Some simple battery questions from a beginner

Hi
I'm starting to understand more about flashlights thanks to this forum, but I still have a lot to learn about batteries. I have read some of the threads and graphs on here but to be honest, it can be a bit like taking maths, physics and chemistry lessons all at the same time - and I was never any good at those subjects at school!

Can you please confirm if I have got this right regarding volts and amps:

1. Does a typical AA Alkaline battery (1.5 volts) gradually lose its voltage in use - in other words if it is connected to a bulb without a regulation circuit will its voltage eventually drop to almost zero (so therefore after 50% of its life be delivering 0.75 volts?) Is this the same for NiMh rechargeables and Lithium primaries, or do these types of battery hold their full voltage for longer and then suddenly 'die'?

2. Does the number of amps directly relate to runtime? i.e. if an appliance requires 500 mAh to drive it, will a 500 mAh battery power it for an hour and a 2000 mAh battery drive it for 4 hours? In other words if my flashlight is stated to run at 100 lumens for 1 hour with a 2000 mAh battery, will it deliver the same output for 1.5 hours with a 3000 mAh battery?

davecroft said:

Hi
I'm starting to understand more about flashlights thanks to this forum, but I still have a lot to learn about batteries. I have read some of the threads and graphs on here but to be honest, it can be a bit like taking maths, physics and chemistry lessons all at the same time - and I was never any good at those subjects at school!

Can you please confirm if I have got this right regarding volts and amps:

1. Does a typical AA Alkaline battery (1.5 volts) gradually lose its voltage in use - in other words if it is connected to a bulb without a regulation circuit will its voltage eventually drop to almost zero (so therefore after 50% of its life be delivering 0.75 volts?) Is this the same for NiMh rechargeables and Lithium primaries, or do these types of battery hold their full voltage for longer and then suddenly 'die'?

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Yes, a battery's voltage is a measure of how much energy (or charge) it contains. More charge, more volts. However, the graph of this is not a straight line. An alkaline battery is fully charged at about 1.55 volts, but is empty for most practical purposes below 0.9 volts. There is some remaining charge from 0.9 volts down to zero volts, but it is very little and not practically useful. You can look at a discharge graph on a battery data sheet to see how this works (for example: http://data.energizer.com/PDFs/E91.pdf).

With NiMH rechargables the discharge is similar, but with different voltages. A battery is fully charged at about 1.35 V and is empty for all practical purposes at 1.2 V. Lithium primaries are different again (see, for example: http://data.energizer.com/PDFs/l91.pdf)

2. Does the number of amps directly relate to runtime? i.e. if an appliance requires 500 mAh to drive it, will a 500 mAh battery power it for an hour and a 2000 mAh battery drive it for 4 hours? In other words if my flashlight is stated to run at 100 lumens for 1 hour with a 2000 mAh battery, will it deliver the same output for 1.5 hours with a 3000 mAh battery?

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In general, yes. However, batteries are less efficient at higher currents. So if you double the current you will get less than half the run time. Another way of saying this is that a 2000 mAh battery only has 2000 mAh at very low currents. If you increase the load, the available mAh will be less.

If you model the battery as a voltage source with a series resistance the voltage drops and the resistance rises as the battery discharges. That's why the battery testers use a load resistor, to pull some current from the battery during test.

For amps vs. runtime vs. capacity, see
http://www.csgnetwork.com/batterylifecalc.html

With Excel and discharge curves you can zero in on your own coefficients for this equation by trial and error.

Just a note to Mr. Happys explanation: The voltage for Alkaline and NiMH is measured when the cell is delivering current. Measuring without load will give a reading very close to maximum voltage, even on a nearly empty cell.

See also
http://www.google.com/search?client...w.,cf.osb&fp=fa8a22845779779&biw=1318&bih=667

davecroft said:

1. Does a typical AA Alkaline battery (1.5 volts) gradually lose its voltage in use - in other words if it is connected to a bulb without a regulation circuit will its voltage eventually drop to almost zero (so therefore after 50% of its life be delivering 0.75 volts?) Is this the same for NiMh rechargeables and Lithium primaries, or do these types of battery hold their full voltage for longer and then suddenly 'die'?

Click to expand...

The voltage of alkaline cells will decrease more proportionally as their capacity decreases than NiMH cells. Their effective capacity is relative to the specific device that they're being used in, its minimum operational voltage, and drain rate.

I don't use lithium primaries because they're not worth the price when compared with rechargeable LSD NiMH cells. The only reason I use Alkaline AA and AAA cells any more is because they come standard or installed in a lot of devices.

NiMH cells vary more in type than most alkaline cells. But generally they do hold their voltage under load better than alkaline cells and maintain a higher proportional voltage longer during their discharge, though alkaline cells will start at a higher voltage when full.

For an example of the voltage discharge curve of first generation Eneloop NiMH cells at a 170mA drain rate, see this graph I made: http://static.rcgroups.net/forums/attachments/2/7/0/8/7/5/a3143676-72-Eneloop T6EX Test Graph.jpg (Divide by 8 to get the per cell voltage levels.)

davecroft said:

2. Does the number of amps directly relate to runtime? i.e. if an appliance requires 500 mAh to drive it, will a 500 mAh battery power it for an hour and a 2000 mAh battery drive it for 4 hours? In other words if my flashlight is stated to run at 100 lumens for 1 hour with a 2000 mAh battery, will it deliver the same output for 1.5 hours with a 3000 mAh battery?

