Parallel charging allows you to charge multiple LiPo batteries simultaneously using a single charger. Parallel charging is an effective way to quickly charge the large amount of LiPo’s that mini quad pilots use. When you do it properly, it’s quite safe.
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?
Parallel charging is a fast and cost effective way to charge a lot of LiPo batteries at the same time. You can charge multiple batteries in parallel with just one single charger.
Parallel charging is also very convenient as it saves you from having to constantly unplug one battery to connect another one to the charger.
Is Parallel Charging Risky?
When you plug two batteries in parallel, their voltage will attempt to equalize. It’s like connecting two barrels at the bottom with a pipe, water from the fuller one will rush into the emptier until the two barrels have the same amount of water.
The same happens with batteries when you connect them in parallel. But when voltage different is too much, you can risk burning out the wires, overheating the batteries, or even causing fires.
To safely charge batteries in parallel, you should only connect batteries of similar voltage level. As a guideline, a difference within 0.1V per cell should be fine, that’s 0.4V for a 4S battery. The smaller the voltage difference, the safer it is.
This is what I normally do when I want to charge a large number of batteries at the same time. I put batteries in groups based on voltages, so I won’t forget.
Getting a Parallel Charging Board
The safest way to parallel charge is to use a dedicated “parallel charging board”. These boards are designed to connect and charge multiple LiPo batteries simultaneously.
Ideally you should get one with built-in protection – inline automotive fuses or poly fuses that protect against excess current flow.
Normally parallel charging boards have enough slots to connect at least 4 to 6 batteries. A LiPo battery should have a discharge lead (the main plug), and a balance plug. You have to insert both connectors into the parallel charging board.
You will see this type of cheap and basic options often, but they don’t have any safety features built-in.
I personally recommend using a parallel board with fuses, like the ones below. This is what I personally use for years and it’s been working great.
What Charger Do I use?
The Basics Rules/Tips of Parallel Charge
So parallel charging is a great way to charge your LiPo, but it’s also more prone to error than charging battery individually. Please do your research and ensure you charge your batteries safely.
Here are some of the important rules you should understand and follow.
1. Safety and Precautions
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 prevent fire from spreading.
2. Same Cell Count
Your batteries have to have the same cell count. E.g. you should only charge 3S batteries with 3S… Charge 4S with 4S… so on.
3. Capacity and C-Rating
It should be fine parallel-charging batteries of different brands and capacity together. However to ensure maximum safety, we do recommend only charging batteries of similar spec, and capacities should be within a reasonable range. Use your common sense.
4. Check Voltage!
As mentioned earlier, you should check the voltage of each battery with a LiPo voltage checker before parallel-charging.
Get a voltage checker for LiPo batteries, so you can check the voltage of each cell by plugging in the balance lead. These are some of the cheapest options:
I personally would make sure cell voltage difference is within 0.1V, that’s 0.4V for a 4S battery. If you want to play safer, you might want even smaller voltage difference.
If you have a battery with vastly different voltage than the rest, you should just charge it alone.
5. Connecting Balance Leads
Always use balance leads when parallel charging. This is no different than charging batteries individually. This prevents the charger from overcharging a particular cell.
6. Charge Current
When charging a single LiPo pack, I usually just charge at 1C to be safe. The charge current can therefore be calculated as
Current = 1C x Capacity.
For example, to charge a 1800mAh pack, I will use 1.8A as the charge current; For an 800mAh pack, that’s 0.8A. Some batteries allow to be charged at higher C rating.
The same principle applies when charging multiple LiPo batteries, the only difference is that the capacity is now the sum of all batteries connected in parallel. For example if I have three 3S 2000mah batteries, the total capacity is now 6000mAh, and I can charge them at 6A (1C).
7. Don’t Charge Unhealthy Batteries
We crash a lot when flying our mini quads, and inevitably we batter and bruise our batteries.
You should never charge unhealthy batteries – not to mention parallel-charge it. Any time you charge an unhealthy LiPo, there is a risk of fire. My advice is to bin those damaged batteries and don’t risk it.
Further Reading: When should I throw out LiPo?
Deformed LiPo batteries could be a sign of defected cells that results in higher internal resistance and significant increase in temperature during charging and discharging. And heat is the main cause of many LiPo fires.
Further Reading: How to Protect LiPo from Physical Damage?
8. Watch Your Batteries While Charging
Never leave LiPos charging unattended. You might be able to catch some signs of fire before it happens. For example, overheating, swelling and crackling.
A lot of LiPo fires could have been avoided if the person was there to stop the charger immediately and get the battery to a safe location.
Tips on Plugging in Batteries to Parallel Board
Always connect the discharge leads first, and then the balance leads.
When you first connect the batteries together, voltage difference will cause current flow as they are trying to equalize. The larger the voltages, the higher the current that will flow.
The balance leads are not rated for high current and could overheat and get damaged. The current surge could even melt the traces on a parallel charging board, as that happened to me before, 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 all the cells in every battery are 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 can still make contact with the wrong pins and cause sparks.
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 individual batteries are connected in parallel, they combine and become one big battery.
As we mentioned earlier, the moment you connect them, there could be a surge of current flow in order to equalize the voltage differences. Seconds later, the voltages will become the same across all the packs.
But how does cell balancing work during charging? With the balance leads connected, all the individual cells in each battery are also connected. For example cell1 in battery1 is connected with cell1 in battery2 (in parallel). These cells will have the same voltage, and to the charger, it’s no different than one single battery.
There are chargers with multiple channels (separate outputs). These are designed to charge multiple batteries at the same time without connecting the batteries together. Some people prefer this over parallel charging because they believe it’s safer. Also they allow you to charge batteries of different type, cell count and voltages at the same time, so arguably it’s more convenient.
Check out this basic and affordable 4-channel charger we recently reviewed – SkyRC E4Q.
- Dec 2014 – Article created
- Oct 2016 – Updated equipment options
- Nov 2017 – Article revised
- Jan 2019 – Article updated
- Mar 2019 – Added image how I group batteries based on voltage