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I am looking for the cheapest, smallest way of charging a 50-100 mAh 3.7V Li-Ion battery through micro USB. Fast-charging is not necessary, especially if it would cause an increase in price or size.

To give you some more background on my ECE education. It has focused primarily on electrical circuits and their biological applications. A really huge gap in my education is that I have almost no knowledge regarding Li-Ion batteries, let alone how to safely charge them. In fact, I have stupidly just been soldering these kinds of batteries to my board for a DIY project without putting them through any kind of regulator. Therefore, I really am recognizing my ignorance now as I begin to research charging these kinds of batteries, and would like to pose this question to those with a more traditional ECE background to make sure I don't make such a stupid, dangerous, and wasteful mistake again.

I have found a number of interesting example circuits, but most of them involve regulators designed for much higher current uses. My circuit requires about 1-5mA. The ultimate design is a heartrate monitor, so safety is also a concern. Charging time is not a concern.

How can I find an effective battery and charger design for this particular use case? Is the MCP73831 the most appropriate IC for this design? What kind of parameters and search terms should I use to find the cheapest possible product on a site such as DigiKey?

Ideally, I would prefer something that could be easily described to a foreign circuit manufacturer should I decide to go that route. That is, I don't just want a simple, overpriced charger from Sparkfun. The ideal answer would be a number of individual components (USB connector, charging circuit, battery), or a pre-made charger that is extremely cheap and needs only a battery to connect to. I want to build it myself, in a way that can be mass manufactured later on, should I be fortunate enough to go that route.

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  • \$\begingroup\$ MCP73831 sounds like a good candidate for your project. Cost of MCP73831 on AliExpress is under 10c in small quantity of 100. How cheap is your cheap? \$\endgroup\$ – Ale..chenski Jan 10 '17 at 1:14
  • \$\begingroup\$ Something on the 10 cent order of magnitude is what I'm looking for, yes. Although, the MCP73831 does not appear to have as many pre-made boards as the TP4056 listed below, meaning I would have to pay a little more than that for a microUSB port (plus shipping from china). Really, I am looking for some sort of price compromise from the $9 boards sparkfun offers, which something like the TP4056 appears to have. \$\endgroup\$ – nick carraway Jan 10 '17 at 4:32
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There are many charger ICs out there. MCP73831 is one of the cheapest ones.

I have learned a lot from the site below over the years (and I'm involved in making charging, discharging, emulation circuits for Li-polymer batteries).

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

The typical parameters for many lithium-ion and lithium-polymer cells are below. This is just typical specs. You need to read the datasheet for your cell.

  1. Use a constant voltage source at the specified voltage (e.g. 4.2V or 4.35V) with built-in current limiting set no higher than the manufacturere's specified max charge current
  2. Output voltage must have 1% absolute accuracy
  3. Charge at a max rate of 1C (C = Ah capacity of battery). Example: A 100 mAh battery can be charged at 100 mA.
  4. Stop charging (full charge is reached) when charge current falls below 1/10C. Example: A 100 mAh 4.2V battery is fully charged when you apply 4.2V to it and the current is less than 10 mA.

Again, these are just typical numbers.

There are critical safety issues that you must also address:

  1. Do not allow the battery to below some minimum voltage, often 2.5V. If battery ever goes below minimum voltage, throw it away because it is damaged. Do not charge it afterwards as it can overheat, puff up due to high internal pressure, and there is a high risk of rupture and fire. (Personal experience.)

  2. Do not exceed the manufacturers specified discharge current or charging current. The battery can overheat and go into thermal runaway.

  3. Do not charge it higher than specified voltage + 1%. You can plate lithium metal onto the electrodes and cause a dangerous condition.

Commercial batteries have a safety IC controlling MOSFET switches that disconnect the cell to try to prevent conditions 5, 6, 7. Hobby batteries do not have such safety circuits.

Also, battery compartment design is important. You need to:

A. Leave enough room for the battery to swell slightly. Over its life it will "puff" slightly. A proper datasheet will tell you how much to expect. Cheaper cells often puff more over time.

