Parallel Charging Your LiPo Batteries

by panther3001 | January 23, 2013 | (23 Ratings) Posted in Tips

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Parallel Charging Your LiPo Batteries

By Gabriel Staples

Initially written 12 April 2011

Preface & explanations added on 22 Jan. 2013

Most Recent Update: 25 Jan 2013

(added one more thing to list of Do's & Don'ts, & clarified "But what about cell balance..." section)

 

 

A newer version of this article, with additional information and rules of thumb for parallel charging (ie: that I keep more up-to-date), is available at its original posting location here: http://www.ElectricRCAircraftGuy.com/2013/01/parallel-charging-your-lipo-batteries_22.html. I recommend you check it out.

Other articles and general RC information are also available on my website, including many I've written on Arduino microcontroller programming. Visit me here:  www.ElectricRCAircraftGuy.com.

Why Use Parallel Charging?

I use a Triton 2 EQ 100W balance charger (http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVZA8&P=ML [Note: I don’t really recommend this charger, I just bought it before I knew much about chargers.  This thing is ridiculously expensive for what it does and has some silly quirks, occasional bugs, and outdated firmware]) to regularly charge up to 8 2S 500-1000 mAh packs simultaneously, all in just over 1 hour! I also regularly charge 3S LiPos and micro 1S Lipos (I’ve done up to 14 of those at once) simultaneously using that one, single-port charger. This is called parallel charging. Essentially, parallel charging allows you to plug in many batteries at once, into one port in a single charger, and, if your charger is powerful enough, charge them all in ~1 hour or less.  All at once—boom, done!  No more messing around buying many chargers or setting up the charger many times to charge multiple batteries.  I use this charge board (shown above) plus a couple other parallel adapters I plug into it.  If you use Deans connectors, use this board instead.  The board by itself is designed for only up to 6 batteries at once, so I added a couple parallel harnesses to get 8. However, YOU MUST KNOW WHAT YOU ARE DOING OR PARALLEL CHARGING IS DANGEROUS. For example, if you plug a 2S battery in with a 3S battery, the 2S battery will be destroyed and catch fire if you leave it there.

 

Background:

One thing that annoys me very much about virtually all local hobby shops is that as of today, 22 Jan 2013, I haven’t found a single one that uses parallel charging or sells parallel charging equipment yet, and yet I’ve been into about 12 local shops in the past few years and looked specifically for parallel charge equipment.  This charging technique has been around for probably 6 years, and is very effective and safe when done right, so I have to assume that most local hobby shops are either A) completely oblivious about parallel charging, B) do not want to promote it because it is better for them to sell another charger rather than a parallel charge board (ie: they’ll make more money selling another charger), or C) they secretly use this technique at home but simply don’t want to be responsible for user error if a customer destroys their equipment or property if they don’t know what they’re doing while attempting it.


(photo below: parallel charging in progress--6 2S LiFe batteries plugged into a parallel charge board)

[photo source: http://www.buddyrc.com/])

 

Despite this, I am a strong proponent of parallel charging and have used it nearly every charge I’ve ever done since I discovered it in the early part of 2011.  I am always recommending to people that they use parallel charging, as it allows any single port LiPo charger to charge many batteries at once instead of just one battery at once.  With the right harness, even a cheap $5 2-3S LiPo charger can do parallel charging!  (However, this is less useful since these low-power chargers don’t have enough power to maintain a 1C charge rate anyway).  So, let’s get started. 

 

First off, parallel charging can be done with LiPo, LiFe, or Li-Ion batteries ONLY.  Do NOT attempt to use this technique with NiCad/NiMH batteries, etc., as they use a peak voltage (ΔV) detection charge technique rather than a constant current/constant voltage (CC/CV) charge technique.  A battery chemistry capable of being charged via the CC/CV technique is a must for parallel charging to be safe and effective. 

 

What is parallel charging?

Parallel charging means that you plug many LiPo batteries into each other via a special board or harness so that all of their negative leads are connected to each other, and all of their positive leads are connected to each other.  Now, the entire battery packs are in parallel.  In order to balance the cells with your charger, however, the balance wires of all of the batteries must also all be connected in order to put the individual cells in parallel with each other.  WARNING: YOU CAN ONLY PARALLEL CHARGE BATTERIES OF THE SAME CELL COUNT (or the lower cell count battery will catch fire), AND SIMILAR STATE OF CHARGE (or damage to the lesser-charged battery will occur).  When many batteries are connected in parallel, the charger “sees” all of them as a single, large battery, with a capacity equivalent to the sum of their individual capacities. 

