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I have a GSM module (Particle Electron) which has pins for powering it from a LiPo battery. The pins accept voltages from 3.6V to 4.4V. The module can also be powered from 5V supply through another pin (Vin).

I have a solar charger based on the MCP73871 LiPo battery charger IC from Microchip. This charger has a BAT+ output for charging LiPo batteries, and we have added a regulator giving a regulated 5V at LOAD+. However, this has current limitation of 1A.

Now, normally we would connect the GSM module's Vin pin to the 5V output. However, since the module has current surge requirements of 1.8A, we are thinking of connecting the Li+ pin of the GSM module to the BAT output of the solar charger.

I want to know what could go wrong in this setup? Could this setup somehow damage the battery, or the solar charger, or the GSM module?

schematic

simulate this circuit – Schematic created using CircuitLab

To put it simply:

Can we connect in something in parallel to a LiPo battery while it is being charged by a LiPo charger? Can the charger or the LiPo get damaged in such a case?

Some essential info:

  • GSM module has surge current requirements of upto 1.8A
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  • \$\begingroup\$ Does the charger feature any timeout? \$\endgroup\$ – winny Jun 23 '18 at 18:42
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I don't think this is good idea. The charger measures voltage and current to determine proper charging profile for LiPo. It cannot distinguish between too high battery current and normal current + additional load. Plus, in your case it is simply too weak to support combined current (max battery output is 1A).

That is the reason for modern BMS having "power path" technology, which does tricks like charging battery while feeding the system with extra power if it is available, or supplementing weak power source with some battery discharge. Your MCP73871 is exactly that kind of BMS! I'd recommend using it as intended.

Keep in mind, that

  • the GSM modules themselves are either 3.3V or 1.8V devices. No matter how you supply power to it, it will go through regulator.
  • I don't know what solar panel you use but I doubt it can supply 2.8A, which is combined max charging current and max GSM requirement.

One more thing. You said your charger provides "regulated 5V output". The MCP73871 does not do this. If external power provided it goes directly to OUT pin. It will be 5V from USB and 6V from solar panel. So basically, there are no "losses of the regulator" you are trying to avoid, except those in the GSM module that you have no control over anyway.

UPDATE:

If you draw enough current to drive supply below battery voltage (which is not hard to do with solar panel) then (and only then, per Kirchhoff) the battery will start discharging. At this point MCP will go into shutdown mode (see UVLO in datasheet) which will cut the input off and (hopefully) protect the charger. So, yes, you will get your surge current.

However in all other conditions you will have charge curves off and nice precise chip doing fools errand. All that to solve a problem created artificially by bad choice of LDO. I'd recommend using ultra-low LDO (ISL80103, LD39200, NCP59300) for as little as 0.12V drop and let the chip do its job.

UPDATE 2

Come to think of it, why do you need LDO at all?

In most cases the output voltage is at or below 5V. Only the combination of fully charged battery, bright sun and GSM in some low power mode will result in higher voltage, and even then only 1V over!

What you need is a voltage clamp. Something as simple as zener circuit below or a bit more complex with FET like this. Note that both have exactly zero loss under normal conditions, unlike LDO which will always drop some voltage, even if input is below regulation level. I even think you can find high power 5.05~5.1V zener with enough internal resistance to get by with just diode alone, assuming GSM module has permanent connection to output.

enter image description here

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  • \$\begingroup\$ Thanks for the reply. The idea that you raised about charger determining charging profile is spot-on! However, I would like to point a few clarifications. First, our LiPo battery can provide surges upto 2.5A easily. Second, during surge current, why would solar panel need to provide 2.8A? Even if the solar panel is providing, say, a meager 300 mA, then the rest would be provided by the battery, wouldn't it? Finally, the regulator I am taking about is an external regulator (which is not in the IC). \$\endgroup\$ – shivams Jun 23 '18 at 4:27
  • \$\begingroup\$ I thought about external regulator and did a search for "solar charger MCP73871". From many hits not one board had additional LDO, so I dismissed the idea. If you have something with it a link in the question would be beneficial. The battery can provide surge current only when it is charged. If the battery is low it is being charged, and the charger must provide for it and the GSM. It is simple question of voltage differentials, after all. If battery voltage is below external power source it will be charging, not supplying \$\endgroup\$ – Maple Jun 23 '18 at 7:05
  • \$\begingroup\$ In the case of surges, despite low voltage, I was assuming that part of the surge would be provided by the charger and partly by battery. Hence, battery would NOT be getting charged at that point. \$\endgroup\$ – shivams Jun 23 '18 at 9:54
  • \$\begingroup\$ I added reply to the answer. It's just my opinion, if you don't like it you still can do whatever you want to do, of course. \$\endgroup\$ – Maple Jun 23 '18 at 12:01
  • \$\begingroup\$ You are welcome. Added one more. \$\endgroup\$ – Maple Jun 23 '18 at 21:20

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