Confused on 6 volt 2700mah packs and Wall Warts....

Confused on 6 volt 2700mah packs and Wall Warts....
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Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 4:33:47 AM

les2097

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From: Lafayette, LA, USA
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I fly only 6 volt NiMh, 2700 mah packs in my planes... I've been charging them with wall warts that have 4.8 volts as output and 50 mah output... These wall warts came with older Futaba radio systems...

My friend tells me that I can't charge a 6 volt pack on a 4.8 out... but my 6 volt packs charge to over 7 volts regularly... ???

I usually plug a pack in at night and fly the next afternoon... usually a 14 to 16 charge....

Should I continue to do what I'm doing ... or change to some other method of charging ... ??

Another question is, "What is the design voltage for Futaba receivers ... like a R-138DP... or a R-149DP ... or even a R-127DF... ??

Thanks

Larry

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 4:43:05 AM

guver

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The voltage is probably close enough and will follow the battery's voltage. It will , however not do any better than 50 ma and should take over 4 days to charge a dead 2700 x 6 volt battery.

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 4:49:38 AM

les2097

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From: Lafayette, LA, USA
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How does the voltage of the pack get over 7 volts with only 4.8 wall wart output .... ??

2700 mah divided by 50 mah == 54 hours or 2 days 6 hours to full charge.... from a dead pack ... Is this correct....???

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 4:59:30 AM

guver

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The output will "follow the battery" up to certain point and the rate will be cut-back as the output voltage goes up and is why I estimate that the rate will not be quite 50 ma. Also it is necessary to use a multiplier because the rate is so low compare to the battery capacity.

I estimate a mutiplier of 200% and guessed that it will take over 4 days. You must've been operating on just a portion of your total capacity.

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 1:46:24 PM

Dr1Driver

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How does the voltage of the pack get over 7 volts with only 4.8 wall wart output .... ??

You can't charge a nominal 4.8 volt pack with a charger that only puts out only 4.8 volts. The charger voltage must always be greater than the pack voltage. Just how much depends on the manufacturer's design criteria. If the charger is putting out over 7 volts, then it can charge a 7 volt pack.

Dr.1

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 3:00:45 PM

Geistware

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From: Locust Grove, GA, USA
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You friend is wrong, you can charge a 5 cell pack with a wall wart.
You can and should continue to do it.
Slow charging a pack will make it last longer than fast charging it. (less stress)
The design voltage of Futaba receivers is 4.8 and 6.0 volts.
The R148DP and R149DP has two regulators to regulate the voltage to the electronics in the receiver. The R138DP only has one. You can supply a higher voltage to teh RX than the servos can safely handle with no problem to the RX electronics. What you have to make sure of in high voltage applications is to let the RX breathe so it can dissipate the heat due to the excess voltage.

quote:

ORIGINAL: les2097

I fly only 6 volt NiMh, 2700 mah packs in my planes... I've been charging them with wall warts that have 4.8 volts as output and 50 mah output... These wall warts came with older Futaba radio systems...

My friend tells me that I can't charge a 6 volt pack on a 4.8 out... but my 6 volt packs charge to over 7 volts regularly... ???

I usually plug a pack in at night and fly the next afternoon... usually a 14 to 16 charge....

Should I continue to do what I'm doing ... or change to some other method of charging ... ??

Another question is, "What is the design voltage for Futaba receivers ... like a R-138DP... or a R-149DP ... or even a R-127DF... ??

Thanks

Larry

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http://www.rcaircraft.net/
Information on the building and flying of Radio Control Aircraft.
Site for hobbyist in the R.C. Aircraft modeling world.

(in reply to les2097)

Post #: 6

RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 3:03:58 PM

Geistware

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From: Locust Grove, GA, USA
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DR1,
The statement below is very wrong.
You cannot charge a 4.8 volt pack with a 4.8 volt source.
Batteries charge from the current supplied.
To force current into a pack, you have to increase the voltage.
A 5 cell 6.0 pack, can have a voltage as high as 8.5-9.0 volts depending on how fast you are charging the pack (charge current)
We can go into more detail offline if you would like.

quote:

ORIGINAL: Dr1Driver
How does the voltage of the pack get over 7 volts with only 4.8 wall wart output .... ??
You can't charge a nominal 4.8 volt pack with a charger that only puts out only 4.8 volts. The charger voltage must always be greater than the pack voltage. Just how much depends on the manufacturer's design criteria. If the charger is putting out over 7 volts, then it can charge a 7 volt pack.

