Trigger relay at different voltages with voltage comparator

Question

TL;DR - I need help on making a relay with voltage comparator which accepts two reference voltages.

Aim

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My project needs to run a Raspberry Pi on solar power. Solar panel will be connected to a power bank which will then be connected to Raspberry Pi.

Challenge

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Running on solar means there won't be enough power left in the power bank for the Pi. In this case I will shut it down and turn it back on when there is enough power. This process needs to be automatic since the project will be installed at a remote place.

Since Raspberry Pi doesn't have ability to detect power from the power bank directly I'm planning to install a circuit between Pi and the power bank. I will use a MCP3008 - Analogue to Digital Converter to detect the voltage level of the power bank. The schematic diagram will look like this. I will run a script to shut down Pi if it detects only 5% of power is left.

Problem

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After shutting down Pi, I want to close the circuit between Pi and the power bank completely so I'm planning to use a relay for this. I think Relay with LM393 like this will do the job.

The problem is I want to switch off the relay when the voltage equals to 5% power of the battery and switch it on automatically when the battery reaches 25% of its power. The module I consider can set only one value of voltage.

So is there any way I can set one value of voltage for the relay to switch off and another value for it to switch on?

Answer 1

So is there any way I can set one value of voltage for the relay to switch off and another value for it to switch on?

You can use a comparator with hysteresis like this: -

This circuit was designed to be a low power (sub 1 uA) comparator that works around 3 volts. It uses R5 to set the hysteresis: -

V3 (the battery) is ramped up from 1.5 volts to 4 volts and the comparator output switches high at about 3.1 volts and then switches low at about 2.9 volts.

If you fiddle with the values a bit you'll get what you want but concentrate on the mid-point between 5% and 25% for the actual set-point and let the hysteresis give you the margins you want.

For instance, if I change R5 to 1 Mohm I get this: -

Answer 2

It's easy to compute hysteresis for any 2 thresholds, but it's hard when to detect battery power [capacity level] or State of Charge (SoC) if you do not have an accurate means of measuring this. Using voltage alone means you need lots of hysteresis. This is because of battery ESR and a double-layer charge effect in all batteries which results in memory of returning to a resting voltage after being charged or discharged. It may be pretty accurate using the resting voltage but that requires switching off your load and so it may cause a cycling problem. However, if your experience using est. 5% and 25% for Off & On works that must be measured in opposite states. i.e. Turn Off when On at 5% SoC. then Turn On when off being charged at 25%.

It is due to the difference in ESR for the primary and resting capacitance and the operating current that causes this apparent voltage drop and the memory effect to return to the resting voltage. This ESR rises with age and rapidly below 5% SoC, which makes a simple voltage measurement delicate to choose for these thresholds vs a Coulomb counter method, perhaps.

Also you can consider a power FET instead of a power Relay, which also has hysteresis and EMI effects.

Can you define the actual thresholds?