Parallel charging allows you to charge multiple LiPo batteries at once using a single charger. Parallel charging is a more effective and faster way of charging your batteries, and it allows you to go out and fly more often, which is what this is all about.
If you are new to LiPo batteries used on quadcopters, make sure to check out our guide first: Quadcopter LiPo Battery Explained.
Why Parallel Charge LiPo Batteries?
Charging your batteries in parallel is nearly always faster than charging them individually. Parallel charging is also convenient as it saves you from having to unplug one battery to connect another to the charger.
Apart from convenience and time saving, there is also benefit to the batteries and the 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 become easier to charge (according to Ohm’s Law, if you connect two identical resistors in parallel, the total resistance halves).
The best way to parallel charge your battery is by using dedicated “Parallel Charging Boards” (para boards). These boards allow you to connect and charge multiple LiPo batteries simultaneously.
Good quality para boards are equipped with fuses which mean they cut off the connection if there is a problem. This is an essential safety feature that minimizes the risk of damaging your battery and causing a fire.
Before You Parallel Charge, Read This
So parallel charging is a great way to charge your LiPo, but it’s also more prone to error than charging a single battery. Please do your research and ensure you charge your batteries safely.
Here are some important points you should understand and follow.
1. Safety and Precaution
It doesn’t matter how you are charging your LiPo, always keep a fire extinguisher near your charging station. More importantly, always use a metal box (or Ammo box) to store your batteries while charging which can minimize the chance of fire spreading.
2. Same Cell Count
Your batteries have to have the same cell count. For example, you should only charge 2S batteries with 2S, 3S with 3S and so on.
3. Capacity and C-Rating
We recommend only parallel charging batteries of the same capacity and C Rating to ensure maximum safety (preferably the same brand and model).
However slightly different capacities within a reasonable range should be okay. For example, you might be able to get away from parallel charging a 4S 1500mAh with a 4S 1600mAh.
Slight differences in C-Rating should also be okay, some LiPo batteries advertise that they can be charged at a higher rate than 1C, but we still only recommend parallel charge Lipo batteries at 1C or lower to minimize risk.
4. Voltage Level
Voltage in a battery is like water in a barrel, if you connect 2 barrels together, water from the fuller one will flow into the emptier until the 2 barrels have the same amount of water. If you have 1 full barrel and 1 empty, the water will flow with the weight of a full barrel behind it which could burst a pipe.
Likewise, before connecting the batteries together, you should always check if their voltages and ensure they are at a similar voltage level. They don’t have to be the exact same voltage, small differences are allowed, generally a difference within 0.1V is acceptable.
When connecting multiple batteries together, the voltage difference between them will be neutralized. But if the voltage difference is too large, it will cause a high current to flow between battery packs as they try to equalize, like bursting a pipe, this could burn your parallel board, the battery wires, or even the batteries.
If you have a battery with vastly different voltage, you should isolate it and charge it alone.
5. Charge Current
When charging a single LiPo pack, I usually just charge at 1C, and the charge current can be calculated as
Current = 1C x Capacity.
For example, to charge a 1800mAh pack, I will use 1.8A; Or for an 800mAh pack, I use 0.8A. You can choose to charge at higher C rate if your batteries allow, but it increases the risk.
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 2000mah batteries, the total capacity is now 6000mAh, and I can charge them at 6A.
How to Parallel Charge LiPo Safely
Now you understand the requirements for parallel charging, let’s talk about how to do it.
Getting a Decent Para-Board
Firstly you will need get a parallel charging board (or para-board). Normally parallel charging boards have enough slots to connect 4 to 6 batteries. Each battery has one discharge lead (main plug), and one balance plug, you need to insert both connectors into the para-board.
The following para-board is the most basic model and it’s the cheapest one. It’s just basically just a PCB with connectors and wires soldered on.
I would personally recommend using para-boards with built-in fuses like the one below. They cost a bit more, but the extra safety is well worth it.
Here are some good ones:
How to use para-board?
