Battery Charger Design
thorin92
I am trying to design a battery charger for this battery:
http://uk.farnell.com/varta/55615605940/battery-pcb-mount-6v-5-v150h/dp/863981
I'm familiar with voltage regulators and current limiting etc. but am
unsure what parameters to use?
If I simply provide 6v limited to 70mA will this work?
Thanks in advance
mike
spec. Not likely.
>
> Thanks in advance
To design a battery charger, you have to disclose a LOT about
how the battery is used. Simplest thing is to charge it at 4.2ma.
Spec at the link says it will last 6-years being abused like that.
But that may not meet your cycle requirements, that you haven't stated.
MK
"thorin92" <thor...@gmail.com> wrote in message
news:36597a80-dd0f-4e96...@u41g2000yqe.googlegroups.com...
There is a lot more to charging batteries than that !
70mA is the fast charge max current and you must not charge continuously at
that level. Varta recomend voltage controlled charge. Look at www.linear.com
and find the LTC4060 data sheet - this may not be suitable for your
application but it will give you some ideas.
The Varta data on the Farnell web site is very sparse - look at some other
battery data and apps notes to find out more.
Michael Kellett
thorin92
> thorin92 wrote:
> > Dear group,
>
> > I am trying to design a battery charger for this battery:
>
>
> > I'm familiar with voltage regulators and current limiting etc. but am
> > unsure what parameters to use?
>
> > If I simply provide 6v limited to 70mA will this work?
>
> NO unless you comply EXACTLY with the conditions of that number in the
> spec. Not likely.
>
> > Thanks in advance
>
> To design a battery charger, you have to disclose a LOT about
> how the battery is used. Simplest thing is to charge it at 4.2ma.
> Spec at the link says it will last 6-years being abused like that.
> But that may not meet your cycle requirements, that you haven't stated.
Hi Mike, thanks for the reply.
The battery will be discharged at around 50mA over 3 hours, then re-
charged as fast as possible. Its for a portable device running a load
of LEDs (like a bike light)
A long life needs to be maintained because the battery will be
permanently attached to the pcb.
Does that inspire any more help? Cheers :)
mike
Often, the most difficult part of a project is writing the spec.
Words like "as fast as possible" will get you loads of input that
may be VERY bad tradeoffs.
Still insufficient information, but...
Unless you have some SERIOUS size/weight constraints, your battery
is probably too small.
What's your definition of "discharged"?
If you mean voltage cutoff at a level that guarantees none of the 5
internal cells gets reversed, you have a chance of success.
If you mean run it 'till the light goes dim, your battery will NOT have
long life.
Charging is problematic. Depending on the NUMBER associated with
"as fast as possible", you may have no option but to use a smart battery
charger chip.
I'm assuming this is a production device that requires agency approvals
and you care about customer retention. If it's an advertising giveaway
device, put lithium coin cells in it and ship it.
Replaceable AAA-cells that the customer has to recharge externally
will save you a LOT of grief. Customers cuss the internal battery
when the light goes dim at midnight when they're half-way home.
thorin92
The battery is for a research project that involves small plastic
glowing cubes, so the size/weight constraint is beyond AAA batteries.
By "fast", I'm only referring to a sensible charge time that would be
convenient. e.g. 8 hours is acceptable.
The cubes only need to operate for around 3 hours between charges, and
this is an AVR uC application so the system will switch off when the
voltage drops below 4.5v. Therefore the battery would not be fully
discharged (I assume).
The charger chip approach sounds the most plausible, what would you
recommend?
Thanks
John Fields
wrote:
>Dear group,
>
>I am trying to design a battery charger for this battery:
>
>http://uk.farnell.com/varta/55615605940/battery-pcb-mount-6v-5-v150h/dp/863981
---
That site's don for maintenance.
What's the Varta part number?
JF
thorin92
> On Tue, 5 Jan 2010 04:34:21 -0800 (PST), thorin92 <thori...@gmail.com>
> wrote:
>
> >Dear group,
>
> >I am trying to design a battery charger for this battery:
>
>
> ---
> That site's don for maintenance.
