Parallel charging allows you to charge multiple LiPo batteries at once using a single charger. Parallel charging is also a more effective and faster way of charging your batteries.
Article first created in Dec 2014, last updated in Oct 2016.
Why Parallel Charging LiPo Batteries
Charging your batteries in parallel is nearly always faster than charging them individually. Even if your charger was not powerful enough to provide high current, parallel charging saves you from hooking/unhooking them up every charge cycle.
Apart from time saving, there is also benefit to the LiPo batteries and charger. The way I see it is, every battery has internal resistance, and the charger has to work hard to “pump” electricity into the battery. When you connect batteries in parallel, the total internal resistance is reduced, and the batteries becomes easier to charge (this is the Omh’s Law. For example, if you connect two identical resistors in parallel, the total resistance becomes half).
Because all the packs are hooked up as one big pack, you are actually charging them at a lower C rating. (Assuming you are charging them at a fixed amperage)
Furthermore, some decent para boards equipped with fuses which mean they might cut off the connection if there is a problem, to minimize the risk of damaging your battery or causing a fire.
Before You Parallel Charge, Read This
So parallel charging is an awesome way of charging your LiPo, but it’s also an advanced technique that requires some basic understanding of electricity.
Here are a few basic concepts and requirement that you need to understand first.
Cell Number – Your batteries have to be the same cell count number. You can only charge 2S battery with 2S batteries, and 3S with 3S and so on.
Capacity and C-Rating – Batteries connected in parallel may be of different capacities, but within reasonable range. For example 1000mAh pack can mix with 1500mAh. They can also be different C Rating (Lipo batteries can be charged at 1C safely)
Discharge State – The batteries should be at similar voltage level to be safe, but they don’t have to be at the exact same voltage, some tiny differences are allowed. Many people consider the difference within 0.1V is acceptable. The voltage difference will be neutralized when you plug them in. If the voltage difference is large, it would cause a very large current going between packs and could possibly burn your parallel board, the wires on the battery, or even the lipo itself.
Charge Current – When charging a single LiPo pack, I usually just charge them at 1C, which means the charge current = 1C * capacity. For example, for an 1800mAh pack, I charge it at 1.8A; and for an 800mAh pack, I use 0.8A. Of course you can choose to charge at higher C if your batteries allow.
The same principle applies when I charge multiple LiPo, the only difference is that the capacity is now the sum of all batteries connected. For example if I have 3 x 3S 2200mah batteries, the total capacity is now 6600mAh, and I can charge them at 6.6A.
How to Parallel Charge LiPo Safely
After you’ve understood what the requirements are for parallel charging, let’s talk about how to do it.
Getting a Para-Board
Firstly you will need get a parallel charging board (para-board) or adapter. Normally parallel charging boards have enough slots for 4 to 6 batteries. Each battery has one discharge lead (main plug), and one balance plug. You need to insert both connectors when parallel charging.
What I usually do is, I plug 2 or 3 batteries on each side, and put them into two separate LiPo Safe bags (3 packs each bag). They help reduce the risk of fire, although LiPo burns up in flame while charging is very uncommon. I have been using LiPo for over 5 years now and have never had a single fire incidence during charging. Anyway, better safe than sorry, right?
Depends on what kind of discharge leads your batteries have, you need to make sure they match the ones on the board (The ones in the picture are XT60 plug). If your batteries have a mix of different types of connectors (e.g. JST, T-connector), you could also get adapters for them instead of getting a whole new board.
Calculate Max Charging Current
Nowadays, smart chargers can automatically adjust charging current for you, depends on the maximum charging power it’s allowed and battery voltage. However if your charger doesn’t have this feature, you might want to manually calculate what the safe charging current limit is.
Assuming that I was using the Turnigy Accucel-6 charger (rated at 50 W) and I have 3 x 1300 mAh 3S LiPo batteries.
And I wanted to charge my batteries at 1C, which is 3.9A total charge current.
Charge current can be calculated by dividing power by voltage, I = P/V. But as you can see, voltage is changing all the time as the batteries get charged up, so will the current.
The voltage of each cell can reach as high as 4.2V during the charge cycle, which is 12.6V for a 3S pack.
The max power required is going to be: 12.6V * 3.9A = 49.14W. And Luckily, our charger is rated at 50W, so it meets our requirement, all good :)
However, what happens if I had four 1300 mAh 3S batteries, can I still charge at 1C?
1.3 A * 4 = 5.2 A
5.2 A * 12.6 V = 65.52 W
As you can see, I cannot charge 4 batteries at 1C in this case, as it will exceed the power limit of the charger.
Odds are, the charger might over heat and blow up if I insisted and set it to charge at 5.2A. To avoid burning my house down, I would have to select a lower charging rate: 50 W / 12.6 V = 3.968 A.
In reality, most smart chargers these days can automatically calculate this for you (including the Accucel-6), and will set a upper bound limit to the charger’s current. Even if you set it to a higher value, it won’t go above that limit. Therefore I would recommend everyone to get a charger that comes with this feature, it’s just so much safer and easier!
I am currently using the iSDT SC620 charger, and absolutely love it! (04/Oct/2016)
Tips on Plugging in Batteries to Para-Board
Remember to always connect the discharge leads first, and then the balance leads. Because when you first connect the batteries together, the difference in voltage will cause a large current flow before they are equalized. The balance leads are much thinner and it could get damaged more easily, as they don’t take as much current as the discharge leads. It could even melt the traces on the parallel charging board like this.
Always balance charge when doing parallel charge, and remember to plug in the balance leads on every pack so that every cell in every pack are balanced before, during and after charging. You might also notice the current going into each pack might be different at a given time, that’s because nearly every cell has different internal resistance and therefore some batteries can be charged up faster than others. But since you have the balance leads connected together, the difference should be equalized.
Also make sure you are plugging in the balance lead in the correct orientation. Even though they are designed the way that you can only plug it in one direction, I had been able to make enough contact with the wrong pins and cause sparks.
Before you start parallel charging, make sure you do your research, read up and understand how to do this correctly.
The Science Behind Parallel Charging
So when all the lipo batteries are connected to the parallel charging board, they will be charged up and balanced nicely. You might wonder how it works though.
When they are connected in parallel, their voltages will become the same. As we mentioned earlier, the moment you connect them, there is a surge of current flow to equalize the voltage differences. They have now literally become a “huge single lipo battery”, with capacity combined. So basically we are just charging up one big lipo pack instead of smaller, separate packs in a sense.
But how does cell balancing work during charging? Similar to the above, the cells are also connected in each battery with the balance leads, for example cell1 in battery1 is connected with cell1 in battery2. These cells will also have their voltages “evened out”, again it acts like a single lipo pack.
It’s not unheard of that LiPo catch fire while charging. It could get quite dangerous especially when you have multiple batteries sitting next to each other. If you decide to go ahead with this, you do so at your own risk. If you are feeling uncomfortable or unsure, simply do not attempt it. Hopefully this short guide gave you some insight into what parallel charging is, and how to do it safely.