[TriEmbed] TriEmbed Digest, Vol 16, Issue 18 - Power regulator questions

Thu Sep 18 14:13:35 CDT 2014

Charles,

As you said in a previous email, "As with all things electronic, there is a
trade off."

How much is it going to add to your BOM cost is the bottom line. Once the
part is on board, measuring and transmitting the data is virtually free,
and for your prototypes, can provide useful information. Bench testing
batteries and extrapolating their lifetimes gets you close to an answer,
but ambient conditions on your bench are a far cry from real world. Getting
the metadata and watching it graph would be interesting to ME. But now we
start down a slippery slope again - how does temperature affect that
battery life? Let's add a temp sensor to your system... (Do I remember that
there is a temperature sensor in the 328? Google says yes:
http://playground.arduino.cc/Main/InternalTemperatureSensor ). So assuming
that the 328 is sleeping most of the time, the die should be pretty close
to the ambient temp inside your enclosure. Temperature might very well
affect trail usage - so reporting that regularly might be useful for the
primary function as well.

Collecting a graph of battery voltage and temp vs time may be of value for
other projects that you come up with in the future. It may not be of much
interest to Parks & Rec - except to know when a sensor may need
maintenance. It might be nice if a sensor went unexpectedly off line to be
able to look at the last reported voltage. If it is close to nominal,
chances are something else happened. Close to the built in cut off voltage
of the battery would lead me to suspect things are still intact, but have
no juice.

Personally, I think I would add a spot for the component on the board, with
a jumper setting or code support to know if it is populated. Early deployed
units would be populated and the data watched. If I discovered that the
data was boring, future units would lack the sensor.

Glen

On Thu, Sep 18, 2014 at 2:27 PM, Charles McClelland <chip at mcclellands.org>
wrote:

> Rodney,
>
> Agreed, it is overkill for my current non-connected sensor for exactly the
> reasons you state below.  I should have mentioned that this is for the GPRS
> based connected sensor which has much higher and more variable power
> demands.
>
> I was reading this article
> <http://e2e.ti.com/blogs_/b/fullycharged/archive/2014/08/12/where-39-s-the-gauge-part-1.aspx?DCMP=fullyc&HQS=pwr-bms-gmap-wheresmygaugept1-fullyc-20140812-myti-en> from
> the TI power folks who - unsurprisingly - thought that fuel gauges should
> be included in all battery designs.  I started thinking about how emotional
> this engineer was getting with words like “baffled”, “excited” and
> “flustered”.  I figured there must be something here.  Full disclosure - TI
> also sent me a sample of their BQ27410-G1 Gas Gauge chip
> <http://www.ti.com/product/bq27410-g1>
>
> I am planning on putting a connected sensor in the local Ulmstead park to
> measure vehicle traffic.  I was thinking about the “metadata” I could send
> with each transmission and I realized that including the battery voltage
> might be a good thing.  I could watch the battery levels and know when I
> needed to swing by the park to change the battery.  I am using the Sparkfun
> one you pointed out below so, it is not about under voltage but about
> remote monitoring.  Over time, I will probably get a sense of how long a
> battery lasts so perhaps this is just for the learning phase.  According to
> the RC blogs, cold weather can affect LiPO performance so, the battery life
> could change significantly as we go from Fall to Winter.
>
> Does this make sense or am I making this more complicated than it needs to
> be?
>
> Thanks,
>
> Chip
>
> On Sep 18, 2014, at 2:09 PM, Rodney Radford <ncgadgetry at gmail.com> wrote:
>
> The subject of Lipo batteries and battery fuel guage were discussed at a
> previous TriEmbed meeting.  For your application, I think it is overkill as
> what you are doing is constant - wake up, take a measurement, log it, and
> go back to sleep - so you should be able to determine the length of time a
> charge will last without it.
>
> The issue with Lipos is that you should not overuse them so they go
> undervoltage, but why not go with one with builtin overcharge, overcurrent
> and undervoltage circuitry built in, like this one from Sparfun:
>
> https://www.sparkfun.com/products/341
>
> They have a variety of sizes from from 40mah to several thousand mah, so
> you could first test your circuit with the smaller unit, determine how long
> it will last, and be able to better extrapolate out how big of a battery
> you would need to last for the amount of time you would need between
> charges.
>
>
> On Thu, Sep 18, 2014 at 1:11 PM, Charles McClelland <chip at mcclellands.org>
> wrote:
>
>> Michael,
>>
>> Thanks for taking a look at the power supply thread,  as my projects run
>> on batteries, I have put some time into this.
>>
>> Here are a few thoughts:
>> - Like you, my circuits draw a relatively small amount of current.  I
>> developed these tests at a higher current so they would not take a month to
>> run
>> - your current regulator (MCP1700-3302E
>> <http://ww1.microchip.com/downloads/en/DeviceDoc/20001826C.pdf>) is
>> Linear so it will dissipate - read waste - power equal to the product of
>> the difference between Vin and Vout time current (Eqn 6-1 in data sheet)
>> - This is where I started but I decided to move to a switched power
>> supply for three reasons:
>> - They are battery omnivores - since the TPS63031
>> <http://www.ti.com/lit/ds/symlink/tps63030.pdf> is both boost and buck
>> and is over 90% efficient over a large range (1.8-5.5Vin) - I can throw all
>> sorts of batteries at it
>> - As they are more efficient, they should throw off less heat -
>> potentially a big deal since I put them in a box and bury them
>> - This supply can handle from very small 25uA to 800mA currents
>>
>> As with all things in Engineering, there are some trade-offs I had to
>> accept:
>> - more expensive
>> - QFN packaging - ugh
>> - weird and exotic new component required - an inductor - no really!
>>  There are a bewildering number of choices here!
>>
>> Some suggestions:
>> - I would be happy to loan you my test rig which you could use to test
>> your current power supply and compare it to the three I made.
>> - I added the Excel spreadsheet with the full set of tests and results to
>> the posting on the Triembed.org <http://triembed.org/blog/?page_id=234>
>>  site
>> - I could give you a TPS63031 breakout board so you could test the supply
>> in your circuit without the pain of soldering - I have some extras
>> - You could look at commercial options such as the little breakout boards
>> from Pololu - I have one of these you could borrow
>> <http://www.pololu.com/product/2122> if you like
>>
>> Hope this helps.
>>
>> BTW, my next step in my power supply obsession is to add a battery fuel
>> gauge to the circuit so my Arduino can check it and so I could use LiPO
>> batteries with more confidence.
>>
>>
>> Chip
>>
>>
>>
>> On Sep 16, 2014, at 5:00 PM, triembed-request at triembed.org wrote:
>>
>> MCP1700-3302E
>>
>>
>>
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>
>
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