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Last year I brought a thermoelectric cooler for camping. These aren’t the best for cooling items down from room temperature but they will kept items cold if they come out of a fridge. The big issue is that the cooler will quite happily drain the battery overnight if left plugged in. There are a number of ways to solve this problem:

Dual battery setup - Great idea for running all sorts of accessories but you need to mount a second battery and charging circuit that cuts off when the car isn’t running. I am not intending on keeping the car for long so didn’t want to invest to much in it.
Use an outlet that is only on when the car is running - We found that you need to keep the cooler running for as long as possible to keep the temperature low. If the temperature rises too much the cooler will take significant time bringing it back down.
Unplug the fridge when stopped - Same issues as above and has the risk that you may forget (I did…) and discharge the battery.
Monitor the battery voltage and turn off cooler before the battery discharges too much. This option maximises the run time of the cooler, is relatively simple to build / install and is cheap.
This instructable will document the fourth option (nicknamed “the Battery Hero”) and provide some additional information on things that I learnt whilst constructing the device. Step by step instructions are not given as everybodies implementation will be slightly different. All concepts and stages of the design will be outlined so it should be relatively easy to develop your own unit.

Supplies:
The battery hero was built whilst the world was in lockdown due to COVID 18 and hence I had to make do with what I had at hand. There are a couple of changes I would suggest and these are described below.

ATtiny 85
L7805 voltage regulator
0.33uf ceramic capacitor, I only had an electrolytic but a ceramic should be used
0.1uf ceramic capacitor
Momentary push switch
WS2812 LED
47k resistor
15k resistor
1k resistor
2N2222 transistor
IN4007 diode
102 Trim pot, this should be replaced with something with a smaller range so that changing the voltage doesn’t take so many turns.”

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