Powering a cellphone/tablet without a li-ion battery? - Page 1
Forum section: electronics / beginners

author: Mpegger
author_rank: Contributor
timestamp: May 24, 2022, 01:17:45 am
content: What I am trying to do:Using old cellphones/tablets as digital photo frames, or screens for monitoring PC/network.How I am trying to do it:Using the original li-ion battery protection board and running 4.0VDC through it to power the cellphone/tablet.Results so far:Mixed. Occasionaly it works, most time it doesn't.What I've done/learned so far:So I'm trying to have some cellphones/tablets powered up and running 24/7, and I know just leaving the li-ion battery in the device and having it plugged into a USB charger all the time is NOT good for the battery, and could eventually lead to killing the li-ion battery in a flashy, sparkling, destructive manner. Since I want to avoid that at all cost, I want to power the cellphone/tablet device without the li-ion battery. Most modern cellphones/tablet will have a li-ion battery that has it's own protection board attached to it, so hooking up a 4VDC source straight to the devices' positive and negative voltage inputs will not power up the device because there is no li-ion protection (control?) board attached and communicating with the device. So I've removed said protection (cotrol?) board from an original OEM battery and tried to feed the 4VDC through the board into the cellphone/tablet with very mixed results.1. It won't power up the device if the board is placed into the device first, then power is applied. Power must be applied first to the board, then the board placed in the device. However, even with that...2. Half the time if the device powers up, it'll boot like normal, get into the OS and main GUI, then shut off as if it lost power. Sometimes when this happens, it'll start going through a reboot/shutdown loop, as if it's experiencing a brown out every time, but I know that voltage supply I am providing can supply more amperage then the device could need (I've only measured around 1.2A spikes @ 4V, and the power supply can easily deliver 3A+).3. The times I have gotten it to power on and stay on, the voltages detected by the OS (Android) are way off from the actual supplied voltage. To me, it seems the li-ion protection board is ignoring the actual voltage on the "battery" side, and going by some internal counter on how much battery capacity has been used or recharged. Using a power supply with a fairly solid 4.0VDC output even under load, the voltages displayed by the Android system are all over the place. Using 18650 batteries, the Android will detect voltages totally different then the actual battery voltage. I even tried charging the 18650 battery through the cellphone, and when the Android OS indicated 100% charge, it was reading 4.385V, while the actual 18650 battery was only 3.874V measured.I really want to use these old cellphone/tablets, but cannot figure out a reliable way to power the device because of the control board. I thought it would as simple as just supplying 3.8-4.0V, but I've only had bad results so far. I've tried searching for anything battery related to getting devices powered without a li-ion battery to no avail. I'm hopeing someone with experience in doing so could point me in the correct direction to get this working.

author: Peabody
author_rank: Super Contributor
timestamp: May 24, 2022, 04:01:19 am
content: Quote from: Mpegger on May 24, 2022, 01:17:45 am"It ain't what you don't know that gets you in trouble. It's what you know - that just ain't so."---Mark TwainCharging circuits for lithium batteries terminate charging when the battery has been fully charged. They do not "trickle". So there should be no problem leaving the charger connected to the battery all the time. If you have a setting that lets it terminate at 80% of full charge, that would be desireable, but not essential.Other than that, I can't offer any suggestions for doing what you're attempting to do. I suspect the processor is involved in monitoring the battery's state, and would be surprised if everything worked properly without the battery and its protection circuit being present.

author: inse
author_rank: Super Contributor
timestamp: May 24, 2022, 05:20:46 am
content: When cellphones power up, they draw a 'huge' burst of current in comparison to the average current draw.So maybe your power supply is not sufficient for that, either get another one or try to adding electrolytic caps until they can buffer long enough.Lithium batteries usually have an integrated thermistor for temperature monitoring.Altough this is only relevant for charging, you can connect a 10k resistor to GND if the device is still bitchy.

