External Lithium Battery Banks

Thread Starter

Keith55555

Joined Aug 11, 2018
6
Does anyone know whether or not these external lithium battery banks people use for charging their phone or as a battery back-up, have a regulated output. I'd like to use one as a power source for a device I'm making.
 

KeithWalker

Joined Jul 10, 2017
882
Does anyone know whether or not these external lithium battery banks people use for charging their phone or as a battery back-up, have a regulated output. I'd like to use one as a power source for a device I'm making.
The power bank USB chargers contain a Iithium ion battery and two electronic circuits:
The first is a charger input circuit which regulates the charge of the internal battery from an external USB 5 volt source.
The second circuit is a voltage boost converter which converts the battery voltage (3 to 4.2 Volts) into 5 Volts DC output usually with a limit of 1 Amp.
The output is fairly well regulated as long as you don't try to draw more current from it than the specified limit. I have used these to successfully power Arduino circuits in a number of applications.
There is some high frequency (40 to 100 KHz) ripple on the DC output voltage which should be filtered if there is a possibility that it could cause problems in the circuit you are powering.
 
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Thread Starter

Keith55555

Joined Aug 11, 2018
6
Thanks Keith. I'm taking the 5volts and running it through a 3.3v regulator for a microcontroller and a sound module. But the 5 volts I need for a servo. Do you think the ripple would effect the servo's function (the micro is pulsing the servo) if I left it unregulated? Total worst case current will be about 500ma. for short periods.
 

dendad

Joined Feb 20, 2016
3,517
The ripple will be ok.
One thing to look at though is to ensure your circuit draws enough current to keep the supply turned on. I don't know what current is needed, but if it drops below a certain level, the supply turns off.
 

Thread Starter

Keith55555

Joined Aug 11, 2018
6
Thanks dendad, that was something I was unaware of. My micro is super low current and the quiescent current for the sound module is quite low. Not sure what the quiescent is on the servo but since I don't know the cut-off threshold, I'd be guessing anyway.
 

KeithWalker

Joined Jul 10, 2017
882
Thanks dendad, that was something I was unaware of. My micro is super low current and the quiescent current for the sound module is quite low. Not sure what the quiescent is on the servo but since I don't know the cut-off threshold, I'd be guessing anyway.
Servos draw a lot of current for a short time when they are activated, often more than one amp. This can cause drop-outs in the 3.3 Volt supply which will seriously interfere with the operation of the microprocessor.
Check that the servo you have will work satisfactorily from the power bank before you invest much time or effort in the rest of the project. If it does, I suggest you use a series diode from the positive of the 5 volt source to the positive input of the 3.3 volt regulator, with an electrolytic capacitor across the regulator input (10 to 20 uF ). This should maintain a steady supply for the processor.
 

Thread Starter

Keith55555

Joined Aug 11, 2018
6
Thanks Keith. I have included a 10uF cap across the 5 volt input using regular batteries as a power source and successfully controlled the servo albeit without much of a load on the servo which I know will make a difference although the load ultimately will not be extremely demanding. Also have a very low noise, LDO regulator for the 3.3V's with in and out caps and a pin that increases noise reduction with another smallish cap.
You include the diode because you think the servo might get 'spike-y'?
 

KeithWalker

Joined Jul 10, 2017
882
Thanks Keith. I have included a 10uF cap across the 5 volt input using regular batteries as a power source and successfully controlled the servo albeit without much of a load on the servo which I know will make a difference although the load ultimately will not be extremely demanding. Also have a very low noise, LDO regulator for the 3.3V's with in and out caps and a pin that increases noise reduction with another smallish cap.
You include the diode because you think the servo might get 'spike-y'?
The instantaneous current drawn by the servo could cause the 5 Volt supply to momentarily drop below the input threshold voltage of the 3.3 Volt regulator.The diode will block this and the capacitor will maintain the input voltage until the supply recovers.
 

Thread Starter

Keith55555

Joined Aug 11, 2018
6
So I hooked up a battery bank I had and got a solid 3.3V out of the regulator and ran the servo without it glitching anything however, this time out between the circuit in action (servo moving) where there is a auto-shutdown in the battery bank looks like a real problem. When there is no servo activity for a minute or so, battery output is lost. Quiescently the circuit draws 33ma and I guess that's not enough for the battery pack to remain engaged.
 

KeithWalker

Joined Jul 10, 2017
882
So I hooked up a battery bank I had and got a solid 3.3V out of the regulator and ran the servo without it glitching anything however, this time out between the circuit in action (servo moving) where there is a auto-shutdown in the battery bank looks like a real problem. When there is no servo activity for a minute or so, battery output is lost. Quiescently the circuit draws 33ma and I guess that's not enough for the battery pack to remain engaged.
If you are going to use that battery bank you have several choices:
You can open it up and replace the USB 5 Volt output circuit with a simple 5 Volt regulator.
You can add a load to the 5 Volt output to keep it active.
You can program the microprocessor to switch in a load for a short time at suitable intervals. This will use less power than a permanent load.
 
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