Lab phone/tablet battery charger design idea.

[Dannyx]

Good day folks, hopefully I'm posting this in the right category. Working in a GSM service, I'd often get phones/tablets with totally dead batteries whose voltage has dropped so low (usually lower than 3.5V), that the built-in charging circuit no longer starts up and I have to "manually" charge the battery by connecting it to a small variable bench power supply that we've got and slowly increasing the voltage until the battery reaches around 3.7 volts, which is the threshold where the device can usually charge normally by itself. This method is fairly effective, except that, as the battery's voltage increases, so does its resistance, which in turn causes the current to drop, so one must stand near the control dial to slowly and constantly increase the output voltage of the supply.
To overcome this, I was thinking of designing my own charger which ramps up the voltage by itself until a set threshold is reached, let's say 3.8v just for good measure. I made up a circuit in my own head, before actually looking it up and the closest one I could find was this thing which seems to be a good starting point, however there are 2 limitations that I immediately spotted: 1, the charging current is limited to 63mA, which is too "slow" (I'd like something like 1.5a, which is what we usually pump them at with no worries) and 2, it's not adjustable, meaning I can't put a 9v battery on it (like some tablets have - rare, but I did have one which gave us a bit of a hard time trying to charge it). To make it adjustable, I was thinking of replacing R1 on the inverting input of the OP amp with a pot, essentially setting the voltage that the output must reach. Since the battery is connected to the inverting input, once its voltage reaches that of the resistor divider, it SHOULD cause the op amp to shut off the transistor. Let me know if this is too far fetched or there is some potential to it...I know you could just BUY a charger, but where's the fun in that ? )

[sam_sam_sam]

Re: Lab phone/tablet battery charger design idea.

I find that with tablets if voltages drops below 3.00 the charger may not charge the battery but this is because of the battery protection board

I find that when this happen it is better to take the battery and slow charge it

Now if the battery voltage is below 2.5 volts you have to charge these batteries very slow like 50 mill-amps until there voltage comes up and pass 3.5 volts

( but you may have to bypass the battery protection board but do this with caution be careful when doing this I have had mishaps doing this)

Other wise I have found that these will run some what warm if they are charged to fast when the battery voltage is that low

I do not like to charge lithium ion batteries too fast if they even if start to become even a little bit warm but that just me

Now with phone I find this is a little bit different in that if the battery voltage drops to low the battery icon come up and the phone will not power ON but if you let charge at least 30 minutes then most of the time the phone will power ON if it dose not power ON then I would NOT trust this battery any more or the charger may have issues

[sam_sam_sam]

Re: Lab phone/tablet battery charger design idea.

Let me know if this is too far fetched or there is some potential to it...I know you could just BUY a charger, but where's the fun in that ? )

You can buy power boards that have current limiting and voltage limiting all one board you have pot adjustment for each to me this would be the best way to go

Here is a link to the type of board that I am talking about and they work very well
I hope this help

http://m.ebay.com/itm/5A-LED-Drive-P....c100408.m2460

[gabiz_ro]

Re: Lab phone/tablet battery charger design idea.

Most of dedicated chargers IC and even if by design charging part doesn't have separated IC and is supervised by PMIC (power management IC) have so called trickle charge.
If battery is too low will trickle charge it with low current until voltage reach some point where real charge start.
Problems encountered.
- voltage to low,slowly increase,but really really slowly becouse of big battery.
some times due to long time required even trickle charge stop so you need to disconnect and reconnect charger after let say one hour.As result a new session of trickle charge is started.
In other words,dead battery,connected charger,voltage increase from (just example) 2.8V to only 3.0V in one hour,maybe charger IC shut down if not started in one hour.Disconnecting and reconnecting charger,this time voltage will increase to let's say 3.3V in some cases may be enough in other maybe not.Charger IC is again in shut down state.Reconnect charger again and maybe this time voltage will be enough to pass trickle charge and start charging normal.
Small batteries like in phones will charge more with low current and be available to start charging sooner but in case of tablets with thousands of mAh time until voltage increase is longer.
Not all chargers circuit shut down after some time some of them will do that until battery is enough charged to start but I encountered few cases when shuting down.
In this case is useful to have voltmeter and ampermeter on PSU.
So you can see what is happening.
Connect charger your measurement will show 5V and 0.XXA
If measure battery you'll see that voltage slowly increase.If after some time indicators will show 5.0V and 0.00A then trickle charge has stopped so you need to restart it by reconnecting charger.

