Trinket issue when switching to battery power (2032 batter)

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JJ82
 
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Trinket issue when switching to battery power (2032 batter)

Post by JJ82 »

Apologies if something similar has been answered here already, but I did a search and couldn't find anything that addresses the issue I'm having.

My set-up is a Trinket M0 with a Piezo buzzer and a pin set up to basically create a continuity sensor such that it buzzes when the circuit is broken. It functions perfectly when the power source is usb, but stops functioning when I switch to battery power. Battery set-up is a single cr2032 coin cell soldered to the JST input on the back.

Specifically when I switch to battery the green power LED is on, the dotstar turns magenta, and the red LED flashes.

Any advice would be appreciated!

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adafruit_support_bill
 
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Re: Trinket issue when switching to battery power (2032 batt

Post by adafruit_support_bill »

Coin cells are typically optimized for long operation at very low discharge rates. A typical nominal discharge rate for a CR2032 is 0.2mA which is a couple orders of magnitude less than what your circuit needs. https://cdn-shop.adafruit.com/datasheet ... asheet.pdf

When discharged at higher rates, you can expect both the voltage and the effective capacity to drop significantly. You can try running with 2 cells in series to counteract the voltage drop as in this project: https://learn.adafruit.com/neopixel-tiara

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JJ82
 
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Re: Trinket issue when switching to battery power (2032 batt

Post by JJ82 »

Thank you! I'll try the two CR 2032 or find some other appropriate supply.

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ericfont
 
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Re: Trinket issue when switching to battery power (2032 batt

Post by ericfont »

I just want to chime in that I have success powering with a (rechargable) lithium LIR3032H (which are nominally rated at 3.6V):

The LIR3032H measures 3.79V when I'm not powering the board with it, and only drops down a bit to 3.73V when powering the Trinket M0+ for 5 minutes. I haven't tested how long this will last though...

edit: after waiting 10 minutes, I see the voltage is down to 3.67V. I think I'll try leaving it on overnight to be curious, but sounds like the safer solution is the earlier mentioned two batteries. And fyi, CR2032 are only nominally rated at 3.0 V according to https://www.batteryequivalents.com/lith ... ments.html
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ericfont
 
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Re: Trinket issue when switching to battery power (2032 batt

Post by ericfont »

Well that single Lir2032H ran worked for a couple hours but after leaving 9 hours overnight ran out of juice trying to power the trinket running active code, and was at a 1 V when I checked in the morning. I guess the 50 mAh battery capacity wasn't enough.

But I have a question about the recommendation of using two 2032 in series. According to the schematic, it uses AP2112 linear regulator which has a low dropout voltage of 250 mV. The schematic also shows an unspecified schottkey diode, whose voltage drop I measured to be 150 mV. So I guess as long as the battery voltage remains safely at or above 3.3 V + .250 mV + 150 mV = 3.7 V, then the regulator will provide a clean 3.3 V regulated supply level.

But my understanding is that linear regulators work by burning excess voltage above the desired regulated level as waste heat. So I think the downside of putting two nominally 3 V batteries in series to produce 6 V is that the excess voltage (6 V - 3.7 V = 2.3 V) would be burnt as heat by the regulator. I guess since the board has low current draw that that the excess heat is not a big deal...but if are powering additional components off the 3.3 V and/or if we are worried about not wasting any heat so can maximize battery life, then wouldn't it make sense to use as low battery voltage that is at least safely 3.7 V? Like if I know one Lir2032h seems to work for a couple hours, then I think maybe I could place multiple Lir2032h in *parallel* so I'm not having the voltage be much higher than 3.7 V. Instead of placing the batteries in series or going for higher voltage batteries. Sorry I'm not trying to hijack your thread.

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Re: Trinket issue when switching to battery power (2032 batt

Post by adafruit_support_bill »

I think the downside of putting two nominally 3 V batteries in series to produce 6 V is that the excess voltage (6 V - 3.7 V = 2.3 V) would be burnt as heat by the regulator.
Quite true. Although that stack of coin cells is not capable of actually delivering 6v at anything more than a couple of millivolts. You are correct that a significant amount of power will be dissipated as heat. But much of that will be dissipated in the batteries themselves due to their high internal resistance.

The bottom line is that coin cells in general are not well suited for driving anything requiring more than a few millivolts at most. You can stack cells to compensate for the voltage drop. But the usable run-time will still be short.

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