Lithium Polymer Cell Balancer
These
are the plans for my Lithium Polymer Cell Balancer / Charger
Goals/Background
The goal with this simple inexpensive little device is to make it
easy to check the balance of the cells in 2S and 3S packs. If it
was not simple to check, chances are that I would not check then as
often as I needed to. I also wanted an easy way to balance the
cells once I found one that was out of balance. This little device
does both in one simple and elegant
design.
There
seems to be no standards to how each battery manufacture handles
their balancing taps. I also did not want to be tied down to one
manufactures batteries. Hence I came up with my own system! Another
goal was to keep connector count down as low as I could, and use
inexpensive readily available connectors stocked by my LHS.
A little background on my battery set ups. I have standardized on 2
connectors for all my batteries. I use Deans Ultra on all packs
that I will be using at 10 or more Amps, and Deans Micro Polarized
on all smaller packs. I only have and only plan to have 2s or 3s
packs. If I need something more, I will simply add packs in
parallel or in series to get to the larger amps or volts as needed.
This makes my charging and cell maintenance very simple. If a plane
is big enough to need a larger cell count, the additional weight of
the connectors should not make a
difference.
This
also makes ordering packs simple, as I can order them with the
Deans Ultra's and only have to add the JST connectors. I used the
male JST connector for the taps as they work better under the heat
shrink.
All my
chargers also have Deans Ultras male connectors on them. I use
adapters that I make to go from the Deans Male to anything else.
This simplifies charger cord management, and also makes bringing
cords into fire proof containers to charge all my LiPo's much
simpler.You are free to substitute what ever connectors you like,
or you can adapt my personal preferences if you
like!Building
My
Balancer / Charger is a simple device. The only components that you
need to construct it is a Double Pole Triple Throw (2P3T) roatary
switch (SW1), SPDT Switch SW2, knob, case, banana jacks, and
connectors of you choice. Depending on the switch you use you might
need a small circuit board from Radio Shack like that one in the
photos. I used a switch I had on hand, Some other possible
substitutes are listed at the end. One thing you want to make sure
is that you get a "Break before Make" type switch. This simply
means that if the switch is in position 1, Position 1 will be
disengaged from the common connector before position 2 is engaged.
Most Rotary switches work this way.
Unfortunately manufactures don't always list this with their
switches. I used a 4P3T (4 pole 3 throw) switch I had on hand.
allelectronics.com has a switch listed that should work well for
this project, and does not need a circuit board.
Link to the switch (You will only use 2 of the 4 poles on the
switch) Don't forget to order a knob and a small project
box.
Radio shack
also sells a 6 position switch that would work. You would only use
the first 3 positions. It is part number 275-1386 and is listed as
a 2-pole, 6 positions. Non- shorting. Rated 0.3A at 125VAC. This is
$3.29 and stocked by most Radio Shack stores SW2 Selects between 2 and 3 cell
operation. Be sure to set the switch for the proper set of cells
before connecting. If you have it in the wrong position you will
get wrong meter readings. Nothing bad will happen.
Using
Testing:
Set the 2 or 3 cell
switch to the correct positionPut the tester in position 1
Hook up your volt meter to the
banana jacks.Hook up
the battery to be testedRead the voltage on meter for each switch
position.
(NOTE:
On a 2s cell you only use the
first 2 positions)Please note that the size and length of
the wire on the balancer/charger could give you slightly different
voltage reading due to the resistance causing a small voltage drop
in the length of wire. When I built mine I tried to use
approximately the same size and length of wire for the leads going
to the packs under test.
Out Of
Balance:
Normally you will find that 1 cell reads a
little low, and 1 cell reads a little high. In most cases the high
cell will be more than the 4.2 volts if you charged the pack to its
full capacity and have a pack that is out of balance. This is one
of the reasons that packs fail, by ballooning or bursting into
flames. This is one of the reasons that it is important to check
and fix the problem on a regular basis. A typical charger only
looks only at the voltage of all the cells in series! If one cell
is exceptionally low, it will allow one of the other cells to be
charged to a higher than recommended voltage to make up for the
voltage in the lower cell.
