Current Monitor for 12V battery

A rather unpleasant discovery yesterday: the car refused to 'start.' Just like a typical ICE, the car uses the 12V battery to initiate the start up, closing the relays and main contactor, which connects the main pack to the motor controller and the DC-DC converter.

But if the 12V battery is dead, then the whole thing won't start.

Why did I do it this way? It seemed simpler at first. The original 12V was already in place, and there was even space for a second one on the other side of the car. (Not enough space for one of the pack batteries - physically too big!)

Plus, I am running the battery monitor and vehicle interface 24/7 off this battery. Which is the problem. The drain (I now know) is about 550mA. This adds up! if this was drawn from the battery in an ICE car, it would die too, as the charging from short trips is not enough to replace the energy used to start the engine and the 24/7 drain.

What to do? First get a handle on the currents into and out of the battery. I had seen this post a long time ago and thought it a pretty neat creation. So I designed one with two sensors, one for the current in from the DC-DC, the other for current out to the car.

The sensors are Allegro ACS756 which make use of the hall effect and have 3kV of isolation (nice but not necessary in this application.)

For electrical integrity, the terminals have some 14ga copper wire soldered around the pads.

For physical integrity, the whole board is through bolted to a alsab of 6mm teflon from a small kitchen cutting board.

The cpu used is an Atmel ATtiny45 in a DIP. I think these things are awesome: for about $1 there are 4 analog inputs and 2 GPIOs. It reads each current sensor, and sends the data out in a PWM code to the vehicle interface (an Atmel ATMega168)

Thus I learned that those 24/7 loads are between 0.5 and 0.6 amps.