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Basically, yes, but don't confuse mAh which is a measurement of capacity (how many effective electrons worth of charged capacity the cell holds) with mA which is a measurement of current rate (how many electrons are flowing through a circuit per time).

One amp (ampere, A, 1000mA) equals about 6.241×1018​ electrons per second.

One amp-hour (Ah, 1000mAh) equals about 2.247×1022​ electrons worth of charged capacity. (While essentially true, I should probably point out that this is a little misleading in regards to how battery cells actually electrochemically work. While discharging or charging, cells aren't actually emptied or filled with electrons. Rather, they move from one part of the cell to another through the circuit or from one cell to the next in series in relation to the chemical changes within the cell or cells.)

davecroft said:

1. Does a typical AA Alkaline battery (1.5 volts) gradually lose its voltage in use - in other words if it is connected to a bulb without a regulation circuit will its voltage eventually drop to almost zero (so therefore after 50% of its life be delivering 0.75 volts?) Is this the same for NiMh rechargeables and Lithium primaries, or do these types of battery hold their full voltage for longer and then suddenly 'die'?

Click to expand...

depending on the "bulb" connected, most alkalines tend to quit around 1.0-1.1v but before that an incan bulb can start to get dim and rather orange colored on the light output. A bulb that draws more current (like 700ma) can cause an alkaline to not adequately drive the bulb at a higher voltage than a bulb drawing perhaps 300 or 500ma.

2. Does the number of amps directly relate to runtime? i.e. if an appliance requires 500 mAh to drive it, will a 500 mAh battery power it for an hour and a 2000 mAh battery drive it for 4 hours? In other words if my flashlight is stated to run at 100 lumens for 1 hour with a 2000 mAh battery, will it deliver the same output for 1.5 hours with a 3000 mAh battery?

Click to expand...

Amps is a part of ampere hours so yes the current draw in amps will effect the runtime in hours related to the capacity in ampere hours. Your calculations are correct but fail to take into account the battery itself as with high current loads the batteries internal resistance can affect the runtime and some larger Ahr batteries can have lower internal resistance but it is not a given. Some batteries can also hold a higher voltage under load which can help them run devices at a higher power level with less loss which affects runtime.

Wrend said:

ttp://static.rcgroups.net/forums/attachments/2/7/0/8/7/5/a3143676-72-Eneloop T6EX Test Graph.jpg

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For your battery, is this true?

20 >rated discharge time in hours

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2 >rated capacity at that rate in A-h

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If you mean 20 hours as a standard discharge time to test a capacity rating, then I suppose, but am not sure that the Eneloop cells are rated this way.

0.17 >actual discharge current in amps

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Yes, but this was based off the specifications of the radio transmitter which has a listed drain rate of 170mA. I did not actually test for the value. The purpose of the test was to accurately estimate the expected run-time of those cells in that transmitter. I posted it here to illustrate the voltage discharge curve of some NiMH cells.

10 >actual battery discharge time in hours

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The radio stayed operational for 12 hours and 31 minutes down to the low voltage alarm at about 8.4V (1.05V per cell) and then lasted for an additional 7 minutes until the minimum operational voltage of the transmitter was reached. The graph only lists the voltage level every 15 minutes, and the test did not last until 12 hours and 45 minutes.

Assuming the drain rate of 170mA as a give, the cells had an effective usable capacity of about 2128mAh to when the low voltage alarm sounded at 1.05V per cell in that transmitter. A little more if you include the time until the transmitter "died."

Keep in mind that this is under a 170mA (or so) "load," so the voltage is going to sag a little below the resting voltage levels.

Wrend said:

The radio stayed operational for 12 hours and 31 minutes

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OK, so the Peukert number for this battery is 0.89 and using this equation you can figure the runtime at other discharge levels [at least in principle].
sorry about the formatting

Well there you go! I think we just moved from physics / chemistry to advanced mathematics in 10 posts!
But seriously thanks for the advice. I'm learning - slowly.

davecroft said:

Well there you go! I think we just moved from physics / chemistry to advanced mathematics in 10 posts!
But seriously thanks for the advice. I'm learning - slowly.

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Dave, it is inevitable in this forum. Follow Mr. Happy's post which was direct, and succinct.

If you want more of a foundation, you can take it in bites reading the incredible resource at Battery University here. It is very readable, and if you were to even read 1/4th of what's there, you would most likely know more about batteries than 98% of the members of this forum.

Yeah, you probably won't ever need to know how many electrons are involved. (I only know from looking it up.) I just used the values to give you an idea of what was physically represented.

For a basic understanding of electricity, the water analogy works really well. I'll resist boring you with some of the details since you can just google "electricity water analogy" to find more info on it, if you want.

What "xul" was basically talking about is that the usable capacity of batteries can vary depending on their discharge rate, and that that variation can be roughly accounted for to help determine how much usable capacity a battery has at a certain discharge rate. For even rougher estimates, people generally just divide the capacity by the discharge rate to figure out the run time (usually measured in hours). That's easier to get away with with some kinds of batteries than others.