I need some advice about powering a project with recha

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bobsone
 
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I need some advice about powering a project with recha

Post by bobsone »

Hi
I am putting together a word clock and would like to be able to run it from batteries rather than a permanent 5V wall supply. I think the max consumption would be between 200 and 250 mA with its sleep state (when there isn't anyone in the room) draw being less than 80mA. I (optimistically) want a 5-7 day run time per charge.

So what to do, I haven't done anything with a project running off a battery power supply.
Is connecting a group of 18650 batteries in parallel with a suitable charger board feasible? Is it possible to run a device from two batteries/battery packs with each battery pack having its own charging board (like the PowerBoost 1000) and both outputs connecting to a common bus (perhaps with a diode on each)?

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adafruit_support_bill
 
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Re: I need some advice about powering a project with recha

Post by adafruit_support_bill »

Building your own battery packs out of loose lithium cells is not recommended. Minor manufacturing variations in charge/discharge characteristics can result in dangerous imbalances. Better to go with a factory-assembled pack. These are constructed of cells from the same lot that have been precision matched and balanced.

https://www.adafruit.com/product/5035
https://www.adafruit.com/product/353
https://www.adafruit.com/product/354

You could considerably reduce your current draw further by going with a 3.3v processor and Neopixels powered direct from the 3.7v - thereby eliminating the need for the 5v voltage boost.

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bobsone
 
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Re: I need some advice about powering a project with recha

Post by bobsone »

Thanks for the reply.
I hadn't considered 3.3V, unfortunately some of the modules I am using require 5V (one requires the common supply of 5.5V)
I wasn't keen on the idea of manually building a battery pack, which is why I was thinking about an Adafruit battery controller and battery combo. But if I need/want some more capacity, is there a way to use more than one Adafruit battery pack? E.g use two battery packs with each battery pack having its own controller and connect the outputs connecting to a common bus, perhaps with a diode to isolate their outputs from eachother?

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Re: I need some advice about powering a project with recha

Post by adafruit_support_bill »

Diodes will just assure that the supply with the highest voltage output will be powering the system. They are not a replacement for an active current sharing circuit.

If you can be more specific about what components you have and their voltage and current requirements, we may be able to offer some suggestions.

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bobsone
 
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Re: I need some advice about powering a project with recha

Post by bobsone »

I don't have a complete working model yet but I have a mockup running on a breadboard.
It is a word clock with one LED per letter, the number of LEDs running changes from minute to minute with the max number being about 60 which (at max brightness) I have guesstimated to have a total device current draw of 200-250mA. The clock also has a small movement sensor board which turns the display off after a set time when no movement is detected reducing current draw to a bit less than 80mA.
Other than a power supply, switches, a couple of resistors and one 20k variable pot (for setting brightness) the following are the clocks components.
  • LEDs, 256 x 5mm flat top White 2.5V
    LDR, 1 x 1 5 mm GL5516 Light Dependent Resistor
    Touch sensors, 4 x TTP223B Capacitive Touch Sensor
    Temp sensor, 1 x JZK DHT22 AM2302 Temperature and Humidity Sensor
    Clock module, 1 x Real Time Clock RTC DS3231
    Motion sensor, 1 x RCWL 0516 Microwave Radar
    Matrix modules, 4 x Dot Matrix MAX7219
    Arduino Nano

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adafruit_support_bill
 
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Re: I need some advice about powering a project with recha

Post by adafruit_support_bill »

You could swap the RCWL 0516 for a PIR and the MAX7219 for an HT16K33. Or even simpler, just use neopixels instead of discrete LEDs. Then you could run everything direct from 3.7v.

Keeping your existing components, I would probably go with a 6v NiMH pack and a buck converter - or even a 6v SLA since weight is not likely an issue for a clock.

My reasoning is this:
The laws of physics require that a voltage increase comes with a proportional decrease in current. So 100mA @ 3.7v is at best 74mA @ 5v. The reality is that no voltage conversion process is 100% efficient. Assuming a very efficient (90%) boost converter, that 74mA will be more like 67mA. (The remaining power will be dissipated as heat). So you will need to scale up your mAh estimates by 150% if powering from boosted LiPo cells.

The inverse applies to a decrease in voltage. Only there is a huge difference in the efficiency of different types of voltage regulators. Linear regulators (like you find on just about every processor board) are about as inefficient as you can get. ALL of the excess voltage is simply burned off as heat. There is no current boost at all.

On the other hand, buck converters can be quite efficient (typically > 90%) and you do get a proportional current boost. So 100mA @ 6v ends up at about 108mA @ 5v.

Both NiMH and SLA are less finicky to work with than LiPo and there are lots of high-capacity packs and chargers available from battery specialist and RC hobby sites.

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