Circuit to get 1S Lipos to OCV 3.85V for Storage

Discussion in 'The Projects Forum' started by ElectronMan1, Jan 26, 2016.

  1. ElectronMan1

    Thread Starter New Member

    Oct 24, 2015
    3
    0
    Hi,

    I am wanting an easy way to get my RC Lipos - 1S Lipos - 3.7V nominal - to 3.85 open circuit voltage for storage quickly and easily. This 3.85V equates to roughly a 50% state of charge I believe. I have 1S Lipos with different capacities and discharge "C" ratings. From what I have read, storing Lipo cells fully charged is bad, and storing them discharged is bad also. After flying the voltage is about 3.6 or 3.7 volts resting OCV. 3.85 OCV is the agreed storage voltage it appears.

    My charger charges the cells all the way to the top, 4 individual charging circuits. No voltage readout. Just an LED that blinks when it is nearly charged and go out when it has finished charging.

    Possible solutions I have ruled out:
    Balance charger - Why - Because in Parallel the cells would need to be similar in state of charge to avoid damage to the cells. In Series the voltages would need to be very similar to avoid damage to the cells. I would like this to be an "easy" process where I just plug in and don't worry about state of charge across cells or voltage, etc.
    Commercial charger - Why - They tend not to have storage mode or if they do are expensive for what they are ($330+) and not really designed for 1S Lipos
    Cheap Adjustable Lipo voltage alarm - Why - Because from what I have seen they only work from 2S lipos upwards
    Simple Voltage Comparator circuit - Why - From my tests with a multimeter, discharging with a 10ohm power resistor, the load voltage is not directly related to the OCV and seems to differ from cell to cell and changes with C ratings and mah capacity. The closest I could get was a load voltage of 3.6V with a 10ohm power resistor load, which seemed to leave it at about 3.85V at rest. But for some cells 3.6V under this load, it shot back up to 4V when the load was removed.
    Manual multimeter / simple timing - Why - takes too long. Same applies with adding voltmeters to the charger, still need to wait around and not miss it. Also simple timing is not reliable and hard to keep track with cells of different capacities.

    Current idea:
    My idea is to use a standard Lipo Battery Gauge Integrated Circuit to accurately determine the state of charge, and then use this to activate a relay if the state of charge is within the parameter (about 50% charge) to either stop charging, or stop discharging.. depending how I implement the circuit.

    The problem:
    I have no idea about these Lipo Battery Gauges ICs, which ones to use, if this is a feasible idea, or if I have missed something.

    Look forward to your thoughts and ideas
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Do you want a circuit to charge them upto 3.85V, or to discharge them?
    A TL431 zener and led can be used to discharge them, as for charging a deadicated chip like TP4056
     
    Last edited: Jan 26, 2016
  3. ElectronMan1

    Thread Starter New Member

    Oct 24, 2015
    3
    0
    Thanks Dodgydave,

    I should have mentioned some of these Lipo cells are 500mah capacity, and soon adding some 750mah ones.

    I like the idea of the adjustable zener, but if I understand correctly the TL431 has a max continuous cathode current of 150ma (datasheet: http://www.ti.com/lit/ds/symlink/tl431.pdf) which means it would take a while...

    Ideally on the charge up, so I can reduce the number of cycles of the cells (to avoid charging the cell only to then discharge it). But if it is simpler I would go for discharge..

    For the TP4056 it appears the voltage is fixed at 4.2 V - datasheet https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf

    If I can find an IC that I can change that voltage to 3.85V, I think I'm in business...
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Ok use a Lm317, you can set the charger voltage to 3.85V by omitting R2 with a link and adjust the 1k pot to set the output voltage to the desired level,,the 0.5 ohm resistor is for current limit of 1.4amp, you can increase it upto 5 ohms to drop the limit to 140mA.

    http://circuit-zone.com/ediy_blog/313/battery-charger.jpg
     
    Last edited: Jan 27, 2016
    ElectronMan1 likes this.
  5. ElectronMan1

    Thread Starter New Member

    Oct 24, 2015
    3
    0
    Thanks Dodgydave, that looks great
     
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