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I am having trouble designing a 24V battery monitoring system for a solar charger, what i want is a light to come on when the voltage goes below 20-21v and off if the voltage is any higher

Try searching CL for battery monitor or similar.

This is an OK place to start:

https://www.circuitlab.com/circuit/d9jkz3/battery-monitor-motorcycle/

It can be scaled to 24V.

It would be better using open collector comparators such as the LM311 or 2/4 of an LM2901, in which case OA2 has to have:

i) the inverting and non-inverting inputs swapped;

ii) the LED, D2, polarity reversed and its anode returned to the positive supply rail so that it is wired the same way as D5.

The reason comparators would be better is because they are designed to cope with big voltage differences between their inverting and non-inverting inputs which opamps are not and which may damage them.

:)