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'[EE] Sealed Lead Acid charger~~'
2006\01\06@092306 by Russell McMahon

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The perennial "how to charge a sealed lead acid battery realllly
cheaply and in a vaguely OK manner" question. More of what I
discussed/asked a month or so ago. If I meander on about this it may
prove useful to others and may prompt some useful input. I'm not going
to spec this in fine detail as the general requirement is clear
enough.

Short description: Aim is to float a 12v battery at 13.7 volt nominal
at about zero cost. Rate of charge when not at float voltage is not
too too critical and can be adjusted by simple things like eg series
resistor and (gasp) limiting heat sinking on any regulator used to
force it into limiting / shutdown above certain power levels.

Also scale answers down by 2/3 for an 8V battery.

Input voltage is a vaguely constantish voltage which may be set within
reason with a say +/- 0.5 volt margin. battery has capacity in the
3Ah - 7 AH range. C/3 max charge rate is tolerable but as low as
/10  - C/20 for 3/7 AH battery probably acceptable. Typical bottom end
Asian SLA's spec sheets allow up to C/3 charge.

An 'obvious' solution is simply a 13V7 voltage regulator with suitably
low reverse leakage (doesn't drain battery), stable-ish output voltage
and temperature characteristics that vaguely match what the battery
wants. ST make such - their PB137, 13V7 regulator. (Interesting name -
Pb = lead, 137 -> 13V7), now you'll remember it :-) ).

2 V ish headroom needed for a 16V+ supply. Googling doesn't suggest
wide availability. Findchips (20 distributors) lists only Mouser at
$0.50/1000. I've little doubt that others do similar at a similar
price (as opposed to the largish range of 'proper" SLA charger chips
at a too dear $USN.

The LM340/LM7805 or LM317 with scaling resistors that one would
otherwise be tempted to use is cheaper BUT have nasty problems with
good accuracy without adjustment . EG the LM317 adjustment current is
50 uA typical and 100 uA max so even with a stiffish reference divider
the variation in set point voltage at 12v can vary by about half a
volt depending on initial adjustment current. This is far too much too
ignore for SLA float purposes. Also, back feed down reference chain
from battery is far too high too be tolerable so a series output diode
is needed, detracting from regulation accuracy. PB137 is rated at 10
uA draw AFAIR.

An LM358, precisionish cheapish reference, a high side PFET (or pnp)
and a small bipolar driver, add R's to taste, would do better and
probably cost about the same.

I can feel a processor based solution coming on - might as well keep
the processor busy. Vref will be calibrated during setup of the main
system and can then be accurate enoughish for this purpose. Processor
is AVR ATmega but all the above is processor independent (and almost
unrelated). Processor solution has advantage of allowing boost charge
to 14V4 ish and then drop back to float.

Thoughts?


       Russell McMahon

2006\01\06@105050 by sws11

picon face

Also make sure you have some good circuit protection at the output of
the charger -- a short and a reverse discharge into your charger is very
dangerous for a SLA battery charger.  One of my former employer's
competitor had 1800 reports of problems, 22 fires with no circuit
protection.  The 12V battery can dump about ~400 watts into the
charger.  Best to put the protection at the connector.  Their charger
was the most simple of all.  Just two rectifying diodes and a
transformer.  The battery acts like a big filter cap so they just set
the peak to about 13.7 volts - the worst method for the battery.  Also
there is a newish book on charging SLA batteries. *Valve-Regulated
Lead-Acid Batteries (Hardcover) - **ISBN:* 0444507469  - tells you
everything about the charging methods you would need to know and how the
batteries work.  It's too expensive $180, I picked up my copy from the
campus library to read.   I put over 100k SLA chargers in the field last
two years and used a regulator circuit - couldn't afford a micro or
custom chip.   There are now switch mode chargers you can buy at about
~4.00 OEM in 100k quantities.  The regulator circuit caused not one
problem and the batteries last a long time.  We had one regulator fail
out of the first 100k, but everything failed safe.
The down side is that the regulator circuit makes the transformer bigger
and heavy due to heat.

