# Battery life question.

Hi,

I'm currently working on trying to heat two heat pads. The working spec of them is:

Operating Voltage: 5V DC
Operating Current: ~600mA (~8.3Ω)

They are joined in parallel. I'm working off this example :http://etextile-summercamp.org/2013/wp-content/uploads/2013/06/heatcontrolling_breadboard_example2-1024x682.jpg

I understand that parallel resistors combine according to 1/R = 1/R1 + 1/R2

So in theory, 1/R= 1/8.3 +1/8.3, therefor 1/R= 0.12 + 0.12= .24

Therefore 1/.24= 4.16ohms

I'm using 3 x 9v batteries in parallel, so the voltage of the batteries in parallel is 9v, (the capacity is increased by putting them in parallel)

Then using ohms law to calculate: i=v/r,
I=9v/4.16= 2.16amps, which would give 1.08amps per heat pad.

I'm just wondering how long I would get out of the 3 x 9v batteries in parallel? I don't know what the mah rating of the batteries is. The battery type is a kodak 9v xtralife alkaline battery

Any help would be greatly appreciated

Thanks

Liam

Well, you need to know the mAh rating of the batteries is.

Look it up... or, in case you havn't heard of Google, this page: Kodak 9V Xtralife Batteries and click on Technical Information

Oh, and the answer is: not long.

Yours,
TonyWilk

P.S. and take note of "Internal Impedance"

Small 9V batteries are good for about 50mA, they are designed for low current equipment.
Some will handle a bit more, especially rechargables, but 600mA is cloud-cuckoo-land for
a small 9V battery, let alone 1.2A.

Heaters take lots of power, batteries are generally not compatible with heating.

What are you trying to do, this could be xyproblem territory.

I checked the spec of the batteries and it say 460 mAh at 620 Ohms to 4.8 Volts.

I'm only looking to heat it for a short period of time. Enough time to test the prototype that I'm making. Say roughly about 15-30 mins

I checked the spec of the batteries and it say 460 mAh at 620 Ohms to 4.8 Volts.

I'm only looking to heat it for a short period of time. Enough time to test the prototype that I'm making. Say roughly about 15-30 mins

Ok, good.

The datasheet also says: Internal Impedance: 2.87 Ohms (typically)
This means it's like having a 9v battery with a 2.87 ohm resistor in series with it. You are right that the two pads in parallel should be 4.15ohms, but you have to add 2.87 = 7ohms so you would only expect 9v/7ohm = 1.28 amps. With 3 batteries you are getting better because you are sort of paralleling up the Internal Impedances.

With 3 batteries you might expect 2.87 * 3 in parallel = 0.95ohm + 4.15 ohm giving a current of 1.76amps or 0.88A per pad. Drawing 0.58A per battery

However, these batteries are only small and there is a maximum rate that the chemicals inside can generate electricity.

As an example I tried a 9v battery with a 10ohm load and measured:

Not connected to load: 9.2v
Immediately on connection@ 7.7v
After 30 seconds: 7.5

With the spec. saying they have a capacity of 460mAh, and the voltage dropping all the time I would expect them still to be providing reasonable current after half an hour.

Yours,
TonyWilk

Also, why are you running your heat pads at twice their spec?

I checked the spec of the batteries and it say 460 mAh at 620 Ohms to 4.8 Volts.

Yes, that means the battery capacity is 0.46Ah (this is a unit of charge, not current), if
the battery is discharged through a 620 ohm resistor until its voltage drops to 4.8V.

That's a discharge current of about 15mA maximum.

The internal impedance is not the cell internal resistance under heavy load, its the differential
impedance on modest load changes, you cannot infer the heavy current discharge curve from it!

Under heavy discharge you have to contend with depolarization, over heating, capacity loss and
probably other factors. Note the product description "Recommended for long term, small power uses"

Here’s a datasheet for a similar battery that includes a run-time versus discharge rate curve. The chart stops at 100 mA and you are asking for 200 mA, but extrapolating suggests a run time on the order of an hour, though it’s hard to say because linear extrapolation isn’t going to be good as the blue curve suggests. Getting 15-30 minutes doesn’t seem to be out of the question, but you’re pushing beyond the design specs of the battery.

Edit: Re-reading the first post, I see that you have two heating elements, so 1.2 A total and 400 mA per battery, rather than 200 as above. It’s really unlikely 9V alkaline batteries will do that for 30 minutes if they can do it at all. I’d consider using a different battery technology.

El CAron, I'm running them at twice the spec as I need them to reach a certain temperature. I'm using thermochromic ink and it reacts at a certain temperature

If you want lots of power from a small battery, the choice is LiPo, LiPo or LiPo...