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, it also holds its voltage stable at high discharge loads so no voltage sag. 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 (2.5 volts per cell) it will switch the battery off to protect it.Stay out of the knees and you will be pleased it is said !
LFP batteries can be discharged down to very low states of charge so to replace Lead Acid types only about a half of the Lead Acid's capacity is needed . If you have a 400 Amp Hour L/A battery you only need a 200 Amp Hour LFP battery to replace it. People considering switching often get that wrong when considering their price.
Another plus for them is on boats . There is no gassing to worry about if you keep them cool but importantly with a 3Kw inverter and Induction Cooktop you can dispense with the gas LPG system with its inherent dangers of explosion and suffocation too.
Take note it is not Lithium Ion(Li-Ion) Nor is it LiPo It is Lithium Iron Phosphate
The completed battery with compression sides ,BMS and stainless rods was 13.5 Kilos and is 50 Amp hours at 24 Volts . See pictures.
Note that I decided to remove the plywood sides and bottom to assist heat transfer not that I have noticed any but heat is any battery's enemy during charging . It boils the very little electrolyte that's in the plastic separators and that turns to gas occupying ten times the volume so pressure goes up and the cells suffer.
I started running two 250 watt solar cells in parallel for charging at 66 volts through the MPPT charger . This gives me a panel output of 8 amps each normally and this is doubled to 15 amps at about 35 Volts coming in by the MPPT function. This charges the pack back up quickly in the morning. One or two hours only in the sun does the job.
After 17 months at this stage July 2019 there have been a few problems .No terminal corrosion or wire corrosion or anything detrimental really except the solar charger I bought is the wrong one made for Lead acid and it refuses to stop trying to equalise my pack by boosting the voltage to over 30 Volts when its full which forces the BMS to disconnect the battery and the damn thing comes back in 10 seconds and tries to do it again and this keeps up until I disconnect the panels and use up some of the battery capacity. Most bloody annoying because no setting turns it off and when the BMS disconnects the pack the inverter is cut off too and all my power dies . They are advertised as suitable fro LFP and they are not. They know it and any enquiry or complaint to SMK is met with absolutely no response whatever.
I'm using two stainless bolts to connect the inverter to and they are clean as a whistle.
I have found that about 3 cells seem weak at this stage and the cells do not stay in balance properly despite trying top and then bottom balancing so I have added an active cell balancer board, which can be found on ebay and the like, which keeps the cells within 30 mV of each other and as I recharge daily this seems to be keeping the bank capacity higher and stable.
I also have a Cellog8 type meter attached with which I can watch the pack. I have a BMS attached which cost $20 AUD and it has worked exceptionally well throughout .
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Typical Charge and Discharge Curves
I have removed all details of my cells and my recommendation from Mr. Kevin Chan Zhuhai OTE Electronic Technologies as when I tried to contact him about a failing cell I was totally ignored. This is what i put on the last message to him in April 2019
So Kevin it looks like problems get ignored . Warranties are worthless and the products un reliable .
I wil note that across my Instructable in big bold type.
Some of the cells at 21/5/19 are only holding about 40% of their rated capacity despite being carefully balanced top and bottom and watched continually. Don't buy these cells !
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 3.2V 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.Normally about 3.2V 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. I bought 50 Amp hour cells with rectangular vented aluminum cases with connectors for USD $25 each .They came all packed in a protective dense foam inside a heavy cardboard box. From payment date to arrival was only two weeks. Cost AUD $250 plus AUD300 Air Freight. Very good service but the air freight is expensive. I painted some more red on the positive terminals . Its a bit hard to see from the factory for these old eyes. There is an easier way to do the connections that they supply too.I did my connectors as shown because when dealing with the terminal nuts I can favour the side which the connector joins so I don't spark the terminals with associated metal thread damage . I have big clumsy hands . 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 anode cathode and plastic separator disengaging from each other. Difficult to do with foil type packs but simple with the rectangular packs as above . Note;- There are ply pieces around the bottom and sides . That was mistake I realise now. I think I should allow the sides air flow to cool. I am updating that instructable like a blog to add on information and experience as I learn.
I would like to remove the below video but have not found a way to do so once added. Not this Kevin Chan is a corner store like the thousands over there so don't think you are dealing with the manufacturer . You are probably not.
