Sigma EQ Mini to Charge NiMH in DX6i

[David Cooke 1]

After suffering dry cell leak in my dormant DX6i, I fitted a pre-wired 4xAA Eneloop battery pack. My question is how to charge them without the Spektrum SPM9550 charger as I believe its just a simple constant current output and not sensing the battery voltage (to determine end of charge). I say this because poking inside the Dx6i with a DVM and by PCB trace inspection, I can see that there is a series diode from the charge socket to the battery. I guess this is to prevent connecting the charger the wrong way. However, it means that the charger cannot see the battery voltage.

I have a Sigma EQ Mini charger that can charge NiMH (not sure how it knows how many cells there are! It must be sensing the battery voltage and I know that it looks at the battery dV/dt to determine end of charge). I'm thinking I need to therefore short that diode out.

Any electronics guy be able to comment on this?

Thanks

[Denis Watkins]

Just my view David, but stick to the dry cells

They are low cost, more voltage, and last a season trouble free

If cells leak, then they must have not have been up to today's standards.

[David Cooke 1]

I have already performed the modification. The -ve spring contacts were very badly corroded and even caused -ve terminal corrosion on the Molex power connector on the main PCB. The spring contacts had to go so I replaced with a battery pack. NiMH Eneloops last for quite some time! Even with a backlight modification, the current draw is ~ 80mA. The transmitter electronics are powered from a 3.3V linear regulator. I could not determine the minimum voltage drop over the regulator but verified with a PSU that the voltage warning comes on at ~4.1V I think it was.

I have shorted the diode and will try tonight!!!

[Nigel R]

David, your delta-peak charger will be directly connected to the battery pack AND the transmitter electronics.

I'll go on record with this being a bad idea.

Trickle charge using a suitable charger, through the diode. Or remove the pack each time and connect it and only it to a delta peak fast charger.

I'd imagine there are many wall wart chargers that will do the job, e.g. Logic RC FUSION NX83 is about half the price of the Spektrum charger (I should say I've never used either, I just searched for "150mA TX charger".

[David Cooke 1]

I can only see this being a bad idea if the transmitter is switch on. The switch isolates the battery/charger from the electronics. Even then I don't think its that bad since everything goes through that liner reg, then worst case, the transmitter is on and the charger is applying an errant high cell series voltage. You'd be dumping power into the linear reg - and not very much since its only 80mA draw from the linear reg....? Even with a wall charger, you're still exposing it to the electronics but only when the power switch is slid to on position.

Having said all that(!) I think though I will put the diode in as it gives me some protection in that I'm not exposing battery direct to the DC jack. For a constant current charger I would just use a bench power supply and set current limit.

[David Cooke 1]

Thanks for the example charger. Looks just the ticket!

[Peter Christy]

A lot of modern peak-detect chargers will work quite happily with a diode in series.

--

Pete

[McG 6969]

... and if using the original charger socket of the DX6i, maybe take into consideration that the genuine Spektrum chargers have a 'reversed' polarity compared to standard chargers - ie negative (ground) to the center pin...

Cheers

Chris

[Denis Watkins]

Gee whizz McG, thank goodness you remembered that one

With no diode, bits would get fried

[McG 6969]

... well yep, Denis... I'm afraid so.

Cheers

Chris