2 - 3 amps) instead of evaporative 3-ways like I have.

Our propane fridge started leaking, and we could have burned up in a fire as the leak was impossible to find until we took apart the burner assembly.
That has made me pretty nervous about propane fridges in the cabin.

A new 15 cu ft propane fridge is ~$2K

A leak would be scary. Our little Dometic 3-way has been getting the shake, rattle and roll treatment bouncing around in the camper for over 13 years and so far no problems. And I hope it stays that way.

I do notice that some newer campers are being delivered with 12V compressor fridges and solar panels. My buddy's newer Alto has 200W of solar on the roof and a lithium battery. He says he's always gotten enough from the solar panels to run the fridge 24x7 indefinitely. The fridge draws about 2.5A when the compressor is running.

I may never get to an electric fridge stage, due to the issue of isolated cabin invites theft over the fall-winter-spring and even solar panels on the roof could be attractive.

What general area is your cabin in? Sounds great.

Have fun!

 

[NW-Bound]

... I do notice that some newer campers are being delivered with 12V compressor fridges and solar panels. My buddy's newer Alto has 200W of solar on the roof and a lithium battery. He says he's always gotten enough enough from the solar panels to run the fridge 24x7 indefinitely. The fridge draws about 2.5A when the compressor is running...

200W of panel is not much at all. I am surprised that it generates enough for the fridge full-time use.

There are motorhomes now that are all electric, meaning no propane. They have a solar panel or two, but that's not the main source of electric power. It comes from a generator.

They do not have a separate generator either. The engine would start up automatically to run a beefy alternator to recharge the lithium battery if needed. There's a big inverter onboard to supply power to the AC, the induction cooktop, and the fridge. The idea is that it is easier to refill the gas tank than finding a place to refill the propane tank.

How about solar power for propulsion also?

No, it's physically impossible due to the limited size of the vehicle roof. You would need about 1000 square feet of solar panels to generate enough electricity in one day to recharge a Tesla X.

And that's one day of solar charge to drive for perhaps 100-150 miles on a small motorhome, or a Tesla X pulling a trailer. And that's solar power in a summer day in the Southwest.

 

[Sunset]

I notice the PWM controllers often have 1 or 2 USB charge outlets , which is nice.

Often the MPPT ones don't. It seems a waste to use an inverter to plug in a usb converter to charge my devices.

I've looked and a better solution seems to be a 12V usb outlet added to the setup such as:
https://www.amazon.com/dp/B09236JCK8/

It made me think:
However, this costs $21, and for $15 -> $20 I can buy a PWM and just connect it to the battery alone to make use of the usb , and have as a backup controller in case my main one fails. It would be easy to just switch over the PV wires.

What do people do ?

 

[NW-Bound]

When RV'ing, I just left my inverter on all the time, and used all my AC adapters for phones, cameras, etc... It's a Xantrex SW2000 that draws an astonishing low idle power of 6 W. The 3-in-1 MPP Solar draws around 25 W. This is one thing that could have been better.


Check out the inverter you intend to buy. The idle drain may not be that bad.


For USB charging, how about a cigarette socket like this, that can be bought for a few dollars to wire directly to the 12V battery?

You can then plug in a common cigarette-plug-to-USB adapter, also for a few bucks.

 

[NW-Bound]

Or if you are a tinkerer, you can wire this 12V-to-USB device directly to the battery.

Cost: $1.59+$0.30 shipping.

 

[travelover]

Or if you are a tinkerer, you can wire this 12V-to-USB device directly to the battery.

Cost: $1.59+$0.30 shipping.

Was that on eBay?

 

[NW-Bound]

Was that on eBay?

Yes sir. eBay and AliExpress are the best sources for little electronic gadgets like this.

I have been fortunate, and most the things I bought over the years worked. I only got a couple of ICs that were DOA, out of perhaps 100s that I have used.

 

[travelover]

Yes sir. eBay and AliExpress are the best sources for little electronic gadgets like this.

I have been fortunate, and most the things I bought over the years worked. I only got a couple of ICs that were DOA, out of perhaps 100s that I have used.

I notice these are generally rated for 3 amps. Any idea what they really are - i.e. adequate for an iPad?

 

[NW-Bound]

I just came in from the patio out back, where I tested an inexpensive 2000W inverter that I bought off Amazon Warehouse Clearance.

Worked fine with a load of 1600W provided by two portable room heaters with setting of 400W/800W. It complained that the battery voltage was low when both heaters were cranked up for a total of 1600W. But the monster LFP battery voltage is not low. I guess that's a problem with inexpensive stuff. There's always something wrong.

Better inverters/chargers will let you program different settings to your liking.

The efficiency of this one at 1600 W is not bad though, and better than 90%.