Video on construction
Step 2: The Battery Management System BMS
This is what I have ended up using but check around prices can vary from$20 to $30.
Note very carefully the cut off voltages because they look the same for 4.2V models also and they will destroy LFP cells
Step 3: 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 blocking diode which stops any current flow in the opposite direction and 3 bypass diodes which help with shading.. later i was astounded by someone on Gumtree selling me 22 new panels and a 5000 w Grid tied inverter and all the alloy and fittings for $500 AUD .(All new and unused)
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 automatic 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. This can be dramatic and I have seen 15 amps flowing into my battery in full sun from two panels in series.
I attached a recent video of the MPPT changing as it charged. PV input s probably up at about 60V or more.
See the picture , the unit works well and i can change the high and low points and the charging voltages to suit me and LiFePo4.
NOTE : I tried to contact their email at firstname.lastname@example.org with a question on lithium settings and was completely ignored so I do not recommend SMK any more . There is no built in algoithm for Lithium which they claim there is and the settings keep jumping back to lead acid settings . Don't buy their charger. See at the end for the email I sent.
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 and the MPPT actually works .
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 . Don't connect the two types of batteries together as you will flatten your LiFePo4.
The solar controller 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.(MPPT turns that into 15 Amps at about 36 Volts which is the MPPT point but it changes all the time)
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 4: 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.
Update May 2019 - It blew up ! I replaced it with a 1000 watt model for $175.
Step 5: Wire It All Up and Turn on Your TV or Desktop Computer or Whatever.
To wire it all up and get wire sizes etc this video might help Wire up for beginner.
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 security spotlights . Once you are this far you can test the inverter and TV are working off the battery.
Wiring of the BMS-The BMS only goes on the Negative line. The input is joined to the charge controller and output to the negative battery terminal. All the sense wires have to be put on to each cell as directed.
As I added the BMS ,Cellog8 and active balancer sense wires I noticed they all go to set battery positions and found by using a ring terminal I could neaten things up by opening up the crimp and pre-soldering it , then soldering the appropriate 3 wires onto it. This avoids the bulk of 3 connectors and if I want to change one device I can unplug its main plug and leave it off or plug in another . All the main connectors seem to be different.
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.
Solar recharged it all up again quick smart . EDIT I feel the capacity of these cells has fallen to about half now and I get about 5 hours but I'm working on it.
Step 6: 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 .
Step 7: Other Useful Stuff
The pic above shows a foil pack i tried to make but it was a waste of time.The second a great way to see it all at a glance on the state of the battery.
EWT LiFePo4 50AH 12V Review Teardown & Load Test
This video is worth a watch because he cuts right into it to show you what is inside these types of Batteries . About USD $389 each and they have 26650 round case cells the same as Tesla Mod 3 cars.
EWT LiFePo4 50AH 24 volt alternative
Wiki on LFP Cut down Prismatic cell showing the plates etc
An expensive BMS system
A cheap PCM Protection Circuit Module (PCM)- This board connects across Positive and Negative of a cell and shunts excess voltage (over whatever the chosen limit inside the IC) through the resistors as heat.
A cheap PCM
An 8s 25 amp for AUD $41
80 Amp AUD $67 But postage on this kills it to me
Suitable one on Ali-Express Up to 60 amp with all the right cutoff values. This is presently running my system $31 AUD 60 amp charge and discharge Note only one charge /discharge port.
Boating Group using them
Here are 240 Amphour cells cheap
This is a load test on some similar cylindrical cells and is why you should watch how you work around the terminals !
This is an Australian Supplier of cells direct to you
This is a 12 Volt LED headlight that would work well on a 12 Volt system
Update 6 months later January 2019
I checked it over for corrosion and found absolutely none and I am not using any type of additive on the connections at all.
I noticed its capacity was dropping . It would not run an 80 watt light bulb all night as it used to along with the two security lights so I investigated the cells at the point the controller had turned off and found in fact the BMS had turned off . When the solar came in in the morning everything turned back on and the pack recharged fine.
Testing the cells at that shutdown point I found the two cells closest to the positive terminal were down below the others by 0.30 V only but that was enough to trip the BMS early. I used a 5V LED power supply meant to run an LED strip to charge up both cells to match the others and that seems to have fixed the early shutdown .
