Is this correct? @ LiPo MCU charging circuit - Page 1
Forum section: electronics / beginners

author: avc4eva
author_rank: Contributor
timestamp: October 26, 2014, 10:50:39 am
content: Hi all,New guy here, i'm designing a project using the Nordic nrf51822 MCU, one of my first queries is my charging circuit for the device.The device will be powered by a 110mAh 3.7V LiPo battery "https://www.sparkfun.com/products/731", the website offer the following to charge the battery "https://www.sparkfun.com/products/10401".I have adapted the schematic from the second product to use in my own, seen below:The device takes in 5V usb, places it into the MCP73831 charge management device (under voltage protection), outputting the required voltage 3V7 to charge the LiPo battery on CN1.1. I have then placed a 3V3 LDO in parallel with LiPO to supply to my MCU, is this a safe setup?2. I am also wondering if i am able to charge the LiPO and power the MCU at the same time?Thanks all, any comment is welcome!

author: Simon
author_rank: Unknown
timestamp: October 26, 2014, 11:03:11 am
content: I'm not sure how much LiPo's like being over discharged so an undervoltage sense and disconnect might be in order. Yes you can run the MCU and charge the battery, that's what phones do.

author: avc4eva
author_rank: Contributor
timestamp: October 26, 2014, 11:40:53 am
content: Thanks for the reply, i am pretty sure according to the PS that the MCP73831 stops under discharging.So the circuit looks okay?Thanks

author: Simon
author_rank: Unknown
timestamp: October 26, 2014, 11:42:30 am
content: You said you wanted to connect the LDO regulator across he LiPo, if your connecting it via the charge regulator thou will be fine

author: tggzzz
author_rank: Super Contributor
timestamp: October 26, 2014, 02:28:56 pm
content: Quote from: avc4eva on October 26, 2014, 10:50:39 amIf you aren't sure that you understand how to charge your specific type of lithium batteries, you might feel it prudent to charge them outdoors where explosions, flames and chemical spillages will do less harm.If you work out the energy stored in a lithium battery, it is in the same ballpark as that that in a hand grenade. Improper charging can damage the battery internally, resulting in runaway exothermic reactions. Search on youtube, and you will see examples.

author: avc4eva
author_rank: Contributor
timestamp: October 26, 2014, 04:11:13 pm
content: After doing more searching i have found similar designs to what i have done:1.http://electronics.stackexchange.com/questions/42229/3-3v-power-failover-supply-with-usb-li-ion-charger2.http://atomsoft.wordpress.com/2011/01/08/lipo-charger-with-3-3v-reg-output-300ma-max/Would it be a good idea to employ the switch design of the 2nd link? And also provide a switch to remove the battery from the circuit?Thanks.

author: graynomad
author_rank: Regular Contributor
timestamp: October 26, 2014, 11:57:08 pm
content: Pin 2 on the 1824 is not connected to GND.

author: Teledog
author_rank: Regular Contributor
timestamp: October 27, 2014, 01:47:59 am
content: Perhaps I'm way off topic, but the last couple oftinyLiPo cells I've purchased on eBay for "RC purposes" (~100-250mAh) have a tiny regulator board built in.One can see it on the top, through the transparent Kapton tape. (or peel the tape off & check the chips)Tried initially charging them through a LiPo charger before I noticed the boards..the built-in boards obviously seem to have controlled the charging, not the charger...my 2 cents anyway...

author: richcj10
author_rank: Unknown
timestamp: October 27, 2014, 02:46:53 am
content: Quote from: Teledog on October 27, 2014, 01:47:59 amThat board on the battery does not control charging. It prevents Under discharge, over charge, over current condition.

author: avc4eva
author_rank: Contributor
timestamp: October 27, 2014, 05:53:22 am
content: Thanks, must have missed it!

author: mikerj
author_rank: Super Contributor
timestamp: October 27, 2014, 11:41:40 am
content: Quote from: Teledog on October 27, 2014, 01:47:59 amThat is NOT a regulator board, it is only for under and over voltage protection. There will be a couple of back to back MOSFETs to switch off the charge and discharge paths if the battery voltage gets too high or too low respectively, but it will not provide the current limiting that is required from a LiPo charger.

author: avc4eva
author_rank: Contributor
timestamp: October 27, 2014, 07:45:04 pm
content: I've now updated the design a bit:Added:1. Missing gnd.2. SPDT switch to help with load sharing, able to charge the LiPO and supply the circuit separately using 5V usb. The other side of switch allow battery to source when USB disconnected.N.B. i guess a mosfet could be used instead for automatic switching using the 5v USB to switch, not sure about volt drop though.3. Added Stat line out from MCP73831 to GPIO of MCU, at the moment the LED lights when charging, i wanted the signal also to route to GPIO to detect this, will this work?Thanks for everyones help, and ye the battery just protects from under voltage, the MCP73831 controls charging.