Companies like Yuasa, Panasonic and PowerSonic have good application
notes on charging also which show the basic circuits.  In tests we found
Panasonic and Yuasa batteries to be the best - months of testing many
batteries in hundreds of charge/discharge cycles, many vendors.  Even
had some batteries swell and nearly ready to burst from cheap vendors.

One guy returned one where his dog chewed through the cord 4 times, and
he re-connected it - he was lucky because he didn't pay attention the
polarity when he was re-connecting it, but got it right.

I know the processor/custom chip solution is best because you can
measure temperature, and correctly charge the battery in multiple modes
- constant current and constant voltage as required.  The book explains
the best algorithm in great detail.  Undercharging or overcharging the
battery can hurt it and shorten its life.

For many consumer applications a regulator is just fine,  long enough
battery life, low cost, and reasonable safety with circuit protection
(fuses and/or diodes).
You should be able to construct a charger in China for under ~US
$4.00.   Battery connectors are a big problem, however, not much out
there to help -- just two reasonable vendors with adequate pins, Tyco
and Anderson Power Products.  

Steve

Russell McMahon wrote:

{Quote hidden}

2006\01\06@114248 by Carey Fisher - NCS

face picon face


Russell McMahon wrote:
> The perennial "how to charge a sealed lead acid battery realllly
> cheaply and in a vaguely OK manner" question. More of what I
> discussed/asked a month or so ago. If I meander on about this it may
> prove useful to others and may prompt some useful input. I'm not going
> to spec this in fine detail as the general requirement is clear
> enough.
...
> Input voltage is a vaguely constantish voltage which may be set within
> reason with a say +/- 0.5 volt margin. battery has capacity in the
> 3Ah - 7 AH range. C/3 max charge rate is tolerable but as low as
> /10  - C/20 for 3/7 AH battery probably acceptable. Typical bottom end
> Asian SLA's spec sheets allow up to C/3 charge.
...
> Thoughts?
>
>
>         Russell McMahon
>
Iish reallyish loveish Russellsish decriptionish ofish
approximateish solutionish toish designish problemsish.
He'sish soish preciseish andish consistentish aboutish usingish
approximateish valuesish heish almost soundsish likeish
aish chefish addingish aish littleish ofish thisis andish
aish littleish ofis thatish toish theish octopusish stewish.
Iish canish useish serveralish ofish hisish designish
ideasish forish myish currentish customerish.  Maybeish aish
resistorish hereish andish aish resistorish thereish.  Hereish
aish capish, thereish aish capish, everwhereish aish capish.

2006\01\06@120036 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: Russell McMahon
>Sent: 06 January 2006 14:22
>To: PIC List
>Subject: [EE] Sealed Lead Acid charger~~
>
>
>The LM340/LM7805 or LM317 with scaling resistors that one would
>otherwise be tempted to use is cheaper BUT have nasty problems with
>good accuracy without adjustment . EG the LM317 adjustment current is
>50 uA typical and 100 uA max so even with a stiffish reference divider
>the variation in set point voltage at 12v can vary by about half a
>volt depending on initial adjustment current. This is far too much too
>ignore for SLA float purposes. Also, back feed down reference chain
>from battery is far too high too be tolerable so a series output diode
>is needed, detracting from regulation accuracy. PB137 is rated at 10
>uA draw AFAIR.

How about MOSFETs on the output of the regulator to disconnect it from the battery when no input voltage is present? Something with a nice low Rds(on) should (in my head at least!) cause minimal regulation impairment compared to a diode.  Or even, *shock horror*, a good old electro-mechanical relay?

How about the old LM723, cheap with built in current limiting, through will need an external pass transistor for the kind of currents you require.

