Tablet and it's battery's protection circuit. - Page 1
Forum section: electronics / beginners

author: VeryFastSnail
author_rank: Contributor
timestamp: May 26, 2020, 10:30:35 am
content: Hello forum members. I am very much confused...So I have tablet and I want to drive it with 5v usb (stepping it down to 3V7-4V2).Battery has protection board like one attached. Btw battery has only two wires (5v, GND), those are connected directly to protection board. GND thru FETS 5V to BAT+With that board connected it works great. I supply 5v, and tablet powers on, without it tablet draws current, while pressing power button, but after release it stops.But by looking at schematic of that protection board I do not understand how tablet "knows" about BPM (Battery Protection module).What I have tested and thought of:Controller IC somehow communicates with charge IC thru 5v line (pulses?),tested via oscilloscope, no result. plain 5v line.FET1andFET2has body diodes, that might have voltage drop effecting how tablet sees this battery.R2somehowpulls up/downto let charge ic in tablet know that it's ok to chargeI do understand how this circuit works and how two FETS are used to control charge/discharge, but how do tablet know about battery being in vs just plain 4.2v?..

author: kjr18
author_rank: Regular Contributor
timestamp: May 26, 2020, 10:54:32 am
content: Battery protection IC does not communicate with anything. It's only function is to monitor cell and incoming input voltage. I cell voltage falls below set value it turns off mosfets to protect cell before it discharges too much. Also when supplied voltage is too high, it also disables mosfet's to protect battery. It's all it does.Wires on battery are ground and battery voltage, not 5V.Charge ic monitores cell voltage and and stops supplying power when voltage on cell is about 4.2V and current falls below certain value. In more complicated devices things like charging, supplying required voltages, and some other things are done by specialized power management chips.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: May 26, 2020, 11:35:44 am
content: First port of call, lookup up the device's datasheet.https://html.alldatasheet.com/html-pdf/736531/SII/S8240A/112/2/S8240A.htmlThe two FETs are switches to turn the battery on when the battery cell's voltage is within range. Note on battery protection circuits, it's often the negative B- line that's switched. R2 is for reverse polarity protection should the charger be plugged in the wrong way. As @kir18 says, the Laptop knows nothing about the protection circuit. It's the 8240 that does all the thinking.

author: VeryFastSnail
author_rank: Contributor
timestamp: May 26, 2020, 12:24:17 pm
content: Quote from: Syntax Error on May 26, 2020, 11:35:44 amHmm... so how do tablet detect that battery? If there are no communication between those two?Or should I ask what conditions has to met what that board causes so be able to power tablet on.

author: VeryFastSnail
author_rank: Contributor
timestamp: May 26, 2020, 12:28:06 pm
content: Quote from: kjr18 on May 26, 2020, 10:54:32 amMy mistake, sorry.Quote from: kjr18 on May 26, 2020, 10:54:32 amright sowhats the difference between my supplied fully charged battery voltage (4.2V) vs original battery voltage?What about those diodes? they do have some kind of voltage drop, will that affect it? could tablet measure Vdrop somehow?

author: Renate
author_rank: Super Contributor
timestamp: May 26, 2020, 01:05:50 pm
content: The BPM itself is only protecting for over/under voltage/current.The little circuit board often has a 10k thermistor for measuring battery temperature.There is often a fixed resistor or more intelligent digital ID so that the uP knows what kind of battery is connected.How many wires go from the circuit board to the tablet board?It's quite common to see a uP power up, realize that it doesn't like the power (due to voltage, ID, whatever) and shut down.I've often gutted a blown up battery, retained the little PCB and wired up a 4V supply.

author: VeryFastSnail
author_rank: Contributor
timestamp: May 26, 2020, 02:41:29 pm
content: Quote from: Renate on May 26, 2020, 01:05:50 pmQuote from: Renate on May 26, 2020, 01:05:50 pmMine has same circuit as I uploaded (or very similar), I found no thermistor, or I don't know where to look at..Quote from: Renate on May 26, 2020, 01:05:50 pmHmm ,there was few resistors on board... It was 10k and 100k if I rememeber correctlyQuote from: Renate on May 26, 2020, 01:05:50 pmTwo wires RED (BAT+) BLACK (GND)Quote from: Renate on May 26, 2020, 01:05:50 pmDid the same, but on every power up I had to connect USB rail to "Wake up" tablet, because protection kept shutting FET down as for battery voltage becoming 0 in my case (unpluged.)