Maximum AA battery current draw?

[nsayer]

I've designed a "reimagining" of the Minty Boost with SMD components. In a nutshell, the design is an NCP1450 controller, an SI3442CDV MOSFET, a 2A rated 10 µH inductor, a B330A Schottky diode and 220 µF OSCON input and output filter caps.

I've also added the appropriate Apple style USB data line voltage dividers to signal a 1A ampacity. That's where it gets dicey.

I set it up with 3 AA in series and plugged in an iPad. And it worked. The parts got warm, but I've seen what happens to those parts when they get too warm (things like the markings on the top burning off) and that's didn't happen.

What IS happening, however, is that the batteries get quite warm.

I've tried both alkaline and lithium (primary) cells. Both get really, really warm. Not "OUCH!" warm but, like, heating pad warm.

The really interesting part is that as the batteries got hotter, the parts on the board actually got cooler. Eventually the iPad stopped making forward charging progress. It never gave an indication that it stopped charging, but it got about a 10% charge and just sort of stopped doing any better (this was with the lithium cells).

First, is the slowing of the charge an indication of voltage sag? I should have had it instrumented, but I didn't. But I would guess that to produce 1A of output current at 5V, it'd need to draw at least an amp and a half at 4.5V. I know that you're going to sacrifice the batteries' energy capacity somewhat when you draw high current like that, but one of the design goals is the ability to charge an iPad - even if it's only a *little*.

[mariush]

An AA battery can do more than 1A but they'll discharge fast, in about half an hour. They're also not really designed for such continuous use at high current - for applications like flash cameras, with short duration pulses, they work fine.

See http://batteryshowdown.com/results-hi.html  for various batteries and discharge graphs, tested with a 1A load.

There's also available datasheets for batteries, here's for example an Energizer AA alkaline battery : http://data.energizer.com/PDFs/E91.pdf  and here's a Varta alkaline battery :  http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1500_US_CT.pdf

You can see in the Energizer datasheet.. Have a look on first page , continuous discharge to 0.8v... you can see that the 2500mAh becomes a 1500 mAh one, at something as low as  that the capacity goes down to 1500 mAh at a load as low as 500mA.
You also have a constant power performance curve on second page ... you're getting about 2 hours at 700mW ... 0.7w / 1.5v = 470mA

Try simulating the circuit in Spice, I have a feeling you're getting more than 1A pulses from batteries. May also help a bit to use a larger input capacitor. Inductor is also quite important, what other specs does it have?

[David Hess]

The Energizer specifications for AA cells only rate them to the range of 0.5 to 1.0 amps maximum and the discharge voltage drop may limit the output to only 0.8 volts so I think that answers your question.  The voltage sag of the alkaline cells is limiting the power your regulator can provide.

Some NiCd and NiMH cells would perform much better despite their lower starting voltage.

[macboy]

The internal resistance of alkaline and primary lithium batteries is too high for that kind of discharge rate. Power dissipated in the cell will be current^2 x internal resistance, and this heats up the cells. There will also be significant voltage sag of I x R. The capacity (mAH) is also greatly diminished at high discharge rates.

You will do much better to use NiMH (or even NiCd) cells, which have a very small internal resistance. They will have no trouble at 1.5 A, or even at 10+ A for that matter. Their capacity at high discharge rates is also quite good.

[HKJ]

Here is some AA discharge curves:


Alkaline are really bad at high current.

You can find many more on my website: http://www.lygte-info.dk/info/indexBatteriesAndChargers%20UK.html

[David Hess]

Alkaline are really bad at high current.

I would not say that.  They are great compared to the batteries they replaced.

Spiral or stacked cells like those used in NiCd, NiMH, and some lithium cells are just so much better.

[nsayer]

Thanks everyone. I was kind of afraid of that.

Adding a LiPoly charger, battery and a power input jack isn't hard, but I really wanted something able to use primary cells. The big objection to secondary cells is that if they're dead, you're just as stuck as when the battery in your actual device is dead. But if primary cells can't put up with that kind of draw, then it's sort of pointless.

That, and you can get a rechargeable USB power pack for about $5 from the local drugstore.

[HKJ]

Alkaline are really bad at high current.

They are great compared to the batteries they replaced.

That is correct, the Varta Superlife is a Zinc-carbon cell and compared to that the alkaline looks very good.

[Artlav]

But if primary cells can't put up with that kind of draw, then it's sort of pointless.

There is an option to make it work - use more cells and a buck converter.
6 AA cells in series can easily provide 300mA or so it takes to step them down to USB's 5V at 0.5A.
Even 8 AA cells is still a pretty compact solution, and the more the better.

[nsayer]

A buck converter is a thought, but it wouldn't let you use all of the battery before the output voltage began to sag. A SEPIC would be better, but now it's starting to get complicated.

[IanB]

Adding a LiPoly charger, battery and a power input jack isn't hard, but I really wanted something able to use primary cells. The big objection to secondary cells is that if they're dead, you're just as stuck as when the battery in your actual device is dead. But if primary cells can't put up with that kind of draw, then it's sort of pointless.