>
> What's the Varta part number?
>
> JF
The datasheet is here: http://www.farnell.com/datasheets/20247.pdf
cheers
ehsjr
Manufacturer Part No:
55615605940
mike
Doesn't make sense to go to a lot of trouble if it only has to
last for a few months and a small number of recharges.
Just charge it at C/10 and use a timer so it doesn't cook too badly.
You can use the AVR to implement the timer internally.
There are some considerations you may not have included in your analysis.
I'm a PIC guy, so I can't speak for AVR, but assume similar
issues apply.
Have you considered that the maximum voltage on the battery during
charge may exceed the supply voltage your AVR can tolerate? Don't
forget to include the increased internal resistance of an aging cell.
4.5V is 0.9V/cell. That's below what's normally done, but not extreme.
BUT, for such tiny cells at high discharge rates, it doesn't take
a lot of imbalance to create problems. 4 cells at 1.2V can reverse
the fifth one at 4.5V. It's not a matter if IF the cells become imbalanced,
just WHEN.
Depends on your actual discharge profiles, but for a timed charge,
you might set a flag if the 4.5V shutdown was triggered and apply
a longer equalizing charge for the next charge cycle.
If you open up one of the NiMH 9V batteries, you find this type
of cell construction inside. I've never had a good experience
with rechargeable 9V batteries. They always leak or short.
I don't have any experience fast charging such tiny cells, but you might
look into 0deltaV charging. You have a processor, use that to monitor
the charge. There are A/D converter resolution issues.
You're probably gonna have to turn off the charge current to measure
the volts.
thorin92
Hi Mike. 'Thanks for the detailed reply.
I'm not really looking for a sophisticated method of charging, just a
basic Voltage Regulator or Charger IC. Essentially the charger is
external and will simply connect to a 9V power supply (wall-wart).
I'm assuming that to achieve a constant current, the charge voltage is
variable. I've been looking at:
http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF
To implement the constant current source. However, I'm concerned with
any problems this method may involve. e.g.
-will the battery accept a voltage higher than 6v across the
terminals?
-will charging stop automatically?
My apologies but this area is new to me. Thanks
mike
You didn't answer my questions, so any info is a shot in the dark.
>
> I'm not really looking for a sophisticated method of charging, just a
> basic Voltage Regulator or Charger IC. Essentially the charger is
> external and will simply connect to a 9V power supply (wall-wart).
What's your definition of wall-wart? If it's a regulated wall-wart,
you can count on 9V if you comply with minimum load specs if any.
If you mean unregulated like you get from radio shack, the output
voltage can be VERY much different from 9V depending on the load.
>
> I'm assuming that to achieve a constant current, the charge voltage is
> variable.
I've been looking at:
>
> http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF
>
> To implement the constant current source. However, I'm concerned with
> any problems this method may involve. e.g.
>
> -will the battery accept a voltage higher than 6v across the
> terminals?
> -will charging stop automatically?
Putting a constant voltage across a NiMH battery is a good way
to make smoke. Stick in current and let the voltage go where it wants.
The problem is when to STOP charging. You need to provide some method
to stop the process when the cell is charged.
There's a quick and very dirty way to limit the current. Use an
incandescent
light bulb. For a relatively constant line voltage and a particular
wall-wart design,
a light bulb with enough volts across it to make it glow red will give you
constant-enough current over the range of interest.
Takes some experimentation. You can't just look up specs and decide
what to do. The parameters you want aren't specified.
I use #382 light bulbs just because I have
a box of them. This would be considered by most to be a BAD design,
but it can work in well constrained cases.
But you still want to STOP charging somehow.
You didn't respond to my caution about overvoltage.
You didn't say which AVR processor you're using, but all the ones
I looked at are specified for a maximum of 5.5V. Your battery will
be over 5.5V while charging...and during the initial portion of the
discharge. It's not unusual for a used cell to go to 1.6V during
charge. X5 that's 8V. Even at 1.4V/cell, you're still WAY over 5.5V.
