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'[EE] Design Challenge - Simple SLA battery charger'
2005\08\01@103144 by Russell McMahon

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Here we go again ...

I need to provide a battery and charger in a piece of consumer
equipment.
Battery needs to be rechargeable.
Target cost for battery and charger is zero $ :-)

5v supply so 6v battery OK as long as it has minimal droop across
discharge cycle.
Peak current maybe 400 mA for milliseconds and 250 mA for seconds.
150 - 200 mA for minutes.
Used for up to 5 minutes then it is either charged for 5 to 30 minutes
(possibly more) or discharged at under 200 microamps.
200 uA discharge may continue for up to a year. (1.75 AH of delivered
capacity).

Sound inexplicable?
It's exercise equipment.
It powers down in under 5 minutes of non use.
User action provides charging energy.
It can be woken up from standby by operation or a button press.

Battery would ideally have a 1 year life to say 50% self discharge
point.
Available charging current can be sensibly as high as desired although
C1 rate (1 hour charge) is probably sensible max and would be hard on
some battery types.

Battery can be treated somewhat roughly as long as it retains adequate
capacity and rechargeability capability for say 3 years. Longer is
good.
Weight and size (within reason) are not important. Actual battery
capacity unimportant as long as delivered capacity is OK.

At present I'm favouring a 6V sealed lead acid battery solution with
an extremely simple charger. This will limit peak charge current to
some selected value and clamp peak terminal voltage to a float value.
This is less than ideal as it reduces available capacity, but this is
not a major issue here.

NiCd, NimH have far too short a self discharge period.
LiIon tends to be dear and potentially tricky to charge (although
simple scheme that don't develop full capacity may be OK).
LiIon would be acceptable performance wise if cost was acceptable wrt
Lead Acid.

Any comments most welcome. Any suggested ultra simple/cheap charger
circuits also welcome. I have ideas already (of course) but there may
well be some brilliant ideas out there.



           Russell McMahon



2005\08\01@111325 by Howard Winter

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Russell,

On Tue, 02 Aug 2005 02:33:36 +1200, Russell McMahon wrote:

>...<
> LiIon tends to be dear and potentially tricky to charge (although
> simple scheme that don't develop full capacity may be OK).
> LiIon would be acceptable performance wise if cost was acceptable wrt
> Lead Acid.
>
> Any comments most welcome.

Just one:  as far as I know there is *no* simple scheme to charge Li-Ion batteries (like constant current or
constant voltage) - you have to follow the rules and they aren't that simple!  Lead-Acid are by far the most
simple to charge and the most forgiving of abuse.

And (OK, two!) if you're using the charging-effort as the fitness element you will need an alternative energy
dump (say a big hefty heat-sinked resistor) as well because you can't be sure the battery will always take the
energy the punter is putting into the system.

Cheers,


Howard Winter
St.Albans, England


2005\08\01@114004 by David VanHorn

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Just one:  as far as I know there is *no* simple scheme to charge Li-Ion
batteries (like constant current or
constant voltage) - you have to follow the rules and they aren't that
simple!  Lead-Acid are by far the most
simple to charge and the most forgiving of abuse.

Two stage, like SLA but with tighter tolerance, is what most of them use.
TI does some nice simple chips for that.
But, I would not recommend them into this application.


And (OK, two!) if you're using the charging-effort as the fitness element
you will need an alternative energy
dump (say a big hefty heat-sinked resistor) as well because you can't be
sure the battery will always take the
energy the punter is putting into the system.

True. A big battery can take a joke, maybe up to 10% of its rated current
for short periods.  You don't want a car battery for this, definitely a
marine deep cycle or similar.

Generically, lead acid batteries do well with a two stage charger, high rate
with one set voltage, till current drops below X, then low rate with a lower
set voltage.



2005\08\01@121509 by Alan B. Pearce

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>5v supply so 6v battery OK as long as it has minimal droop across
>discharge cycle.
>Peak current maybe 400 mA for milliseconds and 250 mA for seconds.
>150 - 200 mA for minutes.
>Used for up to 5 minutes then it is either charged for 5 to 30 minutes
>(possibly more) or discharged at under 200 microamps.
>200 uA discharge may continue for up to a year. (1.75 AH of delivered
>capacity).
>
>Sound inexplicable?
>It's exercise equipment.
>It powers down in under 5 minutes of non use.
>User action provides charging energy.
>It can be woken up from standby by operation or a button press.

Don't know about charging circuits, but assume you have the load sorted, as
you are not asking about that. However if you haven't, look at the Linear
Technology LT1616, LT1676, LT1776 and some others that you find as "related
chips" regulators as they have a shut down pin, which I suspect would do
your shutdown/power up function.

2005\08\02@050743 by Michael Rigby-Jones

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{Quote hidden}

Actually, it can be quite simple as long as the battery is not abused by e.g. over discharging.  The cheap chinese charger that came with my RC Heli for charging the Lithium-Polymer packs is very simple indeed (though I wouldn't necessarily want to leave it unattended).  Li-Po cells are more dangerous/easily damaged than Li-Ion as well.

>Lead-Acid are by far the most
>simple to charge and the most forgiving of abuse.
>

Not sure I agree, deep discharging kills them, and you still need a current limited constant voltage scheme.  Li-Ion essentialy requires the same scheme, but special conditions apply if the battery has ben over discharged and idealy some form of cell balancing would be beneficial (though I note that none of the laptop batteries I've pulled apart appear to have cell balancing, probably why one pair of cells always dies first).  What is really dangerous is charging to a too high terminal voltage which risks cell rupture (or "vent with flame" as the manufacturers like to call it).

NiCad's are probably the simplest overall, if ultimate life isn't a primary goal as a simple constant current, or even just a resistor can be used to charge them.

Regards

Mike

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2005\08\02@063505 by Michael Rigby-Jones

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>-----Original Message-----
>From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....mit.edu]
>Sent: 01 August 2005 15:34
>To: PIC List
>Subject: [EE] Design Challenge - Simple SLA battery charger.
>
>
>Here we go again ...
>
>LiIon tends to be dear and potentially tricky to charge (although
>simple scheme that don't develop full capacity may be OK).
>LiIon would be acceptable performance wise if cost was acceptable wrt
>Lead Acid.

Russell,

This is how simple a Li-Ion/polymer charger can be, as long as you include under-voltage protection in your design to ensure the cell voltage dosen't drop below ~2.9 volts.

http://www.shdesigns.org/lionchg.html

Regards

Mike

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2005\08\02@090340 by Russell McMahon

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> This is how simple a Li-Ion/polymer charger can be, as long as you
> include under-voltage protection in your design to ensure the cell
> voltage dosen't drop below ~2.9 volts.
>
> http://www.shdesigns.org/lionchg.html

I was pretty sure, from having looked at the charging specs, that
something about that simple was viable.

That circuit is very nice and is OK for manual use by an intelligent
person but still needs addition for full automatic operation. But by
the time you add the essential low voltage discharge protection and
trickle start up for low voltage cells you are probably better off to
also add a cheap processor.



       RM


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