 

But what about cell balance—how does parallel charging really work?
In nature, whenever a gradient exists, a natural balancing process will occur.  A gradient means that there is a high concentration of something as compared to a low concentration of that same thing near it.  For example, if you pour salt into still water, the area of water where the salt is will become very salty.  The rest of the water lacks salt, so this “salt gradient” will naturally cause the high concentration of salt to balance out, or diffuse, into the water with a low concentration of salt.  The same occurs with heat.  Heat will naturally diffuse from a hot area into a cold area, attempting to find a balance.  If the hot area always remains hotter than the colder area, it is not because the heat isn’t diffusing, but rather it is because a heat source exists at the hot spot, and there is a resistance to the heat flow preventing it from fully diffusing.  Pressure also follows this natural balancing process.  Take a blown up balloon, for example, and untie it.  There is a large pressure difference (or gradient) between the air in the balloon, and the air in the room.  The high-pressure air in the balloon will rush out into the lower-pressure air in the room, diffusing the high pressure into the lower pressure until an equilibrium pressure is achieved.  Electricity also follows this principle of diffusion.  The battery with the higher voltage will naturally push its charge (electrons) into the batteries with lower voltage, when plugged in parallel, until all batteries equalize to the same voltage.  Since the individual cells of each battery are also in parallel with the individual cells of all the rest of the batteries, when the balance leads are connected in parallel (by plugging them all into a parallel charge board), all of the strings of cells in parallel will also equalize to the same voltage.  Now, when you plug the entire parallel charge board balance lead into your charger, your charger will balance out the cells of each battery as if it was simply one large battery.  The charger will "see" the first string of cells in parallel as a single "Cell 1," and the second string of cells in parallel as a single "Cell 2," and the third string of cells in parallel as a single "Cell 3," etc., balancing them as if they were individual cells of a larger capacity battery!  The result is that in parallel charging, all cells come out properly balanced so long as the cells are not damaged, your charger is functioning properly, you plugged them all in properly, and nothing else is wrong!
 

The Do’s and Don’ts of Parallel Charging/A Couple Things to Know About Parallel Charging:

Rules of Thumb:

1)      ONLY USE LIPO/LIFE/LI-ION BATTERIES WITH THE SAME CELL COUNT, ***NO*** EXCEPTIONS. Ex: 2S with 2S, 3S with 3S, etc.

2)      Preferably, use batteries of similar capacities. Ex: 500~1500 mah batteries together, or 1300~2200 mah batteries together, but not a 500 mah battery with a 10,000 mah battery.

3)      Use batteries with similar states of charge (how much they are charged/discharged). Ex: do NOT put a 1/2 full or 3/4 full battery in parallel with an empty battery. All batteries must be at similar discharge state.  SEE PLOTS BELOW FOR MORE INFORMATION.

4)      Use batteries of similar ages. Ex: it is not as advisable to put a new battery with a 2 year old battery, but not critical as long as the batteries are similar capacities (mAh ratings), the same cell count, and at similar discharge states.

5)      Always plug in the *main* plug first on ALL batteries, *then* plug in all the balance plugs.  Also, it is good to *wait several minutes* AFTER plugging in the main battery leads into the parallel charge board BEFORE plugging in the balance plugs. This prevents high currents from flowing through the balance plugs as the batteries equalize based on their varying voltages upon plugging them in. The main plugs can take more current. This is also the reason you want to use batteries of similar states of charge/discharge.

6)      Be VERY CAREFUL to plug in the main leads and balance leads correctly.  Attempting to plug them in backwards will cause a short circuit and a spark, and potentially damage your batteries and/or your parallel charge board or battery leads.

7)      Once all batteries are plugged in together in parallel, wait several minutes (3~10 minutes or so) for them to equalize their voltages.  The farther apart the batteries' charge states, the longer you should wait.

8)      To determine the charge rate when charging in parallel (assuming the standard 1C charge rate), ADD all of the battery capacities together, then use that value as the charge current. Ex: parallel charging three 3S 1300 mAh (1.3Ah) LiPo's with two 3S 1000 mAh (1Ah) Lipo's would mean that you should set your charger on the 3 cell LiPo setting at a charge rate of (3 x 1.3) + (2 x 1) = 5.9A. Therefore, in this scenario, a charge rate of 5.9A corresponds to a 1C charge rate, and the charger will consider all of those batteries in parallel to be a *single* 5900 mAh (5.9Ah) 3S Lipo.

