QX7 inside battery charging - mainboard connected

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rcyoda
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QX7 inside battery charging - mainboard connected

Post by rcyoda »

I plan to mod QX7 to charge its battery inside the radio, using external charger.

I was planning simply to solder another leads to the battery and connect them to some socket mounted in the chassis. However original plug would be normally connected to QX7 mainboard, also during charge process.

Is it safe? or is it better to somehow disconnect battery to mainboard connection for charging? (i.e. using socket with integrated switch, able to disconnect hot battery lead to mainboard when charging plug in inserted).

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MikeB
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Re: QX7 inside battery charging - mainboard connected

Post by MikeB »

For NiMh cells, Aloft Hobbies do a charge circuit upgrade that does leave the battery connected to the radio while charging, so this should be OK.

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rcyoda
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Re: QX7 inside battery charging - mainboard connected

Post by rcyoda »

MikeB wrote: Mon Nov 20, 2017 11:14 am For NiMh cells, Aloft Hobbies do a charge circuit upgrade that does leave the battery connected to the radio while charging, so this should be OK.
I wonder if they use DC power socket "sleeve" pin (or something similar) to actually disconnect the mainboard, when charging plug is inserted...
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MikeB
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Re: QX7 inside battery charging - mainboard connected

Post by MikeB »

Looking at the picture of the wiring on their web site, it looks like the charge socket just connects to the two wires to the battery, no switching.

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Re: QX7 inside battery charging - mainboard connected

Post by rcyoda »

MikeB wrote: Tue Nov 21, 2017 9:42 am Looking at the picture of the wiring on their web site, it looks like the charge socket just connects to the two wires to the battery, no switching.
Seems you are right. Thanks!

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Re: QX7 inside battery charging - mainboard connected

Post by jhsa »

Question, I have my own theory, but what would be the problem in charging the NiMh battery without disconnecting the radio circuitry?
As long as we keep the battery connected, the charging voltage will stay only slightly higher than the battery voltage, right?
I don't know the radio schematics but I would guess that if the battery was disconnected during charging, and the charger didn't terminate it (I have at least one that doesn't), some components could be damaged?
For example, Mosfets on the radio's power switching circuit with lower Gate Maximum voltage? Perhaps the voltage regulator(s), because of the max input voltage (in case the battery disconnected and user turned radio ON)?
There are ways of protecting such circuits.
As I said, I don't know the actual radio circuitry,but I am just wondering why shouldn't we be able to charge the battery as long as it remains connected to the radio and charger?

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Re: QX7 inside battery charging - mainboard connected

Post by Kilrah »

No reason to worry. The only risky situation would indeed be if the battery got disconnected but the charger was still connected to the circuitry as it could be briefly supplying 30+V to try and maintain the set charge current.
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Re: QX7 inside battery charging - mainboard connected

Post by jhsa »

Yep, that is what I thought. I believe intelligent chargers should detect the batt being disconnected and terminate the charge. I have a couple of those chargers and will try to test later by measuring the voltage while disconnecting the battery, and check how high it is before the charger detects it and stop..
On other chargers perhaps a protection circuit could be placed on the charger cable. Perhaps something using a TL431 and a Mosfet.. Interesting, will look into it as this gave me an idea for a project of mine :)

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Re: QX7 inside battery charging - mainboard connected

Post by jhsa »

Ok, just gave it a thought and came up with this:
Over Voltage Protection.png
This should probably be external to the TX, but I think it is possible to mount it inside as long as R3 and R4 have quite high values to prevent the circuit from Draining the radio's battery.

Assuming the battery is a 6 cell NiMh battery pack and the charger would supply about 30V if the battery suddenly got disconnected from the radio and charger. For example if we disconnected the battery while charging.. :roll: :shock: :o This could damage the radio.

So, how it works? If I'm wrong, please someone correct me, I am always learning :)

R3 and R4 form a voltage divider that will set the threshold voltage the Shunt Regulator (TL431) will start conducting. Let's say 12V. The resistors need to be calculated so that when the charger voltage reaches 12V there will be 2.5V at the sense pin of the TL431. If the voltage at this pin is over 2.5V, the regulator will start conducting.

When the battery is connected (no Charger), battery voltage flows through the Mosfet transistor's internal diode, through R1, to the base of T1, turning it ON, and also the Mosfet transistor..

Connecting the charger, will start charging the battery. The voltage should now be a little higher than it was before. D1 protects against polarity reversal of the charger.

If suddenly the battery is disconnected/removed from the charger and radio, the voltage will go up to the charger's max, let's say 30V. This is over the 12V threshold set by the voltage divider. TL431 will turn ON, grounding the base of T1, turning it OFF. Consequently the Mosfet transistor will also turn OFF, disconnecting the radio circuitry from the charger..

Note, if the charger max voltage is around 30V, the Mosfet must be selected taking this in consideration.
A resistor between the collector of T1 and the Mosfet's Gate should help keeping this voltage down at the Gate.
The Mosfet must also be selected for a higher current than the maximum charging current..

Please comment and let me know if I am totally wrong here :mrgreen: it wouldn't be the first time nor the last ;) :)


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Re: QX7 inside battery charging - mainboard connected

Post by MikeB »

According to my reading of the TL431, it doesn't output a voltage lower than about 2.5V, so won't turn T1 off.
Also, the circuit will create a continuous current drain on the battery, as you mention, there is a current path through the diode of the Mosfet, the 4K7 resistor and the base-emitter junction of T1. This will cause at least 1.4mA to flow.

I've modded my QX7, I did include a diode so it is protected from the charger having the wrong polarity. The battery then charged perfectly OK, my charger works OK with a diode in circuit, not all chargers do.

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Re: QX7 inside battery charging - mainboard connected

Post by jhsa »

MikeB wrote: Thu Nov 23, 2017 11:01 pm According to my reading of the TL431, it doesn't output a voltage lower than about 2.5V, so won't turn T1 off.
I have just tested this today and you are right, it doesn't turn T1 OFF. I was expecting the TL431 to have a lower resistance between K and A when it turns ON, but it seems it doesn't :( I'm starting to think that the best way to do something like this is with a microcontroller. An ATtiny for example.. It takes less components as well..
Also, the circuit will create a continuous current drain on the battery, as you mention, there is a current path through the diode of the Mosfet, the 4K7 resistor and the base-emitter junction of T1. This will cause at least 1.4mA to flow.
Yes, and something else I have noticed is that because of the transistor, the voltage on the voltage divider at the TL431 sense pin becomes also completely wrong. It should for example read over 2.5V, but it reads about 1.5V, and I don't understand why this was happening..
Disconnected the base of the transistor and the voltage at the sense pin was correct..
I've modded my QX7, I did include a diode so it is protected from the charger having the wrong polarity. The battery then charged perfectly OK, my charger works OK with a diode in circuit, not all chargers do.
With the circuit above I wanted to protect the radio against the voltage increasing too much if the battery was disconnected while charging. Some chargers will increase the voltage to try to maintain the charging current. This could damage the radio??
I will give this a better thought when I have some more time :)

Thanks for your explanation.

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