TYT MD390 / Retevis RT8

First off, TYT (aka Tytera, not to be confused with Hytera) MD390 and Retevis RT8 are the same. They are the big brother of TYT MD380 / Retevis RT3. The biggest difference is that MD390/RT8 are waterproof while MD380/RT3 are not. Both radios can be had with or without the GPS. As far as I have been able to find the GPS to APRS functionality only works on Hytera repeaters right now as the positioning packages are a bit mangled. Mine are Retevis RT8 with GPS.

Index

Charger issues

I found out that the MD380/RT3 chargers have some issues with overcharging batteries. So, I checked my charger and saw something really strange; almost 9.3V on the output, way more than the maximum rating of 8.4V for such a battery. I thought that maybe it showed a too high voltage because it's unloaded and will detect when a battery is inserted and lower the voltage. I put a battery in the charger and measured a while later; 8.6V. Not okay, that will damage the battery over time if it's left in charger longer than necessary. It's not the same charger but the fix work in the same way. They have done another fix thou, they have put a diode in the battery to protect it from shorting out. That diode has a voltage drop wich mean the over voltage isn't quite as bad as it first looks.

CurrentVoltage drop
0mA0.27V
1mA0.527V
10mA0.684V
100mA0.8V
600mA0.92V

I want to be able to use my charger for overnight charging, so I want my battery to get pretty close to 8.4V but not over when charging a half full battery for say 12 hours. It's a lot of variables and I won't calculate them all, but I can take some diode drop into account, say 0.5V. That put us at 8.9V. If you want to have your radio in the charger at all time and not worry about overcharging the battery you could try adding a lower value or even charge the battery to only 80% or something instead.

How do we fix it?

The regulator feedback pin(FB) will try it's best to stay at 1.235V, by using a voltage divider between the output, feedback, and ground it can be "programmed" to any voltage from 1.235V to 37V. In this case the formula is Batt+ = 1.235 * ( 1 + 20000 / 3083 ) = 9.25V, pretty much what i measured.

What we want is to get the voltage to a maximum of 8.9V, and we do it by adding a resistor in parallel with R1, as the missing R1-1 should have been. That will make the ratio in the voltage divider more equal and give less voltage. But what value?

If we shift around the formula from above and do some math: 

            8.9 = 1.235 * ( 1 + R / 3083 )
    8.9 / 1.235 = 1 + R / 3083
8.9 / 1.235 - 1 = R / 3083
     6.2 * 3083 = R
              R = 19134

What we want is a maximum of 19134 ohms between Batt+ and FB, we now have 20k. We need to calculate a good value to put in parallel with 20k to get it below 19134 ohms. 

                            R = 1 / ( 1 / R1 + 1 / R2 )
                        19134 = 1 / ( 1 / "R1-1" + 1 / 20000 )
                    1 / 19134 = 1 / "R1-1" + 1 / 20000
        1 / 19134 - 1 / 20000 = 1 / "R1-1"
1 / ( 1 / 19134 - 1 / 20000 ) = "R1-1"
                       "R1-1" = 441893

The two closest values are 390k and 470k ohm. 390k is the safest but let's check both. 

390k:
R = 1 / ( 1 / 390000 + 1 / 20000 )
R = 19024
Batt+ = 1.235 * ( 1 + 19024/3083 )
Batt+ = 8.86V

470k:
R = 1 / ( 1 / 470000 + 1 / 20000 )
R = 19183
Batt+ = 1.235 * ( 1 + 19183/3083 )
Batt+ = 8.919V

Well, pick your poison. I picked 390k ohm, the unloaded output voltage is 8.87V. I will charge a battery under a few different conditions and see what the result is.

Disclaimer

I don't know if all chargers for MD390 / RT8 are affected, check if your's are.
I don't know how bad the overcharge really is, but I'm not comfortable to overcharge my battery as much as Retevis is. I rather shorten the runtime a little bit for a longer lifetime.
The results are not finalized. I will update this page if I see the battery get overcharged too fast, or the charging get too slow.

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