When it comes to parallel charging there are a couple of ways that you can interpret it:
You have multiple chargers and thus can charge multiple batteries at one time. This is the most obvious version, but comes with the big draw back that you need multiples of everything, multiple power supplies and multiple chargers.
Multi channel chargers: Those are chargers that have multiple dedicated charging circuits, so you can charge two ore more batteries at once. The great thing with those is, that you can charge different batteries (different cell count and different capacity) at once. The bad thing is, the more channels they have, the more expensive the tend to get.
Parallel charging boards: A special board that allows you to attach multiple batteries, usually six, to the same charger. There are some things you need to consider when going this route, but I will explain all of them in detail, since this is the method we are going to look at in detail.
If you don’t know anything about battery specs, I would recommend to first read my article about battery specs and what they mean to get a basic primer.
First of all - why would you want to parallel charge? Well, this one is simple: parallel charging cuts the time you need to charge your batteries into a fraction. You can charge six batteries in the time you used to charge one battery.
Parallel charging 1S Batteries
A lot of burhsless whoops use either one or two 1S batteries.
1S batteries are a little bit different from all other batteries, since there are a couple of dedicated multi channel chargers for 1S batteries. They are cheap and usually can be powered from a 2-4S battery - so you are easily set up for charging your 1S batteries on the go.
I recommend the Ultra Power UP-S6AC. It allows to to charge all types of 1S batteries (Micro, MX, JST (PH 2.0), mCPX), you can set the charge current and voltage (HV charging is thus possible too) for each of the six ports. As input you can either use a 2-4S battery or plug it straight to your wall outlet.
Using a parallel charging board
For all batteries with more than one cell I would suggest using a a parallel charging board. They come in different sizes with different plugs, but in general all of them allow to charge six, 2-6S batteries at the same time.
Further down in the article I describe my “on the go” setup for parallel charging my 2S batteries when outside.
Often using a parallel charging board is considered dangerous, but if you keep a couple of things in mind, you will not have any problems. So lets take a look at those rules first:
1. Do not charge unattended
The single most important thing is to not charge unattended, this does apply to charging in general, not only parallel charging. Do not leave the house while charging. In case something happens it is best you are there to react. Put your batteries in a fire save place while charging - use a battery safe, bag or similar. Whatever you use, make sure it can contain the fire in case the LiPo goes of. It is very hard to extinguish a LiPo fire, so your best bet is to limit the spread of the fire.
2. Use similar batteries
If possible use batteries of the same brand with exactly the same specs. For example, use six 2s, NanoTech 300mAh, 80C batteries.
Under no circumstances mix cell count, so never charge 2S and 4S batteries in parallel.
Technically batteries with different capacities can also be charged in parallel.
3. Same voltage level
Only parallel charge batteries with the same voltage level. When putting batteries in parallel, they want to balance themselves.
So when you put a 2S, 7.4V in parallel with another 2S battery, but at 8.4V the want to balance themselves to their mean value: (7.4 + 8.4)V / 2 = 7.9V each. This is just a difference of 0.5V, but since the internal resistance of LiPo batteries is very low, you draw massive amounts of current in a short time.
On a parallel charging board this can have multiple effects: first of all you will most certainly charge the lower battery with more C than it is rated to be charged with (to have a long battery live, they should only be charged with 1C). The other thing that might happen, is that you burn traces on the parallel charging board.
Both might result in fire.
The higher the cell count, the bigger this voltage difference, and the higher the chances of something going wrong. So before plugging in your batteries, make sure that they are at the same voltage level.
Because of the high discharge that might happen because of voltage level difference, make sure to first plug in the discharge lead and then the balance lead.
Make sure that my batteries are within a 0.1V voltage range. So I will plug in a 7.4V and a 7.5V battery, but will not add a 7.3V battery to this constellation. Usually it works out pretty well when out and flying that I discharge my batteries pretty much to the same level.
Some people might say, that 0.2V are also a safe range - but a fact is, that the closer the voltage difference is, the safer you are. I prefer to stay on the safe side, that is why I go with max 0.1V difference in voltage levels.
4. Choose proper charging current
When parallel charging, you want to make sure that you are still charging your batteries with 1C. Six batteries in parallel will each draw 1/6 of the set current. In order to get the proper current value you multiple the current you would use for one battery and multiply it by six.
For example: You want to charge six, 2S, 300mAh batteries. One of those batteries you would charge with 300mA - so 1C. To charge six of those batteries with 1C you would need to set your charger to 6 x 300mA = 1.8A.
If you want to charge batteries with different capacities, simply add up their capacities to get to the proper charging current.
For example: You want to charge three, 2S, 300mAh batteries and three, 2S, 450mAh batteries. You would need to set your charger to 3 x 300mA + 3 x 450mAh = 2.25 A.
If you are in a hurry, feel free to charge with 2C by doubling the value you got from the previous calculation, but be aware that this might shorten the life span of your batteries.
5. Get rid of damaged batteries
If your battery is damaged, get rid of it. If you want to risk continuing to use a dinged up battery, to not add it to your parallel charging constellation, or you risk to ruin the other batteries that are still well. Charge it on its own, see if it gets hot, check the internal resistance and generally be careful with it.
If you keep all of the above points in mind, you are save and will most probably not have any problems with parallel charging.
Parallel charging on the go
When I am out for a longer session I like to take my parallel charging setup with me.
Instead of plugging my charger into the wall, I will power it from a high capacity 4S battery, it does not have to have a high C rating, since I will only be drawing 1.8A, or 3.6A if I feel frisky and want to charge my batteries with 2C.
With my brushlesss whoop I fly 2S, 300mAh batteries. I have 12 batteries in total, so when I am done with my first six batteries I will start parallel charging them, as mentioned above, sometimes even with 2C to get them done quickly.
I use a 4S, 5000mAh battery to power my ISDT Q6 Pro charger this allows me to fully charge 12 sets (2 x 6) of my 300mAh batteries with this XT30, six port parallel charging board. If your batteries do not have XT30 plugs, simply get some adapters to go with them.
This method of parallel charging is of course only viable if you charge smaller capacity batteries.