- another problem,battery is almost at end of life,trickle charging do the job to charge battery,but when voltage is enough and need to start charging normal,device in case also want to power up.Backlight,cpu and others draw current,for a moment is able to power,you can see that startup sequence but in short time battery drops below working voltage and all shut down.Trickle charge start again but when voltage is enough device will power on again,voltage will drop,all shutdown and restart again.
In time is possible that after few restarts battey will able to mantain startup sequence and then charging continue.

Also one thing I see,on many tablets or phones during startup charging is disabled.
By example dead battery:
Connect charger,on PSU display I see 5.1V 0.14A after some time if tablet auto start I see 5.1V and 0.00A and after few seconds 5.1A 1.2A
Also in phones connected charger,5.1V 0.86A when I press power button for short time 0.00A during startup then after startup start charging again.

[sam_sam_sam]

Re: Lab phone/tablet battery charger design idea.

Problem encountered. ------------>

What you have written here I have seen myself
Have you seen this one thought

I have three Nextbook tablet that for most part do the same thing
When you go turn ON the tablet you have to push the power button and hold it until the Nextbook logo comes up then it will boot up and come ON

If you do not hold it long enough you get the battery icon and then turn back OFF

Here is where becomes a little weird

You are using the tablet

The charge light will be yellow which means that the battery is fully charged
Which is correct it should do this

However if you use the tablet for an hour or more the yellow will be on but the battery level will not be at 100% anymore

I have seen it drop as low as 40% I did not wait and see just how low it need to get before it change to charging mode

Now if you unplug the power cord and hook it back up it ---> now the charge light has changed to red which mean that it is charging

When it work right the the charge light will be yellow then it will change to red and go back and forth sometime it does this by it self meaning that you do not have to unplug the power cord

The other problem with this tablet is that one of them dose the following--->

Sometimes you can not get it to turn ON it just keeps showing the battery icon unless you unplug the power cord then you get it to turn ON

What is up with this

The other Weird thing it dose is that the tablet dose not turn it self OFF or go to sleep --> even if the power cord is unplugged

It dose have a sleep timer but I do not use it because if you are read something and you do not touch the screen it will go to sleep but the longest setting is 30 minutes

What is up with this

Other than these few weird thing that it dose the tablet works good

[gabiz_ro]

Re: Lab phone/tablet battery charger design idea.

Here is where becomes a little weird

You are using the tablet

The charge light will be yellow which means that the battery is fully charged
Which is correct it should do this

However if you use the tablet for an hour or more the yellow will be on but the battery level will not be at 100% anymore

I have seen it drop as low as 40% I did not wait and see just how low it need to get before it change to charging mode

Now if you unplug the power cord and hook it back up it ---> now the charge light has changed to red which mean that it is charging

When it work right the the charge light will be yellow then it will change to red and go back and forth sometime it does this by it self meaning that you do not have to unplug the power cord

Problem is with CPU-PMIC-Charger IC
For some reason,battery status is not updated.Reasons can be multiple.
- CPU think (since charger is present) that charger is main source and battery is fully charged as result doesn't monitor battery,maybe except temperature.But for some reason charger IC or PMIC doesn't use charger as main source but use battery.Or battery reach 100% system know that,command PMIC or charger to end charging sequence,and expect for that session,until charger is disconnected,that power come from charger,but from some reason,not enough power from charger,defective IC,damaged track on PCB,all power or most of power is drawn from battery

You can see that on PSU meters.
Charger connected,tablet running,most current must come from PSU but if meteres read only few or ten's,hundred's of mA then is some mixing of power source,some little part from charger,some more from battery.