Unless you're doing something to regulate it, you risk blowing the thing
up. If you ARE regulating it, there are issues in that department.
Electronic design is a science.
Product design is an ART.
There are many, many issues that are obvious to an engineer skilled in
the art that will bite you in the A$$.
The battery is one of the biggest problems in a battery-operated design.
Sounds like you need somebody local to help you design this.
thorin92
Voltage Regulator before powering the AVR, so the AVR supply is of no
concern.
The DC supply for the charger will be heavily filtered and regulated,
powered from a mains transformer.
I do not wish to use an AVR to manage charging, although if this is a
necessity for implementation of a timer then I guess its compulsory.
In the battery specs it states fast charge 70mA for 3 hours. Thus if I
build the LM317 regulator with current control to 70mA this should
suffice, but how would I know when the battery is charged? Is there
some indication from the terminal voltage with this type of battery?
Alternatively, a trickle charge of 4.3mA implemented using the LM317?
Best
Phil Allison
"thorin92"
The battery is for a research project that involves small plastic
glowing cubes, so the size/weight constraint is beyond AAA batteries.
** You should consider another option - Farnell have 2/3AAA size NiMH
cells (solder tagged too) that are far more rugged than your button cells
and have *double* the mAH capacity ( 300mAh against 150mAh ) and four times
the max discharge rate.
Size and weight are only slightly more at 45gms against 32gms.
http://uk.farnell.com/gp-batteries/gp30aaamhb/battery-2-3aaa-1-2v/dp/206519?Ntt=206519
Cost is about the same for either option.
The cubes only need to operate for around 3 hours between charges, and
this is an AVR uC application so the system will switch off when the
voltage drops below 4.5v. Therefore the battery would not be fully
discharged (I assume).
** At 0.9 volts per cell
- they certainly would be FULLY discharged.
.... Phil
Phil Allison
"thorin92"
I'm not really looking for a sophisticated method of charging,
just a basic Voltage Regulator or Charger IC.
** The problem is that " basic" chargers tend to ruin cells
and batteries very quickly - by overcharging them and
permanently altering the internal chemistry.
I'm assuming that to achieve a constant current, the charge
voltage is variable. I've been looking at:
http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF
To implement the constant current source. However, I'm
concerned with any problems this method may involve. e.g.
-will the battery accept a voltage higher than 6v across the
terminals?
** It really has NO choice in the matter .....
-will charging stop automatically?
** No way will that happen, unless a very clever circuit makes
it happen.
My apologies but this area is new to me.
** Not too hard to tell that one ..... which makes it near
impossible to educate you quickly enough.
So your best bet is NOT to try to be too clever or
"high tech" in your choices - rather go conservative,
use slow ( ie 10 hour) charging and cells sized to have
more capacity that you actually need.
A safety margin can be great asset against the unforseen ...
.... Phil
mike
> In the application, the output of the battery is regulated by an LDO
> Voltage Regulator before powering the AVR, so the AVR supply is of no
> concern.
>
> The DC supply for the charger will be heavily filtered and regulated,
> powered from a mains transformer.
>
> I do not wish to use an AVR to manage charging, although if this is a
> necessity for implementation of a timer then I guess its compulsory.
>
> In the battery specs it states fast charge 70mA for 3 hours.
Yes it does. If you start with a completely discharged battery, you can
charge it at 70ma for three hours...not four or five or six
hours...THREE. You tell by measuring the time. If you overshoot
by too much, the cell will vent and you'll get poor reliability.
If it's not fully discharged, you can't charge it for three hours.
If you know the discharge current, you could time it and put back
what you took out times some efficiency ratio. Could all be done with
internal timers
Charge termination is one of the more difficult parts of using rechargeable
batteries. There is no free lunch. If you fast charge it, you shorten
the life. If you overcharge it, you shorten its life.
I already suggested you look up "zero delta V" charge termination
strategies.
You can get clever, but not unless you (we) know all the details.
