Introduction: Listrik L585 585Wh AC DC Portable Power Supply
For my first Instructable, I'm going to show you how I made this portable power supply. There are many terms for this kind of device like power bank, power station, solar generator and many other but I prefer the name "Listrik L585 Portable Power Supply".
The Listrik L585 has built-in 585Wh (6S 22.2V 26,364mAh, tested) lithium battery which can really last. It's also quite lightweight for the given capacity. If you want to compare it with typical customer power bank, you can do it easily by dividing the mAh rating by 1,000 then multiply it by 3.7. For example, the PowerHouse (one of the biggest well-known consumer power bank) has capacity of 120,000mAh. Now, let's do the math. 120,000 / 1,000 * 3.7 = 444Wh. 444Wh VS 585Wh. Easy isn't it?
Everything is packed inside this nice aluminum briefcase. This way, the Listrik L585 can be carried easily and the top cover will protect the sensitive instruments inside while being unused. I got this idea after I saw someone built a solar generator using tool box, but tool box doesn't look that great, right? So I kicked it up a notch with aluminum briefcase and it looks much better.
The Listrik L585 has multiple outputs that can cover nearly all consumer electronic devices.
The first one is AC output which is compatible to almost 90% of mains devices under 300W, not all of them due to non-sinusoidal output but you can fix this by using pure sine wave inverter, which is much more expensive than the standard modified sine wave inverter I used here. They're generally bigger too.
The second output is USB output. There are 8 USB ports, which kinda overkill. A pair of them can deliver maximum current of 3A continuous. Synchronous rectification makes it very efficient.
The third one is auxiliary I/O. It can be used to charge or discharge the internal battery at maximum rate of 15A (300W+) continuous and 25A (500W+) instantaneous. It doesn't have any regulation, basically just plain battery voltage but it does have multiple protections including short-circuit, overcurrent, overcharge and overdischarge.
The last one and my favorite one is adjustable DC output, which can output 0-32V, 0-5A on all voltage range. It can power very wide variety of DC appliances like typical laptop with 19V output, internet router at 12V and much more. This adjustable DC output eliminates the need to use AC to DC power supply, which by the way will worsen the efficiency because the whole system convert DC to AC then to DC again. It can also be used as bench power supply with constant voltage and constant current function, which is very useful for people like me who often work with electronics.
Step 1: The Materials and Tools
Main materials:
* 1X DJI Spark aluminum briefcase
* 60X 80*57*4.7mm prismatic lithium cells (you can substitute with more common 18650, but I found this cell to have just the perfect form factor and dimension)
* 1X 300W 24V DC to AC inverter
* 1X DPH3205 programmable power supply
* 2X 4 port USB buck converters
* 1X Cellmeter 8 battery checker
* 1X 6S 15A BMS
* 1X 6S balance connector
* 12X M4 10mm bolts
* 12X M4 nuts
* 6X stainless steel brackets
* 1X 6A single pole toggle switch
* 1X 6A double pole toggle switch
* 1X 15A single pole toggle switch
* 4X 3mm stainless steel LED holder
* 4X female XT60 connectors
* 4X M3 20mm brass spacers
* 4X M3 30mm machine screws
* 2X M3 8mm machine screws
* 6X M3 nuts
* 1X 25A 3 pin terminal
* 4X 4.5mm cable spades
* Custom cut 3mm instrument panel
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Consumables:
* Heatshrinks
* Solder
* Flux
* 2.5mm solid copper wire
* Heavy duty double-sided tape (get the highest quality one)
* Thin double-sided tape
* Kapton tape
* Epoxy
* Black paint
* 26 AWG wire for LED indicators
* 20 AWG silver stranded wire for low current wiring
* 16 AWG silver stranded wire for high current wiring (lower AWG is preferred. Mine is rated at 17A continuous chassis wiring, just barely enough)
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Tools:
* Soldering iron
* Plier
* Screwdriver
* Scissors
* Hobby knife
* Tweezer
* Drill
Step 2: The Schematic
The schematic should be self-explanatory. Sorry for the poor drawing, but it should be more than enough.
Step 3: The Instrument Panel
I designed the instrument panel first. You can download the PDF file for free. The material can be wood, aluminum sheet, acrylic or anything with similar property. I used acrylic in this "case". The thickness should be 3mm. You can CNC cut it, or just print it on paper with 1:1 scale and cut it manually.
