BCD436HP: Trickle Charger Mod

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jonwienke

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I found a pad on the 436 front circuit board that is connected to the USB jack 5V power. It's labeled V_USB, the rightmost of the 3 pads in a row near the center of the photo.

The 436's inability to charge the batteries while it is powered on is one of its greatest design shortcomings. (I know, it's because of a BS California environmental regulation.)

Anyway, I realized that a simple trickle charge capability could be added to the 436 by connecting a 20Ω resistor and a SD103C diode between the USB power pad and the positive battery terminal. Given a USB voltage of 5.1V and a battery voltage between 3.1V and 4.1V, that particular set of components will result in a charge current of about 80mA when the battery voltage is 3.1V, tapering off to about 30mA when the batteries reach 4.1V. The idea is to fully charge a set of dead batteries over 36 hours or so, and keep them topped off while the scanner is plugged into USB power without frying them, so that a unit normally plugged into USB power will always have a fully-charged set of batteries.
 

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jonwienke

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I'm going to order the parts and mod my 436 and see what happens. If all goes well, I will offer it as a $5 add-on option when I do a GPS mod.
 

jonwienke

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Evidently there aren't as many environmental Nazis in Australia as there are in California.

Can you get DMR for it?
 

Flywheel17

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Would be interesting to get the WD for the scanner that does charge the batts to confirm the solution.

I am very interested in this as well as the GPS mod.

Great stuff Jon W!
 

jonwienke

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The parts are on order. I will post an update with test results next week.
 

dave3825

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Jon, in the top right of your pic, just right of J403, what is the yellow item? Almost looks like an automotive fuse.
 

jonwienke

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It's the USB jack, which has some kind of amber tape on it from the factory.
 

dave3825

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Lol. Really looked like an automotive fuse before my morning coffee..

Maybe it's shielding tape to help usb noise.. lol
 

jonwienke

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The parts arrived, and I've modded my 436, and everything works pretty much as expected. The diode-resistor combo allows about 80mA of trickle charge when the batteries are completely dead, and charge current tapers off to about 40mA as the batteries reach full charge. The pics below show the diode and resistor soldered together, then covered in heat shrink (to prevent shorting to the shield between the circuit boards) and soldered in place.

My thought is to offer this as a $7.50 add-on when I have the scanner apart for a GPS install.
 

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W4EMS

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Apologies Jon. RR site went down briefly as they were loading. Had to run an errand and didn't delete post in time.
Looks like a simple mod for steady hands. :)
 

jonwienke

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No worries.

The components are a SD103C Schottky diode with the cathode connected to the positive battery terminal to prevent battery voltage from feeding back into the USB port, and a 20 ohm 1/4-watt resistor. The resistor dissipates about 1/8-watt during charging, so going smaller than 1/4-watt is not advisable.
 

jonwienke

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I've been running a couple 436s with this mod overnight, and the batteries are definitely charging, but are not overheating. Using an IR thermometer, the GPS patch antenna measures 76°F, and the batteries are only 72°F. Battery voltage has risen from 3.11V to 4.13V over 10 hours.
 

jonwienke

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So I did some tests with some 800mAH 14500 Li-ion batteries. 3 of them in parallel is roughly equivalent to 3 NiMH cells in series. I did a comparison of battery life, and the NiMH cells lasted about 8.5 hours, and the Li-ion cells lasted about 9.75 hours. The performance benefit is there, but probably not worth the hassle of coverting the scanner to use them.
 
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