I’m using an Adafruit micro-USB Charger connected to a standard 850mAh LiPo battery. I would like to know if it’s safe to charge the battery while also using it. The circuit that the BATT and GND pins are connected to will draw about 60-150mA. The charge current is 500mA.

From what I can tell, the charger board does not implement the MCP73831’s load sharing feature. I believe this means any load will be paid out by the charge current. Is this okay? In testing, I can see so far that the DONE light never illuminates (battery gets to about 4.18V and as soon as I disconnect BATT from the remainder of the circuit, it illuminates).

Schematic, etc: https://learn.adafruit.com/adafruit-microlipo-and-minilipo-battery-chargers/downloads

  • \$\begingroup\$ Can you post a schematic? \$\endgroup\$
    – winny
    Jul 8 '18 at 13:58
  • \$\begingroup\$ Updated with link \$\endgroup\$
    – Andrew M.
    Jul 8 '18 at 14:43
  • \$\begingroup\$ No. Post schematic, not links. \$\endgroup\$
    – winny
    Jul 8 '18 at 14:50

Generally speaking, safety of Lithium batteries have to be verified on the product and charger is only a part of the product.

Your circuit from Adafruit is based on MCP73831/2. This linear charger IC has the usual constant current/constant voltage charge control and, once termination is detected, it will restart charge only when battery voltage decreases under a certain threshold. This kind of regulation, under some circumnstances, works even if the battery has some load applied.

However, MCP73832 has a "charge termination" feature. Charge is terminated when the system current is below a certain percentage of the programmed charge current. If your system drains constant load from the battery, this current is always exceeded, and that's why charge termination is never detected.

I suggest you to have a read at AN1149 here: http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf This application note explains the problem and suggests you some possible solutions. It should be valid also if it is not strictly related to the same charge controller.

  • \$\begingroup\$ Thanks for the response. The note you linked is helpful. Do you know what potential downsides there are to the charge circuit never shutting off? E.g. battery health decline, fire danger, etc? \$\endgroup\$
    – Andrew M.
    Jul 8 '18 at 16:16
  • \$\begingroup\$ Yes, most Lithium batteries are not designed to be countinuously charged, trickle charge could cause lithium deposition on electrodes. This might be a concern for safety. However, the app-note suggests that the power is sourced by the wall adapter while the circuit is plugged instead of the battery. In this way, end of charge is detected properly. \$\endgroup\$
    – Fab
    Jul 8 '18 at 16:34
  • \$\begingroup\$ Thank you. I also forgot to mention that the LiPo has a built-in protection PCB. Do you think this would mitigate the safety concerns? \$\endgroup\$
    – Andrew M.
    Jul 8 '18 at 17:48
  • 1
    \$\begingroup\$ The protection circuit is a last resort solution to avoid damage but it does not offer full protection against improper charge and it should not be the functional principle of the device. Other chemistries are less picky about continuous trickle charge, such as NiMh or LiFePo. Unless you know exactly what you are doing, experimental chargers should never be trusted and left alone without supervision. Probably the battery will never explode, since you are not overcharging it (I don't give warranty) but you may reduce lifespan and damage it if you don't stop charging. \$\endgroup\$
    – Fab
    Jul 8 '18 at 18:31

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