Posted by William P.N. Smith on July 24, 2003, 5:41 pm
shoppa@trailing-edge.com (Tim Shoppa) wrote:
>But I'm looking for the state-of-the-art in charging good old lead-acid
>batteries (i.e. deep-cycle marine batteries and/or gel cells).
Those two different battery types are going to have somewhat different
charge voltages and current limits, but the basic idea is about the
same.
Ask in one of the alternate energy newsgroups (alt.homepower, etc)
about multi-stage battery chargers. They call them three-stage:
Constant current to a specific voltage (say 14.4V)
Constant voltage at that voltage till current drops below some
setpoint (say 200 mA)
Constant voltage float at a lower setpoint (say 13.8V).
Note that the first two "stages" are just a current limited power
supply.
Also, there are a bunch of different types of lead-acid battery, from
the standard type, to those with calcium grids, gel cells, AGM
(absorbed glass mat, essentially Gates Cyclon technology), etc. Each
has peculiarities (you have to be especially careful not to overcharge
gel and AGM batteries, as they can't be topped off) but those are
details.
--
William Smith wpns@compusmiths.com N1JBJ@amsat.org
ComputerSmiths Consulting, Inc. www.compusmiths.com
Posted by Jonathan Kirwan on July 24, 2003, 7:11 pm
On 24 Jul 2003 13:57:04 -0700, shoppa@trailing-edge.com (Tim
Shoppa) wrote:
>Maxim and others now have lots of chips for "smart charging" Ni-Cads, Li ion,
>and NiMH batteries. The "smart" battery packs have chips that seem to not
>only sense cell temperature but have other characteristics (accumulated
>charge/discharge, etc.)
>But I'm looking for the state-of-the-art in charging good old lead-acid
>batteries (i.e. deep-cycle marine batteries and/or gel cells). In particular,
>for long-term UPS usage. The battery manufacturers I've gotten technical
>data from (Panasonic and Gates/Hawker) seem to still recommend constant-
>voltage float charging for this application, sometimes mentioning a temperature
>coefficient for float voltage depending on cell temperature and usually
>specifying a maximum charge current (not important for the 99.9% of the time
>the UPS is just floating the batteries, but certainly important for
>initial start-up and recovery after an outage).
>Is there anything new to know beyond this? Last time I looked into the
>subject (over a decade ago) it seemed pretty much the same. With all the
>other battery technologies up-and-coming, are lead-acids still the same
>old boring story?
I'll add one note I didn't see posted to sci.electronics.design,
yet. Many years ago, there was an article about lead-acid
battery charging for motorcycle and car batteries. In it, the
author spent a lot of time talking about closely watching the
charging process and observing the little bubbles forming on the
electrodes and that these bubbles stay small enough long enough
to interfere with the plating back process for the lead. He
claimed that this cycle, repeated over and over, yields filar
structures, excessive surface area, possible bridging (if
something else isn't there to prevent it), and operation quite
different from design parameters, eventually.
So, the author designed a circuit which provided a hard charging
current interrupted periodically with a sharp reverse to "knock"
the bubbles off the sides. Then forward current again. Like
that.
Made sense to me and so I built it and tried it. Worked okay.
I also saw just what he'd said I see, the little bubbles forming
and slowly getting larger and eventually rising on their own,
without the "bump circuit" operating. And tiny bubbles barely
getting the chance to form and suddenly knocked off and gently
rising up to the surface every few seconds with the reversing
circuit engaged.
Jon
Posted by Dave Platt on July 24, 2003, 7:44 pm
>So, the author designed a circuit which provided a hard charging
>current interrupted periodically with a sharp reverse to "knock"
>the bubbles off the sides. Then forward current again. Like
>that.
>Made sense to me and so I built it and tried it. Worked okay.
>I also saw just what he'd said I see, the little bubbles forming
>and slowly getting larger and eventually rising on their own,
>without the "bump circuit" operating. And tiny bubbles barely
>getting the chance to form and suddenly knocked off and gently
>rising up to the surface every few seconds with the reversing
>circuit engaged.
A similar sort of "burp" charging has been recommended for NiCd
batteries as well, for similar reasons.
Another option along these lines is the a charger with a high-voltage
pulse generator - one which creates an inductively-generated voltage
spike periodically. These seem to be popular as "desulphators" - the
high voltage can reportedly break up the normally-insoluble crystals
of lead sulphate which form on the plates. From what I've seen of the
pulse-generation circuits, its waveform almost certainly goes negative
briefly, and might serve to knock off the gas bubbles as well as break
up the sulphate crystals.
--
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Posted by Peter Gottlieb on July 24, 2003, 11:30 pm
> The "smart" battery packs have chips that seem to not
> only sense cell temperature but have other characteristics (accumulated
> charge/discharge, etc.)
I have a patent on perhaps the first of these, and it goes back a while. I
had a micro which read voltage and current going into a battery pack of
lead-acid batteries. The way it worked was to wait for the voltage to get
high enough and the charge current small enough and then it considered the
battery charged. Then when the pack is discharged the micro integrates
current to determine state of charge. As the pack discharges (there may be
many in parallel) certain voltage "checkpoints" are reached and the micro
then looks at how many amp-hours the pack contributed to get to that voltage
and makes a determination as to whether or not the pack is good (i.e.,
meeting a certain minimum number of AH). An eeprom stores parameters such
as count of charge/discharge cycles and the pack capacity as of last
discharge so that the percent of charge remaining at the next discharge may
be determined. This was a number of years back when I originally filed the
patent (more than 10, maybe 15, I'd have to go look) and at that time the
recommended charge method was simple constant voltage float and we were
seeing battery lifetimes which the manufacturers said were all we could
expect to achieve.
Peter
Posted by Tim Shoppa on July 25, 2003, 7:53 am
> They call them three-stage:
>
> Constant current to a specific voltage (say 14.4V)
> Constant voltage at that voltage till current drops below some
> setpoint (say 200 mA)
> Constant voltage float at a lower setpoint (say 13.8V).
>
> Note that the first two "stages" are just a current limited power
> supply.
I've seen others recommend something similar to the first stage, but
they then apply that higher voltage once a week or so during float to
"equalize" charge. I'm not sure exactly what's unequal that requires
this... I do realize that not all cells will be identical but I'm not
sure how these differences are overcome by overcharging, as all the
currents in the series string are guaranteed to be identical.
And if at all possible I'd like to see these recommendations rise from
"old wives tales" to actual documentation by the battery makers. (As
opposed to sales hype by the charger makers!)
Tim.
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>batteries (i.e. deep-cycle marine batteries and/or gel cells).