# Is this the right way to charge a battery?

Discussion in 'Electronic Basics' started by [email protected], Aug 1, 2005.

1. ### Guest

My brother got a Vectron ultralite made by Air Hogs. the battery is a
6v 100mah nimh, the cherger used is a 13v 600ma transformer. This
voltage and power rating sound way to high for a baattery so small.
Will this transformer kill the battery? would a lower power
transformer, like 7v 200ma be a good investment? Thanks in advance!

2. ### RedinkGuest

If you could possible give me the model number and the name of the
charger you are useing i will be glad to research on it and tell you
what is good to charge and not

4. ### Tom BiasiGuest

You don't need a transformer you need a charger.
Tom

5. ### Guest

It was the kind of wall outlet transformer used to charge any battery.

6. ### kellGuest

You are right. The transformer is too hot for the battery.
The crude solution, which is used in cheap chargers, is a series
resistor.
First: does this thing put out dc or ac? Obviously you need dc. If
it's ac, you need to rectify it or get a wall wart that does put out
dc.
Then you need to limit the current with a resistor. You should go for
a low charging current to avoid damage to the battery. That's because
you will have no way of knowing exactly when the battery has finished
charging, so you need a charging rate low enough that you don't have to
worry about how long you leave the battery charging. Nicad and nimh
batteries don't like overcharging. Find out the slow charging rate for
the size nimh you have (google is your friend). If they suggest C/20
for example, you need 5 ma (wow sounds small). Anyway whatever it is,
you can then figure out the resistor value. It's just 7 volts divided
by the charging current (7 volts across the resistor because you have a
13 volt source going into a 6 volt battery). (Of course the actual
voltage from your power source varies with the current, so it might not
be exactly 13 volts.) Then the power the resistor dissipates is going
to be current squared times resistance. Mske sure your resistor has a
higher power rating by at least a factor of three.

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)

8. ### ehsjrGuest

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

9. ### JazzManGuest

Speaking of battery charging circuits, I'd like to build
a nice NiMH charger for single cell AAs rated at around
1700-1800mAh. These are the Energizer cells that came with
a 4-cell charger, but I'd rather charge them with something
that actually monitored cell temperature and actively
altered charging current as needed to get them charged as
fully and reliably as possible.

can handle this? I'm not very good at design, but I
can build like a bad dog.

I've got a couple dozen cells here now at various
states of discharge, and don't want to use the supplied
charger since it seems that after only a dozen or so
charge cycles the NiMH batteries quit holding a charge.
I'm guessing this is the cae because the charger doesn't
shut down after full charge is reached?

If someone can point me to a few decent schematics I
can do all the board layout, etching, etc myself.

Thanks!

JazzMan
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10. ### Bob MonsenGuest

Here is a fast charge IC for NiMH/NiCD from maxim:

http://www.maxim-ic.com/getds.cfm?qv_pk=1666&ln=en

There are some example circuits in the datasheet.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

11. ### Guest

Yes, the power cube gives out dc power. the toy has a light on the
hovercopter itself that tells you when it's done chargeing, which with

12. ### William A. BongGuest

|
| Yes, the power cube gives out dc power. the toy has a light on the
| hovercopter itself that tells you when it's done chargeing, which with
|

It's typical that the equipment (hovercopter) has an on-board electronic
charging circuit which regulates the charging current and voltage, and the
external DC power source is rated a margin higher than the controlled
charging voltage and current.
Thus the capacity that you note in your power source is just a little
over-kill, and of no concern.

13. ### William A. BongGuest

| > | >
| > 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.

I'd follow the advice of another poster who references design specific IC's
such as those from Maxim.

14. ### Kitchen ManGuest

Which makes it a battery charger, not simply a transformer.

15. ### ehsjrGuest

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
This is even worse than the other statement. You are
recommending that he use 13 parts instead of 2, and understand
a 17 page datasheet in order to determine parts values. Think
about it - the poster doesn't even know that his proposed "zener
regulator" will do nothing but blow up the zener. Do you really
think he can follow the datasheet? Did you even look at it?
What do you think his chances are of determining the proper
charge rates, determining the correct values for R1 and Rsense,
and building and testing the circuit? Can he determine what
kind of heatsink, if any, to use for Q1?

Ed
P.S. I would have responded sooner, but I was travelling.

16. ### Bob MonsenGuest

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
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.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

17. ### ehsjrGuest

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