Also that may indicate bad charger,it can deliver few hundred's of mA for charging,but if tablet require more to run,unable to sustain power from charger will switch to battery.

The other problem with this tablet is that one of them dose the following--->

Sometimes you can not get it to turn ON it just keeps showing the battery icon unless you unplug the power cord then you get it to turn ON

What is up with this

Seem like battery is considered too low when charger connected.
But also,that depends on how is designed power and charger IC,if PMIC must charge battery and provide system with all necesary power,maybe voltage is too low to be able to start,except charging or when you try to power up,even is not visible there may be some kind of reset.Charger is able to charge battery but when you require more amp's just drop voltage,PMIC stops some outputs,voltage increase and charging continue.

The other Weird thing it dose is that the tablet dose not turn it self OFF or go to sleep --> even if the power cord is unplugged

It dose have a sleep timer but I do not use it because if you are read something and you do not touch the screen it will go to sleep but the longest setting is 30 minutes

What is up with this

Other than these few weird thing that it dose the tablet works good

That depend from power settings of OS.
Once you disabled any sleep or shut down only way is to use power or sleep wake button.
Would by nice to have settings like notebook on windows or linux ,personalized for each state,running on battery or on charger,but that is from manufacturer fault.

[sam_sam_sam]

Re: Lab phone/tablet battery charger design idea.

Problem is with CPU-PMIC-Charger IC
For some reason,battery status is not updated.Reasons can be multiple.
- CPU think (since charger is present) that charger is main source and battery is fully charged as result doesn't monitor battery,maybe except temperature.But for some reason charger IC or PMIC doesn't use charger as main source but use battery.Or battery reach 100% system know that,command PMIC or charger to end charging sequence,and expect for that session,until charger is disconnected,that power come from charger,but from some reason,not enough power from charger,defective IC,damaged track on PCB,all power or most of power is drawn from battery

You can see that on PSU meters.
Charger connected,tablet running,most current must come from PSU but if meteres read only few or ten's,hundred's of mA then is some mixing of power source,some little part from charger,some more from battery.

Also that may indicate bad charger,it can deliver few hundred's of mA for charging,but if tablet require more to run,unable to sustain power from charger will switch to battery.



Seem like battery is considered too low when charger connected.
But also,that depends on how is designed power and charger IC,if PMIC must charge battery and provide system with all necesary power,maybe voltage is too low to be able to start,except charging or when you try to power up,even is not visible there may be some kind of reset.Charger is able to charge battery but when you require more amp's just drop voltage,PMIC stops some outputs,voltage increase and charging continue.



That depend from power settings of OS.
Once you disabled any sleep or shut down only way is to use power or sleep wake button.
Would by nice to have settings like notebook on windows or linux ,personalized for each state,running on battery or on charger,but that is from manufacturer fault.

Update

It turns out that what you have here got me thinking about what could be causing this problem I have a five volt power supply that has a voltage adj pot on it

I also have one of the things that you can plug in to your USB port and read the voltage and amperage

I have ten foot USB cable and here is where trouble being the voltage from most USB power supplies are not high enough for ten foot USB cable
The voltage is any where from 5.0 to 5.25 volts and the amperage draw is only 450 to 480 mill-amp

So I thought let see what happens when I turn the voltage to 5.50 volts and the amperage went to 850 mill-amp but this still did not keep the battery fully charged and me using it at the same time so i took the amperage to 1.12 amps now the battery stays fully charged but I only use the 10 foot USB cable on this power supply

The voltage drop on these cables are very bad I cut the end off one of these cables and could not believe how thin this cable is

Thanks for your help with this problem this really help