Ni-Mh Battery charger - Ohms law

[suraj]

I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
Now I want to make a charger for it . If I have a 6Volt Transformer
and After Rectifying, what Resistance Should I use to obtain 160 mA @
3V DC?

Also what else Should I do to ensure that the Current that I give to
the Batteries is clean and Best for the Batteries?

Also If I fast charge the Batteries (500 mA for 5hrs) what is the
disadvantage ?

Many Thanks,Suraj

 

[default]

On 2 Oct 2003 01:20:49 -0700, surjones@yahoo.com (suraj) wrote:

I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
Now I want to make a charger for it . If I have a 6Volt Transformer
and After Rectifying, what Resistance Should I use to obtain 160 mA @
3V DC?

The easy answer is 19 ohms at two watts. The power line can vary
plus/minus 10% and 19 or 20 ohms is good if cost is the consideration.

Also what else Should I do to ensure that the Current that I give to
the Batteries is clean and Best for the Batteries

The typical technique in cheap battery chargers it to use a
transformer with a lot of magnetic leakage to limit current. The
better way is to add a current limiting circuit.

Also If I fast charge the Batteries (500 mA for 5hrs) what is the
disadvantage ?

The batteries should be designed for fast charging if that is what you
want to do. The disadvantage is usually shorter life. Left
unattended, without automatic circuitry, the cells can overheat and
rupture.

Many Thanks,Suraj
Good NiMh chargers watch the voltage as the cell charges. Great

battery chargers watch the voltage and temperature of the battery. By
monitoring battery voltage, the charger can throttle the charge
according to what the battery can use. NiMH batteries have a
decrease(!) in their voltage when fully charged, good chargers watch
for that and shut off when they see it.

1.2 volts per cell, so you want to charge two cells? Are they a
standard size and can they be separated? Rayovac makes a nice charger
that does AA and AAA NiMH batteries and alkaline. Monitors and shuts
them down as each of four cells charges. Cost in the US is ~$20 and
worth it. Walmart has cheap Quest chargers that cost ~$20 with a set
of four AA batteries included.

See> http://www.powerstream.com/NiMH.htm
http://www.sensorsmag.com/articles/0303/battery/main.shtml
http://www.extremetech.com/article2/0,3973,1155273,00.asp

Several IC manufacturers have chips designed expressly for charging
batteries.



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[Chang]

As your line voltage may change +/- 20% and the voltage drop on the
resistor depends on the current I recommend you built a current
source.

Answer to the question : 6V * SQRT(2) = 8.5 V. Across the resistor :
5.5 V with the current of 160 mA you need an R = 33 Ohm ( 1 W ).
(R=U/I)

 

[Arachnoid]

I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
Now I want to make a charger for it . If I have a 6Volt Transformer
and After Rectifying, what Resistance Should I use to obtain 160 mA @
3V DC?

Also what else Should I do to ensure that the Current that I give to
the Batteries is clean and Best for the Batteries?

Also If I fast charge the Batteries (500 mA for 5hrs) what is the
disadvantage ?

Many Thanks,Suraj

R = V/I?

you need to include the internal resistance of the battery before you
do the calculation.

NiMH cells are far more sensitive to overcharging than NiCads

If you are planning to fast charge you should measure the voltage /
cell and when it descends below a certain limit you should stop
charging.

If you are cycling NiMH you need to check that the voltage output /
cell doesn't go below a threshold value.

Can't remember all the numbers, but they are out there on the web
somewhere

 

[Harry Glinos]

"Arachnoid" <radicalfish@msn.com> wrote in message
news:70938d8d.0310020807.1d897e12@posting.google.com...

I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
Now I want to make a charger for it . If I have a 6Volt Transformer
and After Rectifying, what Resistance Should I use to obtain 160 mA @
3V DC?

Also what else Should I do to ensure that the Current that I give to
the Batteries is clean and Best for the Batteries?

Also If I fast charge the Batteries (500 mA for 5hrs) what is the
disadvantage ?

Many Thanks,Suraj

R = V/I?

you need to include the internal resistance of the battery before you
do the calculation.

How would you go about measuring that?

 

[Dr Engelbert Buxbaum]

suraj wrote:

I have Ni-Mh Batteries that say Standard Charge : 160 MA for 14 hrs.
Now I want to make a charger for it . If I have a 6Volt Transformer
and After Rectifying, what Resistance Should I use to obtain 160 mA @
3V DC?

You need a constant current source. The easiest way to achieve something
close is a light bulb in series with the accu. The resistance of the
glowing wire in the bulb increases with current, this will tend to keep
the current constant (measure current as a function of voltage through a
bulb to see the effect). Of course, electronic circuits can do a better
job, see for example a book on standard OP-Amp-circuits.

Also If I fast charge the Batteries (500 mA for 5hrs) what is the
disadvantage ?

Provided that you use accus certified for that, and that you keep an eye
on the accus temperature (reducing current as temperature increases),
none.

 

[Fritz Schlunder]

"Harry Glinos" <harry@NOSPAMMMMglinos.com> wrote in message
news:bIlfb.24404$ev2.5531299@newssrv26.news.prodigy.com...

R = V/I?

you need to include the internal resistance of the battery before you
do the calculation.


How would you go about measuring that?

You do not need to know the internal resistance for this type of charger.
The internal resistance of a AA size NiMH cell will typically be something
in the milliohms range, which is dramatically smaller than any external
current limiting resistor that you will attach. Therefore the interal
resistance of the cell will have a truly negligible impact.

The interal resistance of many AA size NiMH that are in good shape and just
recently charged/cycled will be something like 10-20 milliohms. AAA sized
cells will have slightly higher internal resistance. If the cells haven't
been cycled recently (say sitting on the shelf for a year and then just
charged up once but not yet used) the internal resistance gets much larger
(until you cycle it at reasonable rates a couple of times) but still far
less than 1 ohm. In almost all normal cases the effective resistance in
series with the cell is dominated by the battery holder contacts and wires.
In a typical battery holder the contact/wire resistance might limit the
short circuit current to something like 7Amps for a common 4 AA size battery
holder. If you just take the cell out of the holder and put a hard short
circuit (you should use a short piece of 18AWG wire or larger) on a
good/recently cycled cell you can achieve short circuit currents in excess
of 30Amps.

Really though dumb chargers are sub optimal. You should make a smart
charger using something like the MAX712 for instance:

http://pdfserv.maxim-ic.com/en/ds/MAX712-MAX713.pdf