What is Parallel Charging and why is it for me?

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What is parallel charging?

Parallel LiPo charging allows you to charge more than one battery at the same time (with a computerised RC battery charger). The big advantage of parallel LiPo charging is it saves time. You can charge your entire collection of (same cell) LiPo batteries on one charge cycle.

Parallel charging will only work on same cell count batteries, but they can have different capacities, within reason. They can also be at different states of discharge, again within reason. We wouldn’t recommend trying to parallel charge a pack that was 5% discharged with one that was 50% discharged for example.
 
No more taking one pack off when it is finished and then hooking the next one up and starting the charge cycle all over again. The important thing to remember is the battery packs all have to be the same cell count. You CANNOT parallel charge a 6S pack with a 4S pack for example. All packs in the parallel charge circuit must be of the same voltage rating.



Warning and Safety


This is an advanced method of charging and requires a certain level of electrical understanding. If you are not comfortable with charging calculations, wire sizing, your ability to build safe parallel charging and balancing boards or harnesses, please don’t attempt it.

As was discussed on the LiPo battery Page, Lithium battery chemistry is a volatile mixture. One battery starting on fire because of unsafe or improper charging is bad enough, never mind several batteries all hooked up in close proximity to each other!

We would also advise that you read and completely understand the Lipo Safety guidelines found here before you start.
 
How does parallel charging work?

When you place all the packs in parallel, they all carry the same voltage, while the capacity of “C” rating of each battery pack is added together. I.e parallel connecting 3 x 2S 7.4 Volt 2000 mah battery packs will result in one “single” battery pack with a voltage of 7.4 volts and a capacity of 6000 mah (2S3P).

When using a good computerised balance charger with correct parallel wiring (for both main leads as well as balance leads) the current would flow between both packs and they would actually all adjust voltage until they perfectly match each other. The charger only "sees" one larger pack and charges it as if it were a single pack.

Example Diagram of Paralale Balance Charging:



But if the charger only sees one pack, how does it balance all the packs?

When you use a good computerised balance charger all the cells in each pack will be balanced by the charger. The first cell in each pack would be “tied” to the first cell in the second pack and the first cell in the third pack. The second cell in the first pack would be “tied” to the second cell in the second pack and the second cell in the third pack, so on. The balance charger will charge each cell until they have the same voltage. In the case of parallel balance charging each cell in parallel over all the packs would carry an average voltage.

Potential drawbacks and warnings

If something were to go wrong during the charge cycle it could affect multiple packs. For example if the charger was setup incorrectly, it would affect all connected packs instead of just the one.

If packs are at vastly different voltages when you connect them together in parallel, it is possible that a large amount of current can flow between the packs as they equalize themselves. In most cases this will not damage the batteries, but it can damage the wiring. We will cover more on this later in the article.

Parallel charging large packs on less capable chargers will give little to no benefit, aside from only having to set up the charger one time, due to small output capability. Parallel charging is really best on larger chargers or a charger with adequate power rating. More information on Power Ratings can be found on another article on this website.

Again, if you haven’t read this already, it is strongly advised that you read our Lipo safety guidelines Located Here.

Requirements and recommendations for parallel charging


A parallel wiring adapter for the main leads and a parallel adapter for the balance leads. This can be for as few as 2 packs or as many as 6, your choice. You can find these online or most good hobby suppliers. We will also show you a few basic examples to make your own below.

All the packs you charge in parallel must be the same type and cell count. There is no need to match the capacity, C-rating, age or brand for parallel charging. For example, you can charge a 3s 2200mAh 15C lipo, a 3s 3200mAh 25C lipo and a 3s 850mAh 35C lipo together, but you CANNOT charge any 3s lipo in parallel with a 4s lipo.

Balance the charger with adequate power! Just because your charger states that it can output 10amps, it is not necessarily the case. Read More.

High Current Flow Concerns


Many people are greatly concerned that when charging multiple packs in parallel, they be very near in voltage when you connect them or the current that flows between them will be very high and could harm the Lipos or cause greater problems. It is true that large currents can flow, but the size of these currents are often greatly over calculated. Here is some math to show the worst case scenario; connecting 2 high performance 6s 5000mAh packs, one fully charged and the other discharged.

The voltage of the charged pack is 25.2V. The voltage of the discharged pack is 22.2V, so the voltage difference is 3.0V. The internal resistance of each pack is .018 ohms (3mohm per cell) for a total of .036 ohms.

Volts = Amps * Resistance
or
Amps = Volts / Resistance = 3.0V / .036ohms = 83.3A

If we do this simple math and go no further, we find the initial current flow to be 83.3A or nearly 17C. That looks bad on paper, but in reality it is nowhere near that high because of a simple concept that the straight math fails to take into account. That is that the voltages of both packs will change under load. Basically the more you load (discharge) a battery, the greater voltage drop. This means that the charged pack is no longer at 25.2V when it is loaded. In fact a 6s 5000mAh lipo will likely drop to more like 22V under that high of a load. Notice that it is actually lower than the discharged lipo. Likewise, the discharged lipo's voltage will rise under the charging load. The actual initial current flow is not easy to pin down without testing it in real life and will change with different packs, at different voltages, and so on.

Basically, it is never a good idea to connect two packs in parallel that are at vastly different voltages, but in this case the likely end result will be nothing other than 2 packs will end up half charged.

There is one concern that is still very real though. If the pack's balance wires are connected first, this high current could damage the balance wires and/or the balance adapter. As such, the main leads should always be connected first!

How to Build a Simple Parallel LiPo Charging Board or Plug

Deans Parallel LiPo Charging Board. As you can see on the back side of the board, all the plugs are wired and soldered in parallel.

 

This can also be done by simply wiring the positives and negatives together in a cable harness or the board as show above. The reason we suggest building a board over a wire harness is so the male ends of the Deans plugs won’t short against each other (one thing that is a huge oversight in the Deans connector design).

After you build your board, make sure you insulate the back side of it.

How to Build a Simple Parallel Balance Board

 

As with the charge plugs, you generally need no more than 5 or 6 balance plugs to keep things in reason. As the name suggests, this board is also wired in parallel, as you can see in the above photo.

This board is a 6S JST-XH balance plugs since this is the most common balance connector. You can obviously use other balance connectors if all your packs, ESC and charger use different balance connectors.

Just like the parallel charge board, ensure you insulate the back side of the parallel balance board, ensuring none of the points touch each other or short circuit when placed on to a conductive item or surface.

Some examples of parallel charging uses?



450 sized heli
Many times people start with a 450 sized heli and in the beginning they cannot get enough flight time. This is what we call the "Welcome to Your New Addiction" stage. Parallel charging can be a great aid to the new pilot. For example, with a 350W charger a 6x parallel lead set, a new pilot could charge 6, 450 packs in 30min.

700 sized heli
Larger electric helis are becoming very popular and they come with large needs in terms of power. A common battery pack for a 700 is a 12s 5000mAh made up of (2) 6s 5000mAh packs wired in series. Due to the lack of 12s chargers available, a need for charging pairs of large 6s packs has arisen. Of course you could use 2 chargers, or a dual port charger, but there is another solution, a solution that really brings out the best on the new powerful chargers offering 1000W or more of output. By using one of these new 1000W chargers and parallel charging, you can charge a pair of 6s 5000mAh packs in 25min or less.