I found a device to tell me at a glance what the state of the string was. Its a Celllog8 . See the pic above .
It tells me that the string is almost full at 26.89V ,that the highest cell voltage is 3.366 Volts and the lowest is 3.358 V and there is 8mV between high and low cells .
There are other settings for display but I leave it on there and it barely seems to use any current.
They are $ ? Ill post that when I find them again.
The user manual for the logger is above as a pdf file.
Update 8 months later March 2019
I decided to try bottom balancing my cells today so I discharged them by running my TV until the pack cut off. It only gave me 5 amps for 5 hours so 25 Ah instead of 50Ah.!!
Looking at the cells as I was checking voltages I noticed one cell that kept falling fairly fast even though no current was being taken out as everything was disconnected . OOPS . The cell was the negative take off point for the BMS. The Negative terminal . All electrons exit from this terminal.
Not good . It looks like this particular cell has some sort of self discharging problem and is not holding its capacity and is pulling the pack down early . I will have to replace that cell . I hope no more go the same way . If they are stuffed at 8 months then they are useless as cells.
Will Prowse video on bottom balancing
This is what I asked SMK solar controller.... email@example.com
In the advertising for this product on Ebay you state that it is
suitable for Lead Acid type batteries and also Lithium . You mention "user settings" also.
I have studied this model in use and there is no Lithium setting at all .
There is a user setting but it also uses the bulk, float, absorption and Equalise algorithm which is not at all suitable for Lithium.
I have a Lithium Iron Phosphate battery of 24 volts . It needs a constant current charge of 20 Amps until the cells reach 3.4 Volts/cell then a constant voltage of 3.4 Volts/cell until the current taken drops below 5 amps.
There is no such setting on your product . How may I attain that charge cycle?
Step 8: This Is Some Info Added on About Using Aluminium Can Type Containers Around the Cell As Opposed to Plastics
Quote.... Aluminum Encased Batteries Have TWICE the Life of Plastic Encased Batteries
Measuring the life expectancy of a lithium battery is difficult to pinpoint. So we thought that we would like to expound on the subject to help an anticipated lithium user understand the issue and conclude how to get the maximum life from their battery pack. Understanding Charge Cycle Life – Lithium vs. Lead Acid
Typically the life of any battery is measured in the number of charges the battery has before it deteriorates to a point where it can only hold 80% of its capacity when it was new. This number is called the batteries ‘Charge Cycle Life’. As a comparison I would like to start with a lead acid battery. Lead acid batteries have a charge cycle life of between 350 charges all the way up past 600 charges. With the lower charge cycle life usually found in car starter batteries and the higher charge cycle life found in laboratory or solar storage applications. But here is where things can become a little misleading. The honest way of measuring a battery’s life can be manipulated. Because of the Peukert effect which exists on all lead acid batteries you cannot discharge more than about 55% of the batteries new 20 hour capacity rating. A 100 Amp Hour ‘Ah lead acid battery will only yield about 55 Ah before it is considered fully discharged. Lithium batteries are not affected by Peukert’s Law to the extent that lead acid batteries are so you can discharge a lithium battery down to 20% of its full State of Charge ‘SOC’ before the battery is considered discharged. With many lithium batteries if the discharge is more that 80% DOD it will not hurt the batteries life but is generally accepted that the lithium battery is fully discharged at 80% DOD. To run honest tests to determine the exact life of a lithium battery the battery must be fully charged and then fully discharged to 80% DOD. Charging and discharging thousands of times takes a lot of time therefore most companies will do it for a number of complete cycles and then extrapolate the remaining life expectancy based upon some typical known curves for the chemistry of the battery. Lithium Titanium Cells and Polymer Battery Drawbacks
Lithium Titanium cells have the longest life. But they are 3 to 5 times more expensive than the common Lithium Iron Phosphate ‘LiFePO4’ battery. Lithium Nickel Cobalt Magnesium (also called a polymer battery) or NCM batteries exhibit about half the life of the LiFePO4 cells. NCM batteries are lighter in weight and lower in cost and are often seen in electric vehicle applications where cost and weight are paramount. How Companies Cheat on Charge Cycles
