I am trying to design/deploy temperature sensors to each room of a non-profit retreat center. I’ve settled on LoraWan for networking and I am prototyping with the xiao nrf and xiao esp32s3 boards plus the seeed sx1262 addons to handle the network.
I understand little about electronics although I am very handy with a soldering iron. My background is software development.
My nodes need to be battery powered. they will take readings via a thermistor every 15 minutes and send that info back to the gateway. The readings are working great and now I need to figure out the battery power aspect of things.
Searching the web I see conflicting info about soldering directly to the BAT pads on the xiao’s and I don’t have he knowledge to understand what to do.
I’ve been advised to go with an 18650 wired to the bat pads and that it will all work out fine. I’ve also been told that I could wire traditional AA’s to the 3V and ground pins and I’d be fine, My power usage should be very small (I am driving the thermistor/voltage divider circuit from a GPIO pin, so only draws power during read). Recharging 100 units with 18650’s seems like a big ask for the volunteer staff so std AA’s might be better.
Questions:
where do I solder the batteries to if I go with a reputable 18650
where would I solder to if I go with std AA’s and how many/do I need a boost converter would I need?
Sorry for being so clueless. I saw a big scary warning about the xiao’s lack of safety when charging attached lipo’s and now am a bit of a wreck.
What is the overall distance from sensor to gateway?
A good set of AA’s is a nice (cost effective) solution. I would recommend a means to manage the voltage however.
I tend to use rechargeable AA’s and have banks of chargers for when batteries are swapped out. Access is easy(ish) but power consumption is (potentially) quite high. Customers require some level of “green”, hence recharge.
I am hoping to keep things at or under the size of a pack of cigarettes. They will be affixed to the room doors for various reasons. It would be a big pain point to have to visit each room / have a charger hooked up for over an hour… originally was going to attach battery component with jst connector to the xiao for ease of replacing batteries. Don’t really know where to go from here. Charging in place will be a pain point and having a bank of chargers will be a pain point since the nodes should only need recharging once a year.
Temps are only sent to the gateway once every fifteen minutes and the thermistor circuit is being driven by gpio power so it only uses power for a few seconds while being read.
Since the power consumption is very low, the LiPo route may be the simplest, ie just using the VBat connection on the XIAO. The actual battery size (mAh) would depend on your budget.
JST connection is “neatest” and you could get a battery with mating connector.
Thermistor could be wired to GND and an analog input using internal pullup (requires temperature calibration initially). No need for a PCB
One option is to use a single 18650 lithium-ion battery. Connect positive to the BAT pad. Connect negative to a GND pad. Use only protected 18650 cells for safety. Never solder directly onto the battery terminals. Use a proper battery holder with leads instead. The board regulates voltage internally from the BAT input. Charging can occur through the USB port. This requires careful handling and monitoring during charging. It offers long runtime between charges per node. However, managing many batteries becomes operationally difficult.
and this is what I know. The short answer is that while 18650 NCM (Nickel Cobalt Manganese) batteries and standard LiPo (Lithium Polymer) batteries share the same chemistry family, their voltage thresholds are the primary reason for the “No” in the context of the XIAO’s built-in charger.
In summary;
“The XIAO charging circuit is optimized for standard 3.7V/4.2V LiPo batteries. While NCM is a similar chemistry, many 18650 NCM cells have different ideal charging voltages (some up to 4.35V) and vastly different internal resistances. Additionally, the XIAO’s low charge current (approx. 50-100mA) is too weak to efficiently charge a high-capacity 18650, potentially triggering safety timeouts or causing overheating in the charging IC due to the prolonged load.”
Long form … Seeed-AI “Hermes bot” local LLM
response from your prompt..
" The Problem: Voltage Cut-off
The charging circuit on the XIAO ESP32S3 and XIAO nRF52840 is typically tuned for a 4.2V max charge voltage.
Standard LiPo: Charges to 4.2V. This matches the XIAO charger perfectly.
18650 NCM: While many NCM cells are rated for 4.2V, some high-capacity variants (like certain LG or Samsung cells) are designed for 4.3V or 4.35V.
If you use a 4.2V charger on a 4.35V cell, you won’t damage it, but you’ll never reach full capacity.
The Risk: If the user has an older or specific “High Voltage” NCM cell, the discharge curve is different. The XIAO’s protection/management might cut off too early or, more dangerously, the charger might not handle the internal resistance of a large 18650 cell as gracefully as a small flat LiPo, leading to excessive heat during the constant-current phase.
Charge Profile Differences
You can tell them: Yes, the charge profiles are slightly different.
Termination Current: 18650s are much larger (e.g., 3000mAh) than the small LiPos (e.g., 100-500mAh) usually used with XIAO. The XIAO’s charge current is hard-coded (usually around 50mA to 100mA). Charging a massive 18650 at 50mA would take 60+ hours, which can trigger “safety timers” in some charging chips, causing them to shut down before the battery is full.
Safety: Standard 18650s often lack the built-in protection circuit (PCM) that small LiPo “pouches” have. Since the XIAO battery pads connect directly to the charging IC, using an unprotected 18650 is a fire hazard if the software crashes or the chip fails.
"
