Pygate: 0.76V voltage drop between LiPo battery and Vin?

[jand]

I was testing how long a pygate can run on a LiPo battery because I intend to include such a battery as a backup for when mains power is interrupted. To my surprise, I measure a 0.76V voltage drop between the LiPo battery and Vin (on the wipy module inserted in the pygate). Is this normal behavior? What is causing this? That voltage drop means that about half of the capacity of a LiPo battery remains unused.

I measured this with a multimeter after I noticed that a LiPo battery lasted only half the time that I estimated based on pygate power consumption and battery capacity (4500mAh). I had also noticed that the Vin voltage measured by the wipy was much lower than expected for a fully charged LiPo battery (specsheet: 'P14 = VCC Voltage Monitor (100k + 100k voltage divider); so multiplied measured voltage on P14 with a factor 2).

This is what I measured with a multimeter:
at LiPo battery: 3.914V
at Vin of wipy: 3.150V
That means a voltage drop of 0.764V

I also monitored the gateway during battery operation and noticed that it starts failing when the monitored Vin becomes around 2.95V. So that is when the battery voltage reaches 2.95V + 0.76V=3.71V. That is halfway in the typical discharge-curve of a 3.7V LiPo battery!

[Gijs]

I must say for what I've seen in (only) the past 6 months, you're absolutely correct! I know in the background we really want to improve our quality control, but indeed lack the manpower on that front. I'll be sure to pass on the message!

[jcaron]

@Gijs Pycom really, really need to have a better QA for both hardware and software. Software issues are bad enough, but hardware issues will kill you. And it’s not like it’s the first time (by far), sadly. I understand you want to hire a ton of new developers. Divert part of that for hardware and software QA. You’ll notice that some issues are recurring on the forum, and many of them are related to QA issues, many related to software, which is bad enough, but too many related to hardware.

[jand]

@Gijs Thanks very much for this solution. I will try it.

[Gijs]

Hi,
It seems to be a flaw in the hardware unfortunately with no easy fix. Though we did some work on exploring different solutions, and we have the following available:
Solder a schottky diode with low forward voltage from the positive (+) terminal of the battery to the VCC pad (see picture)

Basically any schottky diode with low forward voltage and ~1A of current will work for this purpose. From digikey, we were able to find the following diode as best candidate, but there are definitely more options out there:
https://www.digikey.com/en/products/detail/microsemi-corporation/LSM115JE3-TR13/1634473

Best,
Gijs

[jand]

@Gijs
Thanks! I measured it while the gateway was active. I look forward to more information on this.

[Gijs]

Hi,

The voltage drop you are experiencing is quite high. I just measured my own Pygate to be sure, and get around 0.5V (not operating the Pygate itself, but simply connecting a battery and module), which we also think is quite high. We were not aware of this previously, and my colleague will do some tests to find the cause and I'll let you know of the results! (Please remind me if I forget!)

Best,
Gijs