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 9V rechargeable lithium battery in GMC-300
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delikostidis

Netherlands
5 Posts

Posted - 07/25/2016 :  14:42:08  Show Profile  Reply with Quote
Hi Everyone,

I have just installed an EBL 9V 600mAH rechargeable lithium battery into my GMC-300 and it doubled its run time, which is awesome!

The question is, what would be the simplest way to charge this type of battery inside the unit?

The original 9V Ni-Mh is a 7-cell 8.4V battery (7 x 1.2V) while the 9V lithium ones are 2-cell 7.4V (2 x 3.7V). Thus I am wondering if I can modify the battery's voltage sensing resistor network to stop charging at 8.4V (4.2V x 2) instead of the current 9.8V (1.4V x 7) so that I can use the lithium battery.

Or, could a firmware update accommodate this option? (That would be the best solution by far).

Or else, as a last resort, I can still use a separate charging circuit (preferably squeezed inside the GMC-300 box), together with a Schottky diode in series with the battery connection to the unit's PCB. The latter will disallow charging from GMC-300's current charging circuit (but allow that from the add-on charging PCB instead). This would be the most cumbersome solution but I am willing to do it if needed and post the procedure/results as I have done before here..

Would be great to get any feedback on that :)

Edited by - delikostidis on 07/25/2016 14:43:45
Reply #1

Distelzombie

Germany
176 Posts

Posted - 07/29/2016 :  02:43:27  Show Profile  Reply with Quote
Hi
The new units ship with an AA akku inside with lower voltage. I assume they didn't rework the entire pcb but rather just changed a voltage limiting resistor.
But... I think you can simply use a diode or resistor as a voltage dropper/limiter in series with the battery. There are some diodes that can do that, aren't there? Anyway a resistor should work, but also limit the outgoing voltage. If the unit still works with this low voltage (should) you're fine.

GMC-300E+ V4.20 with sbt-11 alpha tube

My statements are to be taken as stuff-a-hobbyist-says.
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Reply #2

ZLM

1055 Posts

Posted - 07/30/2016 :  18:35:36  Show Profile  Reply with Quote
The GMC-300 V3.XX version used 9V battery. And changed to 3.7V lithium battery since V4.00.

The main problem is the stop charging voltage on your 9V lithium battery. The original circuit stopped charging on about 9.6V. That is the stop charging voltage on 9V Ni-Mh(7-cell 8.4V) battery. If your lithium battery has higher stop charging voltage than 9.6V, then the battery will not be fully charged. If your lithium battery has lower stop charging voltage, then it will be over charged, which is not good.
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Reply #3

Distelzombie

Germany
176 Posts

Posted - 07/31/2016 :  03:19:34  Show Profile  Reply with Quote
Could you provide us with a circuit diagram, ZLM?

GMC-300E+ V4.20 with sbt-11 alpha tube

My statements are to be taken as stuff-a-hobbyist-says.
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Reply #4

delikostidis

Netherlands
5 Posts

Posted - 07/31/2016 :  13:37:13  Show Profile  Reply with Quote
quote:
Originally posted by Distelzombie

Hi
The new units ship with an AA akku inside with lower voltage. I assume they didn't rework the entire pcb but rather just changed a voltage limiting resistor.
But... I think you can simply use a diode or resistor as a voltage dropper/limiter in series with the battery. There are some diodes that can do that, aren't there? Anyway a resistor should work, but also limit the outgoing voltage. If the unit still works with this low voltage (should) you're fine.



I have thought about e.g. putting a 1.2V zener diode in parallel to a shottky diode (anode to anode and cathode to cathode connected) in series to the connection between the battery and PCB. Thus the PCB will be powered with around 8.4V - 0.1V = 8.3V through the schottky. When now the battery is being charged, the voltage from the PCB to the battery will be 9.6V - 1.2V = 8.4V (through the zener). I still need to try it though to see if it works in practice without problems.

ZLM: could you please give us an insight on the voltage charging circuit changes from v.3 to v.4?

Edited by - delikostidis on 07/31/2016 13:52:14
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Reply #5

ZLM

1055 Posts

Posted - 08/06/2016 :  10:18:41  Show Profile  Reply with Quote
The GMC-300E 9V battery charging circuit used a 34063 IC for stepping up charging voltage. The 34063 chip output ~10.2V and then connect to the battery positive end via a diode. To increase the charging voltage, need to adjust the output voltage of 34063.
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Reply #6

delikostidis

Netherlands
5 Posts

Posted - 08/08/2016 :  17:45:21  Show Profile  Reply with Quote
quote:
Originally posted by ZLM

The GMC-300E 9V battery charging circuit used a 34063 IC for stepping up charging voltage. The 34063 chip output ~10.2V and then connect to the battery positive end via a diode. To increase the charging voltage, need to adjust the output voltage of 34063.



Awesome, thanks for this!
I will then modify the 34063 voltage sensing resistor values (actually only one resistor needs to be modified as it is a voltage divider) to produce ~9V instead of ~10.2V now. This will bring the charging voltage to the battery down to 8.4V (9V - 0.6V) using the existing blocking diode.
I will also probably change the 34063's current sensing resistor to be able to supply more mAs (e.g. 200mA) to the Li-ion battery and charge it faster.

I will make the mod when I find a little time and will post photos with the results.

Cheers!

Edited by - delikostidis on 08/08/2016 17:46:00
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