| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 592.1 | | SNDCSL::SMITH | TANSTAAI | Thu Jun 30 1988 12:46 | 6 |
| I would guess that it would do more damage to the charger than it
would to the battery. However, a [lessee] 15 ohm, 1/2 watt resistor
in series should do the trick...
Willie
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| 592.2 | Not really... | LEDS::LEWIS | | Thu Jun 30 1988 15:40 | 24 |
|
RE .0
Maybe, but it's not worth trying. When you say a 9v 120mA battery
charger, I assume what you really mean is that it is designed to
charge a 9v 120mAH battery. Is that right? If so, the charger
probably puts out about 12ma of average current into 9v.
To overnight charge a 1200mAH battery, you really want 120ma average
current out of the charger. This would probably require changing
the series resistor inside the 9v charger, and chances are it will damage
the transformer in the charger (there could actually be a risk
of fire) because it is designed for only 1/10th of that current.
It can actually hurt the battery to charge it with too low a
current, so I wouldn't recommend trying to charge a 120mAH battery
with much less than 100ma.
So, go out and spend the bucks for the right charger, a good investment
considering you pay 20 bucks for these batteries. If you want to
build your own simple charger to save a few bucks, you can
do it with a transformer, diode and resistor. Need any more details?
Bill
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| 592.3 | <> | AKOV11::KAUFMANN | | Thu Jun 30 1988 17:03 | 9 |
|
Re .2
Well the sticker on the charger says OUTPUT: DC 9V 120mA
So, do your comments in .2 still apply ?
|
| 592.4 | You can use this charger with a simple mod | LEDS::WATT | | Thu Jun 30 1988 18:55 | 14 |
| You can use this charger if you drop the additional voltage across
either a resistor or a handful of diodes. Put an ammeter in series
and measure the current, then add diodes until you get 120 mA.
Use any 1 amp diodes like 1N4004. The battery will take 16 to 18
hours to fully charge at that rate. I charge my pack with a power
supply and a resistor since I haven't invested in a suitable charger
yet. I just put a meter in series and adjust the power supply voltage
to give me 120 mA. The battery voltage stays pretty constant so
the current doesn't change much. If it changes, it tends to drop,
so nothing will get smoked. If you use a resistor, make sure that
it has a sufficient power rating.
Charlie
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| 592.5 | You're ok with 120ma rating | LEDS::LEWIS | | Fri Jul 01 1988 00:16 | 9 |
|
>> Well the sticker on the charger says OUTPUT: DC 9V 120mA
>> So, do your comments in .2 still apply ?
Nope, that means it has a transformer that can put out the
current you need. The simple mod Charlie mentioned is fine.
Bill
|
| 592.6 | Constant V and Constant I | SNOC01::BROWNTONY | Tony Brown Sydney, Australia | Fri Jul 01 1988 03:10 | 27 |
|
I agree in general with Charlie, burt woild like to point out that
most of these simple converters are (very roughly) designed to be
constant voltage sources. Ideally, nicads require constant current
sources. Hence, you require a source of 120 ma for 14 -16 hours,
depending upon the initial state of charge of the battery. I would
guess that your charger is designed to give a constant 9v output,
for currents up to 120ma. Thus, as the battery gains charge, its
voltage will rise and the current delivered by the charger will
drop off, probably significantly. The easiest way is to adopt
Charlie's approach, but to be prepared to do some tweaking during
the charging to keep the current up to a reasonable value.
I think I've said this before, but please be careful and don't skimp
on battery quality or charging procedures. My preference is for
crashes to be caused by pilot error, as it is not too hard to minimize
most other causes. Why wreck many $$$$ and many hours work for
the sake of a flight pack which isn't 100%? This is hard for me
to say as I've just shorted out a cell while making up a brand new
pack. I'm tempted to go ahead, but I will definitely use the cells
for something else and buy four new ones. Cheap insurance.
End of diatribe!!!!
Tony
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| 592.7 | A _BAAAAD_ PLACE TO SCRIMP..... | PNO::CASEYA | THE DESERT RAT (I-RC-AV8) | Fri Jul 01 1988 11:43 | 12 |
| I couldn't agree more. Tony. The battery pack is the very heart
and soul of the radio system and the very survival of you plane,
boat or car depends on its health. To shortcut battery pack quality
and or charging in the interest of saving a buck or two is inviting
disaster and will be _ultimately_ more expensive in the end than
the coupla' bucks saved.
|
| | 00 Adios, Al
|_|_| ( >o
| Z__(O_\_ (The Desert Rat)
|
| 592.8 | Testing the charge. | IOENG::JWILLIAMS | Zeitgeist Zoology | Fri Jul 01 1988 11:53 | 12 |
| I would also add that it is a good idea to periodically test the
battery for capacity. You can do this by charging it, then discharging
it through a resistor. Keep a voltmeter across the resistor and
record the voltage every minute or so. Since I = V/R, you can figure
out the current. Integrate this over time, and voila, you have
mAH. Now compare what you measured to what the battery advertises.
