Guys, I'm planning to build a battery discharger for a single sub C type battery. I need an adjustable cut-off circuit that ranges from .9V down to .6Vdc. Any help would be gladly appreciated.

 

What is your discharge rate?

 

Ron H, I usually do it 5 to 10Amp discharge rate per cell.

 

How high is the voltage prior to discharging?

 

That's around the level of a fully charge sub C cell, usually 1.3 to 1.45V I think!

 

Here's a circuit that should work. I have simulated it, but it has not been built. You can substitute any MOSFET with an Rds(on) of 30 milliohms or less. You'll need a heatsink for the MOSFET, as it dissipates several watts during discharge.

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battery_discharger.GIF (13.4 KB, 837 views)

 

Ron H, Thanks for your help man... But, I felt so stupid not mentioning that the circuit I'm looking for is to be used as an equalizing tray for a stick pack(6 sub "C" cell connected in series side by side). Sorry about that man.... ops: ops: ops: Need to equalize each cell to a certain level before recharging them again. Hope you get what I mean. ops:

 

I looked at a product description for the Novak Smart Tray Digital Equalizing Discharger. It seems to me that you can build 6 of these circuits, all powered by the same 9 volt battery, controlled by the same pots for current level and discharge voltage, and started by the same switch. The batteries cannot be connected in series or in parallel. They must be discharged individually. If you use 6 of these circuits, they will all stop discharging at exactly the same voltage, independent of time, which I gather is what you want.

 

For cell equalizing i use this circuit.

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disch.GIF (1.6 KB, 805 views)

 

Ron H:

Yes! you're right... I'll try to finish it this week and inform you later for some updates. Thanks again man...

Sebi:

That's the simpliest equalizing circuit i've ever seen 8) 8) 8) But, unfortunately what i preferred is the one built with cut-off circuit. Thanks man....

 

Scott, build one and test it before you jump in with both feet.

 

I use this circuit, one for each cell. The resistor sets the current and the diode sets the final voltage. If you use a “normal” diode the final voltage is 0.6 and with two schottkys 0.7 . These voltages can vary but you can find schottkys with 0.25 – 0.55 per diode. The life of the cells is much longer when discharging the cells one by one.

Ante :roll:

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Discharge_110.gif (1.0 KB, 769 views)

 

I use this circuit, one for each cell. The resistor sets the current and the diode sets the final voltage. If you use a “normal” diode the final voltage is 0.6 and with two schottkys 0.7 . These voltages can vary but you can find schottkys with 0.25 – 0.55 per diode. The life of the cells is much longer when discharging the cells one by one.

Ante :roll:

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Discharge_198.gif (1.0 KB, 770 views)

 

I like the simplicity of the diode-resistor circuit. Whether it meets you needs is another question. See the plot below of a simulation of both methods. The diode-resistor discharge starts out at 1 amp, then decays pretty much exponentially. The constant current discharge is 1 amp. The time scale is arbitrary, depending on the battery, and I have no idea how much charge they actually hold. This sim is actually used two 1000 Farad capacitors to simulate the batteries, each charged to 1.5 volts. In the diode-resistor circuit, I used a 0.8 ohm resistor and an MUR120 rectifier, which was a diode that I had a spice model for.
You can change the discharge rate to meet your needs in either circuit. The cutoff voltage is set-and-forget with the constant current circuit. Here, I set it for 0.7 volts.

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batt_disch.gif (10.8 KB, 764 views)

 

Ron,
Interesting test you have made there. I have not tried something like that, but you gave me an idea when using a big cap for battery. I have to see if ISIS will do the work. Many years ago I experimented with a cut off load version but didn’t get it right. The problem I encountered was after cutoff the cell regained power (different for each cell) and this caused the cells to differ even more before they were charged again. Then I discovered that the diode resistor method was the right one for the job. I just leave them on till the next day (sometimes I forget them for days) and they survive and stay equal for a long time.

BDW sorry for the double post ops:

Ante :roll: