# Relevance of amperage in a voltage divider circuit

I'm not very competent on the electrical engineering, so forgive me if this is a dumb question. I'm trying to make a voltage divider circuit to monitor the battery level of a project I'm working on. It utilizes a 12v scooter battery and will be monitored by an Arduino and I'm going to be using the circuit defined here (also attached below) using the table values for the 15v max voltage to do it. I don't want to fry my Arduino because I cant really afford to replace it. This battery is pretty big; does its amp/hour have any effect on whether it'll fry my Arduino?

You only need to worry about the voltage. The ampere-hour capacity of the battery has no effect on the operation of the voltage divider.

• Wouldn't the Ah capacity of the battery have a large effect on a, say 1:1 voltage divider of 1 ohm to 100 ohm resistors? Dec 12, 2013 at 1:54
• @Passerby: Picky, picky - the circuit the OP referred to suggested 2K and 1K resistors - but I do agree that that would be a somewhat heavy load on a stack of AAA cells. Dec 12, 2013 at 2:03

Amperage in a voltage divider is absolutely relevant, especially in a battery powered situation. While your setup is a large scooter battery, and this is more relevant to small battery powered design, a resistor voltage divider is still a load, of R1+R2, to ground. Too large a resistor value, and you are wasting power through the resistor pair. At 7.5Ω + 7.5Ω, a 15Ω load, you would be wasting 1 AMP of power. At 1k + 1k, a 2k load, you are wasting 7.5 mA. A 2k + 1k pair is 3k load, 5mA wasted.

Additionally, the larger values will need a larger Wattage rating. At 1k + 1k and 15V, you are wasting 7.5mA * 15v = 0.1125 Watts. Since this is through two resistors of equal value, they share the power wasted, 0.1125 / 2 = 0.0565 Watts. A typical 1/8 (0.125) Watt resistor will work. At 375Ω + 375Ω, or 20mA load, each resistor will waste 7v * 20mA = 0.140W, so a 1/4 Watt resistor would be needed. Too low wattage a resistor can burn out. Too large a resistor can waste too much power and catch on fire or burn you.

You should choose a higher pair with the same ratio (2/1). A 24k R2 and 47k R1 will work the same, while providing only a 0.2mA load (210 microAmps). The Ratio stays the same (1/3 of Battery Voltage).

But no, the amperage is not relevant to the Arduino's Analog input, or the Arduino itself. Unless you connect something wrong of course. The only concern is making the voltage divider too small (can't be read correctly) or too big (drains battery quicker).

• Passerby's answer is the one you should be reading three times & accepting. If your 12v scooter battery is going anywhere near an actual scooter or other vehicle you should also reconsider your 15v ceiling, see: electronics.stackexchange.com/questions/73857/… Dec 12, 2013 at 9:20
• Issue: "too large a resistor value ..." - large makes me think large resistance (>1,000,000Ω) but I suspect you mean large current (low resistance, <1000Ω)? Dec 12, 2013 at 10:53
• @RedGrittyBrick you know, if I said too small a resistor value, someone would say small makes them think a small resistance (>1MΩ) too. Dang subjective language. Dec 12, 2013 at 15:04

This battery is pretty big; does its amp/hour have any effect on whether it'll fry my arduino?

Lets try to clarify what is and what is not important here.

## Battery capacity

Battery capacity is usually given in amp-hours that's amps x hours NOT amps / hours. This is a measure of energy stored in the battery. It tells you how long the battery can provide a specific current at close to its rated voltage. It isn't an especially good indicator of whether it will fry your Arduino, you can fry an Arduino with a 9V battery which has a tiny capacity in amp-hours.

## Voltage

The voltage that you can apply to an Arduino pin is the most critical battery criteria. Arduinos come in 5V and 3.3V flavours. You shouldn't connect to any Arduino pin a voltage higher than the normal supply voltage (Vcc) provided to the Arduino's microcontroller chip.

## Current

If you connected a 5V battery across two Arduino pins you could harm the Arduino because the Arduino pins cannot safely source or sink much current (perhaps 40 mA max typically). Therefore you should take measures to limit the possible current flow. Often a current-limiting resistor is used.

Note: Current is measured in amps so some people write "ampage" or "amperage" instead of "current". I personally don't like this usage but it happens. Don't confuse AMperage with Amp-hour capacity - they are very different things.

## Non-Battery issues

You also need to be careful about static electricity and about back-EMF from motors - but these are not characteristics of your battery.

## Voltage dividers

If you use resistors with low resistance (e.g. 10Ω) a lot of current will flow through the divider and be converted to heat. This will drain your battery faster (not an issue for a high capacity battery). This is waste.

If you use resistors with a very high resistance value (e.g. 1,000,000Ω = 1MΩ) the current flowing through the divider will be small and any current drawn by your ADC will divert a large proportion of the current and this will distort the operation of the voltage divider.

mAh (milli-amp hours) is just the capacity of the battery, or how long it will last. It has no effect on the power passing out of it. However, any power that is not coming from the arduino should be controlled via a MOSFET or other transistor, because it very well could damage the arduino if more than the 5 volts that it uses passes through it.

But amps are just a measure of how much stuff can be powered on your 12/15 volts. If you had it at 0.5 amps, it would definitely be unable to power a scooter. but 9 million amps (just a arbitrary huge number) wouldn't break your scooter or make it explode, because that's just it's maximum capacity. The thing that the arduino needs protected from is the voltage.

• The Ampere-hours rating is shown on most rechargeable batteries, as an example. a AA battery at 2500 mAH is going to last longer than one rated at 1700mAH before needing recharged, but both should theoretically output the same voltage when fully charged. It was a good question, though; It took me ages to find out what amperes actually were, since I knew next to no one with electronic experience when I was starting out. Dec 12, 2013 at 1:08