If there's a significant difference, it's a fair guess that the
battery is reaching the end of its life. You should do this maybe
once every 5-10 charges, or whenever it's convenient. It is never
convenient when the battery is in flight.
John.
|
| 592.9 | Know your charger! | LEDS::LEWIS | | Fri Jul 01 1988 12:32 | 68 |
|
>> most of these simple converters are (very roughly) designed to be
>> constant voltage sources. Ideally, nicads require constant current...
>> Thus, as the battery gains charge, its
>> voltage will rise and the current delivered by the charger will
>> drop off, probably significantly.
Just about every charger sold with radio systems today use this
simple scheme - if you don't believe me open up your Futaba or
Airtronics charger someday. You'll see something like this :
-----|>|-----/\/\/\/-----
+
~ - (Batt)
-------------------------
Where ~ comes from a transformer and the resistor determines the
charge current. Effectively this _is_ like charging with a constant
voltage source that depends on battery voltage to give you the proper
average current.
So it sounds like Tony is saying that people shouldn't use the charger
that comes with the radio system - I don't agree, but do suggest
that everyone check the charge current because some of them come quite
a bit off from the ideal charge current of 1/10 C.
Now as far as the question about using the 9v charger, it would
actually be less susceptible to battery voltage variations (assuming
it has a higher voltage transformer). Take the two simple examples :
---/\/\/\----
+ +
Vin Vout
- -
-------------
Let Vout = 7.2v and Vin = 10v (which you might have with a charger
designed for charging a 7.2v battery. The resistor you would want
for 120ma is
(10 - 7.2) / 120m = 23.3 ohms
Now for Vin = 12v (which you might have with a charger designed
for charging a 9v battery), the resistor is
(12 - 7.2) / 120m = 40.0 ohms
Ok, now lets say Vout rises 800mv during charging. The charge current
in the first case will drop to
(10 - 8.0) / 23.3 = 85.8 ma. (29% drop)
And in the second case the charge current drops to
(12 - 8.0) / 40.0 = 100.0 ma. (20% drop)
So if you're going to use a constant voltage charger you're better
off with higher source voltage and higher series resistance.
I can't argue that the best case is a constant current
charger, but people have been using the chargers that come with
their radio systems for many years without any trouble - and using
the 9v charger presents no additional risk if properly modified.
Bill
|
| 592.10 | I re-iterate... | SNDCSL::SMITH | TANSTAAI | Fri Jul 01 1988 13:49 | 10 |
| > However, a [lessee] 15 ohm, 1/2 watt resistor
> in series should do the trick...
This will drop the voltage from 9 volts to 7.2 at 120 mA, keeping
the current reasonably constant and disippating about 1/4 watt (so
use a 1/2 watt resistor). You don't need to fool around with diodes
and such....
Willie
|
| 592.11 | great feedback | AKOV11::KAUFMANN | | Fri Jul 01 1988 15:41 | 5 |
| Thanks for the feedback, it's been very educational.
I will experiment with the 1/2 watt resistor mod.
|
| 592.12 | RC chargers | SNOC01::BROWNTONY | Tony Brown Sydney, Australia | Sun Jul 03 1988 19:53 | 12 |
|
re .9
I'm not criticizing the chargers that come with the radios as I
haven't looked inside any of the Japanese ones. I'm just advocating
a little caution when using what appears to be a non standard item.
Good luck.
Tony
|
| 592.13 | Chargers that come with radios are nothing special | LEDS::WATT | | Tue Jul 05 1988 13:07 | 21 |
| I'd like to reaffirm what BIll said here. The chargers that come
with radios are nothing special. They are very simple and cheap.
Almost any other charger is as good if you monitor the current and
find it to be correct. For example, let's say your line voltage
is a little high or low. None of the chargers that come with radios
have regulation in them, so your charge current will be high or
low. Nicads can be damaged due to overcharge if you charge them
at a current greater than .1 times their mAH rating. Also, if one
cell in your pack shorts out, the charger will now overcharge the
remaining cells. If you are not monitoring your battery voltage
occasionally, this can happen to you! Your transmitter and your
reciever will still function normally with one cell shorted. You
will not know that you have a problem until another one goes.
Therefor it is wise to check your charger's charge current frequently
and to check your battery voltage and capacity at least a couple
of times during the flying season. Our radio equipment works very
well until the batteries fail. Don't have it happen to you while
in flight.
CHarlie
|