Thus if I
> build the LM317 regulator with current control to 70mA this should
> suffice, but how would I know when the battery is charged? Is there
> some indication from the terminal voltage with this type of battery?
>
> Alternatively, a trickle charge of 4.3mA implemented using the LM317?
>
> Best
>
>
You're all over the map with your requirements. 4.3ma won't get you
8-hours charge time.
>
I'm trying to help you, but you're not answering some critical questions.
How long does this thing have to work?
You say it's a research project.
If it needs to function for six months,
you have more options than if it is expected to run reliably for years.
Expected life under various overcharge conditions are right
there in the spec.
Your choice of battery is causing you more grief than if you could
tolerate a bigger one.
You're trying to make this much simpler than it is.
Wishing won't make it so.
Stuff should be as simple as possible, but no simpler...
Excuse me, but I feel a speech coming on...I can't stop it...here it
comes...
System design is an interactive process. You've asked for help
on a specific part. That's ok, that's what you're getting.
It is very common for an engineer to design a system that he thinks
should work...but creates some difficult problems in one area or another.
Often, eliminating the tunnel vision and re-examining the overall solution
results in much easier overall implementation.
But not knowing what you're doing prevents us from suggesting things
like using three cells large enough to give you the run time
you need AND accept overcharge that eliminates most of your
recharge issues, eliminating the ldo and using one port on the AVR
to implement a boost converter to drive the leds.
But there may be other issues that prevent that.
Don't mind me...I'm easily frustrated...
Spehro Pefhany
wrote:
>Dear group,
Find some actual manufacturer's recommendations with algorithm for
these NiMh cells.
dV/dt charge termination does not work if the current is too low
(typically trickle charging is not recommended)
dT/dt charge termination doesn't work if the current is too low
(typically trickle charging is not recommended)
You really need to look at cell voltage *and* temperature and control
current at least to one of several levels (pre-charge, fast charge,
and top-up) to reliably handle these things. If the cell voltage is
too high or too low, or the temperature is too high or too low, you do
different things, and if the rate-of-rise of cell temperature is high
enough you can terminate fast charge.
Spehro Pefhany
>Putting a constant voltage across a NiMH battery is a good way
>to make smoke.
Perhaps not smoke, but a nice spray of caustic electrolyte. Past a
certain point there is an internal "meltdown" and the reaction
continues without externally applied current, at least with some
cells.
thorin92
Hi, no problem and thanks for the help.
Unfortunately, the Battery has been decided and it cannot be changed.
I have tested the discharge rate/capacity for the spec and all is
fine. The device will operate for 4-5 hours until the AVR brown-out
kicks in and shuts down the system at 4.5v. As I stated before, I
assume the battery is not fully dis-charged at this point.
I would like the Battery to last as long as possible, and a trickle
charge at 4.3mA is acceptable.
Despite the years of experience I am obviously missing, I do want a
simple system, regardless. Since this is for a research project, the
charger design is a secondary priority, thus I'm not interested in its
sophistication provided that it works, safely, reliably, and charges
the battery.
My basic question at this point therefore, is; will the 317 reg
current limited to 4.3mA work? even if it takes 24 hours? Or is there
something I should be aware of using this method? That is the only
question I am asking.
Best
Phil Allison
"thorin92"
Unfortunately, the Battery has been decided and it cannot be changed.
** A choice made in ignorance of the consequences is NOT engineering.
You have painted yourself into a very small corner.
I have tested the discharge rate/capacity for the spec and all is
fine. The device will operate for 4-5 hours until the AVR brown-out
kicks in and shuts down the system at 4.5v. As I stated before, I
assume the battery is not fully dis-charged at this point.
** And you have already been told that is a WRONG assumption.
I would like the Battery to last as long as possible, and a trickle
charge at 4.3mA is acceptable.
** You have no idea what you are crapping on about.
Is a 60 to 80 hour charge time really OK ????
Despite the years of experience I am obviously missing, I do want a
simple system, regardless.
