HP Tablet Battery Elemnation - Page 1
Forum section: electronics / projects

author: BanyBan
author_rank: Newbie
timestamp: July 06, 2021, 01:54:03 am
content: Hi! First post, hope this in the right place!I've got an old HP Stream 8 Tablet - fairly low end, but the screen is okay. I want to use it as an always on display / controller and was hoping to remove the battery and connect it to a power supply (I don't really want the battery constantly charging and want to use the only USB port for Ethernet). Unfortunately its not as simple as just connecting 3.7v to the battery connection. There is also a "C" pin that you need to do something with, but I'm at a bit of a loss as to what its doing.The battery has a small PCB with some mosfets and a 6 pin chip I can't identify. Wondering if I could just cut the cell off and connect the voltage there? or if it'll know the cell was cut off and get upset. I don't want to go cutting the cell off without understanding it more as I only have the one battery.The C pin, when the battery is connected appears to just be continuous 500us pulses at 100Hz.When the battery is not connected, and the + - pins are supplied 3.7 volts, we see similar 500us pulses at 100Hz, however at a higher voltage, but they stop after about 100ms. I think this makes sense as when measuring the resistance between the C pin and ground on the battery its 10kohm. Putting a 10k resistor between C and ground the voltage appears the same, however still no boot, and the pulses still stop after about 100ms.Any ideas on how the battery and board are talking?

author: gabiz_ro
author_rank: Regular Contributor
timestamp: July 06, 2021, 06:38:26 am
content: White wire may be internal thermistor of battery,usually 10kohms at around 20 degree CelsiusJust solder 10k resistor to GNDBattery voltage is between 3.7-4.2V or on some newer 3.85-4.35V or little higher to 4.4VSo at 3.7V it may be not enough.Also during startup,there are spikes of drawn current even high as 2A maybe,that depends on tablet,not sure about that but I see on some phones, by ex 1.6A during power on sequence (no sim inside so no GSM power)So if you use thin and (or) long wires for power,voltage drop on their resistance will be high enough to make power management to detect low battery and switch off.Use short and thick wires for power,rise voltage to 4.2V,use multimeter connected to board (not on PSU) to see what voltage drop have on wires,sometime even thick wires can have some high resistance value.Also connect a high capacity value capacitor on board where battery was connected.Something 2200-4700uF /10Vlater editIt may be possible,but I don't think there is the case,that on white wire top be thermistor but also some one-wire communication.To check that just charge battery,desolder white wire from board,add 10k resistor on board in place of white wire to GND and check if tablet works fine.

author: BanyBan
author_rank: Newbie
timestamp: July 06, 2021, 09:04:34 am
content: While I had tried a 10k resistor between C and ground, I hadn't considered the current draw over the small wires I was using. The battery connector is tiny, and I was using some tiny little clips to connect to it. Tried again with a larger contact area and boom! Happily booted without the battery. Thanks for pointing me the right direction.