switch load depending on battery voltage

Question

I have the following set up;

^{simulate this circuit – Schematic created using CircuitLab}

MPPT charge controller: https://www.sparkfun.com/products/12885

Load: Raspberry Pi Zero

Battery is: http://ie.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&urlLangId=353&langId=353&partNumber=2484233&storeId=10163#techDocsHook

Battery Datasheet: http://www.farnell.com/datasheets/2017383.pdf?_ga=2.166173694.1332281990.1495362248-227627644.1495362211

The battery datasheet states:

Over discharge detection voltage: 2.5V

Over discharge release voltage: 2.85V

Release condition: cut load, release after charging up

What happens is that the load at night drains the battery to 2.5V thus triggering the protection circuitry, upon daylight the battery does not charge nor does the load power up from the MPPT charge controller.

The only way to overcome this is to physically disconnect the battery from the MPPT controller and all works fine again. I assume then that the battery is getting stuck via it’s charge protection circuitry.

I want to avoid the above battery low voltage triggering & manual intervention.

Can someone suggest a circuit/solution that will provide power to the battery & load when there is voltage from the MPPT and disconnect the load when the battery voltage drops to say 2.8v and reconnect the load when the battery voltages charges to say 4V?

How can I implement the following switch in circuitry?

^{simulate this circuit}

Answer 1

The function that you describe is commonly performed by a comparator circuit which typically a specialized version of an op-amp. The circuit works by comparing the monitored voltage to a reference voltage (typically from a zener). When the voltage falls below the set point that is programmed with a simple resistive divider network, the output of the comparator changes state.

The ability of a comparator to change state at one voltage but not revert back until a higher (or lower as the case may be) voltage is measured is called hysteresis. You have asked for 1.2 volts (4.0 - 2.8 volts) of hysteresis although you could probably get by with much less hysteresis.

In your case the comparator circuit must at all times be powered by the battery it is trying to protect so you need a very low power comparator. The output of the comparator could drive a MOSFET switch that disconnects the load from the battery when the comparator trips.

There are many low power comparators on the market, some of which are integrated into brown out protection devices and other special functions. Take a look at the spec sheet for the LTC1540 (here). This is an ultra low current device that includes the zener voltage reference source and it can operate down to a 2 volt supply. This IC with two divider resistors and a MOSFET should suit your application.