Introduction: CHEAP Working System LiFePo4 Battery and Spotlights With Solar Charging.
This instructable is dedicated to my beautiful Daughter Kristian.
First things first . The battery
The battery i have made here is a Lithium Iron Phosphate (LiFePo4)or also known as (LFP) chemistry type and each cell has a nominal voltage of 3.2 Volts because its charge / discharge curve is very flat at that voltage so it puts out current for a long time at about 3.2 Volts and at the end of its discharge when the Battery Management System (BMS) decides it has reached the low point it does not want the cells to go below and switches the battery off to protect it.
Take note it is not Lithium Ion(Li-Ion) It is Lithium Iron
The completed battery with compression sides ,BMS and stainless rods was 13.5 Kilos. See pic.
Step 1: Typical Charge and Discharge Curves
I had my cells air freighted from China . 8 cells at 3.2V is 25.6
nominal. Each cell must be charged to 3.55 Volts firstly for my batteries .Some manufacturers suggest higher voltages but all of these refer only to the first charge and balance.
This is called "top balancing" .If you look at the graph at the point where the curve rises quickly it can be seen as a good indication that the battery is full. If your cells were all showing 3.4V you would not know whether each cell was a quarter full A , a half full B, 3 quarters full C or nearly full D!
There are a couple of methods of doing this. The one I chose was to charge each cell to 3.55V and then connect them all in parallel for a day so they all settle at the same voltage pressure.
By getting them all to that point you have "equalised " your cells and as current is drawn out they should all deliver a similar amount of energy before one cell gets too low and it all switches off. When recharged a similar amount of energy should go back into each cell and they should all fill up at the same time. In practice this seems to stay pretty close for LiFePo4.
This is where I got my cells . "email@example.com" ....Chen. Tell him I sent you.
He sold me sample cells for USD $25 each to help me test his cells and I will be going back for more soon.
50 Amp hour rectangular vented aluminum cases with connectors came all packed in a protective dense foam inside a heavy cardboard box. From payment date to arrival was only two weeks. Cost with air and local freight AUD $250 plus AUD300 Air Freight. Very good service but air freight is expensive.
Video on construction
There is an easier way to do the connections that they supply too.
Australia has just imposed an extra 10% GST on items below $1000 so they will be that much higher now .
Each cell weighs 1.35 Kilos and this is important because there are frauds in the battery market and weight is the only indicator you have that the cells will be genuine fully filled cells. For all Lithium cells you need some sort of compression to stop the electrolyte/plates gassing and disengaging from each other. Difficult to do with foil type packs but simple with the rectangular packs as above .
Step 2: The Battery Management System BMS
Now this piece is still a work in progress. I seek a 24Volt BMS which will charge my LIFEPO4 cells to 3.55 no more and will cut off at 2.5 low voltage .
The best control for value I have seen so far is a small board that goes across each and every cell to protect them. They are tied in to another control which switches relays on and off. The first board and the last are both different and expensive They should be easy and cheap but they are not on ebay yet and the whole system purchased as a full complete system is up near AUD$ 400-500. Too steep for this penny pincher so I have been using two 12 Volt systems of AUD $8 each connected across half of the battery to get the top balancing done with a 12 volt car charger .
You will see them in the first photo They have 5 sense wires , Positive red and black negative Go on the ends of the 12 Volt pack , The other three go to each joiner between the cells. See the circuit on the devices above.
So I balance one side then the other . I attach the positive lead of the charger to the positive battery terminal and the charger negative goes on the negative black wire on the BMS marked negative charge C-. I don't use D- as yet .
When I do the other half I have to put the Positive charger lead onto the 5 th battery terminal positive and the negative on that units negative charge lead C-.
So I am working with 12 Volts only on the top balancing charging. All other charging comes through on the normal solar controller direct to the battery.
Step 4: Get a Couple of Old Solar Cells and a Controller.
I bought some cheap second hand 250 watt solar cells that had been taken down by someone who was installing a leased system. AUD $20 each with all the fittings and a main switch. Ten of them .They have a built in diode which stops any current flow in the opposite direction.
Solar connectors are cheap from ebay ,buy 30 or so its just cheaper.
Those panels put out 30 Volts max and 8.5 amps approx. (30V x 8.5A=250 watts near enough.)
I bought a 12/24 Volt auto solar charge controller to attach to them for about AUD $236 . It has MPPT which simply drops the input voltage to match the battery voltage at a point where the maximum amps flow into your battery. See the pic , the unit works well and i can change the high and low points and the charging voltages to suit me and LiFePo4.
I noticed there are many many more on the market now and a much cheaper version will do the job as long as you can set up the top charge voltage points .
This instructible below would let you build your own cheaply based on an Arduino Processor chip read of battery voltage and switch off precisely Arduino Solar Charger
Old lead acid systems do not work here neither do the controller settings .
It needed at least 50 volts or so in to function so a single panel would not do . I put two panels in series giving an output of 60 volts max which works it well and still gives me 8.5 amps max from the panels. These panels are pretty safe and you can supposedly keep putting them in series increasing the voltage to a maximum of about 1000 volts nowadays.
That said its legal in Australia to connect and play with voltages up to 90 Volts DC . Above that it starts to get dangerous so I just don't go there . Arc Welders put out 50 to 70 odd volts DC thats why they are safe to use. The higher the voltage the lower the current so higher end voltage means thinner wire can be used .
Step 5: Buy a 24 Volt 500 Watt Pure Sine Wave Inverter
I bought mine from ebay. Cost me AUD $115 and it has a plug on it that fits a lot of different countries plugs.
Step 6: Wire It All Up and Turn on Your TV
There is wiring from the panels to the controller .
Wiring from the controller to the battery
Wiring from the controller to the load ,which in this case is my Inverter/Tv and my LED spotlights . Once you are this far you can test the inverter and TV are working off the battery.
Left like that i was able to run the 170 watt LCD Plasma 55 inch TV for about 10 hours before the inverter's cut off stopped everything due to low voltage. I intentionally did not have the solar panels connected at that point to see how long the battery would last. Run through the day with solar connected I guess it would run 24/7 if the solar input was sufficient.
Wiring of the BMS
Solar recharged it all up again quick smart .
Step 7: LED Spotlights , Movement and Darkness Sensors.
I bought 4 of 20 watt LED lights for AUD $20 each some years back . They need 24 volts and have the circuitry to up this to about 32 volts in the lights to get super brightness. None of the silly little things . These draw about 3/4 of an amp at 24 Volts so two use one and a half amps from my battery.
LED Light example These are 24 Volt but I could not find the ones I have so look around .
Additionally I bought a heap of ebay AUD $4 street lighting darkness sensors , one only needed here, and a heap of movement sensors for AUD $4 each ,two here,which I suspended from just below the light on a bent piece of aluminium.
Light Sensors 24V ebay example
Movement Sensors 24v ebay example
They must be 24 Volt to work on this battery. Because these are run by battery the movement sensors I got had to be altered to make the delay shorter or my battery would drain some. I replaced a resistor with a lower value to do this successfully . See the pictures above .
i messed about with it for some time and I think I settled on the picture at the end with R14 removed and replaced with a normal resistor to the bottom of the variable pot that controls delay . The resistor was 2.2 KilOhms . See a discussion here
You will also find the capacitors used in these are 25Volt . I get trouble from time to time and I think the caps need to be changed to 40 Volt or so.
Thats about it . Its taken a long time to perfect but changing to Lithium Iron Phosphate was the big plus .