You're just as stuck if your primary cells are dead when you come to use them. If you use a quality NiMH cell like the Eneloop there is no reason for these to be dead when you need them. If course you do need to keep a stock of charged cells on hand against emergencies.

[nsayer]

Adding a LiPoly charger, battery and a power input jack isn't hard, but I really wanted something able to use primary cells. The big objection to secondary cells is that if they're dead, you're just as stuck as when the battery in your actual device is dead. But if primary cells can't put up with that kind of draw, then it's sort of pointless.

You're just as stuck if your primary cells are dead when you come to use them.

Not at all. You can buy more at any store.

[Artlav]

A buck converter is a thought, but it wouldn't let you use all of the battery before the output voltage began to sag.

Huh?
0.8V * 8 = 6.4V.
Still enough all the way to the cut-off, no?

[nsayer]

A buck converter is a thought, but it wouldn't let you use all of the battery before the output voltage began to sag.

Huh?
0.8V * 8 = 6.4V.
Still enough all the way to the cut-off, no?

Oh, well, with 8, sure, but .8v * 6 = 4.8v

[mariush]

The USB standard allows 4.75-5.25v, so he could just configure the output at 4.8v (to allow for voltage drop on the small cable) and be done with it.

Besides, by the time your battery is down to 0.9v or so, it will probably no longer be able to provide enough current to keep the buck regulator happy or the device charging may stop charging due to low current.

[David Hess]

Linear Technology used to have ads about how to "suck a battery dry" which compared the total energy available with different configurations of switching or linear regulator and 9 volt or AA cells.  Unfortunately they were on the cusp being being pre-internet so I cannot provide a link.

[nsayer]

FWIW I'm going to make a companion board for this with an MCP73831 LiPoly battery charger and a micro-USB receptacle. The two together, plus whatever 3.7/4.2v LiPoly battery you wish to procure can be fashioned into a rechargeable power pack.

I'm also going to make a board with the two circuits together shaped to fit into an Altoids tin. I believe I can make around a 9.25 W-hr (~1500 mA-h @ 5V) LiPoly pack in that form factor.

[Fraser]

For info only.... I just purchased a Sony 10W Halogen video light that runs from four AA size Ni-Mh cells. The instructions warn that the batteries will get "hot" and must be allowed to cool before removal is attempted 

Sony state that the unit must be fitted with >=2000mAh Ni-Mh cells only and the heating of the cells is normal. A low voltage cut-off circuit prevents over discharge. Seems like abuse of the cells to me but hey, Sony normally know what they are doing ? Sony kindly include a cell over-temperature monitor that illuminates an "overheat" LED if things get too hot in the battery compartment !

10W/4.8V = 2.08A continuous current draw with the video light on.

I have seen many equipment's that also state that they are unsuitable for Alkaline or Zinc Carbon batteries, even in emergencies. They are normally high current requirement devices

[IanB]

10W/4.8V = 2.08A continuous current draw with the video light on.

2-3 A is like a walk in the park for a quality cell like the Eneloop. But yes, the cells will get warm at that load. Internal resistance may be low, but it is not zero...

(E.g. 2.5 A through 100 mOhm would be 0.6 W per cell.)

(And that's not even considering the 10 W output of the very hot halogen lamp sitting right next to the battery compartment...)

[vk3yedotcom]

Was thinking about this today when leafing through the Jaycar catalogue.

http://www.jaycar.com.au/productView.asp?ID=AA0487&CATID=85&form=CAT2&SUBCATID=1009#4

The above amplifier claims 2 x 15w RMS and claims it can run off 8 x AA batteries. 

It reminds me of those computer speakers that claimed 400 watts output with a tiny plugpack for power.

Did someone get the decimal point in the wrong place, or maybe the power output is switched down when on the AAs? 

[SeanB]

Note the 15W RMS is likely at 20% distortion.15W is doable on 12V though battery life is going to be measured in minutes there, and you will definitely have issues with running it long time on batteries. Class D output stage will not complain though, though the output filters likely will have been skimped on and will probably overheat. At full power it likely will be drawing 5A plus from the batteries, and it is a toss up as to what will dissipate the heat, the cells themselves or the output filter inductors. Of course they likely also used the cheapest OHL capacitors in the power stage filtering so the ripple will be a lot higher after a year of operation.

[mikerj]

Note the 15W RMS is likely at 20% distortion.15W is doable on 12V though battery life is going to be measured in minutes there, and you will definitely have issues with running it long time on batteries.

15W*2 RMS at 12v is 2.5 Amps.  You won't be delivering that level of power continuously with music or voice however so battery life should be reasonable from decent AA NiMh cells.

[Fraser]

But your equation does not consider the efficiency of the amplifier as that will draw more than 30W RMS to provide the stated output.

[Conrad Hoffman]

One other thing to consider is that when you fully discharge an alkaline, it will quickly leak and corrode everything around it. Always remove discharged alkalines as soon as possible.

[IanB]

One other thing to consider is that when you fully discharge an alkaline, it will quickly leak and corrode everything around it. Always remove discharged alkalines as soon as possible.

There, fixed that for you