9)      It is recommended to use a fire-proof LiPo-Safe Charge bag when charging, such as this one here.  I like to place the entire charge board with all of the attached batteries, if possible, inside of the same charge bag.  If the batteries are very large, and this is not possible, feel free to separate the batteries into separate charge bags.  Note: charge bags have a special slit in the side, near the velcro, to allow the cables to come out of the bag, so placing the entire charge board, or individual attached batteries, into a charge bag is not a problem.

 

Feel free to Google for more info on "parallel charging" of LiPo packs. Again: this method is NOT recommended for NiMh or NiCad cells, as it may cause them to catch fire, though for LiPos it works great!

 

Also see these links as additional sources:

 

To purchase parallel charging equipment:

I recommend HobbyKing (www.hobbyking.com), ProgressiveRC (http://www.progressiverc.com/), or BuddyRC (www.buddyrc.com).

 

Parallel charging all these batteries at once with only 3 chargers!

[photo source: http://www.rchelicopterfun.com/parallel-lipo-charging.html]

 

Useful Plots:

PLOT IS FOR LIPO BATTERIES ONLY,

And is only an ***approximation***


 

Parallel Charging Note: 

AS LONG AS THE BATTERIES ARE WITHIN ~25% (OR LESS) STATE OF CHARGE OF EACH OTHER, THEY MAY BE PARALLEL-CHARGED TOGETHER.   If they are more than ~25% apart, the higher-charged battery will push a current into the lower-charged battery equal to or greater than a 1~2C charge current, which is bad. 

(To know for sure how much current the higher-charged battery is pushing into the lower-charged battery, and to see when they are nearly equalized, I like to hook up a power meter in between the parallel charge board and the *lowest-charged* battery, then I plug the remaining batteries into the parallel charge board, once at a time, beginning with the one that is least-charged and plugging in the one that is most-charged last.)

 

Also, after plugging in all main battery leads in parallel to each other, it is best to let the batteries’ voltage equalize for a few minutes before plugging in the balance leads in parallel and beginning the charge.  The chart below is just a rough estimate of how much time is recommended to let the batteries equalize their voltages prior to plugging in their balance leads and starting the charger.  Note: State of charge range = most charged battery State of Charge – least charged battery state of charge.  Ex:  If the most full battery is at 50% and the least full battery is at 30%, the state of charge range is 50% - 30% = 20%. 

 

 

For additional general LiPo information:

See my extensive article I wrote, titled “The Details of Electric Radio Controlled Aircraft,” under the “Battery” section of the document (approximately pgs. 22-35), found at my main website here:  https://sites.google.com/site/electricrcaircraftguru/.  Also, again, a more recent version of the above article can be found on my website here:  http://electricrcaircraftguy.com/2013/01/parallel-charging-your-lipo-batteries_22.html.




COMMENTS

Adib Vahedi on January 26, 2013
Nice man you must have tooken a long time to ma that!!!! :)
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Ak Flyer on January 26, 2013
Well written and informative. Thanks for taking the time.
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rctestbench on January 26, 2013
great article, lots of supporting information
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NorseHammer on January 27, 2013
Thanks for the charging article, I say that because you refreshed my knowledge, its a good thing to do when you have a regular task that is at times boring and mundane, for safeties sake. Charging is mundane and if done without care and attention can have unfortunate results, thanks again for the article.
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panther3001 on January 29, 2013
thanks for the comments! And yeah....it did take me a REALLY long time to write. :)

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Arx on January 30, 2013
The reason hobby shops don't typically sell the stuff is because most people don't understand enough about what's going on with their batteries and are pretty likely to have a mishap at some point.

Yes, it's not a problem to do if you're careful, and know what you're doing, but IMO if you need a step-by-step how-to you should probably just stick to standard charging for now.

I'm not suggesting it's something that should be kept a secret or anything, just that it's not something I think beginners should be encouraged to do.
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panther3001 on January 31, 2013
(My response to "but IMO if you need a step-by-step how-to you should probably just stick to standard charging for now.")
.....Well....even the simplest computerized charger requires a step by step how-to (and instruction manual) to have any clue what you're doing or why--and to be safe for that matter. Anyone with the cheapest charger capable of user-set currents and voltages is expected to do some reading in an instruction manual, so I don't really buy this argument. I hope more people, including beginners, and hobby-shop owners and operators, become informed about this technique, as it is so useful, and I am more inclined to think that most hobby shop owners/operators/salespeople just don't know about parallel charging, or how/why it works, and that's why they don't have the equipment or know-how to do it. So I hope this article has been informative to them as much as to anyone.

