Greetings:

I am interested in finding a source for schematics and/or a kit for
constructing a battery recharing unit.

I'm new to "homebrew". So kits and/or schematics with explicit
directions is appreciated.

The battery pack I am looking to recharge is a collection of approx 40
1.5v NiMH cells for about 28V overall. In fact, any
kits/plans/suggestion on making the battery pack itself would also be
appreciated.


Thanks!!

Having that many cells in series is a bit of a challenge. By my math, it's
about 2 dozen cells to get to 28 volts, so if you're looking for a tad more
capacity then maybe 48 cells, 2 sets of 24. (NiMH cells are about 1.2 volts
fully charged.)

In principle, it's pretty simple business to charge NiMH cells, just put a
little more voltage on them than they normally produce, and limit the
current. If you want to charge them fast, you need to monitor the
temperature of the cells and manage that temperature. If you are patient,
though, just charge them slowly and no need to watch the temperature.

However, when you get cells in series, slight differences between the cells
will cause one cell to take more of the charge than others. Eventually,
this leads to the failure of the battery, but for small numbers of cells,
it's typically not a problem. With your 24 cells in a line that could be a
problem, but again., if you charge them really slowly there's less need to
be concerned.

In use, you need to have the 24 cells in series, but for charging, you might
consider some sort of a scheme where there are fewer cells in series and
more in parallel. When charging the cells in parallel keep in mind current
limiting on each parallel branch individually, but that could allow you to
use something a little more common as a voltage source, say the station
supply or a 15 volt wall-wart. Older cell phone batteries used to use this
sort of scheme, where the connector was arranged so that the batteries were
connected to the charging connector differently than to the discharging
connector.

...

"Quiet Voice" wrote in message
om...
Greetings:

I am interested in finding a source for schematics and/or a kit for
constructing a battery recharing unit.

I'm new to "homebrew". So kits and/or schematics with explicit
directions is appreciated.

The battery pack I am looking to recharge is a collection of approx 40
1.5v NiMH cells for about 28V overall. In fact, any
kits/plans/suggestion on making the battery pack itself would also be
appreciated.


Thanks!!

Having that many cells in series is a bit of a challenge. By my math, it's
about 2 dozen cells to get to 28 volts, so if you're looking for a tad more
capacity then maybe 48 cells, 2 sets of 24. (NiMH cells are about 1.2 volts
fully charged.)

In principle, it's pretty simple business to charge NiMH cells, just put a
little more voltage on them than they normally produce, and limit the
current. If you want to charge them fast, you need to monitor the
temperature of the cells and manage that temperature. If you are patient,
though, just charge them slowly and no need to watch the temperature.

However, when you get cells in series, slight differences between the cells
will cause one cell to take more of the charge than others. Eventually,
this leads to the failure of the battery, but for small numbers of cells,
it's typically not a problem. With your 24 cells in a line that could be a
problem, but again., if you charge them really slowly there's less need to
be concerned.

In use, you need to have the 24 cells in series, but for charging, you might
consider some sort of a scheme where there are fewer cells in series and
more in parallel. When charging the cells in parallel keep in mind current
limiting on each parallel branch individually, but that could allow you to
use something a little more common as a voltage source, say the station
supply or a 15 volt wall-wart. Older cell phone batteries used to use this
sort of scheme, where the connector was arranged so that the batteries were
connected to the charging connector differently than to the discharging
connector.

...

"Quiet Voice" wrote in message
om...
Greetings:

I am interested in finding a source for schematics and/or a kit for
constructing a battery recharing unit.

I'm new to "homebrew". So kits and/or schematics with explicit
directions is appreciated.

The battery pack I am looking to recharge is a collection of approx 40
1.5v NiMH cells for about 28V overall. In fact, any
kits/plans/suggestion on making the battery pack itself would also be
appreciated.


Thanks!!

