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# simple question about batt chargers

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Hi all my friends

When somebody build a batt charger that stop charging when batt full, he
must use a comparator to compare batt voltage to some referance voltage..

when i conect a 3 volt batt to 5v power supply and wat to measure the
woltage across the connection, what would it be?

i ask this question to know at what level of charge i must stop the process

if i have four 1.2v batts (total = 4.8v), now these batts are not fully
charged, its voltage = 4 v or less.
i want to recharge them using 6v power supply. if i connect a voltmeter
across the connection, what would itsreading at thenbegining and at full
charge

i hope that u uderstand me

yours
walid

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Hi.

This is not a simple question. Actually we need more information:

What kind of battery You are using? NiCad/NiMh etc...

For example with NiCad the voltage is not so good indicator, as it stays in 1.2volts but still getting more and more charge. Finally the voltage goes down a little, indicating a fully charge.

Other indicator is that when the battery is full, the battery temperature has gone up to about 40-degrees centigrate.

Others here have answer with other battery-chemistry I believe.

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Hi Walid,
Read the tutorials about rechargable batteries at www.energizer.com . On the 1st page, click on Technical Info near the top, then on the 2nd page select Ni-Cad or Ni-MH type. Then look at the PDFs of charging and discharging.
The voltage changes with chemistry, temperature and the amount of charging current. Tne voltage reaches a peak near full-charge then drops a little. Some battery charger ICs detect the voltage peak or temperature rise to shut-off the charging.
Here is a typical voltage curve for a Ni-MH cell:

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audioguru i go to where you point me but nothing

my question is very simple
forget that it is charger
assume that u connect a fully charged 3v batt to a 5v ps and measure the voltage across this connection

thank you

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my question is very simple
forget that it is charger
assume that u connect a fully charged 3v batt to a 5v ps and measure the voltage across this connection

You cannot do that!
A Ni-Cad or Ni-MH battery is charged with a current. If you connect a fully charged two-cell battery (about 3.0V) to a 5V power supply then the battery will overcharge with a huge current. The huge current will cause the battery to overheat and vent its chemicals or explode.

I showed a curve of battery voltage vs amount of charge. It is more complicated because the voltage changes with the battery's temperature and its temperature is changed by charging.
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ok ok ok

i want to tell you that i recharge unrechagable batteries for years and reuse them
it is never explode
i have not faced any problem with them and after recharge them the last long time specially in remote control and sound toys

i want that info to stop the charge process whe it must stop
can you help me?

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Walid,
This was the 1st time that you said you are charging non-rechargable batteries.
I will not discuss it because they are not supposed to be charged.

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Hello Walid!
I think temperature is the most critical criteria to stop charging.  If u have lot of experience charging non-rech bats than u can tell us the avg time to charge common cells like AA, AAA or D.  It will be usefull

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I'm sorry, lets forget what i said before and start again ....

the emergency neon lamp has a rechargable 6v rectangular batt

if i want to charge it using 7.5 to 9 volt power supply and want to stop the charging process when the batt is fully charged using a voltage comparator

at what voltage i must stop

thank you

Hello Walid!
I think temperature is the most critical criteria to stop charging. If u have lot of experience charging non-rech bats than u can tell us the avg time to charge common cells like AA, AAA or D. It will be usefull

Hi Mr Muhammad Abu Bakar, in fact i want to learn about batt charging and how the designer make a decesion in chossing his components and not to measure temperature and ave time

yours
walid
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Hi Walid,
Rechargable batteries have different chemicals inside and need completely different charging circuits to match the type of chemicals. For some of the batteries, if it is fast-charged then a temperature sensor is recommended instead of a voltage sensor to stop charging, to avoid an explosion or a fire.
Ni-MH?
Li-Ion?
Li-Po?

The battery is probably lead-acid. Old lead-acid batteries in cars had a fully charged voltage of 13.8V, so half is 6.9V.
New car batteries have a fully charged  voltage of 14.4V, so half is 7.2V.
Some lead-acid batteries have a liquid electrolyte and others have a gel.
Some have calcium with the lead.
Do you know which type is your battery?

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Do you know which type is your battery?

yes, its a 3 cells Lead-acid batt
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I have never charged a lead-acid battery. My car does it without my attention.

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Look at

http://www.national.com/ds/LM/LM138.pdf

The old data books had a much better write-up, but there is a schematic for a 12V charger near the end. I have built a 40A and  5A version of this circuit and both have worked very well for over 20 years!!

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I have never charged a lead-acid battery. My car does it without my attention.

also my car does it without my attention, but i discuss for theory and then for using this theory as a designer, i told you that i live to learn

To indulis
thank you very much

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