Step 4: The Case (Painting and Mounting Brackets)
For the case, I used an aluminum briefcase for DJI Spark, It has just the right dimension. It came with foam thingy to hold the aircraft so I took it out and painted the inner part black. I drilled 6 4mm holes according to the hole distance on my custom cut instrument panel and installed the brackets there. Then I glued M4 nuts on each brackets so I can screw the bolts from the outside without holding the nuts.
Step 5: The Battery Pack Part 1 (Testing Cells and Making Groups)
For the battery pack, I used rejected LG prismatic lithium cells I got for less than $1 each. The reason why they're so cheap is just because they have blown fuse and tagged as faulty. I removed the fuses and they're good as new. It might be a bit unsafe but for less than a buck each, I can't really complain. After all, I'll use a battery management system for the protections. If you're going to use used or unknown cells, I have a good Instructables on how to test and sort used lithium cells here: (COMING SOON).
I've seen a lot of people using lead-acid battery for this kind of device. Sure they're easy to work with and cheap but using lead-acid battery for portable application is a big no-no for me. A lead-acid equivalent will weighs about 15 kilograms! That's 500% heavier than the battery pack I made (3 kilograms). Should I remind you that it'll be bigger in volume too?
I bought 100 of them and tested them one by one. I have the spreadsheet of the test result. I filtered it, sorted it and end up with the best 60 cells. I divide them equally by the capacity so each group will have similar capacity. This way, the battery pack will be balanced.
I've seen a lot of people built their battery pack without further testing on each cell, which I think is mandatory if you're going to make a battery pack out of unknown cells.
Test showed that the average discharge capacity of each cell is 2636mAh at 1.5A discharge current. On lower current, the capacity is going to be higher due to less power loss. I managed to get 2700mAh+ at 0.8A discharge current. I'll get an extra 20% more capacity if I charge the cell to 4.35V/cell (the cell does allow 4.35V charge voltage) but the BMS doesn't allow that. Also, charging the cell to 4.2V will prolong its life.
Back to the instruction. First, I joined 10 cells together using thin double-sided tape. Then, I reinforced it using kapton tape. Remember to be extra careful when dealing with lithium battery. These prismatic lithium cells have extremely close positive and negative part so it's easy to short one.
Step 6: The Battery Pack Part 2 (Joining the Groups)
After I finished making the groups, the next step is to join them together. To join them together, I used thin double-sided tape and I reinforced it with kapton tape again. Very important, make sure the groups are isolated from each other! Otherwise, you'll get a very nasty short circuit when you solder them together in series. The body of the prismatic cell is referenced to the cathode of the battery and vice versa for 18650 cells. Please keep this in mind.
Step 7: The Battery Pack Part 3 (Soldering and Finishing)
This is the hardest and most dangerous part, soldering the cells together. You'll need a soldering iron that's at least 100W for easy soldering. Mine was 60W and it was a total PITA to solder. Don't forget the flux, a hell ton of flux. It really helps.
** Be extremely careful at this step! High capacity lithium battery isn't something you want to be clumsy with. **
First, I cut my 2.5mm solid copper wire to the desired length then peel off the insulation. Then, I soldered the copper wire to the cell's tab. Do this slow enough to let the solder flow, but fast enough to prevent heat buildup. It really requires skill. I'd recommend to practice on something else before you try it with the real thing. Give the battery pack a break after several minutes of soldering to cool down because heat isn't good for any kind of battery, especially for lithium battery.
For finishing, I sticked the BMS with 3 layer of double-sided foam tapes and wire everything according to the schematic. I soldered cable spades on the battery's output and immediately installed those spades to the main power terminal to prevent the spades from touching each other and causing a short.
Remember to solder a wire from the negative side of the balance connector and a wire from negative side of the BMS. We need to break open this circuit to deactivate the Cellmeter 8 (battery indicator) so it won't turn on forever. The other end goes to one pole of a switch later.
Step 8: The Battery Pack Part 4 (Installation)
For the installation, I used double-sided tape. I recommend to use high quality, heavy duty double-sided tape for this case because the battery is quite heavy. I used 3M VHB double-sided tape. So far, the tape holds the battery pack very good. No problem whatsoever.
The battery pack fits really nice there, one reason why I picked this prismatic lithium cell over cylindrical lithium cell. The airgap around the battery pack is very important for heat dissipation.