Be careful, NiMH batteries are more difficult to charge than this. There
exists a charging algorithm for NiMH that is rather complex. You
basically charge it as you stated, but when the current draw
changes a certain way, you must detect it then charge at a different rate for a
predetermined time. There are some IC controllers aware of this
algorithm, so your homebrew charger may be best to employ one of these
IC's...otherwise I suppose you could roll your own with a PIC or BASIC
STAMP and the right monitoring hardware. WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!

Good Luck.
--
Michael

To combat spam, I've put my address into the following gif. Sorry
for any inconvenience. http://mswindustries.us/email.gif

Be careful, NiMH batteries are more difficult to charge than this. There
exists a charging algorithm for NiMH that is rather complex. You
basically charge it as you stated, but when the current draw
changes a certain way, you must detect it then charge at a different rate for a
predetermined time. There are some IC controllers aware of this
algorithm, so your homebrew charger may be best to employ one of these
IC's...otherwise I suppose you could roll your own with a PIC or BASIC
STAMP and the right monitoring hardware. WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!

Good Luck.
--
Michael

To combat spam, I've put my address into the following gif. Sorry
for any inconvenience. http://mswindustries.us/email.gif

The best thing to do is check the Texas Instruments (www.ti.com) AND Maxim
(www.maxim-ic.com) websites. They have application notes with complete
schematics and their chips incorporate the proper charging algorithm.

"michael" wrote in message
news
Be careful, NiMH batteries are more difficult to charge than this. There
exists a charging algorithm for NiMH that is rather complex. You
basically charge it as you stated, but when the current draw
changes a certain way, you must detect it then charge at a different rate
for a
predetermined time. There are some IC controllers aware of this
algorithm, so your homebrew charger may be best to employ one of these
IC's...otherwise I suppose you could roll your own with a PIC or BASIC
STAMP and the right monitoring hardware. WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!

Good Luck.
--
Michael

To combat spam, I've put my address into the following gif. Sorry
for any inconvenience. http://mswindustries.us/email.gif

The best thing to do is check the Texas Instruments (www.ti.com) AND Maxim
(www.maxim-ic.com) websites. They have application notes with complete
schematics and their chips incorporate the proper charging algorithm.

"michael" wrote in message
news
Be careful, NiMH batteries are more difficult to charge than this. There
exists a charging algorithm for NiMH that is rather complex. You
basically charge it as you stated, but when the current draw
changes a certain way, you must detect it then charge at a different rate
for a
predetermined time. There are some IC controllers aware of this
algorithm, so your homebrew charger may be best to employ one of these
IC's...otherwise I suppose you could roll your own with a PIC or BASIC
STAMP and the right monitoring hardware. WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!

Good Luck.
--
Michael

To combat spam, I've put my address into the following gif. Sorry
for any inconvenience. http://mswindustries.us/email.gif

michael wrote:

WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!


Of course you can! I suspect you left off the word
"indefinitely".

I've been trickle charging NiMh's successfully for years.
A properly designed, time limited trickle charger works just
fine on NiMh. What you should not do is leave NiMh's on trickle
charge indefinitely. Some manufacturers claim you can leave NiMh's
on trickle charge for up to a year! See the following quotes:

"Charge your batteries fully, and then leave them in the charger
to maintain a continued full charge condition and your batteries
will always be ready for use. (Nexcell or GP brand NiMH batteries
can be left on trickle charge for up to a year with no damage to
the cells!)"
from: http://www.mahaenergy.com/download/mhc204freview.pdf


"Star Batteries sells only the highest quality NiMH rechargeable
batteries available. All of our batteries:

Meet or exceed their labeled mAh ratings.
Have a "button" top to fit perfectly into your device
Are compatible with "fast" chargers
Will accept a "trickle" charge without damaging your batteries"
from:http://shop.store.yahoo.com/starbatt...1mahaanir.html


"Fast Charging: All our NiMH series can be fast charged at rates
from 0.5C to 1C. Fast charge termination measures should be
employed. These include: Temperature Cut-Off ( TCO ), Rate-of-
temperature rise (dT/dt ), negative Delta V ( -dV ). A timer
control set at 105% of nominal capacity can be used as an
additional protection.
For maximum capacity, a trickle charge can be applied after
fast charging. We recommend top-off trickle charging at 0.1C
for 2 hours or 0.2C for 1 hour."
from: http://www.thomas-distributing.com/nimhbattery-faq.htm

michael wrote:

WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!