Regards

Mike

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2006\01\06@133959 by Enrico Schuerrer

picon face

----- Original Message -----
From: "Michael Rigby-Jones" <spam_OUTMichael.Rigby-JonesTakeThisOuTspambookham.com>
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam@spam@MIT.EDU>
Sent: Friday, January 06, 2006 6:00 PM
Subject: RE: [EE] Sealed Lead Acid charger~~


| How about MOSFETs on the output of the regulator to disconnect it from the
battery when no input voltage is present? Something with a nice low Rds(on)
should (in my head at least!) cause minimal regulation impairment compared
to a diode.  Or even, *shock horror*, a good old electro-mechanical relay?
|
| How about the old LM723, cheap with built in current limiting, through
will need an external pass transistor for the kind of currents you require.


How about a simple, dedicated IC like the Philips TEA1102? There is a PCB in
the documentation, switchable for NiCd, NiMh, LiIon and SLA (from .5C to
5C)... and it's cheap...

If you don't find the data sheet I can send it...

regards

Enrico

2006\01\06@134438 by Peter

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Why isn't anyone using a smpsu for charging lead acid ? I haven't seen
any of those. Besides price, any other good reason ? It looks to me like
a SMPSU is the answer here. With regulated output voltage and input
power limiting it should be a perfect match to a normal lead acid
charge, no ? Maybe use one with current foldback regulation to
accomodate a really flat battery (which requires forming with low
current). But that should not happen with normal use. So why not ?

Then there is also the problem of charging (or rather not charging) a
lead acid 12V battery with a 12V wall wart (the wall wart does not have
enough volts to put current into an almost full battery). A buck
converter preceded by a current limiter would likely work perfectly
here. The current limiter would charge from flat, then the buck would
start when vbatt=vwallwart and provide final charge and trickle. What is
wrong with this scheme ? One could even use a PIC to control this.

Peter

2006\01\06@140319 by Michael Rigby-Jones

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{Quote hidden}

>{Original Message removed}

2006\01\06@141038 by Carey Fisher - NCS

face picon face


Russell McMahon wrote:
> The perennial "how to charge a sealed lead acid battery realllly
> cheaply and in a vaguely OK manner" question. More of what I
> discussed/asked a month or so ago. If I meander on about this it may
> prove useful to others and may prompt some useful input. I'm not going
> to spec this in fine detail as the general requirement is clear
> enough.
...
> Input voltage is a vaguely constantish voltage which may be set within
> reason with a say +/- 0.5 volt margin. battery has capacity in the
> 3Ah - 7 AH range. C/3 max charge rate is tolerable but as low as
> /10  - C/20 for 3/7 AH battery probably acceptable. Typical bottom end
> Asian SLA's spec sheets allow up to C/3 charge.
...
> Thoughts?
>
>
>         Russell McMahon
>
Iish reallyish loveish Russellsish decriptionish ofish
approximateish solutionish toish designish problemsish.
He'sish soish preciseish andish consistentish aboutish usingish
approximateish valuesish heish almost soundsish likeish
aish chefish addingish aish littleish ofish thisis andish
aish littleish ofis thatish toish theish octopusish stewish.
Iish canish useish serveralish ofish hisish designish
ideasish forish myish currentish customerish.  Maybeish aish
resistorish hereish andish aish resistorish thereish.  Hereish
aish capish, thereish aish capish, everwhereish aish capish.

2006\01\06@160627 by Russell McMahon

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> How about a simple, dedicated IC like the Philips TEA1102? There is
> a PCB in
> the documentation, switchable for NiCd, NiMh, LiIon and SLA (from
> .5C to
> 5C)... and it's cheap...

1.    Discontinued by Philips :-(

2.    Does the job well, but unlikely to have been cheap enough.

3.    Data sheet here

       http://www.semiconductors.philips.com/acrobat_download/datasheets/TEA1102_4.pdf

Product page here.

   http://www.discrete.org/pip/TEA1102.html#support


       RM


2006\01\06@162740 by Russell McMahon

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> He'sish soish preciseish andish consistentish aboutish usingish
> approximateish valuesish ...

