The charging of lithium-ion battery backs, typically found in DIY electric skateboards and bikes, is typically done with a balance charger, like an IMAX B6. These chargers can be set-up for a variety of battery packs, but they typically require an external power supply, and are bulky and cannot charge very quickly.
This instructable is going to go over a new battery charging system for electric skateboards. I'm going to cover how to connect an inexpensive BMS (Battery Management System) to an electric skateboard's battery pack, and how to convert an inexpensive, yet powerful power supply, into a battery charger, by putting a current limit on it.
This battery charging system can be adapted for electric bikes as well, and can even be used with pre-made laptop-brick style charging units, which are a very portable and convenient solution.
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Step 1: Watch the Video!
This video will go over the basics of this Instructable. It's not a step-by-step guide, but it will gloss over the basics of this system.
Step 2: Choose a BMS and Power Supply
Choosing a BMS
The best place to find a BMS for your LiPo pack is on eBay. They are typically around $16, and come in a variety of different cell configurations. You will need one that has a rating of however many LiPo cells you have in series. (I have 6 in series, so I will use a 6s BMS). It is very important that you choose a BMS with a "balance cell" feature. Some BMS units are incorrectly labeled, and are actually just a protection circuit.
These BMS boards typically have a current-cutoff as well. It does not matter what the current cut-off is. These current cutoffs go up until about 30 amps, which is still too low for electric skateboards, which can pull up to 100 amps. Go with the cheapest BMS that satisfies your LiPo cell count, and has a balance feature. Here's the link which I use to search with.
Choosing a Power Supply
These inexpensive power supplies on eBay are fitted with potentiometers, which can adjust the voltage range, give or take ~20%. You will need a power supply that can be set to a higher voltage than the fully charged voltage of your battery pack. For example, my 6s system is fully charged at 25.2 volts. A 24v power supply would be able to provide that 25.2 volts. Here's a list of typical power supplies, their voltage range, and the number of cells they can charge:
- 5 volt PSU (4.5v - 5.5v, 1S)
- 12 volt PSU (10.6v - 13.2v, 3S)
- 24 volt PSU (19.5v - 29.8v, 5S 6S)
- 48 volt PSU (43v - 53v, 10S 11S 12S)
You will need to choose the power supply that fits your cell count. Additionally, the voltage adjust potentiometer can be replaced to give you a bigger voltage range, however, it is not recommended to go above the max voltage because it can damage some of the capacitors. Going below the preset voltage can result in unintended operation. The wattage of the power supply affect the speed at which you can charge your batteries (more on that later).
Using a Preset Power Supply
It is also possible to use a preset power supply to charge your skateboard, which comes in the form of a laptop-style power brick. You will simply need to choose one that is for your particular cell count. These power supplies are typically cheaper on Amazon, but they do tend to have lower current-ratings.
Step 3: Connect the BMS to Your LiPo Cells
Connect your BMS to your battery pack by soldering the connections above. Typically, the eBay seller pages have a diagram which you can follow. However, since we are going to be overriding the current limit of the BMS, you can just solder your ESC wires directly to the batteries. If you want your system to be modular, you can also use gold-plated bullet connectors to connect everything together.
You are also going to need to solder some sort of connector to the input of the BMS. The cheapest option is here, for about $0.70. This kind of jack is used on the E-GO boards. However, I decided to go for something a bit more durable. It's advisable to use a female jack on the skateboard side in order to prevent accidental shorts.
Step 4: Change the Current Limit on the Power Supply
The first step to converting your power supply, is to set the voltage to the max voltage of your battery pack. I'm using a 6S battery pack, so I will set it to 4.2v * 6 = 25.2v.
Unfortunately, these Chinese power supplies are meant for audio applications, and are typically set to ~130% their rated constant output (in order to be able to handle bass-drums). Thus, if we were to plug the power supply into the BMS, the battery would draw current proportional the the total resistance * difference in voltage (AKA that would melt the power supply in a short time).
Thus, we need to go inside the BMS, and find a way to lower the current limit just a little bit. I reverse-engineered part of the circuit near the control chip, and I found that adding a 10k ohm potentiometer to the pins of R35 would allow me to adjust the current. The potentiometer is very sensitive, but it seems to be working well. This resistor seems to be the same on other power supplies, and it is commonly known as the "R33 mod" on the genuine MeanWell power supplies.
I soldered the potentiometer with some very long wires, and brought it outside the power supply so it could be adjusted later.
Step 5: Assemble the PSU (add a Case, Switch, and Ammeter)
In order to make this power supply more into a genuine-looking charger, I decided to add an LED toggle switch, and an ammeter (current meter) according to the schematic above. Both of these items are very inexpensive! I did not use any connections this time, and I just soldered everything according to that diagram. You have to take care when connecting the ammeter, current must only flow in one particular way, otherwise it will short-circuit with the power input!
I also added the power plug that I cut from an old computer cord. I suggest you choose a plug with an earth connection, as it will safely ground the power supply in the event of an accident.
Then I cut using a jigsaw, and hot-glued some high-density foam together to make a nice box for my power supply. It seems to get hot even when the batteries are not being charged, so I also drilled some large holes for ventilation above (just a precaution really, I don't think I actually needed them). I glued the current-setting potentiometer to the side of the case, and used heat-shrink tubing to cover all of the contacts up.
Step 6: Calibrate the Voltage and Current
By now, you should have already calibrated the voltage, but just in case, double-check before continuing. Plug the power supply into the skateboard/bike, and flip the on switch. If you have an ammeter, it should be displaying some number. If you don't have one, you will need to connect an ammeter between the skateboard and PSU.
Then use a screw-driver to set the current limit using the potentiometer. Do not set it to the rated current of your power supply. You will need to take the wattage of your power supply (mine was 120 watts), and divide it by the max voltage of your batteries (120 watts / 25.2v = ~4.76 amps). You will then get the current limit that you should set it to.
Don't worry if your power supply goes over it's current limit for a minute or two. It will simply produce more heat than is desired. You can use the potentiometer to bring the current limit down to 0.
Step 7: Enjoy!
And that's it! Enjoy your new speedy charger! In order to roughly calculate how much time it will take to charge, take your battery capacity, and divide it by the current limit you set. Your anwser will be how long it will take to charge in hours. (Keep in mind, this is from when each cell is 3v. Your actual charge time will actually be less if you only discharge to nominal voltage).
Also, a note about using the LED button. The LED light is power by the batteries when you are connected to the PSU without the PSU getting power. That means that the LED will slowly drain your batteries (but don't worry, the BMS will stop them from draining completely!) The LED only light up when you have the batteries connected, or when you have turned the switch on.