# Figuring out battery life with mA and mAh?

Discussion in 'General Electronics Chat' started by TheLaw, Sep 21, 2010.

1. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
Hello,

I'm working on a project that I am hoping to use rechargeable batteries with. However, I'm trying to figure out the projected battery life of the device based on two figures but I'm unsure as to how to do it.

The device pulls roughly 20mA and the battery is rated for 150mAH. Is there a formula for this? Or does anyone want to simply give me an answer?

Thanks a lot.

2. ### Ghar Active Member

Mar 8, 2010
655
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You should pull up the datasheet and look for a discharge curve.

You might be able to say 150mAh as 150mA for one hour, or 15mA for 10 hours, or 20mA for 7.5 hours but it usually isn't very accurate.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Battery constant current discharge time

Tdischarge = Amp-Hour capacity/ Constant Discharge Current [in A]

Tdischarge = (150 *10^-3)/(20*10^-3)=7.5 Hours

Can you now write the relationship in terms of mAH capacity and constant discharge current in mA?

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
Amp hour ratings are "usually" based on an 8 hour discharge time.

Also, battery capacity is less for higher rates of discharge, and more for lower rates of discharge.

If you discharge 10 mA an hour for 8 hours then an 80mAH battery would be out of juice

If you discharge 20 mA an hour for 4 hours then an 80mAH battery would run out of juice before the four hours was up.

If you discharge 5 mA an hour for 16 hours then an 80mAH battery would still have some juice left.

5. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
Well, I found a 175mAH version of the 9V battery....so it will be 175mAH...

Datasheet is located here:

http://data.energizer.com/PDFs/nh22-175.pdf

Datasheets are very helpful! Going by the charts here, I should be able to get something like 9-10 hours of battery life when it is running at maximum power draw. (This is a headphone amp). So I can probably expect around 11-12 hours typical run time. Quite a bit better than I expected. Voltage is not really an issue. I am going to be running dual 9V in series
so even if they drop to 4V/ea, 8V should still be enough to run the amp effectively.

Now to make a charger circuit for dual 9V....Hmmm.....

Thanks for the pointer to the datasheet. I have a new profound love for datasheets.

6. ### 311499 New Member

Apr 29, 2010
5
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Rechargeable 9V batteries must not be discharged down to 4V. The Energizer datasheet indicates a nominal voltage of 8.4V which means that each battery has seven 1.2V Ni-MH cells in it, connected in series. If the 9V (more correctly, 8.4V) battery is discharged to less than about 7V, one or more cells will get their polarity reversed, i.e., the cell voltage will be negative. This would cause permanent damage to the cells and reduce their performance. It's highly recommended to add a circuit to prevent the battery voltage from falling to less than about 7 volts.

If the amplifier is used with normal low-impedance headphones, I'd rather connect the 9V batteries in parallel (except if a step-down output transformer is used) since parallel-connected batteries can supply more current which is needed when the amplifier drives a low-impedance load.

Last edited: Sep 22, 2010
7. ### Audioguru Expert

Dec 20, 2007
10,741
1,202
Energizer changed their label recently. My Energizer 175mAh "8.4V" Ni-MH battery says, "Made in Germany for .....". Maybe the new ones are made in Japan like their Ni-MH AA and AAA cells.

8. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
Well regardless, I would still get pretty close to what I said for battery life (if in series), and more if I wired them in parallel.

But regarding reverse polarity which I forgot about...do normal Alkaline batteries suffer from reverse polarity? It's a cMoy and as far as I am concerned, I don't think I've heard of any of the circuits having protection.

NiMH thing? Either way, what is something simple that will shut down the circuit or prevent it in someway? IC of a sort or something very simple (and small).

Thanks alot. And I guess I will use Parallel seeing that the AD8620 that I hope to be using can run off 7V effectively.

Thanks.

9. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
I know Japanese batteries tend to be better but does it really matter?

10. ### Audioguru Expert

Dec 20, 2007
10,741
1,202
I guess you did not look at the datasheet for the AD8620 opamp.
It is spec'd with a minimum supply of 10V and its supply current drops like a rock below 7V (when it probably distorts like mad).
Its max spec'd load resistance is 1k so it has difficulty driving lower impedance headphones.

11. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
Well, I did look at it. I thought I saw it with minimum voltages of like 5V. I didn't look at any of the graphs though.

Op-amp aside, (I can figure that out), does anyone want to fill me in on how the reverse polarity thing works and why I haven't seen anything about it for cMoy or any other pocket amp?

Thanks.

12. ### Audioguru Expert

Dec 20, 2007
10,741
1,202
Look again. Plus and minus 5V is a total of 10V, not 5V.

An alkaline battery is not rechargeable so if one cell has reversed voltage when the battery is almost dead then it doesn't matter. It gets replaced.

But a rechargeable battery with more than one cell (an "8.4V" Ni-MH battery has 7 cells) is in trouble if you discharge its voltage too low (less than 7V). The weakest cell drops to 0V first then its voltage reverses as the remaining stronger cells continue to discharge. A cell with reversed voltage will fail soon.

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13. ### TheLaw Thread Starter Member

Sep 2, 2010
228
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I gotchu. Yeah I didn't really read the datasheet over. I just briefly looked at it. I'm still debating between an OPA2134 and an DA8620.

Thanks. That makes sense....

I've tried googling it for the past 30 mins to no avail. Would you mind giving me some pointers as to what kind of circuit can shut off the circuit when it approaches an unsafe voltage? An IC of some sort I suppose?

Thanks Audioguru and all others.

14. ### Audioguru Expert

Dec 20, 2007
10,741
1,202
An OPA2134 has extremely low distortion, is spec'd to drive a load resistance as low as 600 ohms and is spec'd to operate perfectly with a supply as low as plus and minus 2.5V. I would use it to drive headphones if they are not 8 ohm ones.

15. ### SgtWookie Expert

Jul 17, 2007
22,198
1,797
Just a reminder: common headphones for some Apple products are rated for 32 Ohms nowadays; they would also likely be too low in impedance to drive via that opamp.

However, perhaps he could use an impedance matching transformer to drive the headphones. Or simply use an amp that was rated for driving such low-impedance headsets.

Last edited: Sep 23, 2010
16. ### TheLaw Thread Starter Member

Sep 2, 2010
228
2
My Sennheiser CX300s are 16ohms if that means anything...

Might I ask a few questions:

1.) How much wattage can/does Cmoy put out?

2.) Can someone please refer to me to a circuit for rechargeable battery polarity protection?

3.) Do Op-Amps need heatsinks?

Thanks a lot.

17. ### Wendy Moderator

Mar 24, 2008
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2,953
1. I assume you mean CMOS, usually not much. It varies between devices, so only the spec sheet can say for sure.

2. Generally rechargeable have their own slots in chargers. This provides protection. If you put them in series you're on your own.

3. Generally no, but if they are providing enough wattage this might not be the case. Stereos and other high power audio equipment use op amps that have tabs for heat sinks, and I have seen op amps that require it (they make good power supply regulators too). Wookie has pointed me to a few over time.

A more general approach is to put the power handing components (that will require heatsinking) outside the op amp, it is usually cheaper and easier to do.

You probably notice I'm talking in generalizations. There are many ways to do a job in electronics, everyone has their own preferences. Without specifics, such as circuits or part numbers, it has to be vague.

18. ### Audioguru Expert

Dec 20, 2007
10,741
1,202
The Cmoy is a portable stereo amplifier designed to drive headphones. It uses a dual opamp.
Look in Google for the project that says nothing about the wide range of impedance for headphones.

Since headphones are directly against your ears then their power is very low.