Handheld battery question

[RobsSanta]

My brain has totally gone blank for this simple question, I know I should
have paid more attention in school !!!.

I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger by a
friend who is quitting flying. It has been that long since he used the
battery he doesn't remember charging times etc.

So the obvious question is, how long should it take to full charge this
battery using the supplied wall charger ? By the looks of it, the charger
looks like your standard charger, no auto cutofff etc.


Rob.

[Morgans]

"RobsSanta" > wrote in message
news:WNQ2d.43479$9Y5.11833@fed1read02...
> My brain has totally gone blank for this simple question, I know I should
> have paid more attention in school !!!.
>
> I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger
SNIP
how long should it take to full charge this
> battery using the supplied wall charger ? By the looks of it, the charger
> looks like your standard charger, no auto cutofff etc.
>
>
> Rob.

Divide 1200 by 200, and the short answer is 6 hours. That is the short
answer.

The wall chargers are sometimes really far off in the real value vs. the
stated value, sometimes 50%, or more. That means it could take 12 hours or
4 hours. You could rig up a splice to test the output with a meter, or you
could charge for varying times and check the battery voltage. When the
battery is reading about 1.28V per cell, after it has rested an hour or so,
it is pretty close to full.

Or you could charge it 8 to 12 hours, and go with it. ;-)
--
Jim in NC


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[Peter]

RobsSanta wrote:

> My brain has totally gone blank for this simple question, I know I should
> have paid more attention in school !!!.
>
> I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger ...
> So the obvious question is, how long should it take to full charge this
> battery using the supplied wall charger ?

Assuming the charger is actually 200 mA (milliamps) then if things were
100% efficient it would take 1200 mA-hr/200 mA = 6 hr to fully charge.
But allow an extra 25% or so for inefficiencies and about 7 - 8 hours
should do if it starts out fully discharged.

[Chris W]

RobsSanta wrote:

> My brain has totally gone blank for this simple question, I know I should
> have paid more attention in school !!!.
>
> I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger by a
> friend who is quitting flying. It has been that long since he used the
> battery he doesn't remember charging times etc.

For NiMH and NiCd, you take the battery capacity * 1.4 (I don't know
where the 1.4 came from I just read it somewhere) and then divide it by
the charge rate. That gives you the number of hours. So 1200 * 1.4 /
200 = 8.4 hours or 8 hours and 24 minutes. As others have said that is
assuming the charger "really" puts out 200ma. If you have a good volt
meter and can monitor the charge from time to time, when you slow charge
a NiMH or NiCd battery the fully charged voltage should be about
1.45V/cell while it is on the charger. This voltage will drop to
1.3V/cell or so shortly after the charger is removed. So what I would
do is record the voltage during charge every 20 or 30 min and if it
reaches 1.45V/cell stop charging. If it seems to plateau at a slightly
lower voltage then stop there.

--
Chris W

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[Dave S]

Is it proper to multiply the mA rating x the number of batteries/cells?

like.. 2 1200 mA batteries would in theory take 12 hrs minimum?

Dave

Chris W wrote:
> RobsSanta wrote:
>
>> My brain has totally gone blank for this simple question, I know I should
>> have paid more attention in school !!!.
>>
>> I just was given a 12volt 1200mAh battery and a 200mh 3W wall charger
>> by a
>> friend who is quitting flying. It has been that long since he used the
>> battery he doesn't remember charging times etc.
>
>
> For NiMH and NiCd, you take the battery capacity * 1.4 (I don't know
> where the 1.4 came from I just read it somewhere) and then divide it by
> the charge rate. That gives you the number of hours. So 1200 * 1.4 /
> 200 = 8.4 hours or 8 hours and 24 minutes. As others have said that is
> assuming the charger "really" puts out 200ma. If you have a good volt
> meter and can monitor the charge from time to time, when you slow charge
> a NiMH or NiCd battery the fully charged voltage should be about
> 1.45V/cell while it is on the charger. This voltage will drop to
> 1.3V/cell or so shortly after the charger is removed. So what I would
> do is record the voltage during charge every 20 or 30 min and if it
> reaches 1.45V/cell stop charging. If it seems to plateau at a slightly
> lower voltage then stop there.
>

[Chris W]

Dave S wrote:
> Is it proper to multiply the mA rating x the number of batteries/cells?
>
> like.. 2 1200 mA batteries would in theory take 12 hrs minimum?
>

if they hare in parallel yes the mAh rating goes up for the pack, but
normally they are in series, so the pack voltage goes up and the mAh
stays the same.


--
Chris W

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[Peter Duniho]

"Chris W" > wrote in message
news:2n73d.69282$mu.28256@okepread07...
> if they hare in parallel yes the mAh rating goes up for the pack, but
> normally they are in series, so the pack voltage goes up and the mAh
> stays the same.

He's asking about charging time, not battery life.

[Chris W]

Peter Duniho wrote:

> "Chris W" > wrote in message
> news:2n73d.69282$mu.28256@okepread07...
>
>>if they hare in parallel yes the mAh rating goes up for the pack, but
>>normally they are in series, so the pack voltage goes up and the mAh
>>stays the same.
>
>
> He's asking about charging time, not battery life.


I guess I wasn't very clear was I. If they are in parallel the mAh goes
up and therefor the charging time when charging at a fixed mA rate.
But in series the mAh doesn't go up so the charging time would stay the
same. Of course your charger has to be able to put out a high enough
voltage or the battery will never charge.


--
Chris W

Bring Back the HP 15C
http://hp15c.org

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[[email protected]]

On Sat, 18 Sep 2004 23:46:03 -0500, Chris W
> wrote:

>Peter Duniho wrote:
>
>> "Chris W" > wrote in message
>> news:2n73d.69282$mu.28256@okepread07...
>>
>>>if they hare in parallel yes the mAh rating goes up for the pack, but
>>>normally they are in series, so the pack voltage goes up and the mAh
>>>stays the same.
>>
>>
>> He's asking about charging time, not battery life.
>
>
>I guess I wasn't very clear was I. If they are in parallel the mAh goes
>up and therefor the charging time when charging at a fixed mA rate.
>But in series the mAh doesn't go up so the charging time would stay the
>same. Of course your charger has to be able to put out a high enough
>voltage or the battery will never charge.

This thread is getting confusing!

EACH BATTERY (NiCAD or NiMH) requires to be charged at an equivalent
to 40% to 60% more than their capacity, because they are not 100%
efficient. As you have all the batteries in series you chage at about
1200/200 = 6 hours plus 40 to 60% = 2.4 to 3.6 hours extra.

Personally I would use 40% to 50% since any battery overcharged will
have a shorter life if charged at more than 10% of its rated capacity,
i.e. greater than 120mA in your case. I seem to remember that Nicads
can be overcharged for abut 100 hours as long as you don't exceed 7%
of their capacity rating for charge current. That is not usually an
option these days as many fast chargers have built in sensing to
detect when fully charged.



David

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