author: Mpegger
author_rank: Contributor
timestamp: May 24, 2022, 04:11:42 pm
content: Quote from: Peabody on May 24, 2022, 04:01:19 amThe problem with leaving the phone constantly charging is that the charging circuit will keep the voltage @ 4.1V, which for a li-ion battery is not good. Itwillshorten the life of the battery and eventually kill it, during which there is a chance the battery can fail in a spectacluar manner (ie flames/explosion). Not exactly something I want as these units will be left on 24/7, even when I'm not home, and any battery obtained now (OEM or aftermarket) will already not be long for the landfill.It would be nice if in Android it would charge up the battery to 100%, cut off the charging entirely, and wait some % before activating the chargning circuit again, like wait till battery is @ 90%, THEN start chargning again. But instead, it keeps the battery @ 100% all the time. I don't know what the on/off cycle rate of the charging circuit is on Android devices, but what I've seen with my devices, it must be multiple times a second as I never see any of my Android devices while they are plugged in go below 100% charge, or the voltage change more then a couple hundreths of a volt.I could Root the devices and install a battery charge manager app that could cut off the charging @ 80%, but I have tried some of those on some of my devices and they are hit and miss as well. Sometimes they work, sometimes they dont. Not to mention, they don't operate off the detected voltage, but off some arbitrary value of what 80% is for the battery. I have not tried them on these particular devices I am trying to power sans battery, but I don't expect much from those programs as I mentioned, the voltages and charge % reported in Android is way off (Rooting is another issue on these devices as well).

author: Mpegger
author_rank: Contributor
timestamp: May 24, 2022, 04:25:38 pm
content: Quote from: inse on May 24, 2022, 05:20:46 amI have a 10A bench power supply and I've measured this particular devices at only around 1.2A max, so the power supply is not the issue, at least not in regards to being able to supply enough Amperage. The small Voltage step-down board I was using for converting a 5VDC 3A wall wart to 4VDC may have been too "dirty" of a voltage, which is what seemed to not power up the device 99% of the time, even though the bench supply had better luck.I actually was considering using capacitors as a "battery" (more like a buffer) and leaving the device plugged in via its normal USB port, but seeing how much trouble I'm having just trying to get this powered up directly, I wasn't sure if that was even worth trying. I think I will try it anyway now that you bring that up, but I'll need to order up some capacitors.So far, from all the batteries I have taken apart, there does not seem to be a thermistor attached to the actual li-ion battery. The thermistor is likely on the board of the protection circuit, which means it should just be reading ambient air tempertures, which also means that to the protection circuit, the "battery" is just fine and not overheating.I'm not following you on connecting a 10k resistor to GND. From GND to where? And which GND? The GND on the battery side of the protection board? The GND on the output of the protection board?I had also come across some post that powering a device without a battery would just require resistors between the postive and one or both of the other pins on the battery connectorm, but I never got any combination working so I scrapped that idea.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 24, 2022, 07:15:36 pm
content: I would not dare to assume all those android devices work and behave the same concerning battery management.Fooling the thing to let it think there is a battery in it while there is not could be quite difficult.It's quite possible that has some extra battery "protection" software in the main processor, and that it for example turns itself of when it tries to charge the battery and the battery voltage rises too quickly. (this is just a wild guess).Oops, it can not start to charge the battery if it's only powered from the battery (or battery connector)

author: glentek
author_rank: Regular Contributor
timestamp: May 24, 2022, 10:41:49 pm
content: I'm using an old Nexus tablet as a dedicated GPS in my car. I've removed the battery and have two power supplies - about 3.8V powering the battery circuit permanently and 5V feeding the charge port and switched by the car's ignition. I have the tablet set to boot on charge in Developer Settings, and use Tasker to detect charge power and wake/sleep the tablet with the car's status.

author: Mpegger
author_rank: Contributor
timestamp: May 25, 2022, 12:29:50 am
content: Quote from: glentek on May 24, 2022, 10:41:49 pmI've been doing some more experimenting and I'm finding that I need to power both the battery protection board, and have the USB charger plugged into the cellphone as well. For some reason, the device just isn't staying on with just the battery power alone, which is strange because when I first started experimenting with it with the bench power supply, it stayed on for hours on end just fine with only the bench power supply powering the phone though the protection circuit. It's possible I damaged the board somehow, but it appears to work just fine in certain situations. Maybe if I buy some cheap aftermarket battery and use the board from that I would have better luck, as those cheap batteries tend to have a much simpler protection board that only feeds the device the data it expects in order to operate properly.I may also try your solution as the 5V 3A wall wart I have should be enough to power both the USB port and the battery protection board together. I also wanna try capactiors as well.