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Old 6th February 2013, 02:41 AM   #1
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Unhappy NiMH battery discharger

Ok guys here I am again. I have several 6V 5AAcell NiMH battery packs that I use for model airplanes. My problem is discharging them. I have used a 6V 50mA light bulb and watched over them the correct amount of time but really want to build a simple circuit to cut the discharging off after 5.5V has been reached. Variable voltages and discharge rates would be great since I use several size and voltage packs but simple is the best. I have scoured the internet but nothing gives me what I need except expensive balancers. I did see one USB PC software ran balancer but it would not match my needs. Any ideas or links would be great especially a simple schematic. I keep thinking pots and LEDs for dicharge rates but how do I make it cut-off when 5.5V is reached? I could use a 7805 and a bulb but that would drop me to 5V and that is too low. I always get great feedback here and it is sooooo appreciated.
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Old 6th February 2013, 02:44 AM   #2
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NiMH Battery Discharger - NiCad discharger Circuit
Could this be reworked and used?
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Old 6th February 2013, 02:59 AM   #3
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Quote:
Originally Posted by Bradford336 View Post
What needs rework? There is a chart with component values for 3-10 cells. NiCad and NiMH are very similar in use and can often be swapped though you shouldn't mix them together. The main problem with actual batteries (multiple cells) is that the cells may not be matched and the weakest one will be die first and then reverse polarize as the other cells discharge through it - hence your 5 cell 5.5 Volt cutoff. Reverse polarity is BAD but running the cells down is good - self discharge while not in use.

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Old 6th February 2013, 04:14 AM   #4
nattawa is offline nattawa  Canada
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Just sketched up a simple circuit. trim R7 to get it to cut off at 5.5V. D2 lights up when discharge is done.
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File Type: jpg 5v5.jpg (34.9 KB, 99 views)
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Old 6th February 2013, 08:10 AM   #5
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Maybe this could help... with some mods to fullfill your needs
http://bas.elitesecurity.org/discharger.html
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Old 7th February 2013, 01:37 AM   #6
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Thank you so much as usual guys. I am in business once again!!
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Old 7th February 2013, 01:47 AM   #7
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nattawa: Does C9 polarity not matter? What would the V and A specifics be for D2? I am also very needy about how the Q1-A is actually put into the schematic. The datasheets I found on it are greek to me. I know I know over my head. Sorry. I do like this setup since I can trim it for different packs that I use though and the learning curve is great too.

Last edited by Bradford336; 7th February 2013 at 02:04 AM.
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Old 7th February 2013, 01:50 AM   #8
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The schematic I posted confuses me about a couple things. D2 = rosu? I am sorry but I have no clue. I don't understand the 3,9 V D1 in the table either. I am new to electronics and picking up on it quickly with a general understanding. I do not know what some of these in depth things equate to.
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Old 7th February 2013, 02:06 AM   #9
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That circuit seems to be from an old Elektor magazine - the Romanian edition.
D2 is a red LED, and the D1 is a 3V9 zener diode. The table lists different zener and resistor values for various battery packs.
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Old 7th February 2013, 02:22 AM   #10
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thanks that makes sense to me
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