:-)

Aim was to TRY to avoid the pointless pillorying and digression that
occurs when some few take exceptions to statements that they feel
imply unwonted precision in specs OR lack of necessary precision. This
sought to make it very clear that it was a
request-for-rough-design-discussion for a
simple-but-workmanlike-and-cheap-as-they-come SLA(ish) charger.
Seems to have worked so far. Feedback has been excellent and useful.


           Russell McMahon

2006\01\06@164320 by David VanHorn

picon face
I think you're overestimating the required precision.
LM-317 / 338 based chargers are all over the place, and working fine.

2006\01\06@173647 by Enrico Schuerrer

picon face
----- Original Message -----
From: "Russell McMahon" <EraseMEapptechspam_OUTspamTakeThisOuTparadise.net.nz>
To: "Microcontroller discussion list - Public." <piclistspamspam_OUTMIT.EDU>
Sent: Friday, January 06, 2006 9:59 PM
Subject: Re: [EE] Sealed Lead Acid charger~~


| > How about a simple, dedicated IC like the Philips TEA1102? There is
| > a PCB in
| > the documentation, switchable for NiCd, NiMh, LiIon and SLA (from
| > .5C to
| > 5C)... and it's cheap...
|
| 1.    Discontinued by Philips :-(
|
| 2.    Does the job well, but unlikely to have been cheap enough.
|
| 3.    Data sheet here
|
|
www.semiconductors.philips.com/acrobat_download/datasheets/TEA1102_4.pdf
|
| Product page here.
|
|     www.discrete.org/pip/TEA1102.html#support
|

look at this alternative...

http://homepages.which.net/~paul.hills/Batteries/BatteriesBody.html

--> 11.2.2 --> UC3906

regards
Enrico

2006\01\06@182536 by Russell McMahon

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>I think you're overestimating the required precision.
> LM-317 / 338 based chargers are all over the place, and working
> fine.

Perhaps I am.
*BUT* battery suppliers and all who talk at any length about their
maintenance make a big deal about the float voltage being correct to
something around +/-0.1 Volt. I really don't want to have to trim this
manually during setup (pot/labour/training/Murphy). And the eg LM317
spec sheet guarantees that you can't get anywhere near the required
specs without adjustment.

The semi processor-controlled system in my next email has the
advantage that the processor's voltage reference calibration, which is
automated, looks after the battery charger voltage calibration. Cost
is comparable to simple LM317 schemes and has some advantages.

Lest anyone think I raise topics like this solely to tell you what I'm
doing (ta da !) rather than actually ask questions - the input from
people in the last 12 hours has significantly influenced my current
intentions and helped clarify existing thinking.


       RM


2006\01\06@182536 by Russell McMahon

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> look at this alternative...

> http://homepages.which.net/~paul.hills/Batteries/BatteriesBody.html
>
> --> 11.2.2 --> UC3906

Does the job. But, as I said, cost is important and specialist ICs
like this tend to be several times more than the whole parts budget
alone.

What I'm most liable to do at present is to have a simple resistor
charger from a slightly higher supply (whose voltage I can set with a
little latitude) switched with a high side FET with the battery
voltage monitored by the processor. I could easily PWM this for linear
charge control but shouldn't need to. Resistor is dimensioned to
provide an acceptable range of charge currents across the normal range
of battery voltages and allow battery to be both boost charged and
floated if desired.

The saving grace is that the maximum designed charge is low enough
(for other reasons) that the series resistor will survive long enough
under battery terminal short circuit for the processor to detect this
and turn off the charge circuit.

Battery short circuit is unlikely due to construction method and
environment but can also be made tolerable with a small series
resistor as a fuse under extreme (and probably non existent)
conditions. This latter resistor is NOT intended as a fuse per se.

Principal parts are PFET, small npn, 3 small R, 1 x 5W R (maybe 1W)
and a diode. And a processor drive pin. Plus, of course a processor
ADC port - but I was measuring this rail already. (Actually a
downstream point which can be higher when another supply cuts in but
it serves the same purpose). The really enthused could probably work
out how to measure and drive with the same pin but I don't need to.


