Hey guys,
I’m looking for the smallest possible battery that will let my core connect to wifi. I don’t have any strong preference between rechargeable or single use. I’ve tried 2 CR 2025s and 3 LR44s, both of which seem to be too weak. I’m trying to fit both the core, and the batteries inside a reasonably small bottle opener. I also don’t need the batteries to last very long. The core wakes from sleep when a bottle is opened, makes a spark.publish, and then goes back to sleep. Any thoughts, suggestions, or help would be greatly appreciated.
Thanks,
Brett
For size/shape reference: This is the bottle opener I’ve been playing with.
I will go for a Lipo since the average current draw is around 80mA but definitely much lower with your device in sleep mode most of the time
Hi, how do you plan to wake-up the core from sleep? Is there any sensor or accelerometer in this bottle opener?
Hey Fabien,
Good question. Sorry I forgot to post. No sensor, accelerometer, or anything else, the metal on the bottle cap connects 2 wires leading from the interrupt pin to a 3.3V pin.
Thanks for the advice. Any particular Lipo you’d recommend that will generally fit in the package size? How many mAh should I shoot for? Links, if you’ve got any, would be super helpful as I’ve never actually worked with Lipo batteries.
@BrettA, it might be more practical to decide how long a battery life you are shooting for first?
Having a physical constraint already narrows down your offerings. Also. might want to decide also charging of the device is required.
Looking at the size, these two might be your best bet:
Hey @kennethlimcp ,
That looks like it’ll work. Thanks so much for the help.
Having never worked with LiPos before how does charging work? Can I just plug in the spark core to a usb while the battery is plugged into Vin and ground and it’ll charge? Or do I need a separate charger like this: https://www.sparkfun.com/products/11231 . If I do–is the one I’ve linked to the right choice?
Thanks again,
Brett
Something even small like this works - https://www.sparkfun.com/products/10217
Awesome thanks–I’ll grab both that and the battery from sparkfun. (I’m assuming that means I can’t just charge from the core itself)
These are really small batteries, both physically and in terms of energy density. Here is the data sheet for CR2025 for instance:
http://data.energizer.com/PDFs/cr2025.pdf
This is 165mAh battery so a pair of those would power a Core for a best case maximum of between 3 and 0.5 hours from the core data sheet numbers of 50mA min to 300mA max. Another thing to consider is the typical discharge rate given in the battery data sheet is for a 15kohm load. In contrast, the Spark Core at its min power usage is a 66 ohm equivalent load.
The LR44’s have even worse specs.
In short, these are not the batteries you are looking for.
I would look at rechargeable Lithium Ion or LiPo batteries and power controller instead.
Hey bko,
thanks for the info and reply. Any particular Lithium Ion or LiPo batteries you’d recommend for a small package size? How many mAh should I shoot for? As you can tell, I’m quite new at working with batteries.
For a bit more background, I’m trying to squeeze the core and batteries inside of a bottle opener (picture below).
As for battery life, I’m wondering if the .5h-3h is actually long enough. All the core does is sleep all day and then an interrupt pin wakes it from sleep, posts a single spark.publish, and then goes back to sleep so 30 minutes of that could very well be like a month+ of use depending on how often you’re drinking a beer. There are no active components/sensors, the bottle cap just connects the interrupt pin lead to the 3.3V pin lead.
Any help would be greatly appreciated.
Best,
Brett
Hi @BrettA
I think you are going to have to become a lot more expert at battery technology before this project is over. I would look for the highest capacity LiPo that fits your form factor. You should be able to get around a 1500 or 2000mAh that meets your needs.
https://www.adafruit.com/categories/574
You still need to think about charging circuits (LiPo’s need somewhat special charging) and how to detect and react to low battery conditions, probably using a power management IC or PMIC.
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How about this kickstarter: https://www.kickstarter.com/projects/1710326172/boostick-small-aa-voltage-booster/description
Should give you enough runtime, and the battery is easily replaced.
Hey @bko ,
Sorry for the delayed reply. I think you’re probably right about needing to become a lot better before the project is over. I just bought a few LiPos and a charger. I’ll keep you posted as I continue working on the project. I really appreciate all the help.
Thanks!
Brett
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