| Electronics > Projects, Designs, and Technical Stuff |
| Looking to (re)build a circuit to discharge my lipo batteries to like 3.9-3.7 |
| << < (4/9) > >> |
| Kasper:
--- Quote from: SiliconWizard on December 29, 2019, 11:16:42 pm ---A relay... that would be absolutely not accurate, not by a long shot. The added fun would be that it would probably get into an oscillating state when the voltage gets close to the min operating coil voltage... furious clicking... Fun stuff. :-DD --- End quote --- The clicking could notify the user the drain is complete ;) |
| not1xor1:
--- Quote from: Siwastaja on December 29, 2019, 11:59:05 am ---It takes ages (days to weeks) to discharge through the small resistor and TL431, so you need to leave it mounted. Then there's the risk you forget it connected, and because the quiescent current is fairly high with that resistor divider in there, there's a risk of destroying the battery by overdischarge during storage. --- End quote --- Days to weeks ? :scared: Lets make a rough calculation (assuming 100% efficiency and constant voltage battery). A 2000mAh cell, at 100% capacity holds an energy of 4.2V * 2A * 60 minutes * 60 seconds = 30.24kJ To discharge it to 60% of capacity you have to take away 40% of energy, that is 30.24kJ * 0.4 = 12.096kJ. A TL431 withstands 100mA of current. Lets make it 80mA of average current (I didn't care much of that in the circuit I previously attached), now that means that at 4.2V and 80mA the needed time would be : 12.096e3 / (4.2*80e-3) = 36e3 seconds, that is 10 hours. So unless I made some coarse mistake and unless you want to discharge the battery pack of an electric SUV I think it would take quite less than days or weeks. :phew: The schematic I posted was just a quick guess of that in the photo. I do not know if they use a dedicated chip or if that is just the usual careless design with a generic shunt regulator. In any case, with just few more components, it is easy to design a few µA leakage circuit (around 1µA in LTspice). It would cost no more than 1-2 euros/USD and might be a good DIY project. 8) notes: - all resistor values should be changed according to the number of cells and the type of LED, do not forget about the maximum MOSFET gate voltage: a voltage divider or zener is needed in case of more than 4 cells. - I just chose the first low Vth MOSFET model in LTspice, I've no idea about the price and other features, but there are plenty of suitable MOSFETs - C2 is there to start the circuit. It might cause TL431 instability (oscillations) and would require a discharge button, so it might be better to just replace it by a (momentary ON) push button with a series resistor (or better just use a bit overrated power resistor for R3) - do not forget to take into account TL431 and resistor tolerances. BTW - AFAIK - Li-ion batteries are not affected by the Ni-Cd memory effect and last longer if the discharge current is in the order of several tenths of mA rather than A |
| magic:
--- Quote from: SiliconWizard on December 29, 2019, 11:16:42 pm ---A relay... that would be absolutely not accurate, not by a long shot. The added fun would be that it would probably get into an oscillating state when the voltage gets close to the min operating coil voltage... furious clicking... Fun stuff. :-DD --- End quote --- Relays have hysteresis, the battery would need a bit of ESR for that. But a relay oscillator is an intriguing idea nevertheless ;) --- Quote from: not1xor1 on December 30, 2019, 04:29:13 am ---saturation voltage should be a little less than 2V --- End quote --- Fair point, it may be limited only by saturation of the reference input transistor. I didn't know. It doesn't change the calculations much, except for my 1 cell version which should definitely increase cathode resistance then. |
| eblc1388:
--- Quote from: Siwastaja on December 29, 2019, 11:59:05 am ---It takes ages (days to weeks) to discharge through the small resistor and TL431, so you need to leave it mounted. --- End quote --- With a slight change, one can also control the discharge current by using different value of R2. The attached example shows a discharge current of 1.2A. The cell voltage would drop very quickly within minutes to hour. Care in the choice of PMOS which should be logic level for it to work. |
| not1xor1:
--- Quote from: eblc1388 on December 30, 2019, 01:28:55 pm --- --- Quote from: Siwastaja on December 29, 2019, 11:59:05 am ---It takes ages (days to weeks) to discharge through the small resistor and TL431, so you need to leave it mounted. --- End quote --- With a slight change, one can also control the discharge current by using different value of R2. The attached example shows a discharge current of 1.2A. The cell voltage would drop very quickly within minutes to hour. Care in the choice of PMOS which should be logic level for it to work. --- End quote --- Apart that you are re-quoting Siwastaja's plain nonsense, there is no difference if the batteries are discharged in half an hour or ten hours. A quick discharge decreases the battery life and increases cost and component size because of more power dissipation while one can forget to disconnect the battery from the discharger circuit in both cases. In the second circuit I proposed power dissipation is in the order of hundredths of mW and once the set voltage is reached the current leakage is in the order of µA. After one month a forgotten 2000mAh cell would lose 0.1-1% of capacity. |
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