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'[OT]: Trickle charging a 6 volt battery pack'
2001\05\07@032136 by Arpit

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From an unstable unknown (greater than 6v) power supple, how would I
trickle charge a 6 volt pack of AA niccads. In this case, I REALLY
mean trickle charge.  THey are being used as a clock radio backup, so
its's ok if they take 1 month to charge fully.

I am looking for a design which will

1. charge tham very slowly, but not overcharge them.

2 uses no transistors or ics

draws no more than 1mA


Thanks. I thought about just a resistor. But I don't know the
complications.

If anyone can offer an alternative solution eg using capacitors I
would be grateful.


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2001\05\07@033208 by David VanHorn

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>
>Thanks. I thought about just a resistor. But I don't know the
>complications.


Works for me :)
You may need a diode or two to manage switchover, but a simple resistor
works here.
I've used this in nicad memory backup systems.
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2001\05\07@033348 by Arpit

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On Mon, 7 May 2001 02:30:53 -0500, you wrote:

>>
>>Thanks. I thought about just a resistor. But I don't know the
>>complications.
>
>
>Works for me :)
>You may need a diode or two to manage switchover, but a simple resistor
>works here.
>I've used this in nicad memory backup systems.
What value do you use?

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2001\05\07@034627 by David VanHorn

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>
>What value do you use?

Short answer:  Large

Long answer:  Find out the battery's self discharge rate.
Make sure that "Large" results in a current that is not less than 2-10* the
self discharge rate.
Very dependent on the exact battery.

If you can't get that info, then just calculate for a low current.
You definitely want it to be <= C/100 in this service.

In either case, you need to know the range of variability of the charging
voltage.


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2001\05\07@035454 by Roman Black

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David VanHorn wrote:
>
> >
> >What value do you use?
>
> Short answer:  Large
>
> Long answer:  Find out the battery's self discharge rate.
> Make sure that "Large" results in a current that is not less than 2-10* the
> self discharge rate.
> Very dependent on the exact battery.
>
> If you can't get that info, then just calculate for a low current.
> You definitely want it to be <= C/100 in this service.

Or forget the calcs and rely on 20 years experience,
the Philips TVs use 170mAh batteries kept topped
by 7mA, some newer models use 110mAh buttons charged
at 5mAh. Charging occurs in standby and on conditions.
That current was measured with the battery full. They
also use a series diode.
I don't think 1mA is nearly enough. Even for 24-hour
trickle I would go for 3% of rated C. :o)
-Roman

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2001\05\07@040119 by David VanHorn

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>
>I don't think 1mA is nearly enough. Even for 24-hour
>trickle I would go for 3% of rated C. :o)
>-Roman

I guess my experience wasn't the same.
We were using SAFT, and a couple other vendors, of a three cell button pack
at something like 30mA IIRC.
Self-discharge wasn't large, and as long as you put a bit more in than
that, they stay up.
More than that, and you're making oxygen and hydrogen, and relying on
recombination to save you, and hydrogen is a slippery gas, gets out pretty
easy.  Those oxygen bowling balls aren't so hard to keep fenced in.


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2001\05\07@040743 by David VanHorn

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>
>Or forget the calcs and rely on 20 years experience,
>the Philips TVs use 170mAh batteries kept topped
>by 7mA, some newer models use 110mAh buttons charged
>at 5mAh.

Had another thought on that one.
The TVs only have to get past the 90 day(?) warranty, and it wasn't
critical if they did loose settings.
In my app, we warranted for a year, more with "special" customers, and the
nicad held not only the application program, but also stored transactions
(AKA money)  A bit upsetting to loose. Figure 3-10 transactions at $50-$200
each? Mr Merchant tends to get upset when this goes "poof".

The battery manufacturer may have specific advise on this, which I'd be
disposed to follow.


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2001\05\07@050217 by Roman Black

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David VanHorn wrote:
>
> >
> >Or forget the calcs and rely on 20 years experience,
> >the Philips TVs use 170mAh batteries kept topped
> >by 7mA, some newer models use 110mAh buttons charged
> >at 5mAh.
>
> Had another thought on that one.
> The TVs only have to get past the 90 day(?) warranty, and it wasn't
> critical if they did loose settings.
> In my app, we warranted for a year, more with "special" customers, and the
> nicad held not only the application program, but also stored transactions
> (AKA money)  A bit upsetting to loose. Figure 3-10 transactions at $50-$200
> each? Mr Merchant tends to get upset when this goes "poof".
>
> The battery manufacturer may have specific advise on this, which I'd be
> disposed to follow.


David, I wish to apologise, on re-reading my
post it sounded arrogant. I didn't mean that.
Just meant to imply that i'd prefer to trust
Philips designers and the track record of their
design that i'd witnessed rather than official
self-discharge rates or Asian datasheets. :o)

The TV's I mentioned trickled the NiCds at
about 4% of their rated capacity. In fairness
these sets are mostly left in standby, which
keep charging turned on. The NiCds generally
last 5 to 8 years, or that's when I replace
them. Pretty good really.

If you were mentioning 30mAh capacity NiCds
I have never seen them this small, and your
1% charge rate might be perfect. I was assuming
the post in question was referring to the "typical"
3-cell backup batteries as used in cordless
phones and other backup apps, typically about
110mAh size. Most times when you need a smaller
capacity than that it's best to use a 3v lithium
cell that doesn't need recharging and lasts for
5 years anyway.
-Roman

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2001\05\07@082002 by Alexandre Domingos F. Souza

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>Had another thought on that one.
>The TVs only have to get past the 90 day(?) warranty, and it wasn't
>critical if they did loose settings.
>In my app, we warranted for a year, more with "special" customers, and the
>nicad held not only the application program, but also stored transactions
>(AKA money)  A bit upsetting to loose. Figure 3-10 transactions at $50-$200
>each? Mr Merchant tends to get upset when this goes "poof".

       10 transactions / day = 3650 transactions / year = 36.500 transactions / 10 years

       Why not use an EEPROM instead? No battery, no nasty chemicals, smaller board, etc?!?

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2001\05\07@082007 by Alexandre Domingos F. Souza

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>If you were mentioning 30mAh capacity NiCds
>I have never seen them this small, and your
>1% charge rate might be perfect. I was assuming

       It's common in videogame machines (comercial ones) to store settings and high scores. It looks like a small cap.

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2001\05\07@093001 by David VanHorn

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At 09:23 AM 5/7/01 -0300, Alexandre Domingos F. Souza wrote:
> >Had another thought on that one.
> >The TVs only have to get past the 90 day(?) warranty, and it wasn't
> >critical if they did loose settings.
> >In my app, we warranted for a year, more with "special" customers, and the
> >nicad held not only the application program, but also stored transactions
> >(AKA money)  A bit upsetting to loose. Figure 3-10 transactions at $50-$200
> >each? Mr Merchant tends to get upset when this goes "poof".
>
>         10 transactions / day = 3650 transactions / year = 36.500
> transactions / 10 years
>
>         Why not use an EEPROM instead? No battery, no nasty chemicals,
> smaller board, etc?!?

At that point in time, they were available but VERY expensive, and with
very limited lifetime.
SRAM was a much more cost-effective solution.

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2001\05\07@121753 by Bill Westfield

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I think we used to use NiCd backed static ram, complete with fancy (dallas)
charge controllers and triple redundancy (three sets of three cells.)  I'm
not sure we ever had good reasons - that was what as on a card we had been
buying "in the really old days", and when we incorporated the same function
onto our own card, we apparently copied elements of the design.  Still, I
think the NiCds were a major source of failures...

In more recent designs, we're pretty much exclusively converted to lithium
backed-up memory (and occasionally EEPROM or Flash; but NO MORE NiCds),
either packaged as "Non volatile RAM" or separate parts ala PC config ram.
Since these are "mostly on" devices anyway, the shelf life of the lithium
batteries seems to be the limitting factor, and its a pretty special NiCd
design that'll work for the 5-10 years that a generic 2032 lithium coin cell
will do with "no fuss" and very little effort expended toward design.

BillW

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