Would an arduino and relay board kill a car battery?

Hey guys so as the title says, I have a SAINSMART 16 Chanel Relay Board, I got 12volts from my battery going to the relay Board, It then powers up the arduino UNO and bluetooth module (hc-06) I am just wondering if they would kill the battery? I am assuming no as It is literraly only powering 2 leds and the bluetooth module, none of the relays are active until the car starts. Just wondering to see. Also should I have the power from the battery to the Relay Board fused? If so with what size amp?

Your word description of your setup doesn't make sense, do you have a diagram of what's hooked up where?

Anything you run straight from your car battery should have an inline fuse near the battery. Amp rating of the fuse can only be figured out by you, since you are the only one who knows what you're running and you are the only one who can stick a meter to it and find out for sure.

If you drive your car regularly, your battery should be fine.

An arduino UNO draws 50mA at 5V so it will likely draw about 50mA at 12V. Will that drain your battery while you're not driving? Not likely. You can do some measurements with a multimeter since you have more than an Arduino UNO. Turn off your engine and measure the battery voltage. Then if you can measure again after a day, you can use these two voltages to estimate how quickly the voltage drops. If you are concerned with short circuit, then pick the fuse to match what your circuit regularly draw. Say it draws up to 1A at 12V when everything is running, have something like 2A or so to protect your battery. Your battery can output very large current. Without a fuse if something short circuits, your wires between battery and arduino will melt.

A schematic drawing would be a big help. As far as fuse, you will have to determine that by the sum of all the 12 amps being used by the equipment. If ANY of your equipment is connected to the automobile chassis, and you have a wire from the battery - (negative) terminal, you need to also fuse that negative wire with the same size fuse as the positive.

Are you going to start the engine with this equipment connected? You may get transient voltage pulses from the starter motor, or you may get voltage drops to 8 volts or so, or both. Best to have the Arduino and other boards powered off while starting the engine. On many cars, the electronic equipment is turned off at the key switch when the engine is started, then back on when the key start is released.

Unless you have a good car battery, the charging voltage may exceed 14 volts. My Diesel truck has exceeded 15 volts, (160 amp alternator) when the batteries were about about shot.

A schematic of the connections, please.


The 16-channel Sainsmart board I know has an onboard 5volt buck converter.
Current draw when all relays are on, plus Arduino, plus BT module could be about 700mA.
An inline (with the + wire) 2Amp slow blow fuse will do.
The buck converter will have an idle current.
Educated guess is ~50mA for the Arduino and BT module.
That could ofcourse drain a car battery over time (more than a week).

An arduino UNO draws 50mA at 5V so it will likely draw about 50mA at 12V.

If you use a regulator, yes. This would be a good time to spend the $2 on a buck converter!

Arduino has low-power modes but I wonder why keep it awake?

If you can live with lower clock frequencies you can also reduce power consumption, run at 3.4V and connect directly to SD (same voltage) at 8 MHz or less.

Other option: get the Arduino its own laptop battery and charge that when the engine RPM > idle + 500.

As said before, there IS already a 5volt buck converter on a 16-channel board.

A typical car battery has a capacity of 40AH. At 50mA it would last 40/0.05 Hrs.. = 800. Thats about 4.5 weeks. You wouldn't be able to start the car at more that half that, I guess.

A key remote receiver takes around 10mA.

for interest


Well, if you already HAVE the parts, why don't you measure the current and see for yourself?

Remember that 'standard' car batteries - like SLA's don't like deep discharge cycles.
So unless you manage your discharge / recharge profile - the overall battery life will probably be severely degraded.

Are any relays energised when the battery is not charging (you are asleep?)
Of course as suggested above - sleep mode strategies will help - but the relays (and their controlled loads) will be the biggest hogs.

If you have 12V going to the 16 Chanel Relay Board with the ignition OFF and the Arduino UNO is powered off, then ALL relays would be energized. This would draw about 1.5 amps continuously from the battery.

Rather than guessing, could you show us your circuit/connection diagram?

Do you have the relay board version without the 5 volt buck converter installed … like this?