simple NiMh battery discharger

Thread Starter

unclearty

Joined Dec 24, 2017
2
I race small RC cars that run on 4 AAA NiMh batteries. I am trying to build a discharger that would discharge at about 1 to 1.5amp...but in series...then shut off completely. I don't want a parasitic drain left on them. I want to bring each cell down to .9 volts and stop. I understand that afterwhich, the batteries will spring back up a bit, but that is not a problem. Currently, modern dischargers discharge each cell individually and frequently seem to discharge lower than .9 volts. Discharging a pack of 4...in series..down to 3.6 volts and then removing the load seems to be more like the way they would discharge by running in the car.
I have done this manually by using a series battery holder...3 6 volt bulbs ...and sitting watching them with a multi-meter across...and waiting till they hit 3.6 volts....a bit time consuming. I would really like to build something that would automate this system.
Thanks
 

Alec_t

Joined Sep 17, 2013
14,280
Welcome to AAC!
Currently, modern dischargers discharge each cell individually
The reason for that is to ensure (hopefully) that each cell is discharged to the same state of charge. The problem with discharging cells in series, as you propose, is that cells do not all have exactly the same Ah capacity, so the same discharge current through all the cells will leave the cells in different states of charge. Subsequent charging could result in some cells being under-charged and some being over-charged.
That said, a circuit to discharge 4 cells in series, at a constant current, down to a set voltage is certainly doable.
 

ian field

Joined Oct 27, 2012
6,536
Welcome to AAC!

The reason for that is to ensure (hopefully) that each cell is discharged to the same state of charge. The problem with discharging cells in series, as you propose, is that cells do not all have exactly the same Ah capacity, so the same discharge current through all the cells will leave the cells in different states of charge. Subsequent charging could result in some cells being under-charged and some being over-charged.
That said, a circuit to discharge 4 cells in series, at a constant current, down to a set voltage is certainly doable.
Its worse - the cells still going reverse charge the one that gives up first. That pretty much finishes it off.
 

Thread Starter

unclearty

Joined Dec 24, 2017
2
Welcome to AAC!

The reason for that is to ensure (hopefully) that each cell is discharged to the same state of charge. The problem with discharging cells in series, as you propose, is that cells do not all have exactly the same Ah capacity, so the same discharge current through all the cells will leave the cells in different states of charge. Subsequent charging could result in some cells being under-charged and some being over-charged.
That said, a circuit to discharge 4 cells in series, at a constant current, down to a set voltage is certainly doable.
How would you suggest I set termination point to shut off at 3.9 volts...accurately?
 

Alec_t

Joined Sep 17, 2013
14,280
Use a comparator with a TL431 regulator configured to provide a 3.9V reference. The comparator output would control a power stage to discharge the cell stack. But I think you would need an independent supply of >3.9V to run the circuit.
 

crutschow

Joined Mar 14, 2008
34,285
Here's the LTspice simulation of a constant-current battery discharge circuit using a TLV431 (not TL431) shunt reference regulator that uses the battery voltage as a supply.
The discharge constant-current is determined by the value of R_CC (≈0.68 / R3).
The cutoff point is determined by the setting of pot U2 (giving a simulated cutoff of ≈3.6V for the pot wiper setting shown).
After cutoff, the battery current should be less than 1μA.
Capacitor C1 allows startup of the circuit when the battery is initially connected.
The P-MOSFETs must be a logic level type with a max. Vgs(th) of ≤2V.

upload_2017-12-26_11-15-5.png
 

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