Of course you can! I suspect you left off the word
"indefinitely".

I've been trickle charging NiMh's successfully for years.
A properly designed, time limited trickle charger works just
fine on NiMh. What you should not do is leave NiMh's on trickle
charge indefinitely. Some manufacturers claim you can leave NiMh's
on trickle charge for up to a year! See the following quotes:

"Charge your batteries fully, and then leave them in the charger
to maintain a continued full charge condition and your batteries
will always be ready for use. (Nexcell or GP brand NiMH batteries
can be left on trickle charge for up to a year with no damage to
the cells!)"
from: http://www.mahaenergy.com/download/mhc204freview.pdf


"Star Batteries sells only the highest quality NiMH rechargeable
batteries available. All of our batteries:

Meet or exceed their labeled mAh ratings.
Have a "button" top to fit perfectly into your device
Are compatible with "fast" chargers
Will accept a "trickle" charge without damaging your batteries"
from:http://shop.store.yahoo.com/starbatt...1mahaanir.html


"Fast Charging: All our NiMH series can be fast charged at rates
from 0.5C to 1C. Fast charge termination measures should be
employed. These include: Temperature Cut-Off ( TCO ), Rate-of-
temperature rise (dT/dt ), negative Delta V ( -dV ). A timer
control set at 105% of nominal capacity can be used as an
additional protection.
For maximum capacity, a trickle charge can be applied after
fast charging. We recommend top-off trickle charging at 0.1C
for 2 hours or 0.2C for 1 hour."
from: http://www.thomas-distributing.com/nimhbattery-faq.htm

On Tue, 30 Sep 2003 09:39:08 -0400, "John Walton"
wrote:

The best thing to do is check the Texas Instruments (www.ti.com) AND Maxim
(www.maxim-ic.com) websites. They have application notes with complete
schematics and their chips incorporate the proper charging algorithm.

Indeed they do, but xpyttl's caution about "long" serires trings
remains just as valid a concern with decent controller chips. They
generally have no way to sense the individual cell SOC differences,
and therefore treat the whole string in terms of per-cell-average
voltage etc.

Additionally, any attempt at charge termination using temperature
sensing (absolute temp limit or rate of change) is VERY difficult to
implement on long strings simply because you need to sense at several
points and theses controllers are invariably designed for a single
temp sensing input. Been there, done that in a string 10, and we
gave up. The only other feasible option is to use MULTIPLE charge
controllers, each treating (and sensing) a small part of the total
string. This is often not a cost-effective approach. Sometimes,
redefining/rethinking the requirement leads to alternative approaches,
such as shorter strings feeding switchmode DC-DC boost converters.

"michael" wrote in message
news
Be careful, NiMH batteries are more difficult to charge than this. There
exists a charging algorithm for NiMH that is rather complex. You
basically charge it as you stated, but when the current draw
changes a certain way, you must detect it then charge at a different rate
for a
predetermined time. There are some IC controllers aware of this
algorithm, so your homebrew charger may be best to employ one of these
IC's...otherwise I suppose you could roll your own with a PIC or BASIC
STAMP and the right monitoring hardware. WARNING: YOU CANNOT
TRICKLE-CHARGE NIMH CELLS!

Good Luck.
--
Michael

To combat spam, I've put my address into the following gif. Sorry
for any inconvenience. http://mswindustries.us/email.gif