Charging/discharging your battery and keeping it balanced

Charging batteries is usually something easy in every domain. You plug your charger to the wall, plug your device to your charger and you're done. When building your own esk8, you are in charge of understanding how the battery works and how to charge it properly.

This is a quick overview guide, always refer to your charger manual and battery manual before charging a battery. Never leave your charging battery unattended.

Keeping the battery balanced

As you probably know by now (if you read this page), the battery in electric skateboards are composed of several cells, some in parallel, some in series. Those cells need to stay balanced: they need to all have the same voltage. But when charging or discharging the battery, the cells in series can go off by a bit... and a bit... and a bit... And that's actually dangerous. This applies for any battery type (lipo and li-ion).

Take a 3S lipo battery with cell #1 at 3.6V, cell #2 at 3.9V and cell #3 at 3.6V. When charging that battery, the charger will charge it up to the full capacity: 3 x 4.2V = 12.6V. The charger would charge all cells equally until the entire battery is 12.6V. Problem: the battery will end up with (approximately) cell #1 at 4.1V, cell #2 at 4.4V and cell #3 the 4.1V. The cell #2 is now over charged when the battery pack reaches 12.6V. And an overcharged cell can lead to a fire or explosion!

That's why you need to make sure the battery stays balanced (read more on that). It also helps to keep each the cells healthy (no over-charge or over-discharge) and that means more charge cycles.

But how does it get balanced then?

There are mainly 2 ways to "automatically" keep the battery balanced:

  • using a BMS
  • using a balance charger

The BMS is what you'll find in all esk8 in the market (and other products as well!). A balance charger is much more painful to use.

1. Battery Management System (BMS)

A BMS is a small electronic component that stays connected to your battery at all times. On one end, you have a charging port, on the other you have the battery, connected via the + and - wires and the balance wires. More on balance wires further down the page.

The BMS will continuously check each cell voltage via the balance wires. If a cell goes off by a bit, it'll adapt the charge or discharge to make sure it stays balanced with the others. Most of BMS are built for a specific number of cells; if you have an 8S battery, you need an 8S BMS. They are quite small and are embedded directly with the battery on your electric skateboard.

A BMS for a 10S battery
A BMS for a 10S battery

During the charge, the BMS will charge all cells equally. Then, if a cell has reached its full capacity, it will only charge the other cells that aren't fully charged yet via the balance wires.

A BMS can be used for charge only or charge & discharge. In the first case, the BMS will only balance cells when charging the battery. The later will balance cells during charge and discharge, making it even safer for the battery.

Why charge only? Some BMS won't allow enough current to flow through during discharge, leading to poor performance (slow speed, low torque), over-heating BMS and sometimes even BMS cutoff, meaning the entire board will be turned off. And you absolutely don't want that happening while riding your esk8! Very often, balancing while discharging isn't really needed, so using a BMS only for charging is a good compromise. It's also cheaper, as you don't have to build a BMS that can handle high rate discharge, and charging is a slow process anyway. Less energy flowing means smaller, "weaker" components that cost less then their beefier equivalent.

Charging a battery with a BMS

This is actually quite simple. The BMS is the intermediate between the DC charger and the battery and does everything automatically. One charger, one cable, you plug it in, you wait, you unplug, you're ready.

That's how a lot of batteries for mainstream products work. A laptop battery has the same, a cordless drill has the same, etc...

When building you own esk8, you have to wire your battery to the BMS and find a charger that's the right voltage for your battery, and put a charging port somewhere. Once you have everything setup and wired up, it's as easy as a pre-built eboard.

2. Balance Charger

Balance chargers are programmable for different type of batteries, different capacities and different number of cells. They're often used to charge Lipo batteries.

balance charger
A programmable balance charger

It's a bit more annoying to setup since you have to unplug your battery from your esk8 and plug it in to the charger (+ and -, and the balance wires) each time you need to charge it. If you're running 5 2S lipo batteries, it's gonna take some time.

Balance chargers come in 2 types:

  • AC to DC balance chargers; those are easy to use, you plug it into the wall, plug your battery, program it and you're done.
  • DC to DC balance chargers; they are cheaper but they require an external power supply to convert AC to DC.

Power supply for a DC to DC balance charger

They are different power supply you can use, this forum post is a good place to start (even though it's a bit old).

First, you need to check what voltage your charger is accepting (usually from 11V to 18V).

Then, calculate how much power you need to charge the battery. A common practice is to charge your lipo batteries at 1C or lower (some batteries can be charged at a higher rate, but in general, slower is better). Let's take a 5S 4500mAh battery. Charging at 1C means charging the battery at 4.5A. It's a 5S battery, maximum voltage at full charge is 18.5V. So 4.5 x 18.5 = 83.25 Watts.

Let's say our charger is 70% efficient. So for it to output 83.25W, it'll need 83.25 x 100 / 70 = 118.93W on input.

Now it depends on what voltage your power supply is providing. A lot of power supplies provide 12V DC, so it would need to output at least 118.93 / 12 = 9.91A. If you have a 18V power supply, it'll need only 118.93 / 18 = 6.61A. That's in theory, real-life numbers will vary. And this is if you want to charge your battery at 1C, you can go lower without any problem. It will just take longer to charge the battery.

If your balance charger can do parallel charge, you'll have to double the values (2 batteries = double charge power required).

If you want to go cheap, you can buy a laptop charger. If you're good with your hands, you can buy an old desktop computer power supply, add a few plugs to it and voila (desktop power supplies have several 12V outputs). Again, check the forum post to look at the different solutions.

How to use a balance charger

We're not going into details here as each balance charger is a bit different.

This is a quick overview guide, always refer to your charger manual and battery manual before charging a battery. Never leave your charging battery unattended.

Basically, you need to connect your balance charger to its power input (either an AC-DC power supply or a wall power plug if your charger allows it).

You'll sometime get a balance board with your charger. Some balance boards only have connectors for the battery balance plug, some bigger ones have plugs for the battery power connectors too.

Connect the battery power plugs to the charger or the balance board.

And connect the battery balance plug to the charger or balance board.

Battery connected to balance charger via balance board
The battery balance plug is connected to the charger via a balance board

Power on your charger and let the charge begin.

This is a quick overview guide, always refer to your charger manual and battery manual before charging a battery. Never leave your charging battery unattended. Try to put your charging battery far from flammable objects.

Balance Wires

If you're curious about balance wires, here's a bit more info.

In this first example, I took a Turnigy nano-tech 5S lipo battery.

What you see below is a schematic of how cells are connected to the balance wires and the main wire (XT90 plug).

5S Battery with balance wires schematics
Schematics of a 5S lipo battery with its balance wires

As you can see, there's a balance wire for each positive side of the cells, plus one extra at the end for the global negative side. The cell numbering could have been done the other way around, it doesn't matter much at this point.

With those balance wires, it's easy to check the voltage of each cell, but also individually charge cells as well. Want to know the voltage of cell #3? Measure it with the gray wire (+) and yellow wire (-).