When charging, I always put the para-board and batteries in a metal box (or other fire-proof container of your choice), this helps reduce the risk of fire.
Although LiPo burns up in flame while charging is very uncommon, and I have been using LiPo batteries for over 5 years now and I have never had a single fire during charging, still, better safe than sorry right?
You must make sure that the power connectors on your para-board match the ones on your batteries (the ones in the picture are XT60). If your batteries have a mix of different types of connectors (e.g. JST, T-connector), you can just get adapters for them instead of a whole new board.
Getting a voltage checker
Another must-have tool for parallel charging is a voltage checker, which allows you to check the voltage of each cell in a lipo battery simultaneously. I personally use these and really like them:
Calculate Max Charging Current
Nowadays, smart chargers can automatically adjust the charging current for you, depending on the maximum power of the charger and battery voltage.
If your charger doesn’t have this feature, however, you might want to manually calculate the safe charging current before connecting the batteries.
Assuming that I was using the 50W Turnigy Accucel-6 charger, and I have 3 x 1300 mAh 3S LiPo batteries.
Now if I want to charge my batteries at 1C, which is 3.9A. But can my charger support this current? Let’s find out.
The maximum charge current of the charger can be calculated using this equation
I = P/V
But as you might be aware, battery voltage increases during charging, and the current should decrease as power stays as a constant value. It’s not ideal to manually change charge current all the time, so it’s probably easier to just set the lowest charge current, which is when voltage is at the highest (i.e. 4.2V per cell – for 3S this is 12.6V and for 4S this is 16.8V).
In our example, if we want to set the charge current at 3.9A, the power required would be:
12.6V x 3.9A = 49.14W
And Luckily, our charger is rated for 50W, so it’s all good!
But what happens if we had 4 x 1300 mAh 3S batteries, can we still charge at 1C? Let’s do the same calculations:
Total charge current at 1C: 1.3A x 4 = 5.2A
Power required: 5.2A x 12.6V = 65.52W
As you can see, the power requirement in this case exceeds the power rating of the charger, therefore we cannot charge them at 1C.
What happens if we still set it to charge at 5.2A despite this? The charger could overheat and blow up! To avoid burning the house down, we should select a lower charging current that requires lower than 50W, i.e. 50W / 12.6V = 3.968A.
Most smart LiPo chargers these days can automatically calculate this for you, and it will set an upper limit to the charge current. Even if you set it to a higher value, it won’t go above that limit. Therefore I recommend everyone to spend a little extra to get a smart charger, it’s just so much safer and easier!
Tips on Plugging in Batteries to Para-Board
Remember always connect the discharge leads first, and the balance leads after.
When you first connect the batteries together, a difference in voltage will cause a current flow before they are equalized, the larger the voltages, the higher the current that will flow.
The balance leads are not rated for high current and could easily overheat and get damaged. The current surge could even melt the traces on the parallel charging board (it happened to me a while ago as shown in the following photo).
Always balance charge when parallel charging, and remember to plug in the balance leads on every pack so that every cell in every pack is balanced before, during and after charging.
Make sure you are plugging in the balance lead in the correct orientation. Even though they are designed so that you can only plug it in one way, it’s possible to make enough contact with the wrong pins to 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 multiple LiPo batteries are connected to a parallel charging board, they will be charged and balanced nicely. You might wonder how it works.
When they are connected in parallel, separate batteries become one big battery. As we mentioned earlier, the moment you connect them, there is a surge of current flow to equalize the voltage differences, and their voltages will become the same across all the packs.
But how does cell balancing work during charging? Similar to the above, the individual 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 have their voltages “evened out”, and again it acts like a single LiPo battery.
It’s not unheard of for a LiPo to catch fire while charging, especially when you have multiple batteries sitting next to each other, this can be very dangerous!
It is good practice to remain in the room where your batteries are charging, and always make sure that they are all disconnected from the charger before going out.
If you decide to go ahead with parallel charging, 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.
- Dec 2014 – Article created
- Oct 2016 – Updated equipment options
- Nov 2017 – Article revised