I am trying to charge an 850mah 3.7v battery using a TP4056 circuit that will recieve regulated 4.2v at a maximum of 500ma though the current can drop due to it being regulated solar power. What will occur if the regulated voltage is below what current I set the TP4056 to charge. I am using the TP4056 due to its trickle charge feature and its overcharge protection. Can I just set the current on the TP4056 to the max and let it charge at whatever current it has available?

Is there any chip that justs stop the flow of current once the battery has reached 4.2?

Here is a schematic including the SPV1040 MPPT regulator

  • \$\begingroup\$ You "could" but then the impedance won't be matched to the PV current source and then you won't get max power transfer. \$\endgroup\$ Commented Jul 26, 2018 at 23:58
  • \$\begingroup\$ Since the SPV1040 outputs a a regulated voltage and current, is there some sort of circuit that just acts as a mosfet and stops charging once the battery reaches 4.2v? \$\endgroup\$
    – Julianfer5
    Commented Jul 27, 2018 at 9:22
  • \$\begingroup\$ a BMS card or IC for Lithiium and MPPT for PV's Look up MPT \$\endgroup\$ Commented Jul 27, 2018 at 14:14
  • \$\begingroup\$ I think I will float charge the battery taking in consideration it will have a constant load, the battery only needs to function for about a year and the charger will be nearly constantly operating. \$\endgroup\$
    – Julianfer5
    Commented Jul 27, 2018 at 22:25

1 Answer 1


Is there any chip that justs stop the flow of current once the battery has reached 4.2?

In charging of Li-Ion batteries the chargers do not stop when the voltage is reached 4.2 V (or 4.35 for Li-Po). If the chargers would do so, the battery cell will be charged to maybe 50% of its rated capacity. In fact, the charger voltage is slowly creeping up around 4.2 V for a while when the cell accumulates energy sucking full charge current (CC stage). The charger continues to keep 4.2V until the charge current starts dropping due to internal cell approaching full charge, and [charger] stops [voltage supply] when the current usually reaches 1/10 of main charging current (cut-off current). Because of this very slow slope in voltage the criteria of reaching 4.2V will be very unreliable, and is not used.

Technically every good programmable charger can set the cut-off threshold to much higher level. For example, the BQ2xxxx family of chargers from Texas Instruments can set the termination current limit to a wide range of values.

  • \$\begingroup\$ This is more than a bit misleading, and parts are downright false. The charge termination voltage (which for LiPo is 4.2v for all but the special high voltage cells rated to 4.35) is not something reflected in the charging behavior of the cell; it's a safety rating. The only way that the current would drop at that point, is if the charger limits the applied voltage to that level, which of course proper chargers do. Without that limiting the cell will happily rise to an unsafe voltage and fail. \$\endgroup\$ Commented Jul 28, 2018 at 19:22
  • \$\begingroup\$ In terms of capacity, only a very small amount is in the fractions of a volt around the termination voltage. Your grossly erroneous claim of 50% implies a flatness which just isn't there. \$\endgroup\$ Commented Jul 28, 2018 at 19:22
  • \$\begingroup\$ "Li-Ion cells do reduce the current draw on their own when they approach end of charge." this is ignorantly, dangerously wrong. There is no "end of charge" inherent in the cell - the only reason current falls is because there ceases to be a difference between the voltage applied by the charger and the voltage of the cell. Should the charger fail to do this, the cell will quite happily rise to an unsafe voltage far in excess of rating, while puffing up and then catastrophically failing. \$\endgroup\$ Commented Jul 28, 2018 at 19:24
  • \$\begingroup\$ False. Ohms law applies everywhere, even when the resistance is non-linear or time-varying, it is still true for the values in effect at that point of operation and instant in time. The key misconception in your arguments is the idea that a lithium cell has an end-of-charge behavior of its own. This is false - the only end of charge behavior limiting things to a safe state is that resulting from conditions created by a proper charger. \$\endgroup\$ Commented Jul 28, 2018 at 19:37
  • \$\begingroup\$ Thank you all for your contribution, Both of you have valid points. I am going to go for float charging due to simplicity and the fact It will only need to operate for a couple months. Thanks both and please avoid arguing over slight misconceptions again, you both have valid arguments and errors. \$\endgroup\$
    – Julianfer5
    Commented Jul 30, 2018 at 10:41

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