"Power bank" or UPS for low power device after input power off

[Windarrow]

Situation

I have BT audio adapter in my car.

Car doesn't deliver power to 12V output when engine is not running.

 

My thoughts

I want to attach some kind of "Power bank" or UPS in between, so I can still have music when engine is not running.

"Power bank" or UPS should be timer based after power off.

It should last at least 15 minutes.

Preferably supper capacitor power storage, since it will be in very hot and quite cold in car.

 

Device itself is Firefly BT receiver that should consume about 20mW of power.

 

If anyone knows off the shelf devices for this scenario, I would be happy to look into it, since I didn't find anything useful myself.

Alternately I need to build it myself, but would also like some things to look out for

[mariushm]

Easiest and simplest

 

Step-up regulator board  to boost a low voltage (0.9v ...5v) to 5v : https://www.ebay.com/itm/323297709876

 

A USB cable you have extra so that you could cut the end connector and get access to 5v and GND wires 

If you don't have one, you can buy one, example : https://www.digikey.com/en/products/detail/assmann-wsw-components/A-USB20AM-OE-100BE28/10408407

 

 

A couple of diodes (1n581*  ex 1n5817, 1n5819)

 

A battery holder for 2 AA/AAA batteries :  https://www.ebay.com/itm/144351826790

 

 

usb cable to get 5v from a charger or something  -> diode in series with the +5v output --> step-up regulator board to get 5v

AAA/AA battery holder -> diode in series with the positive voltage output  -> same step-up regulator board to get 5v

 

The diodes prevent voltage from one source to go into the other, for example 5v from the output of the usb cable won't "push" into the batteries   ... and when car's off and the cable doesn't output anything, battery holder won't push voltage towards the usb cable. 

The highest voltage will win and that's what it's gonna be used.

 

So when the usb cable has 5v on it,  the step-up regulator will "see" around 4.5v (because the diode will drop around 0.3-0.5v) and boosts that back to 5v transparently.

When the usb cable doesn't output anything, the battery holder will output  2 x 1.1v .. 1.35v or around 3v (2x1.5v) if you use alkaline non-rechargeable batteries... let's say 2.4v, the diode will cause a drop of 0.3v..0.5v so you'll have around 2v which is plenty for the step-up regulator to boost to 5v and keep the receiver working.

 

A couple AA batteries will do 3v at around 2500mAh ... 20mW at 5v  is around 20mW/5v = 4mA so let's round it to 10mA ... you're looking at maybe 250-300 hours of operation from 2 alkaline batteries.

 

You can replace with  battery holder with a lithium charger board and a small rechargeable battery

 

For example :  https://www.ebay.com/itm/192223970196 or https://www.ebay.com/itm/112461463964

 

This board is more expensive than others, but it has a pass through feature, it auto switches between 5v from input or whatever battery voltage you have when 5v is gone. 

you connect 5v from somewhere (get a usb cable and cut the end to get the 5v and ground wires , or use the 5v to 3.3v/5v linked above)

you connect the battery to the  BATT and GND header

You have your output on the LOAD and GND   - if the 5v is present, the charger chip will pass through 5v while charging battery. If 5v is not present, the chip takes power from battery and outputs 3v...4.2v, whatever is in battery.

So you connect your step-up regulator to this load header, and the charger chip takes care of the switch between 5v and battery.

 

note you don't need that chunky capacitor, such big one is only needed at very big charge currents, you don't need that. A small 100-220uF 10v (or better) capacitor that you can lay flat on the board would be plenty.

Example batteries : https://www.digikey.com/short/z080mrfq

A 6$ 400mAh battery will keep the receiver working for maybe a few hours : https://www.digikey.com/en/products/detail/sparkfun-electronics/PRT-13851/6605199

 

[Windarrow]

Thank you for the reply @mariushm.

Do you have experience with supercapacitors as well by any chance?

