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This article is also posted on FliteTest.com here: http://flitetest.com/articles/Parallel_Charging_Your_LiPo_Batteries
Parallel Charging Your LiPo Batteries
By Gabriel Staples
Initially written 12 April 2011
Preface & explanations added on 22 Jan. 2013
Most Recent Update: 26 Feb 2017
-added a lot of notes about fuse-protected parallel charge boards - 21 July 2015
-added a couple more things to list of Do's & Don'ts, & clarified "But what about cell balance..." section; added another rule about battery chemistry - 5 May 2013
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ridiculously expensive for what it does and has some silly quirks, occasional bugs, and outdated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.
UPDATE (21 July 2015): I have found a hobby shop that now has parallel charging equipment! It is Radical RC. (They've had them for a couple years now, I just haven't updated this article until now). Way to go Radical RC!
(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!
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Update Notes [21 July 2015]:
Fuse-Protected Parallel Charge Boards:
This entire article is written for those of you using raw, unprotected parallel charge boards with NO fuses or current protection circuits of any kind. If anyone tells you that you *must* have protection power fuses and polyfuses and things on your board, they are flat out wrong. I don't own such a board. I still use the raw, unprotected boards. IF YOU FOLLOW MY PROCEDURES THIS IS PERFECTLY FINE AND SAFE. If you make a mistake, however, a fuse-protected board could prevent a battery mishap, so feel free to buy protected boards.
IN THE PAST FEW YEARS, as parallel charging has become more widespread, companies selling parallel charge equipment have added things such as large power fuses to the main leads, and small polyfuses (self-resetting, automatic fuses), to the balance leads. You can recognize these well-protected and more advanced boards usually by their price tag. They are oftentimes very expensive! (Think: "I could buy another cheap smart charger, or I could buy a parallel charge board" kind of expensive).
Here's how a fuse-protected parallel charge board might work/save your butt:
- If you are foolish or inattentive and plug a fully-charged battery into an empty battery, for instance, the main fuse will blow. You'll have to replace it. It may be an automotive-type fuse you can buy at an auto parts store. If you make this mistake on an unprotected board, this large current may damage the main leads, blow the large traces in the parallel charge board, and damage the lower-charged battery, causing it to puff up or even catch fire.
- Note: the main fuse requires a *lot* of current to blow usually, so, you better still follow my guidelines below to keep you and your batteries safe.
- If the batteries are too far out of balance, or if you plug the balance leads in parallel first, before the main leads, an unsafe surge might go through the balance leads. A polyfused parallel charge board will automatically limit this current flow through the balance leads, preventing any problems. The polyfuses will simply close off (passively, but automatically increasing their resistance) to limit the current flow. If you make this mistake on an unprotected board, this large current may heat up and melt, damage, or blow the balance leads, blow the balance traces in the parallel charge board (I've done this by touching the balance leads to the balance board backwards, in reverse polarity, for instance), or cause a high enough current to the lower-charged battery to damage it.
- The same goes for reverse-polarity protection: a protected board may protect against this mistake. An unprotected board.....could cause the same problems as above. You might experience damage to the leads, blowing traces in the board, batteries on fire (if the reverse-polarity contact is prolonged), spark-welding of plugs together, etc.
Summary of the above: I like the raw, unprotected boards. They work well for me. They are very inexpensive. They are easy to find, and parallel charge harnesses can be easily hand-made if desired. If you follow my rules below, unprotected boards work fine. If you are prone to errors or want extra protection, consider buying the protected boards with things like main lead fuses, balance lead polyfuses, and reverse-polarity protection.
The Do’s
and Don’ts of Parallel Charging/A Couple Things to Know About Parallel Charging:
General Rules of Thumb (again, these rules work perfectly well for normal, unprotected [without fuses] parallel charge boards, and they work great for fuse-protected boards as well!):
- ONLY USE LIPO/LIFE/LI-ION
BATTERIES WITH THE SAME CELL COUNT, ***NO*** EXCEPTIONS. Ex: 2S with 2S, 3S
with 3S, etc.
- ONLY PARALLEL CHARGE BATTERIES OF THE SAME LITHIUM CHEMISTRY TYPE TOGETHER. Ex: LiPo with LiPo, LiFe with LiFe, Li-Ion with Li-Ion, but NOT LiPo with LiFe, LiPo with Li-Ion, etc.
- Preferably, use batteries
of similar capacities. Ex: 500~1500 mah batteries together, or 1000~2200 mah
batteries together, but not a 500 mah battery with a 10,000 mah battery.
- 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.
- 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.
- 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.
- Fuse-protected parallel charge board note: this rule of plugging in the main leads, waiting a while, *then* plugging in the balance leads always works if you also follow all of the rest of my advice, *even with fuse-protected boards.* However, people have told me that some polyfuse-protected parallel charge boards recommend you plug in the *balance* leads first, wait a while, *then* plug in the main leads. Why? Well, it seems logical to me that this is because the balance leads are polyfuse protected. Since the balance leads have polyfuses, they act as automatic current limiters to let the charge between the LiPos more slowly balance out. The main leads require too much current to be easily polyfuse-protected, however, so the balance leads are probably the ones with the current-limiting polyfuses. So...if your particular "protected board" recommends this "reversed plug-in order," that's ok. You may choose to follow their advice, depending on how well-designed their board is, or you can just follow my standard advice and order. I like to just stick to all of my standard rules here, because I know they always work, for all boards, whether fuse-protected or not, all of the time.
- 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. (Note: many parallel charge boards require the balance plugs to be in a certain orientation on one side of the board, and in a reverse orientation on the other side of the board, so pay careful attention to the plastic guides on the plugs to ensure you attempt to plug in the balance leads correctly.)
- 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. See plots below.
- 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.
- 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:
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, one
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:
I have been interested in propellers for a very long time. I've also been interested in how they produce thrust, and how forward velocity affects that thrust. Therefore, I've done a lot of thinking about it, and put a lot of time into understanding them better. Here is an equation that I came up with to quantify the thrust produced by propellers. I wanted it to be a simple approximation, with a minimal number of inputs. Therefore, it uses only the propeller's pitch and diameter (from the numbers on the front of the prop), and the RPMs at which the prop is spinning (this can be measured from a basic optical tachometer such as the one shown in the picture to the left). That's it!
