Accuracy of Reported Voltage

Hardware Notecarrier A
1

Hi there. I recently acquired my first few Note Cards and have begun experimenting with them. I have two devices, 1) a Note-NBNAN on Carrier A, and 2) Note-NBNAN on Carrier X… no other hardware. Both have an attached 2000mah lipo, which has been charged by the Note Carrier while connected via USB.

Device 1) is fully charged, and reports 4.31v via NoteHub, but Vbatt (as measured at the battery and on the Vbatt pin is 4.12v.
Device 2) has been running on battery for awhile. It reports 4.22v via NoteHub, while I measure 3.96v at Vbatt pin.

I’ve check this with multiple dvm to make sure my meter is not off. I’m sure I must be misunderstanding something, or doing something wrong. Can anyone help me understand what’s happening?

Secondly, I’m curious why the charge voltage seems to be around 4.1v. Seems lower than what I typically see (4.2)?

Thanks!!!
**
Here is the configuration for both cards:

Set Battery Type
{
“req”:“card.voltage”,
“mode”:“lipo”
}

Set Location Mode
{“req”:“card.location.mode”,“mode”:“periodic”,“seconds”:60}

Tracking Set-Up
{
“req”:“card.location.track”,
“start”:true,
“heartbeat”:true,
“hours”:24,
“File”:“_track.qo”,
“sync”: true
}

2

Hi @DSKUP and welcome to the Blues community!

Due to differing diodes used between the battery and the Notecard on our Notecarriers (which can vary between Blues Notecarriers and certainly with Notecarriers designed by customers) it’s not possible for us to perfectly estimate the forward voltage drop. Therefore the response from a card.voltage request is best used as an approximation. However, if you know the actual voltage drop you can use a card.voltage request like this to calibrate it (not yet documented, but available on Notecard firmware v7.2.2 and up):

{"req":"card.voltage","calibration":1.23,"set":true}
3

Thanks @RobLauer! I was hoping there would be some kind of calibration function available. Just to confirm… is “calibration” is a multiplication factor (such as correcting for resistor tolerance in the measurement circuit or temp compensation) or a voltage addition/subtraction (such as correcting for a fixed diode voltage drop)?

4

Hi @DSKUP,

It’s an addition/subtraction. For instance I believe the default calibration is 0.35, but here I’m setting it to 1:

> {"req":"card.voltage"}
{
 "usb": true,
 "mode": "usb",
 "value": 4.964152266150933,
 "calibration": 0.35
}
> {"req":"card.voltage","set":true,"calibration":1}
{
 "usb": true,
 "mode": "usb",
 "value": 5.609190823553654,
 "calibration": 1
}
5

Thanks Rob. I’m wondering if you can help explain a related phenomenon I’ve observed. Using “calibration”, I’ve been able to get a very accurate and stable reading from a Note Carrier X. Both the heartbeat and motion notes show the same reading, and they are (with small calibration) very close to the battery voltage measured with a meter. However, from Note Carrier A, the heartbeat and motion notes are consistently showing 0.1v difference- heartbeat being higher. As you know, 0.1v is huge when trying to assess the capacity of a LiPo. I recall you mentioned that “voltage” should only be used as an approximation. But why the difference from one Note Carrier model to the next? I’m assuming there is not two voltage reading circuits on Note Carrier A. Could it be timing of celluar/and or GPS reading at the time of “voltage” reading… meaning current draw is affecting the reading?

The internal antennas of the Carrier A is a better form-factor fit for one product I am developing, but I can’t afford to add hardware just to get a decent battery reading. Any thoughts on this?

By the way, I did purchase two Carrier As for testing, but the second one draws down the battery excessively quick when idle (tested with several batteries), so I’m unable to reliably compare to the other.

6

Hi @DSKUP,

It’s entirely possible that active GPS/Cellular utilization could impact the voltage levels to that degree. Just like the onboard temperature sensor of the Notecard, it’s not advisable to take those readings as any more than guidelines when designing a product.

With the Notecarrier A that is drawing down power, I would recommend reaching out via support@blues.com to start an RMA on that one. You definitely shouldn’t be seeing those results.

Rob