Balancing:
Packs should be
balanced when they are fully charged or close to being fully
charged! I charge my cells to 4.1 volts per cell, by setting my
Triton charger to Lithium ION. This should charge the packs to 4.1
volt per cell (.1 volt under the max) This will undercharge a 3S
pack by .3 volts . I then check my pack with the checker. If I see
an imbalance I then charge each cell up the the 4.2 volts
individually. If I find a cell that is higher than 4.2 volts by a
bit, I slowly discharge the pack until that cell is under the 4.2
volts max. Then I charge each cell up to the 4.2
volts.
Hint:
If you do not have the
option of charging to a lower voltage per cell, you can simply stop
the charge early. You want to stop the charge at about 4.0 to 4.1
volts. Then check the pack, and charge each cell up individually to
the 4.2 volts per cell.
Charging:
I use my Triton charger
to charge each individual cell in the pack as needed to bring them
in balance. Select the cell you want to Charge with SW1. Then
simply set the charger for 1s (4.2 Volts) and charge as needed.
Keep in mind that the taps are light wire, and you need to charge
and discharge at very low amperage. I never exceed .25 amps! The
other nice thing here is that you do not have to plug and unplug
you pack while testing each cell, all you have to do is move the
switch!
Triton
Note: You can use the
volt meter in the Triton to check the balance of each cell. No
external volt meter is needed. The internal volt meter is enable by
pushing both the buttons down at the same time on the charger. The
Triton also will have its volt meter displayed when you are done
charging a cell. Simple use SW1 to select the cell you would like
to read.
Optional
Y Adapter to Monitor Cell Voltage While Charging the Pack in
Series:
After building and using the adapter I
wanted the ability to monitor the voltage of each cell while
charging the batteries normally in series. This way I could stop
the charge when 1 cell reached 4.2 volts, as well as check for
gross imbalances in the cells during charge. I thought about adding
another switch to the Checker/Charger, but after working the
schematic out I decided it would be safer to build a simple T
adapter. This would eliminate the error of having the switch in the
incorrect position and charging one cell at the 2S or 3S rate. Even
if your charger auto-detected the mismatch the possibility of the
switch being moved after the charge started would pose too much of
a danger.
To use
the Y adapter Hook up your volt meter to the Checker/Charger. Plug
your charger into one of the female connectors on the Y adapter.
Plug the Deans Male from the Checker/Charger into the other female
plug. Connect the battery to the Male connecter on the Y adapter
and the JST connector from the Checker/Charger to the battery.
Note: that the Deans female connector on the Checker/Charger is not
connected to anything when using the Checker/Charger in this mode!.
Set your charger up to charge the battery in its normal mode. You
may now use the switch on the Checker/Charger to monitor the
voltage of each cell during the charge. If you notice an imbalance
monitor the highest cell untill it reaches 4.2 volts. Then
terminate the charge. Remove the Y adapter, reconfigure the charger
for a single cell charge. Connect the Battery to the
Checker/Charger. Connect the charger to the female Deans connector
on the Checker/Charger, and top off each cell. When you are
completed you should have a balanced pack.
Disclaimer:
If you are not familiar
with the use and abuse of Lithium Polymer cells, and the in and
outs to building and balancing packs, you should not attempt to
build this! You should know what you are doing before playing
around with electronics and Lithium Polymer batteries. This is a
design, that I came up with that works good for me. I am not an
electrical engineer, just a simple hobbyist. Use at your own risk,
as I said I am not an expert This is simply documentation of the
system that I use. Please take all charging and handling
precautions when working with Lithium Polymer Batteries!
More
Information:
Lipo Cells Charge to 4.2 volts per
cell!Never discharge to
below 3 volts!The
resting voltage of the cells should match!
You should check your packs after they have charged, and set for a
bit. If you discharge
and charge one cell you will have to wait for the cell to settle
before taking a voltage reading again.
This is one reason that charging each individual cell up from a not
fully charged pack is a good idea!
Internet
Links:
There is lots of good info at the
rcgroups.com
site.
Check out the battery forums and be sure to check out the Ultimate
Guide while there!Ultimate
Guide to Lithium Polymer Batteries at
RCGroups.com
A Review
of Cell Equalization Methods for Lithium Ion and Lithium Polimer
Batteries
Schematic