Battery Charger



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Nlee1875

Guest
January 26, 2004

Hello...

I have a collection of "wall warts" (class 2 transformers) battery chargers that are normally
included in rechargeable lighting systems.

Question 1: How can one tell if the wall wart is specifically designed for charging NI-CAD, NI-MH,
and/or SLA batteries.

Question 2: What kind of fire hazard/risks are the manufacturers suggesting if you were to
accidently charge a NI-CAD or NI-MH battery pack with a wall wart that is designed for SLA
battery packs?

I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging and
Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want to
regularly use for NI-CAD / NI-MH battery packs.

Any thoughts?

Much obliged. Nick Lee Sparks, NV 89431 carless since January 2000.
 
J

Jacques Bilinsk

Guest
>>
> I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging
> and Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want
> to regularly use for NI-CAD
/
> NI-MH battery packs.
>
> Any thoughts?
>
I don't think you should do this. A 'smart' charger may work for both Ni-Cd and MiMh but SLA
batteries are quite different. The chargers use different methods of determining when the battery is
charged for different battery chemistries, also some chargers are designed for a specific number of
cells. You should probably invest in a smart charger designed for the type of cells you are using,
although if you use low charge rates (overnight or longer) you can get away with dumb chargers
without causing a significant reduction in battery life. NiCds and SLA are more forgiving of
overcharging than NiMh. Some people also use timers to terminate charging. There is a Topica list
called bikecurrent dedicated to bike electronics, where there are knowledgable people re this.
 
Z

Zilla

Guest
Jacques Bilinski wrote:
>> I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging
>> and Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want
>> to regularly use for NI-CAD / NI-MH battery packs.
>>
>> Any thoughts?
>>
> I don't think you should do this. A 'smart' charger may work for both Ni-Cd and MiMh but SLA
> batteries are quite different. The chargers use different methods of determining when the battery
> is charged for different battery chemistries, also some chargers are designed for a specific
> number of cells. You should probably invest in a smart charger designed for the type of cells you
> are using, although if you use low charge rates (overnight or longer) you can get away with dumb
> chargers without causing a significant reduction in battery life. NiCds and SLA are more
> forgiving of overcharging than NiMh. Some people also use timers to terminate charging. There is
> a Topica list called bikecurrent dedicated to bike electronics, where there are knowledgable
> people re this.

Hmmm, I'm not saying to "go ahead and do this" but, isn't current, current, voltage, voltage, and
power, power? Or Power = voltage x current. So as long as the charger can handle the
current/voltage/power requirement for a battery, shouldn't it work?

--
- Zilla Cary, NC (Remove XSPAM)
 
P

Peter

Guest
Zilla wrote:

> Jacques Bilinski wrote:
>
>>>I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging
>>>and Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want
>>>to regularly use for NI-CAD / NI-MH battery packs.
>>>
>>>Any thoughts?
>>>
>>
>>I don't think you should do this. A 'smart' charger may work for both Ni-Cd and MiMh but SLA
>>batteries are quite different. The chargers use different methods of determining when the battery
>>is charged for different battery chemistries, also some chargers are designed for a specific
>>number of cells. You should probably invest in a smart charger designed for the type of cells you
>>are using, although if you use low charge rates (overnight or longer) you can get away with dumb
>>chargers without causing a significant reduction in battery life. NiCds and SLA are more
>>forgiving of overcharging than NiMh. Some people also use timers to terminate charging. There is
>>a Topica list called bikecurrent dedicated to bike electronics, where there are knowledgable
>>people re this.
>
>
> Hmmm, I'm not saying to "go ahead and do this" but, isn't current, current, voltage, voltage, and
> power, power? Or Power = voltage x current. So as long as the charger can handle the
> current/voltage/power requirement for a battery, shouldn't it work?
>
The problem is that the characteristics of the batteries are very different. With a lead-acid
battery (like in a car), the charger can put out a voltage slightly higher than the nominal battery
voltage (your car alternator puts out about 14 V to charge the 12 V battery) and the battery itself
will limit the current to a reasonable level. That approach won't work with NiCd or NiMH
rechargeable cells which would draw too much current and overheat, damaging the cells and possibly
starting a fire. 'Smart' chargers for these cells limit the current while monitoring the voltage and
frequently temperature of the cells. When fully charged there is a plateau or slight dip in the
voltage and the temperature starts to rise. At that point the charger either turns off or switches
to trickle mode where a very small amount of current is passed to the cells to keep them fully
charged without causing damage. Chargers designed for lead-acid cells should not be used for other
chemistries like NiCD or NiMH.
 
