Volts, Amps, Watts, and Hours

So my fish-finder is 12VDC and the manufacturer says the “Power Consumption” is 2.7W RMS.

So I get that it’s 2.7/12=.225 Amps, but is there a time scale?

In other words, how do these .225 amps figure into the amp-hour ratings of batteries?

If the battery is rated at 3AH, does this mean you will get 3/.225=13.3 hours of operation… theoretically?

The amp draw is continuous. It’s always drawing 225mA. (Theoretically. That’s more likely to be its maximum power draw, but for something like that it may always be near it’s max.) As far as battery capacity, you’re exactly right. A 3AH battery is theoretically able to supply three amps for one hour, or 1 amp for three hours, or, as you say, 225mA for 13.3 hours. Of course, real-world performance will be a little less than that, for a whole host of reasons, the biggest of which is that the output voltage will start to drop as the battery runs out of juice, and there will still be some energy left in the battery when the voltage gets so low that your device shuts itself off.

One thing to append : lead acid batteries face drastically reduced lifespan if you discharge them down to 10-20%. You’re not supposed to discharge them below 50% if you want them to last their normal lifespan (which is just 200-550 cycles). See here : https://www.solar-electric.com/deep-cycle-battery-faq.html#Cycles%20vs%20Life

So in this case, you should actually multiply that 3AH by 0.5 to get a more realistic assessment of how long you can use it for this device. It will keep working past that point - but you should think of that as “emergency” power, because it’s going to destroy your battery much faster.

automotive batteries are like that.
Marine and other “Deep cycle” batteries, not so damaged by deep discharge.

Not correct. Automotive batteries you don’t want to discharge more than 5%. Deep discharge, 50%. See here : http://batteryuniversity.com/learn/article/how_to_restore_and_prolong_lead_acid_batteries

if it’s a DC power supply, what does “RMS” have to do with anything?

No, that’s correct, automotive start batteries are damaged by deep discharges. It’s because of the way they are constructed, to be able to produce high amperage for short periods of time. Marine or “deep cycle” batteries are designed to be discharged heavily on a repeated basis. They tend to be larger and heavier, with thicker plates or sponge lead. An automotive start battery pressed into service for a deep cycle application will be ruined in short order, that’s true.

I have always been annoyed when I see “RMS” included with power units.

You can talk about average power. You can talk about peak power. You can talk about apparent and reactive power. But not RMS power. Car stereo manufacturers are particularly guilty of this.

Having said that, when “RMS” is included with power units, I think it means average power was measured using a true RMS voltmeter and true RMS ammeter. (Even then, it’s really apparent power unless the load is purely resistive.)

jz #6.

I have the same question.

The device may use a DC/DC converter for the output pulse. The rating may be RMS on the converter input. Seems like average current makes more sense.

Crane

If the current it draws from the battery is constant (flat line on oscilloscope) then they probably would have simply said the power draw is “2.7 W.” But because they appended “RMS” to the power, I am guessing it means the current it draws from the battery is not constant… it probably has a periodic waveform riding on the DC. To measure the power of such a waveform you need to use meters than measure true RMS. (But as mentioned above, using a true RMS voltmeter and true RMS ammeter will only allow you to measure apparent power unless the load is purely resistive. To measure real power you also need to measure the phase difference between V and I.)

So if I’m sizing an SLA battery, I need to size it for at least twice of what I’m going to need per session, so I don’t run it down too much? So my 3AH SLA battery should really only be used up to six hours or so?

What about sizing rechargeable alkaline battery packs? Are there “don’t run therm dead” considerations there as well?

Be careful. Alkalines are primary cells meaning they can’t be recharged.

I’d think about something like this:
https://www.amazon.com/Booster-ES2500-1100-Peak-Starter/dp/B0002SQTYG
Quite portable and has a familiar automotive accessory (cigar lighter) port right on the front. I can’t find a spec sheet at the moment but I think it is a 17Ah SLA inside.

Thanks for the effort, but your recommendation is overkill by a mile.

I’m talking about powering a fish finder on a float tube. I’m looking at small SLA batteries for like \$15 and was wondering about other options.

…or if the RMS meter doesn’t have a power function, use a thermocouple and DC voltmeter. Knowing some basic characteristics of the thermocouple would allow you to calculate the rms power from the voltage reading.

That’s a better description of the project. You can use an SLA if cost or high capacity is the primary concern like in car batteries. If weight and/or dimensions are the primary constraints (such as mobile phones or laptops), use a lithium type: Lipo, LiFePO4 or lithium-ion.

For sizing an SLA, I’d personally cut the rated capacity in half.
If you get more, bonus.
Your 3Ah battery then becomes 12V x 1.5 Ah = 25 Watt-hours.
The 2.7W fish finder device should run a little under ten hours.

Don’t these fish finders use acoustic transducers?
The 2.7W rating may be only the power for the transducer.
This would explain the RMS specification.
You might need a more or less powerful transducers depending on water characteristics like depth, salt and other mineral content, and temperature.

Thanks for the help!

*Don’t these fish finders use acoustic transducers?
The 2.7W rating may be only the power for the transducer.
This would explain the RMS specification.
You might need a more or less powerful transducers depending on water characteristics like depth, salt and other mineral content, and temperature.
*

You don’t have a choice of transducers - they tell you what it will do and you either take it or leave it.
Wi-Fish Fishfinder (scoll down for specs)

Finding SLA batteries is a snap. Finding other types of batteries for this application is proving difficult.