Thanks everyone for reading by the way. :) I'm super glad people are finding this interesting, informative, and useful. I've put a lot of effort into understanding this myself and doing it, learning it, and teaching it with care.
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Markjohn on January 30, 2013
Great post! My experience is that hobby shop operators have a lot of things with which they need to have a basic familiarity and something as esoteric as parallel battery charging does rise to the "need to know" threshold. Your post provides a conceptual understanding. Well done!!
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panther3001 on January 31, 2013
Thank you very much! I couldn't agree more.
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3cellbrain on February 1, 2013
Thanks for this article! Gives me confidence to do it myself.
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panther3001 on June 21, 2013
glad I could help!
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liveyourdreamsRC on July 6, 2013
Man that is sure helpful! I had no clue about this until I just read it. Now I can charge batteries way faster! keep sharing this type of knowledge!!!
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seansgrandad on May 13, 2014
Great article! Clear and informative! I'll use it as my Parallel Charging Bible from now on!
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jw423 on August 13, 2014
Great article. Thank you. I have a quick question though. While setting up your charger do you select the type of batteries i.e. 1S 2S 3S etc. that you are charging? Or are you basically creating a larger battery, such as; 2- 3S batteries make one 6S to the charger. If I am thinking right it is the same as the type you are charging because it is in parallel. It would be added if it were hooked up in series. Am I thinking right?

Thank you again!
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HanSolo on June 1, 2015
i need a little help. i did some reading for parallel charging, most of the time, if people insist on a certain order, they go main plug first. - some youtube vids show balance first, without mentioning it.
i bought a parralel charging board from china, which says in a clear order: 1. connect balance lead of board to charger. 2. connect main wires from board to charger. and 3. connect batteries to board - allways plug in balance leads first, then connect main lead.
so now i am a bit confused. me three batteries for the first test are pretty close though, all around 11,2v with cell differences of 3,7 to 3,75v... so this time it will be not that important, but for future..?
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panther3001 on June 1, 2015
Based on my experience and knowledge, my logic in my article (Rule of Thumb 5 above) is sound. So, follow what I have stated. Plug in the *main* lead of each battery to the charge board first, give them a little time to equalize, *then* plug in the balance lead. If the batteries are too different in charge state, the current rush though the small balance leads could fry the leads, causing the tabs in the battery, or the connections, to literally melt or disintegrate off.

For a simple, unprotected (ie: no passive or active circuitry involved) parallel charge harness or board, follow what I have stated above.

Can you please post a link to the exact board, or explain exactly which one it is so I can look at it? Can you find these instructions posted anywhere online? I'd like to see the sites.

So, I have a few ideas, however, on why your instructions might say what they say. Here they are, in order of probability (what I think the most probable reason is, to the least probable reason):
1) They are written in Chinglish/Chingrish (Chinese-English). It's simply a translation error.
2) The person made a mistake/typo and meant to say it the other way around.
3) The person legitimately thinks it is this way, but is wrong.
4) The board is specially designed to have current-limiting resistors, polyfuses, or other passive or active circuitry in the balance lead connections in the board to prevent over-current. This way, with this particular board, it is intended to handle batteries of wildly varying voltages. You plug in the balance leads, which have current-limiting built in to the board, and wait several minutes, then you plug in the main leads, which have no protection. The current-limited balance leads A) protect the leads from otherwise frying if they were not protected, and B) allow the batteries to more slowly self-equalize before plugging in the main leads, where large currents could otherwise surge and damage the lower-charged pack otherwise (see my Rule of Thumb 3 above).

Let me know if you can find any links. Thanks!
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panther3001 on June 1, 2015
One more note: it IS correct, as they have stated, to connect the charge board to the charger BEFORE connecting the batteries to the charge board. Otherwise, the charge board main leads could be dangling around touching each other (short circuited), which is very bad when you plug in the batteries.
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panther3001 on June 1, 2015
Another note: To back up my 4th point above, see Post #5 here: http://www.rcgroups.com/forums/showthread.php?t=1781388.

Also, on a side note, my website has a slightly more complete version of My Parallel Charge Article if you want to take a look at it there (link is at the top of this article), though they are both the same content overall.
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