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'[EE]Lithium battery charger with balancer proof of'
2012\05\14@061711 by cdb

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I'm just kicking this around and would be interested in other thoughts.

In the R-Pi E-14 forum someone wanted to know if there is such a thing as a series connected lithium battery charger.

I suggested a module made up of a lithium charger IC,  a switching mechanism that could isolate those batteries in the chain (envisaging no more than 3) once they've charged and some form of electronic load (probably a FET). It has been proposed that each  battery could have its own switch controller and thinking about this a small Pic or Atmel chip could be used to provide the intelligent switching per battery with the charger/battery gauge chip providing the correct charging algorithm, the controller chip would either switch it out of the pack chain or alternatively switch in the active load to maintain that particular battery charge state.
Does this seem workable in theory?

Colin
--
cdb,   14/05/2012
--




spam_OUTcolinTakeThisOuTspambtech-online.co.uk

2012\05\14@083221 by John Ferrell

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LiPo charging is a well developed technology.
Check out:
www.fmadirect.com/support_docs/item_1266.pdf
This handles up to a 10S battery setup Quickly, safely, and babies the batteries. Over kill, I think, for your App but a good place to learn a bit!

On 5/14/2012 6:17 AM, cdb wrote:
{Quote hidden}

-- John Ferrell W8CCW
“During times of universal deceit,
  Telling the TRUTH becomes a revolutionary act”
     George Orwell

2012\05\14@113713 by RussellMc

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> In the R-Pi E-14 forum someone wanted to know if there is such a thing as a
> series connected lithium battery charger.
>
> I suggested a module made up of a lithium charger IC,  a switching
> mechanism that could isolate those batteries in the chain (envisaging no
> more than 3) once they've charged and some form of electronic load
> (probably a FET) ...

> Does this seem workable in theory?

Yes. Almost anything sounds workable in theory :-)

I've thought of simple ways of doing this
One such follows.
This can be complexicated as desired

Assume all cells are roughly similar in performance and that gross
mismatch does not occur. In particular. no cell should ever need to be
below Vmin_usual so that it does NOT require slow trickling up
beforenormal charging can start. This condition can be accomodated but
is not covered below.

Each cell has a simple shunt  regulator placed across it. This can be
as simple as eg a TL431, a transistor and a few resistors. Regulator
is set to Vmax -  say 4.2V.
Cells are connected in series.

Let's have 10 cells to make numbers easier.

Provide 4.2 x N Volts current limited to desired charge rate. If
charge rate is C then I is Ichg is ~= 2A for a string of 18650 cells.
So Vmax = 4.2 x 10 = 42 V. I max = 2A.

Assume cells are all about fully discharged with Vcell ~~= 3V
Apply above supply to string of cells.
Initially cells will charge at 2A.
When any cell reaches 4.2V its shunt regulator will hold the cell
voltage at 4.2V, the cell will take what it wants in constant voltage
current tail mode and  the balance will be passed by the shunt
element.
Dissipation per shunt = 4.2V x 2A = 8.4W = say about 10W/cell.
Individual electronics modules COULD signal that they are in CV mode
but some deduction and a little playing would allow the all-done state
to be detected.

Current tail off and terminate could be added by connecting a series
element between cells and a shunt as before from top of cell to below
series element. Shunt reg works as before but when shunting current
rises to say 75% of max then series element is opened - disconnecting
cell and shunt goes short. This removes power dissipation in charged
cells. Shunted current is sent to next cells and smart smps charger
backs off voltage to suit demand. When all cells are charged the
needed voltage is N x short circuit shunt voltages and charger auto
terminates.

It would work :-).

Cost of the smarter system is two MOSFETS plus simple electronics per cell.
Simpler system = 1 MOSFET, TL431, few R's per cell.


Russell McMahon

2012\05\14@154013 by cdb

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Damn, I always make a problem more complicated, why on earth can I never think simple?

Thanks Russell something to ponder on.

Colin --
cdb, colinspamKILLspambtech-online.co.uk on 15/05/2012
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2012\05\14@154348 by cdb

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On Mon, 14 May 2012 08:32:17 -0400, John Ferrell wrote:
:: Check out:
:: http://www.fmadirect.com/support_docs/item_1266.pdf

That's sort of along the lines I was thinking of until Russell simplified me!

