TYT MD-380 Battery Charger Modification

“8.4 V” Battery Charger: Modified to avoid overcharging the LiIon battery

The MD-380 was shipped with several accessories, including a charger for the radio’s 7.4 V LiIon battery. That charger has a bi-colour LED on the front side, which should indicate ‘red’ while charging, and ‘green’ when fully charged.

According to the label, charging current should be around 400 to 450 mA. Thus a 2000 mAh LiIon accumulator (2 cells in series as a “battery”) should have been charged within 5 hours. My expectation was the LED would switch from red to green after this time.

But it didn’t. In fact, the LED never turned green. After removing the radio from the charger, and checking the voltage directly at the battery terminals (see next chapter), almost 8.6 Volts were measured. Ooops ! That’s a bit much for two LiIon cells in series !
In the interest of lifetime, the voltage for a single cell should never exceed 4.2 Volts. More voltage will not “fill in” significant extra charge, but reduce the battery’s life time. So the charger was modified as described below, to achieve a ‘healthier’ max output voltage of 8.4 V (for two cells in series).

The principle is easy. The charger shipped with the MD380 uses an MC34063 in a switching-mode step-down converter configuration.

 

 

This IC uses an internal reference voltage of 1.25 V, which is compared against the charger’s output voltage (“8.4” V) divided down to 1.25 V. The voltage divider consists of resistors R3A (33 kOhm) paralleled with R3B (1.8 kOhm), resulting in 1.707 kOhm, and R4 (10 kOhm) in series.


                                      
                                       ____
                      R4         ,----|____|----|
                      ____       |  R3A : 33 kOhm
      +"8.4" V  -----|____|------*
     (battery                    |     ____
      voltage)       10 kOhm     *----|____|----|
                                 |  R3B : 1.8 kOhm   
                                \|/
                              1.25 V (to MC34063 control input, pin 5)
                     

The regulator’s theoretic output voltage with the original components (shown above) is

1.25 V * ( 1 + 10 kOhm / 1.707 kOhm ) = 8.57 V .

This ‘surprisingly high’ voltage for a 2-cell LiIon battery pack was confirmed by measuring it at the battery after removing it from the charger !

Almost 8.6 V is quite a lot for two LiIon cells in series, especially if the radio was kept in the charger for longer than necessary. So how to reduce the charger’s maximum output voltage ? The photo below shows the charger’s PCB, component side, with the originally populated resistors (R3A, R3B, R4 labelled in white), and a few voltages measured when the battery was fully charged (but the LED was still ‘RED’ when it should have been ‘GREEN’).


(Click on image to magnify)

A simple method to bring down the maximum output voltage to 8.4 V is shunting R4 (10 kOhm ‘original’) with a 390 kOhm resistor.
The ‘paralleled’ resistance is
1 / ( 1/10kOhm + 1/390kOhm) = 9.75 kOhm,
so with the modified voltage divider, the theoretic output voltage should be:

1.25 V * ( 1 + 9.75 kOhm / 1.707 kOhm ) = 8.39 V .

That’s 4.19 Volts per LiIon cell – ok. And indeed, with this modification, the battery stopped ‘drawing current’ when fully charged, and the LED turned green after a couple of hours (btw, that’s what the LM358 is for, visible in the right half of the photo above). Voila ! An even simpler alternative (instead of ‘shunting’ R4) would be to remove the 33 kOhm resistor (R3A in the photo shown above).
The theoretic output voltage would then be:

1.25 V * ( 1 + 10 kOhm [R4] / 1.8 kOhm [R3B] ) = 8.19 V .

The two-cell LiIon battery would possibly not be “fully” charged very quickly with this voltage, but forgetting to remove the radio from the charger would do even less harm then.

As an indicator for a “charged” battery, if you also don’t trust the on-screen battery indicator symbol: Six hours after a full charge, without load, the battery voltage was 8.10 V. It dropped by 4 mV after turning the radio on (circa 70 mA load current), thus the internal resistance of ‘new’ battery (as shipped from the manufacturer, 2016-12) was about 0.06 Ohms – will keep an eye on this as time passes.

 

Exposed ‘hot’ LiIon battery contacts


The LiIon battery’s charger contacts can ‘provide current’ during normal operation !

Both the LiIon battery, and the charger appear to have THREE CONNECTORS, but the center connector isn’t connected anywhere (at least, not inside the cheap charger).

There doesn’t appear to be a ‘smart’ circuit inside the battery to protect it from excessive loads.
THIS APPLIES NOT ONLY TO THE CONNECTORS THAT ‘FEED THE RADIO’ (not exposed),
BUT ALSO TO THE CONNECTORS FOR THE CHARGER,
which are exposed when operating the radio !

Remember this when dropping the RT3 (or MD-380) into your pocket along with the bundle of keys, or other metal objects
– a short across the battery’s outer, exposed contacts will possibly ruin it, or even worse.

I didn’t want to try how many amps can be drawn from the exposed contacts on the backside of the transceiver. But as you can see on the right, it’s enough to power an ancient filament lamp.

Autor: Wolfgang “Wolf” Büscher, DL4YHF http://www.qsl.net/dl4yhf/RT3/

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