 

If I'd rather go with https://www.digikey.com/en/products/detail/kyocera-avx/SCCU25B256SRB/6165963 or something similar for power storage instead of battery.

I would much prefer to avoid LiIon and LiPo batteries since heat and cold can get quite extreme in the car (32C outside + sun to -30C in the winter).

 

Your recommended logic boards only work with batteries I would assume? Something like that should work? https://www.ebay.com/itm/385434053447

Edit: https://www.ebay.com/itm/115354731398 this board seems to be more appropriate even. First one I have no idea, how the IN and OUT power works .. 

 

Edit 2: Would the diagram with capacitor look something like that?

[mariushm]

Super capacitors don't work well for this and have their own issues. Some types of super capacitors don't like being fully discharged and then charged up fully, and have a limited number of such cycles.

The capacitor you linked to has a 2.7v maximum voltage rating, easiest would be to use a 2.5v regulator to make sure you never exceed 2.7v otherwise you'd damage the capacitor. As the super capacitor discharges, the voltage will lower, and you'll only be able to use the energy in the 1v... 2.5v range, not down to 0v ... so almost half the super capacitor energy can never be used.

 

the boards are not chargers. The second could be used with two super capacitors to get a voltage higher than 2.7v (rating of a single capacitor) but you'd still need to have a power supply to give it a clean 5v.

Super capacitors also self discharge... so don't know, doubt it would power the receiver for more than 5-10 minutes.

 

A Couple  alkaline batteries will last tens of hours as I explained, if you're afraid of lithium batteries. No, you would only have heat problems if you put the small box under the windscreen to have sun cook it. You can have a usb cable go down to the "ups" hidden in the glove compartment or somewhere it can stay cool, and from there a cable can go to the dongle.

 

 

[Heats with Nvidia]

Every car has permanent 12V somewhere, why not find that and connect to it?

[NadiaMayer]

4 hours ago, Heats with Nvidia said:

Every car has permanent 12V somewhere, why not find that and connect to it?

Unless they don't wish to mod the car this would be the best way to go about it.

Not sure about the model but if you go the radio there should be a 12V rail.

Ive not messed with any car radios from a car that was not a least 15 years old tho standards may changed.

the non hacky way to to do tho is get a power bank and cook it power in to the 12V rail and output to the usb A port a cheap but in store power bank from Walmart should do.

[Windarrow]

On 6/25/2023 at 12:24 PM, Heats with Nvidia said:

Every car has permanent 12V somewhere, why not find that and connect to it?

I have E91, it has some "advanced" power management system. I'm even having trouble hardwiring my dashcam there, since those voltages are changing by some weird logic.
I got an offer from shop as well, and they wanted around 200€ just for wiring this stuff. Could do it myself as well ofc, but I'm not that advanced and really don't want to mess around with electric stuff in my car

 

On 6/25/2023 at 4:34 PM, NadiaMayer said:

Unless they don't wish to mod the car this would be the best way to go about it.

Not sure about the model but if you go the radio there should be a 12V rail.

Ive not messed with any car radios from a car that was not a least 15 years old tho standards may changed.

the non hacky way to to do tho is get a power bank and cook it power in to the 12V rail and output to the usb A port a cheap but in store power bank from Walmart should do.

Powerbanks are an option, but some of them don't allow charging and outputing at the same time. I have samsungs power bank at the moment, and it doesn't work like that at least.

Also, I want to integrate some timer into it, since I really don't want it being active for hours or days - will be quite annoying with music playback to other devices

 

 

I was hoping rather, that there is some kind of UPS similar system with timer functionality, but it seems, that I still have to build something like that myself rather.

[Windarrow]

I have come up with updated diagram, that includes transistor to close battery circuit, when 5V rail has power. Transistor power is handled with voltage regulator.
Also added timer unit (didn't find correct diagram), that handles 20 minutes or so of power when getting initial power in (final result depends on unit itself)

 

Does this diagram make sense or have I missed something in it?