Dr.1

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Information on the building and flying of Radio Control Aircraft.
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Post #: 7

RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 3:27:29 PM

les2097

Posts: 125
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From: Lafayette, LA, USA
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Geistware,

Thanks for the information...

I sure would like to know more on how a 5 cell NiMh charges with a wall wart of 4.8 volts and 50 mah ... I know they do... and it's a matter of time... but how does the voltage get to 7+ volts at full charge....??

I understand now that the charging voltage must increase to "jam" more milliamps into the pack ... how does this happen...??

Larry

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Post #: 8

RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 5:36:21 PM

guver

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Maybe to simplify: the output voltage of the 4.8 volt wall-wart can be maybe 10 volts or so.

Remind of garden hose without and end on the outlet (the pressure is low , but as soon as the end starts getting bottlenecked all the upstream presssure increases.)

Ok, maybe that is a bad analogy. Let's just say that IF the output voltage of the charger is only 4.8 and they are hooked together then the battery will be discharged, but the point is that the output voltage of the 4,8 charger IS higher than the battery so the current flows to the battery and charges.

< Message edited by guver -- 1/27/2007 5:42:48 PM >

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/27/2007 5:39:44 PM

Dr1Driver

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Geistware,

The statement below is very wrong.

I know that. Read ALL the posts. The question was from les2097.

That's why I replied, "You can't charge a nominal 4.8 volt pack with a charger that only puts out only 4.8 volts. The charger voltage must always be greater than the pack voltage."

Dr.1

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/28/2007 4:24:16 AM

les2097

Posts: 125
Joined: 12/26/2001
From: Lafayette, LA, USA
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Guver,

I have 10 wall-warts... and I measured the voltage output from all 10 and each was 4.8 or very close... not 10+ as you said.... what's going on...??

I have a Hobbico Multicharger... the one that has 4 outputs ... each with a selectable charge rate.... I measured the voltage on each of the outputs and it was 16.5 volts... irrespective of the charge rate selected....

I'm confused again...

Larry

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/28/2007 4:31:03 AM

guver

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From: LaGrange, OH, USA
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Try measuring them with the battery plugged in and if possible put a dvm measuring current inbetween. You should see the current goping from the charger to the battery with an output voltage of 6 volts or more.

You've already been doing it (according to post # 1) right?

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RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/28/2007 4:34:00 AM

les2097

Posts: 125
Joined: 12/26/2001
From: Lafayette, LA, USA
Status: offline

OK... I'll give it a try in the morning.... and yes... I've been using the wall-warts to charge my 6 volt packs....

Larry

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Post #: 13

RE: Confused on 6 volt 2700mah packs and Wall Warts.... - 1/28/2007 6:19:52 PM

Geistware

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From: Locust Grove, GA, USA
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What I am going to say is NOT true but will give you an idea of what is going on with a wall wart.
A wall wart will only transfer a set amount of power (watts) from the AC wall outlet to the battery.
Lets set this value at 0.24 watts. Equation to use is Watts=Voltage Times Current (W=V*I)
On the AC power side, it is 0.24 = 120 times 0.002 (2ma)
On the battery side it is 0.24 = 4.8 times 0.050 4 cell pack at nominal voltage (50ma)
On 5 cell battery side it is 0.24 = 6.0 times 0.040 (40ma)
As you can see, the current goes from 50ma (4.8 volts) to 40ma (6.0 volts)
This is an analogy. It is not this simple
It should give you an idea of what is going on.
As I stated at first..... THIS IS WRONG but will give you an idea.

quote:

ORIGINAL: les2097
Geistware,
Thanks for the information...
I sure would like to know more on how a 5 cell NiMh charges with a wall wart of 4.8 volts and 50 mah ... I know they do... and it's a matter of time... but how does the voltage get to 7+ volts at full charge....??
I understand now that the charging voltage must increase to "jam" more milliamps into the pack ... how does this happen...??
Larry

_____________________________

Aircraft Proving Grounds
http://www.rcaircraft.net/
Information on the building and flying of Radio Control Aircraft.
Site for hobbyist in the R.C. Aircraft modeling world.

(in reply to les2097)