Two Diodes Fool Charger !

[MrAl]

Hi there,

I've successfully used two diodes to fool a NiMH charger
into thinking there were two cells in the charger instead
of just one. The type of charger was an RS charger.
Before using the two diodes, i tried both 1 ohms
and 1.5 ohms as well as a direct short for one cell,
and all methods failed...the charger would 'pulse'
the cell with a high current but only once per every
two or three seconds and only with a short pulse.
The only thing that worked is two diodes in series.

I used 1N4006 diodes for the test, but they got too hot
because the charger puts out 1.2 amps and the diodes are
only rated for 1 amp, so im recommending 1N5400 (or similar)
diodes instead.
As mentioned, the two diodes are wired in series so that
the total voltage drop is near 1.5 volts. The voltage
is fairly constant, so to the charger it looks like a cell
/ubbthreads/images/graemlins/smile.gif

I cant say for sure if this will work with every charger
out there, but it's worth a try.
I originally wanted to do this because when allowing the
charger to charge two cells in series it shuts off once
only one cell reaches it's full charge. If it's only
charging one cell that cell has to get full.

A drawback to this approach is that, at least with my
charger, if i want to charge two cells i'd have to wait
twice as long (put one cell in each side with a dummy,
where each side normally does two cells) because the
current is only half when charging four 'cells'.

If anyone else tries this please report back here with
your charger name and model number (if you can).
Thanks!
If you report back here any results (good or bad)
your contribution will be used in the future when
other people visit the site to determine if they
can fool their charger, and how to do it.


My charger is RS Cat No. 23-043


Take care,
Al

PS Also tried a single diode and that didnt work either.

 

[Doug S]

For NiCd chargers that terminate on dV/dT you might get a quick charge termination on the falling Vf of the diodes as they heat up if the rising terminal voltage on the actual cell is not enough to offset the falling diode Vf.

 

[MrAl]

Hi Doug S,

Good point. I didnt check but if i have any problems
i'll unplug the one cell and then plug it back in
quick after the diodes heat up.
Im prepared to solder two small heak sinks to the leads
of each diode if need be too. I wish to keep it simple.

Take care,
Al

 

[PEU]

will x (try+tell) /ubbthreads/images/graemlins/smile.gif

Have spare 3A diodes and I will turn a AA sized aluminium body that will act a s a dissipator.


Pablo

 

[RussH]

What a concept. I've done this on 12v chargers to get into my desired 13.4-13.8v (or whatever) charging voltage, but this is extreme. I may try Pablo's approach only I think I'll use copper tubing with end caps (I already have the material).

Doug, I understand what you're saying about the DV/DT, but do you see any drawback to major heatsinking (or bigger diodes) like Pablo is proposing?

 

[MrAl]

Hi again,

Pablo:
Oh yes, that sounds good. I was suggesting the heat sink(s)
to act more as a heat capacitor rather than a dissipator,
so that when the diodes heat up they stay that way for
a few seconds even if the (now) single cell is removed
in order to reset the charger, just in case the -dv/dt
caused by the diodes triggers the end-of-discharge
on the charger. With the extra heat capacitance, the
dv/dt is lower in both directions so unplugging the cell
for one second (only) shouldnt cool the diodes too much.

RussH:
You are aware that i was suggesting 3 amp diodes to begin
with arent you? I used the 1 amp ones just for a quick
test to see if it was even possible to fool the charger.
If it worked (and it did) i would of course go to the
3 amp diodes such as 1N5400.


Take care,
Al

 

[legtu]

I've been using this trick on my MAX712/713 chargers for quite sometime. Delta detection works fine.

I'm using 1n4007's potted in epoxy.

 

[MrAl]

Hi there legtu,

Oh that's good to know. Im probably going to build something
permanent for my charger too. I did the test, but now
need to wire/epoxy like you did too.
I'll probably use the 1N400x series too since the
voltage drop seems about right.

Take care,
Al

 

[legtu]

I've been testing another one using 1n5402's. Seems to work fine too. /ubbthreads/images/graemlins/smile.gif

Here's some pix:
-Potting

full-sized pix

-AA and AAA diode batt

full-sized pix
*yup, it looks like crap. /ubbthreads/images/graemlins/grin.gif

 

[MrAl]

Hi again legtu,

OH yes, very cute. I'll probably use something like
that starting with a drilled out dowel.
Glad you reminded me about the 1N54xx type diodes, as
i want to use those instead of the 1N40xx type diodes
because my charger puts out 1.2 amps and that's too
high for the 1N40xx type diodes which are only rated
at 1 amp. The 1N54xx diodes are rated at 3 amps so
they will work out better.

Take care,
Al

 

[legtu]

*bump*

Might be of some use to others... :)

Do post your experiences if it worked(or not) with your 'smart' charger.

 

[PEU]

I just finished one, and it works as expected, instead of an alu body I machined one in acrylic, it took more than two years from my previous post :D

Used a philips and a slot screw for for marking positive and negative :)


Pablo

 

[MrAl]

Hey there Pablo,

Nice idea with the Phillips and Slot heads to indicate plus and minus...good idea.

Lately i have been using the Energizer 15 minute charger which does each cell
individually, but i may end up using my older charger too in the future as i get
more cells like the Eneloops.

The plastic is a good idea too, as long as the diodes dont melt it or burn up.
Even the higher rated diodes get hot, they just have more surface area and
wider die, but they still have to give off the heat somewhere.
In some designs they wrap a short piece of tin around each lead and solder
it in place (close to the body). The two 'fins' that result from the tin strip
(maybe 1/4 to 1/2 inch wide strips) can exit the plastic from the top where
they can give off the heat...just in case they get too hot and melt something.
I'd watch the first few charges carefully.

Thanks for the pics too, which are very much nicer than pure description !
That one pic says it all !

Wow two years ago already? Geeze, we're getting old :-)