Colin
--
cdb, .....colinKILLspamspam.....btech-online.co.uk on 15/05/2012
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2012\05\14@155110 by alan.b.pearce

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Of course, you can already get ICs that will do all this for you ...

IIRC Linear Technology has them, but I am sure I have also seen them mentioned by another company, possibly TI or Analog Devices.

{Quote hidden}

>

2012\05\14@190413 by Lee Mulvogue

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       Or you could just buy a Turnigy 3S balancer and charger for $4.50,
does pretty-much exactly this.  Back-order at the moment though.
       www.hobbyking.com/hobbyking/store/__7637__Turnigy_balancer_Charger_2S_3S.html
[1]
       > I suggested a module made up of a lithium charger IC, a switching
> mechanism that could isolate those batteries in the chain
(envisaging no
> more than 3) once they've charged and some form of electronic load
> (probably a FET). It has been proposed that each battery could have
its
> own switch controller and thinking about this a small Pic or Atmel
chip
> could be used to provide the intelligent switching per battery with
the
> charger/battery gauge chip providing the correct charging
algorithm, the
> controller chip would either switch it out of the pack chain or
> alternatively switch in the active load to maintain that particular
battery
> charge state.
>
> Does this seem workable in theory?
-------------------------
Msg sent via Webmail - http://hosting.myob.com/

Links:
------
[1]
http://www.hobbyking.com/hobbyking/store/__7637__Turnigy_balancer_Charger_2S_3S.htm

2012\05\14@200726 by Isaac Marino Bavaresco

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Or a Turnigy Accucel 6, charges and balances up to 6S at 5A, discharges
and supports Li-xx, Ni-xx and Pb chemistries.
Just around $23.


Isaac



Em 14/5/2012 20:04, Lee Mulvogue escreveu:
{Quote hidden}

> http://www.hobbyking.com/hobbyking/store/__7637__Turnigy_balancer_Charger_2S_3S.html

2012\05\15@092130 by RussellMc

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>        Or you could just buy a Turnigy 3S balancer and charger for $4.50,
> does pretty-much exactly this.  Back-order at the moment though.

>        http://www.hobbyking.com/hobbyking/store/__7637__Turnigy_balancer_Charger_2S_3S.html


looks good.
Appears to use special magic.
It says it is a balancer.How do you balance 3S LiIon with no centre
taps? That's not to say you can't do it - just that the lead acid
method would be unwise.
With some battery chemistries you can bring up a low cell by charging
the overall battery at a higher than usual voltage so that fully
charge cells are driven into the steep upper tail of the charge curve
while the low cell ramps up to meet them.
LiIon frowns extremely severely on such practices. Normally, when a
cell reaches its upper voltage limit the cell  is put in a constant
voltage mode and accepts whatever current it wishes and the current
"tails off". The tail current is controlled by the most charged cells.
There is no way to apply a higher current that this without pushing
the cell out of CV mode and back into voltage increasing mode - magic
smoke may follow.

What MAY be done is to rely on the fact that charging is less
efficient as the end of CC mode is approached and then also as CV is
initiated. ie for a given number of amp-seconds of charge a cell will
accept a greater percentage of charge if it is more discharged. By
charging the good cells up into the edge of CV and then discharging
and repeating the lowest charged cells are operated in a higher charge
efficiency region and so their stat of charge asymptotes towards that
of the more charge cells. Maybe.



Russell






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>

2012\05\15@095326 by Isaac Marino Bavaresco

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These chargers DO use taps for each cell.
Nowadays all R/C hobby Li-xx packs have a balancing connector ( usually
a JST-XH connector ),besides the heavy duty main wires.
Low power chargers charge the cells only through the balancing
connectors, while discharge is done via the main wires.
High power chargers push current via the main wires and use the
balancing connector just to monitor and balance the charging current.

Some electronic speed controllers for brushless motors (Turnigy Sentry)
connect also to the balancing connector to monitor individual cell
voltages during discharge.


Isaac



Em 15/5/2012 10:20, RussellMc escreveu:
{Quote hidden}

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