4 ms·

Would be 3-4 days on 2000mAH battery

chrismorgan 5y ago
I’m utterly inexpert in this area, but that doesn’t sound right at all; I think it should be more like 2–3 months, and probably even more. 3–4 days on a 2000mAH battery means over 20mAH per hour. You say deep sleep uses <20μA, which would be under 0.1% of the total here, so we can call that rounding error and just focus on the time when it’s actively doing something. Assume it wakes up every hour, takes 12 seconds to connect, fetch and render, before returning to deep sleep. 20mAH in 12 seconds is 6 amps. I’m not familiar with the actual power consumption figures, but https://lastminuteengineers.com/esp32-sleep-modes-power-consumption/ https://lastminuteengineers.com/esp32-sleep-modes-power-cons... suggests an ESP32 chip with everything on is using up to about 260mA, and I seem to recall hearing that e-ink displays used a few milliwatts, but it’s hard to find details. All up, my extremely rough estimates are coming to active consumption more like 300mA, not 6A—so more like three months than three days. (I haven’t been careful with the voltages in these calculations, but my impression is that you’re looking at about 3.3–3.6V on the battery and ESP32 chip. I think the 300mA is probably fairly significantly on the high side, since most of the system won’t be in use for all of that nominal 12 seconds. But I caution you again that I’m not competent in this field; I’m just a layman that thinks the numbers didn’t add up. While I’m talking: why do most of these things, especially the batteries, cite current figures rather than power figures? Is there a genuine reason, or is it some kind of historical mistake, or something else?)
- OJFord 5y ago
  > why do most of these things, especially the batteries, cite current figures rather than power figures? Is there a genuine reason, or is it some kind of historical mistake, or something else? I agree it's not so helpful in practice; I think though it's because they're a store of charge, N electrons = capacity in Coulombs, and 1C = 1As (by definition) or more familiarly 1/3600 Ah. This is true load or no load, so from C (As) and V (= J/C) you know the charge stored and the rate of depletion. Maybe the real error is not Ah instead of Wh, but V instead of J, since the former will drop off non-linearly (in practice) under load and while draining, or with temperature; whereas I think J & C independently 'when new' would give a better picture, or at least one that will more obviously change over the life of the battery?
- robertlangdon 5y ago
  You're right, we made a mistake. I checked with my partner. Thanks for pointing out, it would be at least 1-1.5 months for 2000mAh. Talk about short selling yourself! E-papers have a slow refresh rate, it takes about 5 seconds to display a completely new image. And considering it takes about 15 seconds to connect and download an image (based on network speed), we get current spikes upto 300-400mA during WiFi transfer. We can expect conservatively a 1-1.5 months life from a 2000mAh battery. The mAh unit enables us to compare batteries easier I believe. Standard LiPo are 3.7V so given a mAh rating its easier to know which will last longer. While for a Wh unit you would have to calculate it.
- purplecats 5y ago
  Sounds very reasonable, thank you! I could make it refresh 1/12th of the time (twice a day) and that should get me well past once a year maintenance levels. seems pretty reasonable of a product after all. If I put something like this in my bathroom, where it could be exposed to freckles of water droplets and humidity often, how concerned do I need to be of a malfunction that could be disastrous (such as starting a fire when I'm out of the home)?
robertlangdon 5y ago
Update: Please check my next comment in the thread. It should be around 1-1.5 months for the 2000mAh battery.

I’m utterly inexpert in this area, but that doesn’t sound right at all; I think it should be more like 2–3 months, and probably even more. 3–4 days on a 2000mAH battery means over 20mAH per hour. You say deep sleep uses <20μA, which would be under 0.1% of the total here, so we can call that rounding error and just focus on the time when it’s actively doing something. Assume it wakes up every hour, takes 12 seconds to connect, fetch and render, before returning to deep sleep. 20mAH in 12 seconds is 6 amps. I’m not familiar with the actual power consumption figures, but https://lastminuteengineers.com/esp32-sleep-modes-power-consumption/ https://lastminuteengineers.com/esp32-sleep-modes-power-cons... suggests an ESP32 chip with everything on is using up to about 260mA, and I seem to recall hearing that e-ink displays used a few milliwatts, but it’s hard to find details. All up, my extremely rough estimates are coming to active consumption more like 300mA, not 6A—so more like three months than three days. (I haven’t been careful with the voltages in these calculations, but my impression is that you’re looking at about 3.3–3.6V on the battery and ESP32 chip. I think the 300mA is probably fairly significantly on the high side, since most of the system won’t be in use for all of that nominal 12 seconds. But I caution you again that I’m not competent in this field; I’m just a layman that thinks the numbers didn’t add up. While I’m talking: why do most of these things, especially the batteries, cite current figures rather than power figures? Is there a genuine reason, or is it some kind of historical mistake, or something else?)

OJFord 5y ago
> why do most of these things, especially the batteries, cite current figures rather than power figures? Is there a genuine reason, or is it some kind of historical mistake, or something else? I agree it's not so helpful in practice; I think though it's because they're a store of charge, N electrons = capacity in Coulombs, and 1C = 1As (by definition) or more familiarly 1/3600 Ah. This is true load or no load, so from C (As) and V (= J/C) you know the charge stored and the rate of depletion. Maybe the real error is not Ah instead of Wh, but V instead of J, since the former will drop off non-linearly (in practice) under load and while draining, or with temperature; whereas I think J & C independently 'when new' would give a better picture, or at least one that will more obviously change over the life of the battery?
robertlangdon 5y ago
You're right, we made a mistake. I checked with my partner. Thanks for pointing out, it would be at least 1-1.5 months for 2000mAh. Talk about short selling yourself! E-papers have a slow refresh rate, it takes about 5 seconds to display a completely new image. And considering it takes about 15 seconds to connect and download an image (based on network speed), we get current spikes upto 300-400mA during WiFi transfer. We can expect conservatively a 1-1.5 months life from a 2000mAh battery. The mAh unit enables us to compare batteries easier I believe. Standard LiPo are 3.7V so given a mAh rating its easier to know which will last longer. While for a Wh unit you would have to calculate it.
purplecats 5y ago
Sounds very reasonable, thank you! I could make it refresh 1/12th of the time (twice a day) and that should get me well past once a year maintenance levels. seems pretty reasonable of a product after all. If I put something like this in my bathroom, where it could be exposed to freckles of water droplets and humidity often, how concerned do I need to be of a malfunction that could be disastrous (such as starting a fire when I'm out of the home)?

robertlangdon 5y ago

Update: Please check my next comment in the thread. It should be around 1-1.5 months for the 2000mAh battery.