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Author Topic: Usb Lipo charger from Sparkfun question  (Read 1091 times)
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I use this lipo charger : https://www.sparkfun.com/products/10161

with an Lipo battery of 400mAh/3.7V.

This lipo charger can be configured in 2 ways : one with 500mA the other with 100mA.

To drive it, I don't use the usb connector to charge the lipo but an 7805 with 2 capacitors because it takes power from an professional battery camera which is 12V.

So from, the lipo charger is in 500mA confiuration. I did some test with an external 9v power supply and the 7805 is very hot, around 90° !!!!

If I understand well, from my dc power with the 7805 : 9V - 5V = 4V and with 0.5A means 2 Watt of thermal dissipation. This is a reson why the 7805 is very hot !

I would like to know if I can use the lipo cherger with these lipo battery with 100mA instead of 500mA, so from it will give only : 0.4 Watts so I hope the 7805 will not be to hot.

If someone can help me or suggest another way of doing it. The system is in small closed box that is only :

85x55x30 mm.

The complete system is :

Arduino ProMini 3.3V/8Mhz / Nrf2401 HF transmitter, 1 push Button, 1 small Led, 1 lipo charger and 1 Lipo battery.

best regards.

Grag38
« Last Edit: October 06, 2012, 01:28:50 pm by Grag38 » Logged

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So from, the lipo charger is in 500mA confiuration. I did some test with an external 9v power supply and the 7805 is very hot, around 90° !!!!
Have you actually measured the temperature?  Most LM7805s are rated for operation up to 125C.
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the temp during charging an empty lipo battery is around 90°C/194°F.

 
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Using the 100mA should work.  Obviously it will charge much slower.

Regarding the 90C, remember that electronics tolerate heat much better than humans.
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Using the 100mA should work.  Obviously it will charge much slower.

Regarding the 90C, remember that electronics tolerate heat much better than humans.

You're right and I know that 90°C is not a so much temp. But in a so small box, it's to much IMHO, just because there no air, no hole that would allow heat exchange. I will do some test with the 100mA charging rate, and give you back the temp of the 7805 in that case.

By the way the 7805 will get an 12V on its input, I suppose the temp will be around the 120°C, I can use it in that way. So from 100mA will do the job I hope.
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If you can tolerate the slower charging, then definitely go for the 100mA.  Even though a chip can handle a certain amount of heat, more heat means less life. 
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Ok I did some tests with the 100mA instead of 500mA.

It tooks more time to charge the battery, but I don't mind about it.

Considering an power of 9V through this lipo charger I get now a temp around 55°C that will be ok for my small boxes.

Thanks to you, that helped me in this thread.

Best Regards
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Why don't you try a switching regulator like:

http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781

You can supply up to 1.5 amps without the heat and your camera battery will last much longer as it is not dissipating the excess energy as heat.

Digikey also stock Recom units but they are usually a bit more expensive.
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I never use these one before, I will order some to test them.
thanks for the link of this product.
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