N

N2vx Jim

Guest
On Tue, 27 Jan 2004 13:54:18 -0500, "Zilla"
<[email protected]> wrote:

>Jacques Bilinski wrote:
>>> I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging
>>> and Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want
>>> to regularly use for NI-CAD / NI-MH battery packs.
>>>
>>> Any thoughts?
>>>
>> I don't think you should do this. A 'smart' charger may work for both Ni-Cd and MiMh but SLA
>> batteries are quite different. The chargers use different methods of determining when the battery
>> is charged for different battery chemistries, also some chargers are designed for a specific
>> number of cells. You should probably invest in a smart charger designed for the type of cells you
>> are using, although if you use low charge rates (overnight or longer) you can get away with dumb
>> chargers without causing a significant reduction in battery life. NiCds and SLA are more
>> forgiving of overcharging than NiMh. Some people also use timers to terminate charging. There is
>> a Topica list called bikecurrent dedicated to bike electronics, where there are knowledgable
>> people re this.
>
>Hmmm, I'm not saying to "go ahead and do this" but, isn't current, current, voltage, voltage, and
>power, power? Or Power = voltage x current. So as long as the charger can handle the
>current/voltage/power requirement for a battery, shouldn't it work?

It won't work because the charger has to do sense and adjust voltage/current for NiCad/NiMh.

Lead-acid batteries are happy with a fixed voltage. This will fry NiCad/NiMh.

Take the previous poster's advice and get a smart charger. The MaHa C777-plus is great and there are
all sorts of chargers for the R/C car crowd.

Jim
 
F

Frkrygow

Guest
NLee1875 wrote:

> Question 2: What kind of fire hazard/risks are the manufacturers suggesting if you were to
> accidently charge a NI-CAD or NI-MH battery pack with a wall wart that is designed for SLA
> battery packs?
>
> I have a Nite Hawk Class 2 transformer (smart charger) that has a Red LED indicator for Charging
> and Green LED indicator for charged -- with a note "Caution for 6v SLA battery only" that I want
> to regularly use for NI-CAD / NI-MH battery packs.

You've gotten some good replies, particularly the ones that describe charging in terms of cell
capacity (C/10), and using a timer.

One thing they haven't made clear, I think, is this: the hazard isn't likely to be a fire hazard.
It's just going to be a ruined battery. I suppose if you charge your ni-cads pack with an automotive
battery jump-starter, or perhaps an arc welder, you'll get a fire. But that's not what you're doing.

Another point is, whether a smart charger is worth it or not depends on it's cost relative to your
battery. I have a friend who occasionally gives me ni-cad packs from hospital defibrillators. Given
the cost (zero) it would be silly for me to spend money on a smart charger - because smart chargers
aren't cheap!

I charge these packs exactly the way you're describing: I use a wall wart of appropriate voltage,
just over the pack's voltage. It charges pretty slowly, and I never leave it on as a trickle
charger. I also make sure the batteries don't get very warm.

Admittedly, I don't use those battery packs very much. I prefer my generator light. But if you keep
the charging current and charging time under control, you can do well enough, especially if a forgetfulness-
ruined battery won't break you.

If you are paying list prices for battery packs and you want them to last a long, long time, then
yes, I'd recommend a smart charger.

--
Frank Krygowski [To reply, omit what's between "at" and "cc"]
 
L

Lioninoil At Ne

Guest
> The problem is that the characteristics of the batteries are very different. With a lead-acid
> battery (like in a car), the charger can put out a voltage slightly higher than the nominal
> battery voltage (your car alternator puts out about 14 V to charge the 12 V battery)

The individual cell voltage for the SLA is about 2.2V, and the nominal "12-volt" SLA has six cells,
so its fully-charged voltage is actually 13.2V, and a car's regulator allows a maximum charging
voltage of about 15V, give or take a half-volt, depending upon manufacturer.

The NiCd and NiMH cell voltage is about 1.2V, so a SLA charger is just not going to be a good match
for these batteries. As others have pointed out, however, a "dumb" trickle-charger (meaning that it
has a supply current low enough for a charge rate of 10 hours or more) will work just fine for them
(or any other battery) as long as the voltage is close to correct, and you don't leave the batteries
on the charger for any longer than the time it takes for a full recharge. I've used dumb wall-warts
as SLA and RA battery chargers for years, without ill effect.
 
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