Battery Discharger schematic for Life po4 batterys

Thread Starter

Electrocub82

Joined Aug 13, 2020
5
I have 2s life batteries (1100 and 2200 mah) that I would like to make a discharger that will drain the batteries down to 2.5 volts.( I plan to discharge each cell individually). I plan to use a light bulb for the load and when the bulb goes out, I know the battery is at 2.5 volts. I had a discharger for other batteries, but have lost the design. I have searched online for schematics to do what I want, but the schematics are not designed to discharge to 2.5v. I am very much an amature, so I really can't take an existing circuit and change the values of the components to do what I would like it to do. The circuits that I have seen are very simple with only a few parts, (zener diodes, resistors, transisdtors, etc) Would someone please help me with what I would like to do? Thanks
 

Irving

Joined Jan 30, 2016
1,497
How accurate do you want the 2.5v to be? The more accurate the more parts/cost. What bulb are you planning to use?
 

Irving

Joined Jan 30, 2016
1,497
As already said, no reason to discharge them, but if you really must, this is about as simple as you can get....
The voltage at the junction of R1 & R2 is 1/2 battery volts. When battery >2.5v, U1 is turned on and so is Q1 and the lamp. When battery reaches 2.5v, U1 turns off and so does Q1 & the lamp.

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Thread Starter

Electrocub82

Joined Aug 13, 2020
5
Why do you want to discharge the batteries?
There is no advantage to discharging lithium batteries.
From what I read, the lithium phosphate batteries need to be cycled 20 times to be efficient. It would be nice to have the batteries cycled before I put them in my airplane.
 

Audioguru again

Joined Oct 21, 2019
3,173
How accurate do you want the 2.5v to be? The more accurate the more parts/cost. What bulb are you planning to use?
Lithium rechargeable batteries "wear out soon" if kept fully charged. I discharge my ordinary (4.2V full charge) Li-PO batteries to the storage voltage of 3.7V to 3.8V each winter as recommended in The Battery University.
I have never tried newer LiFeP04 batteries because they produce less power. They are damaged if discharged below 2.5V.
 

Thread Starter

Electrocub82

Joined Aug 13, 2020
5
As already said, no reason to discharge them, but if you really must, this is about as simple as you can get....
The voltage at the junction of R1 & R2 is 1/2 battery volts. When battery >2.5v, U1 is turned on and so is Q1 and the lamp. When battery reaches 2.5v, U1 turns off and so does Q1 & the lamp.

View attachment 238935
I was going to use quartz halogen lamps in paralell. Do you have a recommendation
 

Thread Starter

Electrocub82

Joined Aug 13, 2020
5
Lithium rechargeable batteries "wear out soon" if kept fully charged. I discharge my ordinary (4.2V full charge) Li-PO batteries to the storage voltage of 3.7V to 3.8V each winter as recommended in The Battery University.
I have never tried newer LiFeP04 batteries because they produce less power. They are damaged if discharged below 2.5V.
I am using them for ignition batteries and for flight pack batteries. 6.6v for two cells is perfect for receivers and servos.
 

Irving

Joined Jan 30, 2016
1,497
I was going to use quartz halogen lamps in paralell. Do you have a recommendation
Not really. Those batteries should be discharged at 0.5C, so say 500mA max. That suggests a 2 - 3v, 1 - 1.5W bulb. A 12v 5W bulb might work, but wouldn't light up much. A 5ohm 2W resistor, in parallel with a red LED (forward voltage ~ 1.5v) and 150ohm series resistor might be a better option.

However, you haven't really explained why you want to discharge the cells. Good LiFePO4 cells need no cycling (thats a fallacy from earlier technologies, there is no memory effect with LiFePO4), but should never be stored fully charged or discharged. They are happiest when kept between 30 and 70% charged. The longest life in charge/discharge cycles will occur if they are maintained between 70% and 30%, ideally only recharged once they reach the lower limit. It doesn't hurt to fully charge them as long as they aren't left in that state for any length of time. Since all cells self discharge, a cell at 30% thats not going to be used for a while should be fully charged, then discharged to ~70% for storage. The only reason for cycling LiFePO4 batteries is, when new, to ascertain their exact capacity and approx voltage at 10/20/30% DoD so you can track their ageing and occassionally through their life to recalibrate the 'fuel gauge'. This only is useful if you have mV/mA accurate automated charge/discharge equipment. I do this with my 24v/200Ah packs but then I have full & accurate cell voltage and charge/discharge current monitoring 100% of the time (and I'm obsessive with numbers/stats!) but for most users, and for those small cells, its completely unnecessary.

<~2.9 - 3v would be considered a low point for most LiFePO4 cells. 2.5v is arguably too low and will reduce cell life.
 
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