       Russell McMahon

2006\01\06@183513 by David VanHorn

picon face
>
>
> *BUT* battery suppliers and all who talk at any length about their
> maintenance make a big deal about the float voltage being correct to
> something around +/-0.1 Volt. I really don't want to have to trim this
> manually during setup (pot/labour/training/Murphy). And the eg LM317
> spec sheet guarantees that you can't get anywhere near the required
> specs without adjustment.


Ok, I'll agree at that point.  You would need to trim them.

2006\01\06@184507 by Brent Brown

picon face
> *BUT* battery suppliers and all who talk at any length about their
> maintenance make a big deal about the float voltage being correct to
> something around +/-0.1 Volt. I really don't want to have to trim this
> manually during setup (pot/labour/training/Murphy). And the eg LM317
> spec sheet guarantees that you can't get anywhere near the required
> specs without adjustment.

How close would you get with a 7805, a zener diode in the GND leg, and a  
series diode to prevent current drain from battery when charging power is
off? Pretty cheap. More accurate than resistor divider.

I have done this before and seem to remember it working ok'ish, but I don't
remember the details and can't find my schematics right now!

--
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton 2001, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell/txt: 027 433 4069
eMail:  @spam@brent.brownKILLspamspamclear.net.nz


2006\01\06@225715 by Russell McMahon

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>> *BUT* battery suppliers and all who talk at any length about their
>> maintenance make a big deal about the float voltage being correct
>> to
>> something around +/-0.1 Volt. I really don't want to have to trim
>> this
>> manually during setup (pot/labour/training/Murphy). And the eg
>> LM317
>> spec sheet guarantees that you can't get anywhere near the required
>> specs without adjustment.

> How close would you get with a 7805, a zener diode in the GND leg,
> and a
> series diode to prevent current drain from battery when charging
> power is
> off? Pretty cheap. More accurate than resistor divider.

A zener wouldn't provide either the initial required accuracy or the
longer term stability under parameter variations BUT a "precision
zener" would work just fine. eg a precision shunt regulator such as
LM385 or a TL431. This would provide the thermal shutdown and absolute
current limiting of the regulator and the accuracy of the reference.
The TL431 costs in the $US0.10 - $US0.20 / 1000 and may make a very
acceptable solution. I'm probably wed by now to my processor
integrated solution as it is cheap and allows a reasonable amount of
control BUT I'll do a quick design with TL431 and compare. Unlike uP
based design you have to have absolute and long term accuracy out of
the box if you want to avoid calibration. uP solution allows easy auto
calibration.



       Russell McMahon


2006\01\06@230930 by Russell McMahon

face
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face
> How about MOSFETs on the output of the regulator to disconnect it
> from the battery when no input voltage is present?

Would have to be connected 'backwards" to stop reverse flow via body
diode when off. so when power present the off FEt would be the body
diode and turning it on wuld make it somewhat better. As the blocking
diode is not a major perfomance issue this is not likely to be cost
effective compared toma diode.

\> How about the old LM723, cheap with built in current limiting,
through will need an external pass transistor for the kind of currents
you require.

LM723 is still an extremely useful device. My first ever voltage
regulator was a uA723 in a metal TO5 (?TO39?) can.

Add a cheap pass transistor and not much else and it would handle this
task and wuth a little care you could use foldback to achieve both
boost charging and float.

About $US0.30/1000 for the regulator which is good for 150 mA and
about 7V differential by itself. It's surprising how long sucha  dated
design can still compete. The ootb reference accuracy is only +/-5% so
calibration would be neweded but long term stability looks easily good
enough.

For those who haven't met it the LM723 can implement most forms of
regulators including switching, shunt and series and even floating
negative and positive topologies which exceed the ICs maximum voltage
rating. It does this by providing "building block" access to its
internals - it's the "555" of the early regulatr regulator world. I
had my first introduction to switching regulators in a uA723 app note
and they probably still publish the same app note today - but with a
newer date on it.


       Russell McMahon

2006\01\07@125422 by Vasile Surducan

face picon face
On my alarm systems I've made 5 years ago I've used a simple 14V
stabilizer followed by a series resistor. The charging current was
chosen with 5% greater than the standby current of the system. Once
per month the accumulator was discharged at about 25%. The average
life time it wasn't more than 2 years.
9Ah sealed lead accumulator, 3A siren, 40mA standby, using japanese acumulators.
More of those alarm systems in use today.

cheers,
Vasile


On 1/6/06, Russell McMahon <KILLspamapptechKILLspamspamparadise.net.nz> wrote:
{Quote hidden}

> -

2006\01\07@135734 by Russell McMahon

face
flavicon
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> On my alarm systems I've made 5 years ago I've used a simple 14V
> stabilizer followed by a series resistor. The charging current was
> chosen with 5% greater than the standby current of the system. Once
> per month the accumulator was discharged at about 25%. The average
> life time it wasn't more than 2 years.
> 9Ah sealed lead accumulator, 3A siren, 40mA standby, using japanese
> acumulators.
> More of those alarm systems in use today.

I take that to mean that most batteries die within 2 years.

More than 2 years would be "nice" as customers are liable to get a
little annoyed as the battery would tend to be a "non customer
replaceable item" for many people as it's inside largish consumer
equipment with panels with many screws. (Exercise equipment (again)).

I'm surprised that you don't get longer lifetimes as the 14v is not
far above the recommended 13.7-13.8 V float voltage. Which may
demonstrate that it is as critical as many people say :-). How
accurate is your 14 volts? Is it adjusted for each unit or set by
design?

To set charge current at 5% greater than standby you'd need to assume
some lower terminal voltage than 14 volts as your battery will tend to
zero charge at 14 volts OR the actual voltage is somewhat > 14V in
order to maintain charging.

More information would be of interest as it sounds like a good example
of a simple system which is OK but which highlights the inability of a
really simple system to do a really good job - as many people report
far longer lifetimes.

I'm looking at using a larger than required battery (7AH) as it's
actually cheaper than anything smaller (!) - presumably due to use in
volume in the alarm industry. The extra capacity won't go amiss as it
allows a very long capacity degradation before reaching the point
where startup use will not be possible.



       Russell McMahon






2006\01\07@143253 by David VanHorn

picon face
>
> I'm surprised that you don't get longer lifetimes as the 14v is not
> far above the recommended 13.7-13.8 V float voltage. Which may
> demonstrate that it is as critical as many people say :-). How
> accurate is your 14 volts? Is it adjusted for each unit or set by
> design?


I'd think that the cycling isn't helping.
For most LA batteries, my understanding is that lifetime is pretty much a
direct expression of number of cycles and depth of discharge.

To set charge current at 5% greater than standby you'd need to assume
> some lower terminal voltage than 14 volts as your battery will tend to
> zero charge at 14 volts OR the actual voltage is somewhat > 14V in
> order to maintain charging.


I didn't understand this part.
If the battery is floated across the load, then how do you know the battery
current explicitly?
Setting up the float to push a constant current into the battery is BAD.  An
ideal float charger approaches zero current into the battery.

In my systems, I set the float voltage on a power supply that is current
limited, and just let the load and battery do what they will.
As long as the load is less than the power supply's current limit, and the
power supply is set to the right float voltage, then the battery will be
charged.  The rate depends on the state of charge of the battery, and how
much is being drained off by the load.



> More information would be of interest as it sounds like a good example
> of a simple system which is OK but which highlights the inability of a
> really simple system to do a really good job - as many people report
> far longer lifetimes.


8+ years on my bench system, with few cycling, and other than the initial
settinga and occasional checking on the float voltage, no maintainance.

I'm looking at using a larger than required battery (7AH) as it's
> actually cheaper than anything smaller (!) - presumably due to use in
> volume in the alarm industry.



We called those "Bricks". :)


The extra capacity won't go amiss as it allows a very long capacity
> degradation before reaching the point
> where startup use will not be possible.