B. May need some cooling if you are running at max discharge or charge rate (or exceeding it!).

YOU MUST READ THE DATASHEET TO MAKE SURE YOU MEET THE MANUFACTURERS REQUIREMENTS

Read that link and other references and absolutely read the manufacturers datasheet and you will gain plenty of knowledge. Hope that helps, -Vince

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    \$\begingroup\$ This is a great answer for someone with my educational background. Particularly the comment about "C" referring to the battery's charge capacity, as originally I thought it stood for capacitance. The safety information is extremely useful as wel. \$\endgroup\$ – nick carraway Jan 10 '17 at 4:35
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    \$\begingroup\$ Keep in mind that the charger IC only 1 part of equation. Also critical is to do design in shutdown at low voltage (typically 3 Volts) on the load side to prevent overdischarge. Key is to design all the circuits (not just charging IC) to meet all of the manufacturer's datasheet if you want to be safe. Yes, safety absolutely critical with Li-ion/li-poly for a commercial device. Google "Note 7 battery problems". Also, phone batteries are 30x bigger than your 100 mAh, so 30x more "kaboom" energy. \$\endgroup\$ – Vince Patron Jan 10 '17 at 17:43
  • \$\begingroup\$ As I implement this charger today on a LiPo that just ran out of juice, your comment about not recharging batteries below ~2.5 V is a life saver. So, I'm accepting your answer, even though I did end up using the TP4056 \$\endgroup\$ – nick carraway Jan 15 '17 at 10:45
  • \$\begingroup\$ Scratch that. Going with the MCP73831 since the tp4056 isnt specified for this low of power \$\endgroup\$ – nick carraway Jan 15 '17 at 11:50
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TP4056 is one of the cheapest solutions. Assembled avail online as below, $2cdn ~$1usd on sale with this data sheet . It uses the 3 stage charge standard method 1A-CC, CV , Shutdown @100mA cutoff (10%A-max)

Unlike many other IC's designed for 500mA max with external drivers, this one regulates CC mode at 1000 mA with an internal driver.

https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf enter image description here

Another solution is this similar but non-compatible http://cds.linear.com/docs/en/datasheet/405642f.pdf

The LTC®4056 is a low cost, single-cell, constant-current/constant-voltage Li-Ion battery charger controller with a programmable termination timer. When combined with a few external components, the LTC4056 forms a very small standalone charger for single cell lithium-ion batteries.

Charge current and charge time are set externally with a single resistor and capacitor, respectively. The LTC4056 charges to a final float voltage accurate to ± 0.6%. Manual shutdown is accomplished by grounding the TIMER/SHDN pin, while removing input power automatically puts the LTC4056 into a sleep mode. Both the shutdown and sleep modes drain near zero current from the battery; the shutdown mode reduces supply current to 40μA.

The output driver is both current limited and thermally protected to prevent operating outside of safe limits. No external blocking diode or sense resistor is required.

The LTC4056 also includes low battery charge conditioning (trickle charging), undervoltage charge current limiting, automatic recharge and a charge status output

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  • \$\begingroup\$ To implement something like this in my DIY circuit, I would first have to solder the negative terminal of my LiPo to two places: the negative terminal of my load, and the negative terminal of the charging circuit. For the positive terminal of the battery, I would need a 3-pole DIP switch so that theres an "On" mode, an "off" mode, and a charging mode. Is this correct? \$\endgroup\$ – nick carraway Jan 10 '17 at 4:25
  • \$\begingroup\$ Or, is there another place I should be soldering to on these kinds of boards? I'm assuming the only places to solder on these boards are the BAT+ and BAT- areas, but some other boards use actual system out terminals for the load. \$\endgroup\$ – nick carraway Jan 10 '17 at 4:39
  • \$\begingroup\$ ty @AliChen updated with correct link \$\endgroup\$ – Sunnyskyguy EE75 Jan 10 '17 at 4:43
  • \$\begingroup\$ Still researching. But it seems both these ICs have a minimum charge rate of 100 mA, which would not be appropriate for a 50-100mAh battery? \$\endgroup\$ – nick carraway Jan 15 '17 at 11:01
  • \$\begingroup\$ Yea taking out this SMD resistor is probably harder than just ordering an MCP73831. Plus I'll be outside the manufacturer's specifications \$\endgroup\$ – nick carraway Jan 15 '17 at 11:49

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