Re: Is this the right way to charge a battery?

Bob Monsen wrote: 
ehsjr wrote:

William A. Bong wrote:

"ehsjr" <ehsjr@xxxxxxxxxxxxxxxx> wrote in message
news:4WgIe.6019$%a2.5766@xxxxxxxxxxx
| ChadMan wrote:
| > "kell" <kellrobinson@xxxxxxxxxxxx> wrote in message
| > news:1122946886.804292.80170@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| >
| > I wanted to ask a similare question. I have a bunch of 7.2 volt 600 MAH
bat
| > packs. I want to make a charger for them.
| >
| > In this fellows case wouldn't it be better to use a Zener regulator so
that
| > it wouldn't overcharge?
| >
| > Does anyone know of a schematic for these 7.2 AA packs? I'm thinking
that I
| > could use something like this:
| >
| >
| > Batt (+) ___
| > o-------o----|___|-----(+)
| > | Charge
| > | x-former
| > '-----z<---o---(-)
| > Batt (-) |
| > o------------------'
| > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
| >
| >
|
| That will blow up the zener.
|
| Here's a simple circuit for slow charging a 600 mah
| 7.2 volt pack:
|
| -------
| +12 ------Vin| 7805 |Vout------+
| ------- |
| | [R] 91 ohms 1 watt
| | |
| +--------------+-----> To Batt +
|
| Gnd-----------------------------------> To Batt -
|
| The 7805 will need to dissipate about .3 watts. The TO-220
| package does not need a heat sink at that power, but I'd
| use a small sink anyway. The circuit will limit the charge
| rate to ~ 55 mA, roughly C/10, which is desireable for
| a 14 hour charger. Note that Gnd from the 12 volt supply
| connects *only* to the negative side of the battery.
|
| Ed

Your circuit offers only current control using the 7805 Voltage regulator,
which will certainly overcharge the batteries if their voltage is not
carefully monitored. 



Rubbish.  Where did you ever see a 14 hour charger
with *any* voltage monitoring, let alone "careful
voltage monitoring"?  14 hour chargers have been in use
for years with *no* voltage monitoring, and no problem,
unless the batteries are abused. Just put the batteries in
when they are low, and take them out after 14 hours have
elapsed. If you leave them in far in excess of 14 hours,
or charge them when only a small percentage of the charge
has been used, you're abusing them


Well, it is fairly easy to build a simple CCCV charger. Why burn up your batteries by overcharging them if and when you get distracted, and leave them charging for a week?

Using a TL431 and three resistors with your proposed circuit will limit the voltage to some chosen value. Just use the TL431 to yank on the adj pin of the regulator when the voltage gets to the limit point. Use a 1k resistor between batt+ and adj. You can build in a nice "Done" indicator by putting an LED between the TL431 and the adj pin. Set the final voltage to 8.4V, and you have it made (unless one of your cells is bad, in which case you'll destroy them all).

Also, using an LM317 makes more sense, because the required resistor will dissipate less energy, and the required input voltage will be lower. Your charger will only allow charging up to 7V given 12V input. Using the 317, which is the same as a 7805 except that the drop is only 1.2V, means you need a 22 ohm resistor for a similar current.


Good point about the LM317 and the input voltage. The
7805 circuit won't go over 7 v with a regulated 12v in -
he would have to use an unregulated 12 v supply that
droops to 12 v under its rated load with the 7805 circuit.

While is is fairly simple to use a TL431, it is not as
simple as the proposed circuit: two components.  It doesn't
get much simpler than that. Remember, this poster doesn't
know enough electronics yet to use a zener properly, so
keeping it as simple as possible is the goal.

Ed
.

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