Daisychaining powerbanks observations

[thelayman123]

Hello, i have no knowledge of electrical concepts other than amperage, voltage and wattage and have recently decided to power my dashcam with daisychained powerbanks. I've read many posts that advise against it and decided to figure out why. Using usb power meters and expecting some kind of erratic or abnormal pattern, i found that from whatever charging rate a single powerbank would have put out, once it is being charged by a second powerbank, its output would always be reduced to around 4.6v and 0.45A. What in the circuitry is causing this to happen? Additionally, despite reduced power input into the dashcam, everything seems to work fine. Shouldn't i be expecting some kind of problem with read and write or even basic functions?

Thanks in advance.

 

[davenn]

Hello, i have no knowledge of electrical concepts other than amperage, voltage and wattage and have recently decided to power my dashcam with daisychained powerbanks. I've read many posts that advise against it and decided to figure out why. Using usb power meters and expecting some kind of erratic or abnormal pattern, i found that from whatever charging rate a single powerbank would have put out, once it is being charged by a second powerbank, its output would always be reduced to around 4.6v and 0.45A. What in the circuitry is causing this to happen? Additionally, despite reduced power input into the dashcam, everything seems to work fine. Shouldn't i be expecting some kind of problem with read and write or even basic functions?

Thanks in advance.

Welcome to EP

Would you like to do a photo or a drawing of how you are connecting things up, please
So we can get a better understanding of what you are trying to achieve

 

[thelayman123]

Hello and thanks! The first picture shows 4.881v 0.572A when powering my dashcam with one powerbank. The second picture shows a drop to 4.408v 0.342A when that powerbank is being simultaneously charged. I'm curious as to why this happens, if it's really unsafe to do and why the dashcam works perfectly even in an underpower circumstance.

 

[kellys_eye]

You can 'daisy chain' in series (not a good idea) or parallel (equally bad) or you can use a larger bank equivalent in capacity to the others you would be using. Or, you could use the proper power converter and power the dashcam from the vehicle directly - as it is intended.

 

[Harald Kapp]

Maybe "disychaining" is a misnomer?
My guess is that the op uses the ouput of power bank 1 as the input to power bank 2, then the output of power bank 2 as input to the dashcam.
I observe the same behavior with a (randomly selected) power bank of mine:

• 5.1 V output with no power to the input
• 4,8 V output with power to the input

A look at the datasheet of one typical powre bank chip, the TP5400, shows that the charge circuit and the boost circuit are independent:

Therefore the would be no obvious reason why output voltage drops when the battery is being charged. But since both circuits share a common ground, it may be possible that the current from the charging circuit through the ground pin has an influence on the step-up circuit.

 

[thelayman123]

You can 'daisy chain' in series (not a good idea) or parallel (equally bad) or you can use a larger bank equivalent in capacity to the others you would be using. Or, you could use the proper power converter and power the dashcam from the vehicle directly - as it is intended.

Yes, i'd do that, if only i weren't concerned about running it flat.

Maybe "disychaining" is a misnomer?
My guess is that the op uses the ouput of power bank 1 as the input to power bank 2, then the output of power bank 2 as input to the dashcam.
I observe the same behavior with a (randomly selected) power bank of mine:

• 5.1 V output with no power to the input
• 4,8 V output with power to the input

A look at the datasheet of one typical powre bank chip, the TP5400, shows that the charge circuit and the boost circuit are independent:
View attachment 57357
Therefore the would be no obvious reason why output voltage drops when the battery is being charged. But since both circuits share a common ground, it may be possible that the current from the charging circuit through the ground pin has an influence on the step-up circuit.

Yes that's exactly it, basically both powerbanks are fully charged, the 2nd powerbank powers the 1st powerbank which in turn powers the dashcam. They're huge powerbanks too and i doubt anyone makes economical options larger than them. Not something i do often anyway. What i really want to know now is what are the likeliest consequences of prolonged usage in this manner, even if there aren't any obvious abnormalities in the short run. Battery degradation, meltdown, or something else?

 

[Harald Kapp]

what are the likeliest consequences of prolonged usage in this manner

Probably none. The power bank electronics will (or should, when well done) shut down the power bank if the battery is drained to the limit (below which there is a risk of damage to the battery).
Due to the losses incurred by each power bank this is not an efficient setup, however. A single large powerbank is a better option. For exampel something like this one. Suitable for 8 × 18650 batteries, each with a capacity of your choice, say 2 Ah? Makes for 16 Ah capacity. Of these 16 Ah @ 3.7 V (the batteries' voltage) are available approx. 10 Ah at 5 V (accounting for the voltage conversion from 3.7 V to 5 V plus losses in the steop-up converter. With the dashcam drawing ~ 0.6 A, this setup will last for ~ 17 hours.

 

[thelayman123]

Would battery damage always present in the form of unusual heat production? The power banks i use have in-built capacity displays but i suspect that they aren't too reliable as quick charge can be resumed after reaching trickle charge stage by turning off and off the charger and also capacity seems to vary. They're the Remax 80000mah ones, probably not very well known, but they do have positive reviews with one having tested it with USB testers. almost 300Wh, 33Ah. Mine lasted 10 days at first usage but since subsequent charges have only managed 7 to 8 at most. very odd...

 

[Harald Kapp]

Would battery damage always present in the form of unusual heat production?

Another symptom is complete loss of capacity, voltage almost down to 0 V.

quick charge can be resumed after reaching trickle charge stage by turning off and off the charger

The problem with batteries is that you cannot determine the state of charge simply from the voltage present on the pins. Therefore "correctly" charging a battery without over-charging it is a science per se (see e.g. here). With Lithium batteries usually the charge current is monitored and charging is stopped when the current falls belows a threshold. The charger then keeps the battery charged by e.g. trickle charging.
When you turn off, the charger and on again, the charger "has forgotten" that the battery was fully charged seconds before. It then starts charging but should detect that the battery doesn't need charging and revert to trickle charge within a short time.

The power banks i use have in-built capacity displays but i suspect that they aren't too reliable

The capacity of a battery cannot be measured directly. To get a good estimate of capacity, one will have to discharge the battery to its lower limit, then charge it and integrate the charge current over time.
Much too complicated for a power bank on a limited budget. What these "capacity" displays typically show instead is a very rough estimate of the capacity in % based on the voltage of the cell (see e.g. here). But since the characteristic of a cell depends on many factors like chemistry of the cell, age etc. this type of indicator can really give only a rough estimation. Nothing more can be expected. But that is good enough in many if not most use cases.

Mine lasted 10 days at first usage but since subsequent charges have only managed 7 to 8 at most. very odd...

This can be due to different discharge currents during that time, different temperatures etc. Imho nothing to worry about.

 

[thelayman123]

Great, many thanks for the description, learnt a lot about them already! What component is responsible for detecting that the battery is fully charged in reverting to trickle charge after restarting the charging process?

 

[Harald Kapp]

That is integrated into the charger IC.

 

[thelayman123]

I've left the dual powerbank set up running for 2 days and the secondary powerbank has run almost flat in contrast to the usual 5 to 6 days if it were used alone. Very odd..

 

[steve1418]

It takes time to understand