** You are starting to sound like an arrogant fool stamping its foot.
You must in software design - right ?
Since this is for a research project, the
charger design is a secondary priority, thus I'm not interested in its
sophistication provided that it works, safely, reliably, and charges
the battery.
** But have not the tiniest clue what the issues are.
My basic question at this point therefore, is; will the 317 reg
current limited to 4.3mA work? even if it takes 24 hours?
** It could take up to 60 hours at that rate and be impossible tell what
state of charge the battery is in.
If the pack has dropped to 4.5 volts under load - then it IS fully
discharged.
So you only need to TIME the recharge.
Need help to design a 4 hour timer ??
.... Phil
mike
As previously stated, 0.9V/cell ==> 4.5V is more than fully discharged.
As I stated before, I
> assume the battery is not fully dis-charged at this point.
I think you should check that assumption against the specs.
Never assume when you can use real info.
At this level of discharge you should probably pay some attention
to equalizing the cells. Small overcharge can do that.
>
> I would like the Battery to last as long as possible, and a trickle
> charge at 4.3mA is acceptable.
Not unless you change your charge time requirements AGAIN!!!
Quit using the phrase, "as long as possible." There is no MATH do
deal with that requirement. Everything must have a NUMBER. And pick
the number according to REAL requirements AFTER you've thought about it.
Arbitrary numbers are no better than no numbers.
>
> Despite the years of experience I am obviously missing, I do want a
> simple system,
Yeah, and I want Taylor Swift to show up at my door and call me "daddy",
or in my case, "granddaddy".
But we're both likely to be disappointed. Wishing DON'T make it so.
regardless.
There is no "regardless" in electronic design. REGARD is the essence
of design.
Since this is for a research project, the
> charger design is a secondary priority, thus I'm not interested in its
> sophistication provided that it works, safely, reliably, and charges
> the battery.
That's a nonsense sentence..."It doesn't have to be perfect as long as
it works perfectly". A charger is unimportant until you find that the
project fails because of the charger.
>
> My basic question at this point therefore, is; will the 317 reg
> current limited to 4.3mA work?
A 317 can be made to limit current.
There are "headroom" requirements that you probably meet.
A 317 is serious overkill for this application.
With 9V input and 4.3ma charge, you can get by just fine with a
resistor.
even if it takes 24 hours? Or is there
> something I should be aware of using this method?
Yes, you should be aware of the "division" button on your calculator.
That is the only
> question I am asking.
The short answer to the question you asked is, "the path you're suggesting
will not take you where you said you wanted to go."
It's not because of the 317. It's because of the simple math
you didn't do.
>
> Best
>
This is gonna be my last post...hopefully...on the subject.
If you won't listen, I can't help.
I apologize, but you've completely depleted my supply of tact.
THEMOSTIMPORTANTPARTOFAPROJECTISTHE SPECIFICATION/REQUIREMENTS DOCUMENT.
If you don't know what you're building, you can't design it.
You're all over the map. First it was 3 hours...now 4-5.
You want it to charge "as fast as possible" then 8-hours then 24 hours.
It's very simple math. 150 mah / 4.3ma ==> 34.883721 hours plus some
based on the charge efficiency. If you can't/won't
do that level of analysis, there's not much I can do to help you.
I gotta go to the store now and buy more tact. I'm sending YOU
the bill.
Phil Allison
"mike"
> You're all over the map. First it was 3 hours...now 4-5.
> You want it to charge "as fast as possible" then 8-hours then 24 hours.
> It's very simple math. 150 mah / 4.3ma ==> 34.883721 hours plus some
> based on the charge efficiency. If you can't/won't
> do that level of analysis, there's not much I can do to help you.
** Experience with NiMH cells tells me that at the C/50 rate little if any
charging occurs and at the C/100 rate - no charging occurs at all.
Only charging at current levels above these rates does the business.
So, in the OP's example, the first 1.5 to 3mA does nothing but tread water.
A second issue is being able to tell when the dish is cooked - ie, if one
cell in a pack requires 3mA to tread water and another only 1.5mA, then the
dish will only ever be cooked in parts.