Some companies cheat and they say that their LiFePO4 batteries will last over some ridiculous number - like 8000 charge cycles. Those numbers are not realistic if they fully discharge the battery down to 80% DOD, which they do not. However, there are several other factors that play into the battery life scenario. One of them is the case of the battery. When batteries charge or discharge they create heat. Heat that is trapped inside the battery will cause the battery life to go down. A plastic encased battery of a given Ah rating will have a shorter life than a metal encased cell. That is because the plastic is a poor thermal conductor. The smaller the Ah rating of the battery the easier it is for the battery stack to dissipate the heat away from the core of the stack and therefore will have a longer life. Honest manufactures of LiFePO4 plastic encased batteries which I call a prismatic cell will normally claim that their cells will have more than 2000 charge cycles. A prismatic plastic encased battery is illustrated in the graph that is in this dissertation. Why Aluminum Encased Lithium Batteries Perform Better
A typical aluminum encased battery that has better heat dissipation will start out higher. Dependent upon the cell of the size of the cell the expectant charger cycle life of a smaller LifePO4 cell can be as much as 4000 charge cycles and follow the same trajectory of the plastic prismatic cell shown below. The simple fact is that if you have an application where you want a long life, like for solar storage, and you do not discharge the cells below their nominal 3.2 voltage you will have a cell that will probably outlive you. Depth of discharge has a very profound effect on a lithium battery’s life.
What is the Life Expectancy of the Aluminum Cased Batteries?
- Factor 1: Depth of Discharge…Percentage of Capacity Used Per Cycle.
- DOD, short for the Depth of Discharge, is used to describe how deeply the battery is discharged. If we say a battery is 100% fully charged, it means the DOD of this battery is 0%, If we say the battery have delivered 30% of its energy, here are 70% energy reserved, we say the DOD of this battery is 30%.
-Factor 2: Discharge Rate, The AMPS Divided by the Capacity - To Read the Chart Above, Here are Some Examples
A. All Numbers are at a 1C Discharge Rate, Which is Aggressive…
B. At a 90% Depth of Discharge, Your Batteries Will Provide 2400 Cycle to 80% State-of-Original-Capacity (SOIC)
C. At a 10% Depth of Discharge (Say, Engine Starting) Your Batteries Will Provide 35,000 Cycles Until 80% SOIC. Beyond 35,000 Cycles the Battery is Still Functioning But Gradually Losing Capacity, but not Performance
D. Conclusion: By recharging a lithium ion battery more frequently, thus reducing the DOD (depth of discharge), the battery cycle life is Increased. Solar is very often a beneficial method to reduce DOD and increase life!
Step 9: Bottom Balancing
I found my cells occasionally got a bit too high and triggered an alarm on the Celllog8 I monitor the cells on.
I also noticed my battery pack capacity seemed to be lower than it should be so I decided to try a bottom balance this time .
I drained my battery pack until one of my automatics cut it out and then balanced from there .
Because one cell dropped more quickly than the rest to 2.5V that was enough to trigger the pack off via the controller in my case . I have a cut off at 25 volts on the charger , 24.4 volts on the Inverter, and 20 Volts on the BMS and the alarm on the celllog8 is set to 24.5V ....So in my case I started a bit high .
When I got the cells separated I found I had cells from 3.2 V to 2.8 V . I discharged any over 3.0V down to 3.0V which got me into the knee of the cell discharge curve above ( or the waterfall I prefer to call it ) then made up the connections to join the cells in parallel as in the first picture. I then used the existing connectors to join two cells together in parallel ..see second picture . Then I connected the cut wire across the pairs as per the 2nd pic.
Now I could watch the current flow with a current meter . You can see the reading on the pic. It started at 5 amps max and was down to 4 amps in about 5 minutes and it just kept dropping from there . I monitored the current flow between each pair until the current had dropped to 0.005 amps (5 milliamps) max in any wire and then disconnected everything and reassembled the pack as usual . This was over at least 48 hours.
I think it pays to number each cell to match the celllog8 numbers and put some tape on the BMS and celllog8 sense wires so they don't get mixed up. In my case I just worked through from the positive red wire on the BMS loom and connected each of the blacks in order and with the celllog8 it has a black negative lead and then connect all the reds in order .
I'll update as I watch it but all seems well at this point . The pack is charging back up.