Always On Power Bank for powering low consumption devices


#1

Can anyone tell me if the Little Bird “Raspberry Pi USB Power Banks” will work in “always on” mode i.e. if they have an auto switch off mode to prevent overcharging and if so whether it can be switched off, so that they could be used with a very low power devices.

Thanks Peter


#2

It depends on which one you mean. The only one available from the Raspberry Pi Australia site is

but says there minimum current 100mA otherwise it will switch off. A circuit is mentioned to avoid the switch off, but not linked.

The purpose of the automatic switch off is to preserve the stored energy, not to prevent overcharging. The charging circuit has overcharge prevention built in; a maximum voltage beyond which current will not be delivered to the cells. The cells sometimes have a redundant overvoltage sensor and MOSFET gate as well, but this is becoming less common as manufacturers do their cost-down changes.

I’ve used similar products adding my very low power circuit directly to the cell terminals, thus bypassing the automatic switch off. This also adds risk of discharge below minimum safe voltage, so I can’t recommend it unless you can be sure the system won’t let that happen.


#3

Thanks for the clarification on the purpose of the automatic low current switch off. The unit I was referencing was the Raspberry Pi 4000 listed on your web site as SKU: AF-1565. The description said it could be used for “a Raspberry Pi (or Arduino, or Propeller, or anything else that uses 5V!)” so I thought it might be an always on device but it may just mean that those controllers use enough power to keep the unit switched on. However, I now think this is the revised version you mention and so probably has the auto shutdown. I saw that Little Bird also sell five other power banks. The SKU: SS-313010011, SKU: SS-313010010, SKU: say that “it’s enough to continuously provide electricity for your devices with its 5,000mAh capacity”. The TB-MB3723, SKU: TB-MB3646, or SKU: TB-MB3720 just say they are for recharging batteries. Would any of these be always on devices?

My application uses a CurieNano board or an Adafruit Feather with sensors and BLE. The power bank requirement is to supply USB power that can be switched on and off manually and is rechargeable with a separate solar panel. I’d prefer not to use up power pulsing every few minutes, though that is an option.

Thanks for all of your advice.


#4

I don’t know.

I’ve had another think about this. There’s another reason for the low current switch off, and that’s because it is really hard and therefore expensive to design a 3.7V to 5V DC converter for 2.1A or higher and still have it properly regulated at less than 100mA of load. Factories that make these devices can’t afford to make one that costs too much, so they don’t.

The power bank products are designed for the mobile phone and tablet recharging market, and not the kind of things we want to do with them instead. We’re frankly lucky to have Adafruit go into detail on the low current switch off; that detail raises their risk, and that’s their choice. They will have added that into their price, no doubt. That Jaycar/TechBrands have not is emergent from their restocking practices; minimal product detail lets them buy in a different device later, lowering cost.

Thing is, stock is not guaranteed to be all-the-same outside the product description. It will be functionally identical, but not necessarily internally identical. Unless the product is described as having a minimum, the stock could vary quite a bit on what minimum power must be drawn to keep it on. I’d be worried that the Adafruit product might need you to draw more or less power to keep it on when the temperature is very different to normal.

If the product has a display, or a button, there’s a very good chance it will have a low current switch off feature. If the product contains more than one lithium-ion cell, it will almost certainly have a low current switch off feature, because voltage below minimum is very unsafe for multiple-cell parallel packs; it can lead to cell imbalance with no practical way to overcome it. If the product looks to be the size of a single 18650 cell and doesn’t have a button, the chances are good that it will do what you want.

In your application, if you propose to draw less than 100mA then I suggest a Pololu 5V DC step-up switching regulator attached to the battery terminals, with a separate USB socket. That way it won’t switch off, and won’t be glitched when the charger turns on. You can also build up such a battery subsystem using bare cells with a solar charger.

For remote dark sites, you might also worry about daytime maximum temperature. Lithium-ion cells degrade quite quickly above 40°C, your best bet are the Lithium-Ferrophosphate (LiFePO4) cells, with a lower charge voltage.

Little Bird has a returns policy; you can use it to get a good look at a product, but read the terms and conditions carefully. It has changed since I last used it.

https://littlebirdelectronics.com.au/returns-policy/

Hope that helps!


#5

Thats very helpful and building a supply is a viable option. My present system is a single channel version of that anyway but I need to disassemble it and put the battery in a separate box because it interferes with the magnetometer. The need to replicate power supplies combined with a desire for better angular resolution makes buying a couple of Adafruit Feather 32u4 BLE more attractive (built in battery management and BLE), plus BNO055 IMU boards. Bit expensive but could make an excellent replacement for the now redundant Intel Curie based boards. I did find an ‘always on’ power bank made by Voltaic but your suggestions are making me think much more seriously about the AdaFruit Feather.

Many thanks Peter