Linear Transformer Rated Current

[Krankshaft]

Before I get into my question some background.

So I pulled out my Grundig FR200 today one of those wind up radios that charges a NIMH battery (basically it's exact same style battery pack as in most cordless phones).

I look on the back of it and it has a place for an AC adapter (a 3.4mm one with a negative tip thanks Germans) curious to see if it would charge the battery I look in the manual and it does.

I find an old universal multiple connector switch type wall wart (300 mah rated) (which has this connector and a polarity reversal switch) flip it to 4.5V center negative.

The unit charges fine and I unplug it so I begin thinking whether or not there is a charging circuit so the battery won't fry and I can leave it plugged in and charging all the time like a cordless phone.

So I look at page 8 of the manual and heres where my question comes in:



It says that as long as the maximum rated output current of the wall wart is 80-100 mah you can leave it plugged in with no problems.

But if the maximum rated current is out of the prescribed range only leave it plugged in for a max of 2 hours otherwise disconnect the battery if you're going to leave it plugged in all the time.

What gives here? Just because a transformer is rated at 100ma that doesn't mean that it won't supply more current if it's drawn by the device right?

From my understanding if you draw too much current from an unfused secondary it will lead to the windings overheating and eventually stinky insulation breakdown.

Do you guys think there is any truth to this or is it just hype to buy the transformer from the manufacturer?

[relkin67]

Re: Linear Transformer Rated Current

NiMH batteries should only be given a low amp trickle charge once 80% capacity is reached. The cells become resistive at that point and additional current is sloughed off as heat - eventually overheating and destroying the cells.

The circuit must not have a feedback circuit to taper off the amps when the voltage of the cells signifies that they are approaching full charge.

The manf. recommends a low amp charger because it essentially trickle charges the entire capacity - slow but safe. I would not leave on a 1A current indefinitely because it will cook the cells.

[relkin67]

Re: Linear Transformer Rated Current

By 'the circuit' I meant to say, "the circuit in your radio".

A true charging circuit will either step down current with voltage rise or have an IC that periodically does little capacitance checks to see if the batteries are still accepting charge.

[Wizard]

Re: Linear Transformer Rated Current

Cheap dumb charger relies on intentional high impedance of secondary winding to limit current and careful selection of voltage also.

Cheers, Wizard

[i4004]

Re: Linear Transformer Rated Current

relkin, that makes no sense, according to this
http://en.wikipedia.org/wiki/Trickle_charging
if it stays on it should be float charger and that means it has cutoff circuit.
ie you can't leave even the low amps charger connected indefinitely.
it too will fry the battery...only at a slower pace.

let me tell you about my varta "lemon" chrager and that might explain this:
it's this
http://www.en.varta-consumer.com/con...a-consumer.com
and it really is a lemon! lol!

it does this: charges with particular current (aa with 170ma, aaa with 80ma) but it has a timer that shuts it down if charging lasts more than 15hrs.

it's not completely dumb charger, but i'ts not very bright either, is it?

i believe you have same or simillar in your radio!
it makes sense, doesn't it?

you can leave your 80-100ma charger plugged in because it has self timer(perhaps simple 555 ic) that will shut it off after certain period.
but if you connect more power, then the amount timer does might just be too long, and that can fry batteries.

15hrs of my lemon is too long too, as it can get the batteries rather hot.
but its problem is that all sorts of batteries can go to it.
you have only one battery and charger made for it.

so no, i wouldn't say it's the hype, it's just a cheap design so you must be carefull.
but you can buy 100ma ac/dc adapter that wouldn't be grunding's.
that should be dirt cheap.

but the best way to charge those batteries in that radio would be to have some "smart" charger to do it.
one with some sort of cpu that monitors everything. batteries have some "delta" parameter they need to monitor while charging...

ie liek this
http://www.en.varta-consumer.com/con...a-consumer.com

but i still use my lemon with the "charging current=1/10th of capacity" rule adapted to battery's capacity ie time it needs to charge.
yes, i need to check the clock.

oh yeah, i have smart charger in one cordless phone that is otherwise unusable(keyboard is borked). but it only charges 2 aa batts and it takes about 5hrs.
i must adapt it to use aaa too...hehe...