Battery Charger circuit with BQ24040 - Page 1
Forum section: electronics / projects

author: mwestern
author_rank: Newbie
timestamp: August 13, 2020, 10:28:03 pm
content: Hello everybody, I am working on a project where space is a huge premium. I will only be able to use the front side of the pcb as the backside will be the positive plane connecting to the battery. Just wondering if anybody has used a BQ24040 for an 18650 battery charger circuit, or if you have any suggestions for another route to take. Attached is my current schematic, I am new to all of this so any comments are greatly appreciated. I will also attach the data sheet for the IC.

author: tooki
author_rank: Super Contributor
timestamp: August 14, 2020, 09:40:37 am
content: You need to look at the datasheet more carefully — it tells you which pins are inputs and which are outputs. In your schematic, you've got the two status outputs /PG (power good) and /CHG (charging) with pull-up resistors to VCC as if they were inputs. The resistors shown in the datasheet are simply the current limiting resistors for the LEDs, which it looks you're not using. Having the 10K to VCC won't hurt anything (besides wasting a trivial amount of power), but it's not going to help, and above all makes me a bit worried that you may not understand quite enough to be working with lithium charging, which is (while not difficult per se) very unforgiving of mistakes. Are you using batteries with built-in protection? Otherwise, that's also something you need to add.

author: mwestern
author_rank: Newbie
timestamp: August 14, 2020, 06:04:39 pm
content: Yes I will be using batteries with built in protection. What would you suggest I do with those pins if I don't wish to connect them with LEDs to display PG and CHG? Pull down resistors? My goal is to get this to output the maximum 1 Amp and have as quick of a trickle charge as I can without damaging the battery.

author: tooki
author_rank: Super Contributor
timestamp: August 14, 2020, 08:54:49 pm
content: Quote from: mwestern on August 14, 2020, 06:04:39 pmNothing. They don't need to go anywhere. All they do, in essence, is to connect themselves to ground to turn on the LED. The datasheet even says they go high impedance when inactive. If they needed to be pulled up/down when unused, the datasheet would say so.

author: tooki
author_rank: Super Contributor
timestamp: August 14, 2020, 09:09:42 pm
content: Quote from: mwestern on August 14, 2020, 06:04:39 pmYou can't trickle charge lithium batteries — but I suspect you're using the wrong term and don't actually mean trickle charging.Trickle charging is a charging method where a battery is charged by leaving it on a very small charging currentindefinitely, so that it's always kept at full charge. Of the common rechargeable battery types, only lead-acid, NiCd, and special trickle-charge NiMH can handle this. Lithium, standard NiMH, and LSD NiMH (Eneloop) are all damaged by trickle charging. Trickle charging is basically how car alternators top off car batteries, as well as how lots of old cordless phones, CB radios, flashlights, etc. through the mid 90s charged their NiCd batteries.I suspect instead that you are actually referring to the termination charge, the constant-voltage phase of CC/CV charging where the charging current tapers down to the termination (cutoff) current. Note that changing the termination current setting doesn't change therateat which the CV phase charges at (as that's determined by the voltage and the internal resistance of the battery, which changes as it gets full), it's just setting when to stop. Too high a value and you're not filling the battery to capacity, but extend its life, too low a value and you're overcharging the battery, diminishing its life. Just go with what the battery manufacturer says to use.