Ideal Battery Hardware For Project

jjrr007 used Ask the Experts™
I am developing a remote module (not wired to a computer) that has an Arduino Uno board, 5 volt hall effect sensor, xbee radio and data logger connected together.   On this remote module,the data from the sensor will be recorded to my data logger every 15 or 30 minutes.  The xbee radio though would need to be available the vast majority of the times.

For the xbee, I will configure the sleep mode the best I can to preserve battery.  Ideally, I would like this to last a couple of years (as long as possible ) without changing/charging the battery. For now, I am purchasing the hardware.

I wanted to ask what is the best battery I can purchase for this (i.e lithium, how many volts etc)?     For this much time, I am thinking that I should use a few batteries and have something autotmatically switch or share power among batteries.  Which switching regulator and battery would be good for this project? If need be, I can purchase a voltage regulator if it's needed.
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corvetteguruDBA/Systems Analyist

would this unit be outside? Have you considered a solar recharger?

Lithium would be a great start, but I would go as low in voltage as you can, because the more voltage, the faster the charge will drop.

Take a look at Radio Control car batteries, they are LiPo and are rather immune to abuse.

There are tons of options and you can then test the best one for your needs.

If anything, a couple of those in parallel might just do the trick... You will have to test your discharge rates to get an idea of the mAh you will need. But some of these RC batteries are now into the 8000 mAh range, ahead of many laptop batteries and easier to deal with.

Another option is a laptop battery booster... these pads or chunks of batteries that are designed to plug into the power port and drive the laptop when there isn't a power source. These things are pretty intense. I know Targus and Belkin made some, and another company, Electro-something made them... The last company had good batteries, but sketch customer service. The only downside, they might be too large.
dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.

How big will this completed project be?  if there is plenty of room then a battery is no problem but I suspect you have a space limitation.
corvetteguruDBA/Systems Analyist
that's why i suggested RC car batteries... they give a lot of options for installation.

Check these out:

The one above is 3.7v, 4800 mAh... and look how easy it would be to string a couple of those together and double the mAh! Oh, the rated voltage is 3.7, but it's over 4 if you put a DVM on it.

A couple of these would rock it... 10kmAh and 7.2v... They are about 6" long and 2" wide and 1" high, more or less:

Those are examples... I have used RC car batteries for other projects and things. Used to use a 6 cell sub-C NiCd pack to run my transmitter with. Use a coiled cable from the transmitter to the battery, slip the battery into your pocket and all is grand. Do you know how long an RC transmitter will last with one of those packs attached? MONTHS before recharging.

I have also used them on camera flashes, but the modern flash units are a little more sophisticated and plugging external power into them isn't as easy as it used to be.
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corvetteguruDBA/Systems Analyist

I am doing some rough math... the board looks to draw about 40mW @ 7v ? the radio, another 50mW @3.3v... let's be safe to assume that the unit, at full load, is drawing 200 mW.

7.2v is more than enough it appears.

I am ending up with .02 amps for the radio, and .005 amps for the board... two of those 7.2v packs = 11AH.

I am just adding the two amps together and getting:

11/.0025 = 440 hours of usage. Or, 18 straight days.

If this goes off for 1 minute at a time, it would have 26,400 hits.

Assuming that this happens every 30 minutes, you would get 48 hits a day, it would last 550 days. That's not quite 2 years.

My math is sloppy cause there are variables I don't know... so some are swags... but I don't think I am really all that far off.
Not really much point in going with a rechargeable if you want it to run that long.  It just end's up costing an awful lot more up front and after a 2-3 years the rechargeables will be getting stale and will no longer take a full charge.

A primary (non-rechargeable) lithium battery offers more power and in a more compact form if size matters.  There are actually quite a few lithium variants.  Saft have a line of Li-SoCl2 batteries that are very well suited for this type of long term application in any environment.  The temperature range is pretty wide,  -60 to +85 C.

The Saft LSH20 is a good example.  It is a D size cell with 13 Ah at 3.6 vdc at a cost of roughly $30-40 each.  More than double the power of a LiPo for 1/3 of the cost.

Here's the data sheet on it,

Here's a quickly googled price,

Probably have to buy from a place that specializes in batteries,  shop around battery prices vary.  The LSH20's actually come in some outdoor sensors we use,  the sensors will run up to 10 years on one set.

The real problem you face is not the battery,  you can make up any size bank as needed.  The problem is the power draw of the Arduino Uno.  Apparently even in sleep mode the built in power regulator draws 10 mA.  It does not sound like much but at 24 hours a day adds up fast - to the tune of 87 Ah a year,  and we are not even including the active time.

This guy goes into the details of the problem as well as his work around,

You have to watch power regulators because they are not 100% efficient and can draw power when connected even if there is no load connected.  This is also why you need to test your setup to get an accurate figure.  I would think you are powering the XBee and the 5 V sensor off the board and you are going to have some loss with the onboard regulation.  It might not be that much but you need to know exactly how much power draw you unit has each time it goes through a operation cycle.

The work around in the article is to shut down power to the arduino externally then use the sleep mode on the external power controller which is built to draw a very low power while sleeping.  By getting the sleep power requirement way down then it will be more about the power for booting the arduino up,  taking your measurements,  and running the radio.

Corvetteguru your numbers are off because you have used mW when those ratings are actually in mA.  The actual power comes out much higher.  So 50 mA @ 3.3 v = 0.165 W,  the Arduino can vary depending the the clock speed used (and some other tweaks) but it seems the 'stock' figure is around 40-45 mA,  so 40 mA @ 7 v = 0.28 W.  This brings the total power to 0.445 W which at 7.2 v comes out to 0.0618 A.


sorry for the tardy reply. Thanks

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