Far better to charge moderately fast - like 5 hours at the C/4 rate ( 38mA
in the OP's case ) and KNOW when the dish is fit to serve.
> I gotta go to the store now and buy more tact. I'm sending YOU
> the bill.
** ROTFL !!
... Phil
ehsjr
Yes, the LM317 set for 4.3 mA will work. It is not the best
solution. Here's a simple circuit with a 317 and a TL431,
which is better than just the LM317 constant current circuit.
The 317 is set to charge the cells at C/10, which for your
cells is ~14 mA. The TL431 is set to steal current when the
battery voltage rises approaching full charge.
-----
+ ----in|LM317|out-+
----- |
Adj [85R]
| |
+-------+------+------+-->|---+
| | |
[680R] | |
| | |
__|_/ | |
/ / \ P | +
/431\--->0 1M Battery
----- T |
| | |
Gnd ----------------------+------+-------+
The 85 ohm resistance sets the LM317 to provide about 14 mA.
The pot feeding the 431 is set to make it conduct when the
battery reaches ~6.5 volts. Prior to it conducting, the full
~14mA current, minus the ~ 7uA the 1meg pot draws, goes to the
battery. When the 431 conducts, the 680 ohm resistor limits the
current through the 431 to a max of ~ 9.5 mA, leaving about 4.5 mA
to go into the battery.
Note that you could change the 85 ohms to 18 ohms to get
a charge current of about 70 mA, and the 680 to 100 ohms
to allow about 65 mA through the 431. But the sheet at the
url you posted mentions only the general** 14ma and 4.3 mA rates
so depart from that at your own risk.
**The 70 ma rate is specific for a completely discharged battery
for 3 hours at 20 C
Ed
Phil Allison
"ehsjr"
> Yes, the LM317 set for 4.3 mA will work.
** No it won't - dickhead.
That rate is only the " trickle charge ".
You better go look the term up.
... Phil
robb
"thorin92" <thor...@gmail.com> wrote in message
news:36597a80-dd0f-4e96...@u41g2000yqe.googlegroups.com...
It seems (from the other posts) that this battery charger bit is
not part of the research, just a project hurdle to get the
colored cubes working ?
the battery spec. says it is a drop in replace for other similar
ni-cad & nimh batteries and so i presume there are existing ready
made battery chargers that can charge this battery ?
couldn't one just buy a ready made charger match for this
battery and use that (**or the guts**) to solve the charger
problem ?
I have a box full of various rechargeable battery chargers for
nimh/nicad from 4.5/6v/9v and for a variety of devices like
camera batteries, RC batteries , rechargeable cells, etc. with
many output ranges.
maybe you just want to build one from scratch but that sounds
like a project all it's own.
my $0.02
robb
Phil Allison
"robb"
> the battery spec. says it is a drop in replace for other similar
> ni-cad & nimh batteries and so i presume there are existing ready
> made battery chargers that can charge this battery ?
** Wot a silly presumption.
The battery in question is a *solder in * replacement for MEMORY BACKUP
purposes .
So it fits on a large PCB and is supplied with a trickle charge by the same
board.
>
> couldn't one just buy a ready made charger match for this
> battery and use that (**or the guts**) to solve the charger
> problem ?
** No.
.... Phil
JosephKK
I kind of have to agree. The very poor assumption that you can get a
full 50 mA for 3 hours out of a 150 mAh cell scares me. If you want
more than a few cycles of battery life 200 mAh or 250 mAh is a better
choice.
thorin92
solution and I'll be prototyping this. I contacted VARTA directly and
their recommendation is exactly the same scheme as originally
proposed, using a regulator with constant current, but with a timer.
Thanks for the help here.
thorin92
actual circuit nominally draws around 20mA. When I tested the Battery
lasted for around 5 hours.
ehsjr
You're welcome. You might want to consider adding an LED that
glows when the TL431 steals current, to let you know that the
charge is complete.
Ed