Single Lipo cell discharger

Thread Starter

BJD00000

Joined Sep 6, 2017
3
I have designed the below circuit which works well but, the LM393 must remain connected to the cell after the mosfet turns off in order to keep it off. This will eventually drain the cell. Any thoughts on how I can completely disconnect the cell from the circuit once the mosfet turns off?
 

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ian field

Joined Oct 27, 2012
6,536
I have designed the below circuit which works well but, the LM393 must remain connected to the cell after the mosfet turns off in order to keep it off. This will eventually drain the cell. Any thoughts on how I can completely disconnect the cell from the circuit once the mosfet turns off?
There is a minimum fully discharged terminal voltage that you shouldn't drain the cell lower than - stack a bunch of diode Vf's and include a series current limiter. Aim the sum of Vf's just above the minimum terminal voltage, they cut off and break the current through the resistor.

There will be a tiny leakage current - so you probably don't want to shove it under the stairs and forget about it.
 

Thread Starter

BJD00000

Joined Sep 6, 2017
3
Thanks for the reply. The minimal voltage that I have designed is 3.8 volts. This is where the mosfet turns off. Ideally this circuit would remain connected to the cell indefinitely but draw zero current once that mosfet has turned off. The diodes I believe will continue to sink some small amount of current and eventually drain the cell. There must be a way!!

Thanks
 

AlbertHall

Joined Jun 4, 2014
12,397
Thanks for the reply. The minimal voltage that I have designed is 3.8 volts. This is where the mosfet turns off. Ideally this circuit would remain connected to the cell indefinitely but draw zero current once that mosfet has turned off. The diodes I believe will continue to sink some small amount of current and eventually drain the cell. There must be a way!!

Thanks
Even if there was just a switched off MOSFET there would still be a leakage current, though it would be low.
You could get to zero current if the switching was done by a relay. A 5V relay would likely hold in with a coil voltage of 3.8V but you'd have to check the spec.
 

ian field

Joined Oct 27, 2012
6,536
Thanks for the reply. The minimal voltage that I have designed is 3.8 volts. This is where the mosfet turns off. Ideally this circuit would remain connected to the cell indefinitely but draw zero current once that mosfet has turned off. The diodes I believe will continue to sink some small amount of current and eventually drain the cell. There must be a way!!

Thanks
You should check cell data sheets - I think the minimum voltage is lower than that.

The cut off leakage in a Si diode is *VERY* small - leakage in a series stack of them is ***VERY*** small.

Calculate series Vf and round up any difference from minimum terminal voltage.

You could be looking at years before the leakage makes anything bad happen, you could also add one more Vf just for luck.

All my e-cig cells came from laptop packs liberated from recycling tubs - some were dead flat when recovered from the salvage. They all worked, and with such easy source of supply - longevity and reliability is some way down the list.
 

Thread Starter

BJD00000

Joined Sep 6, 2017
3
I really appreciate the continued feedback. I think the tiny leakage may be acceptable. The 3.8 v floor is an ideal storage voltage not minimum which tends to be in the 2.4 v range. The circuit is for RC use. Currently any charged packs that you may have at the end of the day must be discharged to storage voltage if you don't plan on using them for a few weeks. This can be an arduous task with the bench tools most folks have available. This little circuit would be chained together up to 6 wide (for anything from 2 to 6 cell packs) and attached to the balance plug. Throw it in the box and forget it for a few weeks. If cost can be contained, a person could have 5 or 6 of them for your various packs.
 

ian field

Joined Oct 27, 2012
6,536
I really appreciate the continued feedback. I think the tiny leakage may be acceptable. The 3.8 v floor is an ideal storage voltage not minimum which tends to be in the 2.4 v range. The circuit is for RC use. Currently any charged packs that you may have at the end of the day must be discharged to storage voltage if you don't plan on using them for a few weeks. This can be an arduous task with the bench tools most folks have available. This little circuit would be chained together up to 6 wide (for anything from 2 to 6 cell packs) and attached to the balance plug. Throw it in the box and forget it for a few weeks. If cost can be contained, a person could have 5 or 6 of them for your various packs.
AFAIK: Storage level is a safety thing. Lithium is the slowest self discharge I can think of - but it does eventually. That can violate the minimum allowable terminal voltage.

Some manufacturers recommend storing at 50% charge - whatever that is.

My e-cig is run on recovered cells which experience high current draw in that application. I recently retired a set of battery units that had been in service about 3 years.
 
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