by John Salt - Updated October 2018
I wrote this page to give you an idea of what parallel LiPo charging is and the reasons it is so popular, so you can decide if it's a charging method you may want to use, (now or later).
Parallel LiPo charging allows you to charge more than one battery at a time with your computerized RC battery charger.
I think you can already appreciate the advantage parallel LiPo charging offers us - it saves lots of time! Instead of charging each battery pack one at a time, para charging allows you to charge your entire heard of LiPo batts on one charge cycle at the same time.
Now, this is very important... Charging LiPo batteries in parallel will only work on same cell count batteries.
They can have different capacities (within reason). They can also be at different states of discharge - again within reason. You wouldn't want to try and parallel charge a pack that was 5% discharged with one that was 80% discharged for example.
The general rule I follow is no more than a 0.1 volt maximum difference between cells of packs I hook together in parallel.
This for example would be very close to the voltage difference between a cell that is 80% discharged (around 3.75V) and a cell that is in a 50% storage state (about 3.85V). The larger the voltage difference between packs hooked together in parallel, the larger the amount of current that flows between packs as they all equalize.
Below is a LiPo battery current flow video I made that demonstrates this in more detail.
At any rate, 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.
Once again... The important thing to remember is the battery packs all have to be the same cell count. You can’t 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.
Words Of Caution!
Parallel LiPo charging 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/harnesses, or keep your wits about you when hooking up the batteries in parallel, 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/improper charging is bad enough, never mind several batteries all hooked up in close proximity to each other! If you choose to charge this way, just like charging any lithium based battery, you do so at your own risk and never unattended.
As you may have seen in above video, I now use "ParaBoards" while "Para Charging" and they are the best solution yet in my opinion. They make for a neat, safe, hassle free hookup and they are pretty much "goof proof".
Highly recommended if you parallel charge and don't want to build your own harness &/or boards.
As with any type of LiPo battery charging, parallel charging can be as safe or dangerous as you make it. As I just mentioned in the previous paragraph, this is an advanced method of charging and requires some basic DC circuit and Ohm's law understanding.
I have read and even heard a fair amount of "fear mongering" out there about parallel LiPo charging, but most of it is unsubstantiated and unsupported by factual evidence or theory.
Nonsensical statements like the differences in resistance values of cells will cause voltage variations in each pack & cell. Last time I calculated or measured the voltage across various resistances within a DC parallel circuit, the voltages all were equal.
Again, if you know and understand Ohm's law, this is a proven fact of DC parallel circuits. I have even read a couple articles that state no battery expert would ever recommend parallel charging. Well, I would love to get the data source on statement's like those and know who these "experts" are.
Why? Because out of the half dozen LiPo experts I've talked to, and the numerous electrical engineers in the hobby I've also conversed with; I simply can't find a single person in the know that will say parallel charging of LiPo batteries is more harmful than single pack charging or will reduce the pack's life span by over or undercharging some of the cells.
Provided you have good equipment, healthy batteries, and understand exactly what you are doing.
Perhaps some of these fears were brought about by people who figured they could cut corners and just hook the main power plugs up in parallel and not worry about the balance plugs? That of course will certainly lead to individual cells in the various packs getting over and under charged.
I've been para-charging LiPo packs now for over 8 years and have never once had any issues. My packs are living long healthy lives and I would guess I have had at least 800 para-charging cycles assuming about 100 per year give or take.
Am I just lucky? Well, I would like to think there is more to this procedure than just luck. I will trust the electrical physics in other words over the anecdotal hearsay. Science wins out every time :-)
Of course, if you don't think parallel LiPo charging is a safe charging method, or you are not comfortable with it - don't do it!
This article is not meant to force anyone either way; it's just meant to explain what para-charging is, how it works, and the pros & cons. Hopefully giving those who want to try it a solid understanding based on facts - not fiction.
If you understand how Ohm's law works in relation to DC parallel circuits, you know the basic principle behind parallel charging, how it works, and why it's just as healthy for the LiPo as single pack charging - perhaps even healthier.
I have experienced this almost every time I parallel charge... The voltages of the individual cell branch circuits are almost always in closer balance proximity during the charge cycle and the charger doesn't have to work as hard putting a load on a certain branch to bring it/them down to balance with the rest of the cells/branches in the pack.
Why... Law of averages & sample size statistics.
The more packs you have hooked up in parallel, the more evened out the internal resistance values will be between each branch of cells since it is very unlikely cell # 3 in all your packs has a slightly higher internal resistance than cell # 2.
The more common reality is every cell in every pack will have a marginally different internal resistance. This generally tends to even the resistances out on each cell branch very nicely resulting in the current flowing between branches being very even as well while the packs are charging. This also means your charger generally won't have to work as hard putting a balancing load on the individual branches to maintain a low mV delta between cells.
Again, just simple DC parallel circuit Ohm's law that we all learned back in high school physics - assuming you weren't skipping class that day ;-)
If you were, the video below will show what you missed.
First you will need to build or purchase a parallel charge plug or board that the charge leads from each battery will be plugged into.
The positive from each plug is hooked to positive and the negative of each plug is hooked to negative (parallel circuit). To keep things in reason, most will only build a plug or board that will support up to 5 or 6 batts maximum.
Things start to become a little congested as you can see with that amount of wiring and batteries, especially if you still want to use a fire safe charge container or bag while charging.
The picture above is showing a parallel charge board I built many years ago that supports all my LiPo packs that have Deans connectors. As you can see on the back side of the board, all the plugs are wired and soldered in parallel.
The reason I built a board over a multi plug harness is so the male ends of the Deans plugs can’t short against each other (one thing I really don't like with Deans connectors).
After you build your board, make sure you insulate the back side of it. I for example just used several layers of fabric hockey tape.
Here is the EC3 connector parallel charge plug I built and since EC3 connectors are insulated (better safer plug in my opinion), I didn’t bother using a board and just wired them all up in parallel.
One thing I should point out is the size of the wiring, especially the main wire from the banana plugs (red/black) to the connection point of the 5 EC3 plugs.
I could theoretically be pushing 20 Amps through this wire so it has to be a fairly good size gauge. I am using 14AWG and this seems very adequate. It only gets a little warm at a full 20 amp current load and the voltage drop over the very short length of wire is insignificant. 14AWG is also the size of the wiring used to power my charger, so that is a pretty good way to "gauge" it :-)
Here's another example of a parallel charging harness I built that supports all my smaller LiPo packs that use JST connectors.
Next you will need to build yourself a parallel balancing board or harness.
I can't stress this point enough. If you are parallel charging multi celled LiPo packs, you must hook each cell up in parallel as well.
If you don't, yes, you will have cell voltage imbalance between packs and some cells could very well be getting overcharged!
I am first going to show the board I built and then a harness. Again, just 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.
The board I built here uses 6S JST-XH balance plugs since most of my LiPo packs use JST-XH balance taps and JST-XH plugs have a nice advantage... you can (with a little persuasion) fit different plug counts into the plugs.
For example, a 3 pin balancing plug will still plug into a 7 pin plug. This is nice so you don’t have to build several balancing boards with different plug pin counts.
I also have a few FlightPower and ThunderPower packs that use Thunder Power Taps, so needed to get a few TP to JST converter plugs.
Just like the parallel charge board, ensure you insulate the back side of the parallel balance board so it won’t short out on anything.
The parallel balance board you built will now plug into the Chargers balance board.
Here I have my parallel LiPo charging balance board plugged into the 6S plug on the balance board that came with the charger enabling me to parallel balance everything from a 2S pack to 6S pack. If I ever get into 7S, 8S or higher, I will have to build a new parallel balance board, but for now, I am very content with nothing larger than 6S packs.
Here is another parallel LiPo charging balance method that will work and I find easier to both build and hook up. With a balance harness like this over a board, it is easier to hook up all your balance taps, especially if your batteries have short balance leads.
The first step is to get a number of female balance plugs with wiring on them if possible. Having the wiring in place saves considerable time. The 6S JST-XH ones I am using here came with Kokam male ends (they are actually a balance adapter) but they come in packs of 5 for less than I could get single ends for anywhere.
I just cut the Kokam ends off and went to work soldering all the leads off the 6S JST-XH female plugs in parallel. Mounting them all lined up in the correct +/- orientation in a small vise as I have done here while working on each branch of wiring keeps things neat and mistake free.
Also note I have heat shrink already in place for insulating the solder connections when I put the male balance plug on.
Once I have all the wiring sorted and soldered it is time to solder on the single male balance plug end that will plug into my chargers parallel charge board. As it turned out, all those Kokam male ends came in useful for this.
I just shaved the orientation tabs on the sides off the Kokam plug and swap the black and red wires so the polarity was correct. Now I have a 6S JST-XH parallel balance harness that plugs directly into my 208B charger's balance port making the hookup a little neater.
This is perhaps the question I get asked most often "what charge current do I use when parallel LiPo charging"? It's really simple math here. Basically when you have everything hooked up in parallel like this, your individual packs now have become one single big capacity pack.
I'll say that one more time in case you missed it. YOUR INDIVIDUAL PACKS HAVE NOW BECOME ONE BIG SINGLE PACK! For instance, if you hooked three 2200 mAh 3S packs all up in parallel as I have done here, they essentially become one single big 6600 mAh 3S3P pack.
As per charge current calculations, a 1C charge rate on this pack is now 6600 mA (6.6 Amps). A 2C rate would be 13.2 Amps, and so on. I think you can now appreciate why high power chargers are generally required with this parallel LiPo battery charging method, as the wattage required in this example will be in the 83 Watt range at 1C and about 166 Watts at 2C.
Of course you don’t have to charge at a 1C or 2C rate, lower charge rates are always safest so a 0.5C rate of 3.3 Amps (41 Watts) will work just as good, it will just take twice to 4 times as long to charge.
The same calculations apply to packs of different capacities. Let's say we have one 4S 1500 mAh pack, one 4S 2800 mAh pack, and one 4S 4000 mAh pack all hooked together in parallel. That is the exact same thing as far as the charger is concerned as a single 4S 8300 mAh pack. A 1C charge rate set on your charger would therefore be 8.3 Amps. Again, just simple grade school math :-)
Connection Warning Tip!
One very important caution when you hook all your packs up in parallel is, you MUST plug the main power plugs from the packs into the para-harness, or para-board first.
This way the potentially large current flowing between the packs as the voltages all equalize when they are first connected all together flows through the heavy power wires and connectors without issue. Video demonstration of this shown already near the beginning of this article.
If you were to plug the balance plugs in first, that same potentially high short term voltage equalizing current flowing through the small balance wiring could overheat those small gauge wires, or the small balance plugs. It could even melt the circuit board trace lines on the Para-Boards.
If all the LiPo packs are at the exact same voltage potential when you hook them together in parallel, this is not a concern as there will be little to no current flow between all the packs because they are all at the same voltage potential. In the real everyday world however, that is likely not to be the case so it's always best to plug the main power plugs in first and then the balance plugs.
Hopefully that gives you a little background and understanding on parallel LiPo charging and several methods of how to do it. As I said in the opening paragraph, the main reason I built this page was to show you why a higher power charger, even with smaller battery packs, may be a good long term investment.
You'll know you're a Parallel LiPo Charging junkie when...
Here's a good video of LiPo parallel charging procedure...