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Simple question regarding mAh

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techz

New Member
Hey there,

This might be a very simple and newbie sounding question, but I am just starting out and I've encountered a problem.

If for example I had a circuit that draws 20mAh at 12V and I had a power supply with an output of 1200mAh at 12V. Would I damage any components (both short-term and long-term) due to such a difference in mAh?

I'm sorry for asking such a simple question. Thought this would be a good place to find out.

Thank you in advance.
Regards
 

Dx3

New Member
Look at the top of this page. Click on Home. Read, "The Amp Hour Fallacy" on that page. That should give you the right idea.
 

Sceadwian

Banned
You don't draw 20maH you draw 20ma, your terms are mixed up to start with. mAH is a term used in reference to the capacity of a battery and is different even from the MA rating of a typical plug in power supply.

If your power supply CAN provide more MA than your device requires it can't harm it, as long as the voltage is the same. mAH aren't even directly related.
 
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MRCecil

Member
You don't draw 20maH you draw 20ma, your terms are mixed up to start with. mAH is a term used in reference to the capacity of a battery and is different even from the MA rating of a typical plug in power supply.

If your power supply CAN provide more MA than your device requires it can't harm it, as long as the voltage is the same. mAH aren't even directly related.
I think you just took the bait! Could be wrong but........
 

techz

New Member
Thank you Sceadwian.

I appreciate your reply!

I had no idea that I was using the terms incorrectly. I always thought that the mAh denoted the ma that would be drawn per hour by a particular component. Is that correct in any way haha?

Again thanks for your help.

Regards
 
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Sceadwian

Banned
Mah is a term used (as far as I know) exclusively by batteries to give an estimate of their capacity. For example a 1000mah battery contains the equivalent amount of energy that a device that draws 1000ma would use in 1 hour. Here's the real kicker, if you actually draw that 1000ma from a 1000mah battery you won't make it the full hour =) Because the AH rating of a battery is generally measured at a fixed current draw that is LOWER than it's total capacity. For example a typical lead acid battery that is rated for 7AH's which is the same as 7000mah rating is based on a 20 hour discharge, which 7/20 = 350ma's for 20 hours. If you draw more than 350ma per hour the capacity will be reduced because more energy will be wasted from heating in the battery itself, mind your during the discharge the voltage will drop so the total power available will gradually decrease as the cell is drained.

Find as many wall warts as you can find around your house, you'll note that they're all rated for MA not mah. Pop the cover off your cell phone and look at the writing on the battery, you'll likley find a 800-1100mah battery.

A device will be rated for voltage, and for current (generally in MA) never mah's. The MA rating of a device is it's maximum power it can draw not it's continuous one. Typical power supplys don't force amperage, they provide voltage and allow amperage to flow based on the device's effective resistance.

There are some power supplys that are constant current which will provide a fixed current producing as little or as much voltage (up to a maximum) as they are able to, these are sometimes found in older laptops, the bulk majority of all modern power supplies that you find in use will be simple voltage sources with a maximum amperage rating.

The biggest problem with saying all this is a typical wall wart will never actually produce the fixed voltage printer on it's outside, if you measure the output of a transformer based stupid power supply (wallwart) you'll see that the voltag when nothing is drawing power from it can be quiet high, and as the load increases the voltage will drop. The rating on the outside is supposed to mean the voltage that the power supply will be at when the listed amperage is being drawn, but this can be quiet variable with manufacture variances especially with cheap Chinese made power supplies. Most cell phone and more small device power supplies are small switch mode power supplies, the transformers are much smaller and switching is used to produce the output, they're smaller lighter and better they're voltage regulated unlike typical wallwarts.
 

techz

New Member
Thank you again Sceadwian!

I don't think I've ever had such a comprehensive answer on any forum before.
I understand it completely now. Pretty simple stuff too!

Appreciate it.
Regards
 

Sceadwian

Banned
You're welcome techz.
 

audioguru

Well-Known Member
Most Helpful Member
I never thought about the recovery a battery has after resting following a continuous high discharge current.

My heavy old electric RC airplane used seven AAA Ni-MH cells (8.4V) but did not have enough power to climb and the battery charge did not last long. So I replaced it with two lithium-ion cells (7.4V) from a laptop battery. The power was more so the airplane could climb and the flying time was much more, but the motor got extremely hot and the battery got hot and the power reduced to nearly nothing. After resting for a few minutes there was enough power for the airlane to fly again for a few minutes, and the resting and flying again could be done over and over.

I assumed that the hot motor and hot battery reduced the power then the power returned when they cooled, but now I think it was due to the recovery of the battery.
 

Sceadwian

Banned
The chemical reactions that take place in a battery can only occur so fast, and any 'corners' are going to take longer to re-balance, though I'm guessing modern battery construction tries to avoid the corner effect the chemistry is still going to be limited.

Thanks for the link indulis, provided some new insights =)
Curious though I looked into it further and the only reference I found said that Lithiums don't suffer from this effect, so Audioguru for your lithium cells at least it was probably the heating of the battery increasing the cells resistance which caused it to effectively lose capacity, not the actual Peukert effect. In other words if you could properly heatsink the battery you'd have no issues down to the packs discharge limit. That's one thing I've always been surprised about with Lithium packs in RC applications, I've never seen someone heatsink them. Heat is the number 1 killer of Lithium cells.
 
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