And your depth of discharge proportionally shallower.

2006\01\07@163015 by Jinx

face picon face
> In my systems, I set the float voltage on a power supply that is
> current limited, and just let the load and battery do what they will

The PSU output should be temperature compensated too, as the
float voltage varies with temperature (13.6-ish - 13.8-ish)

And as well as the float scenario, there's the charging to consider.
You can't always use the same method for every battery

eg

www.mil.ufl.edu/projects/koolio/Koolio/offline_webpages/battery_charg
er_info.htm

2006\01\07@165311 by Carey Fisher - NCS

face picon face


Russell McMahon wrote:
>>He'sish soish preciseish andish consistentish aboutish usingish
>>approximateish valuesish ...
>
>
> :-)
>
> Aim was to TRY to avoid the pointless pillorying and digression that
> occurs when some few take exceptions to statements that they feel
> imply unwonted precision in specs OR lack of necessary precision. This
> sought to make it very clear that it was a
> request-for-rough-design-discussion for a
> simple-but-workmanlike-and-cheap-as-they-come SLA(ish) charger.
> Seems to have worked so far. Feedback has been excellent and useful.
>
>
>             Russell McMahon

Russellish
Iish understandish yourish aimish andish agreeish withish
yourish techniqueish.  Iish haveish alwaysish lovedish
readingish yourish proseish andish enjoyish youris wideish
breadthish ofish knowledgeish aboutish soish manyish thingsish.
I'llish tryish usingish "ish"ish inish placeish ofish approximatelyish
andish seeish ifish itish soundsish asish coolish asish whenish
youish useish itish.
Sincereish regardsish
Carey(ish)

2006\01\07@181418 by David VanHorn

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>
>
> The PSU output should be temperature compensated too, as the
> float voltage varies with temperature (13.6-ish - 13.8-ish)


I would if the temperature varied much, but these are shirtsleves
environments.

I've done the TC routine with 317 based chargers though, it's not hard.

2006\01\09@042743 by John Ward

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face
Hey guys,

The Elektor magazine had an article a long time ago on how to use a pwm
output to be used as a reference for a power supply. The circuit also used
the PIC micro to sense current.
This has variable voltage and current adjustments so i'm thinking that if
you had a decent device (or maybe 4066 on decent 10 bit a/d) you could do a
loop and pretty much configure the charger for 8 or so channels easily.
One would simply end up with a few LM324's, shunt and a decent fet per
channel.

I'm about to embark on a similar project as i'm also irritated with
adjusting 10x lm317T's in a multiple SLAcid/gel cell charger.

However, i'm going to try and use an ATMEL with C.
my first choice for reference will be the TL431 seeing as every supplier in
ZA has an issue with service ;)

On the accuracy of the charge voltage, I have somewhat of a large outlay to
protect ...
The batteries themselves are charged, and then discharged until about 11V or
so when "cutout" pic switches the output off and increments a register in
eeprom for stats. THe batteries are often left on 13.8 volt float charge.

There is a diode (A600 iirc ) in series with the chargers output and the
charger is set for 14.2 volts with fluke multimeter.

The important thing is to calibrate it while its heatsinks are not in a
frozen state. Being a charger, it tends to get a bit warm. This being said,
the unit has never varied more than about +- 0.2 volts and this is within
the range of the battery, which they quote as happily being from 13.7-14.2
If you are not floating the voltage, you can even charge up to 15V.

The nice one to have, would be the one where you have the option of a diode
network to sense the battery temperature too. This i know makes a big
difference as on a hot day, the charger current limits alot more than a cold
day.

My guess is that our chargers will be too expensive to make and we'd have
NASA worthy specs :)

I do however think that battery charging is an avenue not well known and
this thread is providing an even more usefull function. The information in
this thread alone was never before seen in anyones reference texts ;)
J



On 1/8/06, David VanHorn <RemoveMEdvanhornTakeThisOuTspammicrobrix.com> wrote:
{Quote hidden}

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