MAX1873 Li-Ion Charging Circuit Query - Page 1
Forum section: electronics / projects

author: Tatey
author_rank: Newbie
timestamp: May 26, 2020, 03:50:26 pm
content: Good afternoon,I am currently designing a lithium ion battery charging circuit for a 3S battery pack using a MAX1873 IC.I have a query about how the system load and battery connections should work with this IC. As I understand it this IC is able to charge the battery while simultaneously powering the circuit (unregulated).The IC is able to measure the system load using a sense resistor and change the charging current to suit the maximum supply current set when designing the circuit. I am looking to implement this functionality but I am confused as to how to wire up the system load connection and the lithium ion battery to ensure I don't connect the system load directly to the battery.I have attached the typical application circuit detailed by MAXIM in the spec sheet for the IC. Obviously I want the battery to be connected to my circuit so when the charging adapter is not plugged in the circuit is powered by the battery. When the charger is then plugged in I want the battery to be charged and for the charging adapter to also power the circuit. I believe this is possible.I have highlighted in yellow the system power path which the spec sheet says will allow the IC to measure the current drawing using the sense resistor Rcss.However if I was to connect the positive and negative terminals of the battery to the circuit and the system load to the positive of the battery then this will effectively bypass the charging circuit mosfet and I believe cause me problems.Do I need to add 2 diodes, one in the system load line after the CL capacitor and one on the battery positive line before connecting this to the system load line? I am thinking this will prevent charge from flowing down the system load line to the battery when the charger is plugged in and will also prevent current flowing from the battery down the system load line to IC when the charger is unplugged. I have attached a schematic showing what I was thinking.

author: IanJ
author_rank: Unknown
timestamp: May 26, 2020, 05:41:10 pm
content: Hi,I use this IC a lot.......you don't need any extra components......so from memory:-- The battery connection and system load interface are two separate conections. Don't mix them about.- RCSS is the incoming supply current measurement (allowing you to shutdown the circuit if it goes over your threshold), RCSB is the actual charge current measurement.- Device runs from battery when charger unplugged.- Charger plugged in will power the device AND charge the battery at the same time. You can monitor the charge status and control the charging from your connected uP.- Via the uP you can disable charging completely if you want so that the charger powers the device and the batteries just sit there (with no load at all).I use it in 4S mode.Make sure you pay attention to the control side so that you can enable charging if the uP is switched off........I don't do this with mine though meaning I need the uP running to allow charging.....and I have my software monitor the charge current and the ability to shutdown the IC if something goes wrong.PS. RS & Farnell state no-stock and a long delivery (last time I checked), but I buy mine direct from Maxim......plenty stock......go figure!Ian.

author: Tatey
author_rank: Newbie
timestamp: May 26, 2020, 07:17:07 pm
content: Thank you very much for the detail reply Ian, I really appreciate it. I am still slightly confused as to how to connect the battery and the supply line by keeping these separate? As if they are both the positive supply for my circuit will they not be connect together at some point?I am also not familiar with the term uP. Can you confirm what this is please? Apologies if it is common sense, this is only my second electronics project.

author: IanJ
author_rank: Unknown
timestamp: May 26, 2020, 07:39:17 pm
content: Quote from: Tatey on May 26, 2020, 07:17:07 pmuP = micro processor, but you don't have to use one.......there's only couple of interface/control lines and you can easily do that manually.The datasheet as you will have already seen is really quite good.....a lot of good explanation.Have fun.Ian.