# Safe assumption for AA and C battery current draw

I have 5 LED candles with built-in timers, but they eat batteries, so I'm hoping to use an AC adapter to power them instead. They are all different with some using AA batteries and others using C batteries, but each of them use a pair of batteries, so I'm thinking about getting a 3V AC adapter and connect all of these candles in parallel.

What I'm unsure about is how much current is being drawn by each of these candles to make sure I get a sufficiently powerful AC adapter. Since it changes throughout a 24 hour period, I don't have the equipment to measure and log that for even one cycle. Are there numbers where I could safely assume that the candles won't draw more than x mA from a AA battery and y mA from a C battery?

• If you run it 24/7 with 2 AA batteries, how many hours do the batteries last? This will give us an idea of how much current it uses. Commented Jun 18, 2018 at 17:20
• It's on the order of several weeks or months. I've never counted unfortunately. The timer does go on a cycle of 5 hours on, then 19 hours off for each day. Commented Jun 18, 2018 at 17:46
• If it also helps, I bought them all from Target last year and I think they still sell them. I'll post the exact models tonight. Commented Jun 18, 2018 at 17:48
• A comment an an aside from my answer, if your only goal is to avoid purchasing and replacing batteries often, you might consider simply using rechargeable batteries. This would avoid the hassle of running wires and connecting them to the candles which I presume do not have a DC input jack. Commented Jun 18, 2018 at 17:58
• Measure it with a current meter, if you don't have a current meter this may not be the best place to pose a question like this. Commented Jun 18, 2018 at 22:44

## 2 Answers

LED candles are often a small microcontroller of some sort (to create a flicker effect) and one or more LEDs. Aside from on (flickering) and off, what other cycles or modes do your candles exhibit?

You size the power supply based on the maximum current that the load will draw at a given moment. Your question implies you might think you need to measure the average current drawn over some period of time, but this would be more for calculating power consumption, not current draw. (See Olin's Q&A about calculating power supply requirements.)

If you measure current when the candle is operating at maximum brightness (or whatever constitutes its maximum setting), this will give you the amount of current required, and can be summed with the other devices to inform your power supply choice. For example, if you measured 100mA for the highest-draw LED candle, you could likely assume that 500mA is sufficient to operate them all. I would always recommend adding some overhead for things like inrush current, etc. Remember, the load determines the current draw, not the capability of the power supply. Therefore, having a power supply rated for more current just means it can handle more than may be needed.

A 3V 1.67A power supply can be found for less than $6 (example at All Electronics). This is just based on a guess -- you'll need to measure the candles to be sure. Keep in mind you could just purchase something like a Meanwell LRS-50-3.3 for less than$15 USD (example at Mouser), and have 10 amperes available for whatever 3V applications you want. (That supply is 3.3V but adjustable from 2.97 to 3.6V.) It's extremely doubtful your candles require anywhere near that much, so for the price, the problem is solved without having to spend too much time measuring and calculating the requirements more precisely.

If you want to measure the current, you only need to do it once. If the batteries have a voltage of 3v, and the current is 100ma, then as long as you supply 3v continuously, then the current will stay at 100ma. The reason the current changes over time is because the batteries voltage is changing, which effects the led circuit. Your power supply voltage will not change over time.

..now that being said. You probably don't have to measure anything. Lets look at the LEDs that run on 2 C batteries. Just ballparking here, an alkaline c battery can supply 8Ah at 1.5v. There's 2 in series, so 8Ah at 3v. Lets say you discharge this completely in 24 hours. That's 8000/24 = 300ma, which is not much in terms of AC adapters, and that's a very high estimate of the current draw. You shouldn't have much trouble finding a 2A 3V supply. That can easily power 6 of the C battery lights. If you measure them I think you will find that you can actually run at least twice as many as that. The AA lights will draw much less.

• That's along the lines of my thinking. However the chief concern I have is that because each is on a built-in timer, the LEDs aren't on all the time (5 hours each day), so I'm unsure if the load suddenly spikes at these state changes. I do have a 3V ~300mA AC adapter on hand, which outside of any spikes should be able to handle all 5 candles in parallel since I only need to change batteries every several weeks, but I want to make sure that I don't fry it or worse if it is momentary expected to supply, say, 5A. Commented Jun 18, 2018 at 18:20
• Oh, I see. You should measure the current when the LED is on and just budget for all of them being in that state. Nothing is going to fry if you draw too much current, the wall wart may get hot, but it will probably have an over-current or over temp shutoff. Basically I was trying to say that 2A will probably be more than enough. I would just try it.
– Drew
Commented Jun 18, 2018 at 18:28