VIN/VBATT Range, using 4xAA

[cyounkins]

I'm looking at powering the CurrentRanger, and I'm confused about the upper limit of VIN/VBATT. The "Power, battery & charging" page states a 3x AAA can be used and "VIN limits: [3.3, 6]V". Where does the upper limit come from?

Me trying to figure it out:

In the schematic (using the PNG version, PDF would be great for text search), I see the JST connector has a FET but is otherwise connected to VBAT and GND nets.

I see the MCP73831 has max 6V on VDD pin, which is labeled VIN and part of VUSB net on the CR schematic. It also has "All Inputs and Outputs w.r.t. VSS ............... -0.3 to (VDD+0.3)V".

When USB is disconnected, VUSB will be pulled to GND. So the VIN/VDD pin will be ~GND/VSS. If VBAT is at even 0.31V, doesn't that violate the second statement above about all inputs and outputs? Since the IC is designed to charge a battery, one would hope it would be OK for the supply to be disconnected and the battery remain!

To be honest I'm a bit rusty and don't fully understand how VBAT becomes VIN, but from there I see it goes to the MCP1703 and TC7660S. The MCP1703 has input range up to 16V. Since it's an LDO a high input voltage might create more heat.

The TC7660S looks to support up to 12V and creates VNEG. Does VNEG have a limit because it's used with FETs with the 3.3V rail?

I was looking at using rechargeable AA/AAAs but they have lower nominal voltage. NiMH cells are 1.3-1.4V max fully charged, 1.2V nominal. So 3x puts that at say 4.2V max, 3.6V nominal. Not far from the lower limit of 3.3V. 4x puts it at 5.6V max, 4.8V nominal. But then if someone used alkalines, 4x ~1.7V max = 6.8V.

So this long-winded post comes down to: where does the referenced 6V limit come from? Thanks!!

[Felix]

I removed any reference to AA batteries, you should use the recommended LiPo type of battery.
6VIN was max on the USB for charging the LiPo battery. But USB is 5V right? So I removed that reference as well.

Please use only as recommended. A small lipo is a few $.

https://lowpowerlab.com/guide/currentranger/battery-power-charging/

[bigjosh]

+1 for PDF version of schematic for searching and not having to physically trace the PCB to figure out mangled labels like `D3` on the SPI connector.

[Felix]

The markings on the silkscreen clearly label what each pin is.
But you are welcome to ask questions if something is not clear in the available information or description of the CurrentRanger.

The SPI header has "D3" on it. That schematic is the previous revision with 5-pins SPI header, the current revision also includes 3V to power an external SPI device.
The SPI header is useful in some cases where certain applications require interfacing to an external SPI device. Some labs and other specialized use cases make use of this header in various ways. Please note the SPI header is not there to back-power the device through that header.

So just to be clear, while the project is not fully open source the older revision schematic in the published form was shared in hope that it would help answer some of the many general questions that came my way. It is there for reference only and should not be considered 100% accurate or basis for reverse engineering the device. While everyone is free to do whatever they like with the device, I cannot endorse and encourage hacking away and modifying the CurrentRanger with any kind of expectation of warranty or support. However I do welcome ideas, suggestions for improvement and new features, and to the extent allowed by personal bandwidth I will implement these in future revisions. And if someone has the knowledge and aspiration to mod it and come up with innovative/useful ways to use it, all the better and I am curious to hear about it.