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My Lithium Ion 21 volt Solar Power System


I had started out with a 12 volt Lead acid system 20 years ago and it evolved over time into 150 watts of solar panels with 225 amphours of batteries. Eventually the batteries were worn out and I decided to rebuild the system using Lithium Ion 18650 cells. The whole story is in this thread for reference . . .

I already had a camping cooler for the batteries so I made a battery that easily fit into the cooler and so it ended up being 5 banks in Series. At 4.2 volts per bank it makes 21 volts full charged. I started out using using recovered laptop cells for this but the used batteries were too difficult to maintain good enough charge balancing for Lithium Cells.

A 13 minute video of building and installing the battery
Last month I bought this set of 80 all new cells for about $350 which is $4.38 a cell. They are Samsung 35E 18650 cells rated at 3.5 amphours. Also using an Active Bank Balancer, instead of passively burning away voltage on the higher voltage banks, it actively moves voltage to the lower voltage banks.

You can get great 18650 battery prices at . . .
or used batteries including Electric Car Batteries at

I had been using the same 6 Harbor Freight Solar Panels for about 15 years now. The Amorphous panels had degraded over time and were only putting out about 75 watts meaning a 3 amp charge in full Sun. It's like charging in slow motion, Found a nice panel about 70 miles away on Craigslist and did a small road trip to get it.
A REC260PE 260 watt panel intended for 24 volt systems. Rated 31 volts at 8.5 amps.
Mounted it down that evening and it was amazing the next day, super bright cloudless day, very good luck for a New Improvement in a solar power system. Was so happy with it, charged up in a couple of hours and I got to see the charge controller actually functioning and working well for the first time. At the point where the Sun was straight onto the panel, the charge controller showed 9 amps Charging Current.

When I was using the laptop batteries I was using a passive balancer with protection and it would disconnect if any cell went over/under voltage. One day it disconnected for overcharge and my system went up to the 35 volt panel voltage killing some buck converters that were regulating voltage and current for parts of the system. I promptly purchased a programmable Charge Controller that could properly charge and protect my 21 volt Battery

Found this Wincong SL-01 Programmable Charge controller on eBay for $25 from China. It can be set to run batteries between 10 and 35 volts with max panel voltage of 50v. all of the battery settings can be set between 8v-35 volts including: Max Charge, Float, Low Volt Disconnect and Low Volt Reconnect. This model is only rated for a Max Load/Charge current of 10 amps though.

All of the Above stuff is new but I can't leave out these 2 older pieces of gear than tie it all together.

On the right-side is a diode combiner which would combine multiple voltage sources through diodes for charging. It has 10 diode inputs with 3 Bridge Rectifier inputs for wind or pedal generators. even experimented with elevated wires to try and Tap the atmosphere. This box is not doing much now that I am only using a single panel for charging, but it will stay here for when I want to experiment more.

On the left-side is the main Fuse Box. All power that goes in or out of the Battery and charge Controller will go through fuses before going to the next device. Has saved me many times over the years.

So I've only had a chance to use this system with the new Solar panel for 1 day so far. It was a perfect cloudless day and still working much better than I thought it would be. Was charging between 4 and 9 amps depending on Sun angle and only took a couple of hours to charge the battery to full by 2 volts. Plenty of excess power for the rest of the day.

Now it's the next day which started out Sunny but has turned to hazy on & off clouds. Not getting so much out of the panel now but the newly rebuilt battery is being generous with the power it has available. Loving the new capacity, making me think I can add many more loads.
More to come

Hi Woof, glad to see that you are still having fun.
Are you going to add an inverter to the system or is it just for low voltage lighting?
I guess that in the event of a power failure on the mains you will at least still be able to have lights.
That is one of my favourite things about having stand alone power. We see the lights go out in the houses near us (2) and still have power and lights.
We just added another set of batteries to our system to bring it up to 24 volts. So far it is great. Teething troubles with the inverter caused by untrained workers putting them together but apart from having to fix the inverter before we used it all is good.
Have fun


--- Quote from: Pete on September 08, 2020, 05:27:02 pm ---Are you going to add an inverter to the system or is it just for low voltage lighting?
I guess that in the event of a power failure on the mains you will at least still be able to have lights.


--- End quote ---

The battery voltage on this system runs between 15v and 21v which to each cell is between 3v and 4.2v
Cant really use an inverter due to the Wild voltage of 21v, it would need to be a custom inverter.
I do have a Buck Converter that feeds 12.6vdc to some Cigarette Lighter sockets, so I do have a constant easily used 12 volts that I use to run a TV & Radio but it's not strong enough to power an inverter. With my 55 amphour battery it's not very practical anyway.

This system is more of a serious hobby than than real power.  It has the side effect of giving me something that I can use during a power failure.
These days I've been making a lot of LED lighting which can work on any voltage between 12 and 32 volts so it's perfect for using on this wild voltage.
Then the TV & Radio on the 12v output, and also some things that run straight off the battery. My Computer Sound is running off of an 15 watt stereo amplifier module that will work on 12v-24v so it gets a direct connection. Then I have a couple of DC fans that have PWM motor speed control/switches on them. One is a 12v muffin fan for personal cooling. The other is a converted window fan using a 90vdc treadmill motor to run it. Those fans connect directly to wild battery power too.

It has been unusually cold around here with Snow and Ice accumulation that is the most in a decade.  Lows have been getting close to 15f/-10c here in North Alabama.
I have my new Lithium batteries in a Picnic Ice Cooler outside which would normally be fine, but  it's best that they don't freeze, so I made a small 5 watt heater out of a resistor network. This is a Series/Parallel setup made with 12 resistors totaling 70 ohms. Leads are left as long as possible to help with heat dissipation. Gets nice and toasty without melting the solder and a nice column of warm air coming off of it. Pulls 1/4 amp from the batteries.


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