About heat dissipation, I'm not concerned too much about it. For charging, I'll use my IMAX B6 Mini which can only deliver 60W. That's nothing compared to the 585Wh battery pack. Charging took more than 10 hours, so slow that no heat is generated. Slow charging is also good for any kind of battery. For discharging, the maximum current I can draw from the battery pack is well below 1C discharge rate (26A) at only 15A continuous, 25A instantaneous. My battery pack has around 33mOhm internal resistance. Dissipated power equation is I^2*R. 15*15*0.033 = 7.4W of power lost as heat at 15A discharge current. For something this big, that's not a big deal. Real world test shows that at high load, the temperature of the battery pack rise to around 45-48 degree Celsius. Not really a comfortable temperature for lithium battery, but still within the working temperature range (60º maximum)
Step 9: The Inverter Part 1 (Disassembling and Heatsink Installation)
For the inverter, I removed it from the case so it'll fit inside the aluminum briefcase and installed a pair of heatsinks I got from a broken computer power supply. I also took the cooling fan, the AC socket and the switch for later use.
The inverter works down to 19V before the undervoltage protection kick in. That's good enough.
One unusual thing is that the labeling clearly says 500W while the silkscreen on the PCB says it's 300W. Also, this inverter has real reverse polarity protection unlike most inverters out there which use dumb diode + fuse contraption for reverse polarity protection. Nice, but not very useful in this case.
Step 10: The Inverter (Installation and Mounting)
First, I extended the input power, LED indicators, the switch and the AC outlet's wire so they're long enough. Then, I installed the inverter in the case using double-sided tape. I soldered cable spades on the other end of the power input wires and connected those to the main terminal. I mounted the LED indicators, fan and the AC outlet to the instrument panel.
I found that the inverter has zero quiescent current (<1mA) when connected to power source but deactivated so I decided to connect the inverter's power wire directly without any switch. This way, I don't need a bulky high current switch and less wasted power on the wire and switch.
Step 11: The USB Module (Installation and Wiring)
First, I extended the LED indicators on both modules. Then, I stacked the modules with the M3 20mm brass spacers. I soldered the power wires according to the schematic and put the whole assembly to the instrument panel and tied it with zip ties. I soldered the 2 wires from the battery I mentioned earlier pack to the other pole of the switch.
Step 12: The DPH3205 Module Part 1 (Installation and Input Wiring)
I drilled 2 3mm holes through the bottom plate diagonally and then I installed the DPH3205 module with 8mm M3 screws which go through those holes. I wired the input with thick 16 AWG wires. The negative goes straight to the module. The positive goes to a switch first then to the module. I soldered cable spades on the other end which will be connected to the main terminal.
Step 13: The DPH3205 Module Part 2 (Display Mounting and Output Wiring)
I mounted the display to the front panel and connected the wires. Then, I mounted the XT60 connectors to the instrument panel using two part epoxy and wired those connectors in parallel. Then the wire goes to the module's output.
Step 14: The Auxiliary I/O (Mounting and Wiring)
I mounted 2 XT60 connectors with 2 part epoxy and soldered the connectors in parallel with thick 16 AWG wires. I soldered cable spades on the other end which go to the main terminal. The wire from the USB module also goes to here.
Step 15: QC (Quick Inspection)
Make sure that there's nothing rattling inside. Unwanted conductive items can induce short circuit.
Step 16: Finishing and Testing
I closed the cover, screwed the bolts and done! I tested every functions and everything works as I hoped. Definitely very useful for me. It cost me slightly over $150 (material only, not including failures), which is very cheap for something like this. The assembling process took around 10 hours, but the planning and research took around 3 months.
Even though I have done quite a lot of research before I build my power supply, my power supply still has many flaws. I'm not really satisfied with the result. In the future, I will build the Listrik V2.0 with a lot of improvements. I don't want to spoil the whole plan, but here's some of it:
- Switch to high capacity 18650 cells
- Slightly higher capacity
- Much higher output power
- Much better safety features
- Internal MPPT charger
- Better material selection
- Arduino automation
- Dedicated parameter indicator (battery capacity, power drawn, temperature and so on)
- App controlled DC output and many other which I won't tell you for now ;-)
Step 17: Updates
Update #1: I added a manual override switch for the cooling fan so I can turn it on manually if I want to use the power supply at full load so the parts inside will stay cool.
Update #2: The BMS caught on fire, so I remake the whole battery system with a better one. The new one boast 7S8P config instead of 6S10P. A bit less capacity but better heat dissipation. Each group are now spaced for better safety and cooling. 4.1V/cell charge voltage instead of 4.2V/cell for better longevity.
