Since getting my hands on a Nordic PPK2 (power profiler) I have been able to accurately measure the operational and standby/sleep performance of my ESP8266 systems that use batteries.
One of my boards I’ve investigated contains the following:
1 x ESP12S
1 x ATTINY1614
1 x MPU6050 module
1 x DS18B20
plus a few capacitors and resistors.
This is powered from 2 x 600mAh LiFePo4 cells in parallel.
The board is set up to wake up every 5-min, send data, go back to deep-sleep.
I like to use the ESP12S in compact designs because it requires no external resistors. The relevant pull-up/down resistors are built in to the module so if its been programmed, all you need is to supply power (and a VCC decoupling cap).
The MPU6050 module is a common version found on ebay. It has a small regulator (5V to 3.3V), a few caps and resistors and a power LED.
To minimise the power consumption I remove both the power LED and the regulator from the MPU6050 module. Not using a regulator avoids the parasitic drain or voltage drop that would normally be involved.
The charge/discharge voltage profile for this type of cell means that no regulator is required for the electronics.
From the ESP data sheet, operating voltage is from 2.5 to 3.6V.
MPU data says 2.375 to 3.46
ATtiny is 1.8V to 5.5V
By using the Nordic power profiler I obtained the following data for a 5-min wake/sleep cycle
The highlighted grey-section contains a complete 5-min cycle (well.., 4:57.9).
(For an actual wakeup event, see here)
This shows max current (when the ESP transmits), average current and the charge used. (25.95 milli-Coulombs)
To translate Coulombs to mA/h is a simple divide by 3.6 which produces 0.00720833mAh for the 5-min cycle.
So 1 hour takes that figure up to 0.0865mAh, 1 day is 2.076mAh, 30 days uses 62.28mAh.
Theoretically with 1200mAh of battery this allows for 13872 hours of operation. Or 578 days (19.2 months!). Mmm… was not expecting that. Seems a bit optimistic, maybe got my sums wrong somewhere.
But even if its correct, its never going to be that good. Not even close.
This battery chemistry has a self-discharge rate of around 5% a month. Some sources say 3% but 5% seems the most common figure so lets go with 5%.
This equates to 60mAh a month just vanishing. Nothing you can do about it.
The electronics actually uses 62.28mAh/month and the battery looses another 60 in self-discharge. This brings the new monthly ‘consumption’ figure to 122.28mAh so the theoretical run-time changes from 19.2 months to 9.8 months.
But wait, there’s more!
The battery capacity figures given by manufacturers are from full charged to fully discharged. If you read lots of stuff about LiFePo4 batteries, charging/discharging, what constitutes fully charged seems to vary. Some advocate charging as high as 4.2V which packs in an extra percent, others say fully charged is 3.65V (seems to be common). If you charge to 3.65V then stop, take it out the charger and next day measure the voltage it has settled to a more friendly (for my purposes) 3.4-ish V.
(Some very good info about charge voltages and percentage capacity at powerstream)
The minimum voltage for LiFePo4 cells is often given as 2.5V. I don’t let mine get that far down. I shut-down at 2.9V which probably looses maybe 8% of the available capacity so the 1200mAh battery is actually 1200-8% so 1104.
Only charging to 3.4V can loose another 3%, so instead of 1200-8%, make that 1200-11% so now only 1068mAh
At last the numbers should be usable. 1068mAh/122.28 = 8.7 months which I’m hoping is a bit more realistic.
Disappointingly different from the first number of 19.2 months (I’d got a bit excited!) but after thinking about it a bit, and reading more, I realised it just was not going to be.
Theoretical-maximum and realism can be very different.
The style of usage has quite an effect on the usable capacity. If I was discharging at 0.5C then I should get close to the theoretical capacity as the self-discharge is not a factor.
By using low discharge rates over a long period, the self-discharge of the battery becomes very significant. In my case, almost half the energy is lost in self-discharge.
I have not managed to find any information about LiFePo4 cell very low discharge rates. 0.1C was the lowest test discharge I found.
Although I’m not sure if this qualifies as low, perhaps pulsed is a better description. It does have high current pulses (~230mA), but of very short duration and 5-min apart to allow for ‘recovery’. It probably makes a difference and in a beneficial way to the user but I’m just guessing here.
Since the self-discharge has such a large effect on my battery life I looked at using 2 AA alkaline cells in series, no regulator. So lets look at some numbers here..
Capacity of a AA alkaline cell seems to be from 1700 to 2850mAh depending on the manufacturer so 2000mAh seems reasonable to use.
Self discharge for alkaline cells is 2-3% a year, so 3%/12 gives 0.25% per month, or 5mAh per month loss.
The ESP8266 needs 2.5V minimum so each cell only gets down to 1.25V which means only about 35% of its 2000mAh is available for use, so 700mAh
Monthly use is 62.28mAh for the electronics plus 5mAh self discharge = 67.28mAh
700mAh/67.28 = 10.4 months. An extra 1.7 months than the rechargeable cells and starting with a lower available mAh. The self-discharge makes the difference.
Its not quite that simple. The circuit would need more capacitance because as the internal resistance in the cells rises (as its discharged), the high current peaks for the ESP transmission need the energy reservoir of the caps to source it. (Excellent article here with measurements and examples)
There is also a degree of waste involved. The AA batteries still have 65% charge left. Maybe OK for a TV remote? The rechargeables are, well, rechargeable. 1000 charge cycles for the LiFePo4 batteries seems quite reasonable, but the alkaline cells probably go in the bin.
Or modify the circuit to have a very efficient micropower boost regulator so much more energy can be extracted from the alkaline cells. I have not looked into this, I’m going for low component count and simplicity. Ease of building at home.
The figures above (apart from the actual measurements) are gathered (and interpreted) from the dreaded internet so don’t take it all as rigorously defined. It should however be ball-park enough to be useful and food for thought.