Author Topic: Outdoor sensor node (Relatively low cost)  (Read 9234 times)

kobuki

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Re: Outdoor sensor node (Relatively low cost)
« Reply #15 on: December 15, 2014, 09:34:13 AM »
The Lithium batteries we're discussing are 1.5V. You can directly power a moteino using 3 of them in series, or employ tricks to be able to use 2  cells at 3V or use a boost converter. I think the simplest would be 3xAA or 3xAAA, depending on the available room in your project box. That would provide 4.5V nominally.

TomWS

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Re: Outdoor sensor node (Relatively low cost)
« Reply #16 on: December 15, 2014, 11:32:07 AM »
Reading the datasheet of a well known brand and emploiyng a 10% safety margin on the textbook value, I'd rather say it's 3 Ah than 3.3 Ah. It's debatable though. What is your real life experience with them?
My real life experience is running PIC-based remote wireless soil moisture sensors in a harsh Texas environment (extreme hot and cold) for several years and getting up to 4 years of operation without a battery change.  As you say, it's hard to tell the difference between +/-10% when the average drain is so microscopically small.
OTOH, I admit that multiplying the Ah capacity by the number of batteries is a little over-simplifying things,
uh, with all due respect, I wouldn't say that is 'over-simplifying', it is incorrect.  Wh is multiplied, yes, Ah is not.  My comment wasn't intended as a dig and I'm sure you understand the relationship, but I just wanted to make sure those who are learning to learn the correct things.

I explicitly mentioned the usability below freezing temperatures in the post you answered to. Please also see the DS I linked above.
Yes, I missed the 'with the same chemistry' reference.  Sorry.

Tom

kobuki

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Re: Outdoor sensor node (Relatively low cost)
« Reply #17 on: December 15, 2014, 12:05:37 PM »
What is the average power draw on your PIC nodes? Have you ever measured it or tried to estimate?

OK, then I'd like to add for completeness that Ah isn't multiplied is not entirely correct either. It's not multiplied in a series topology. It is in a parallel one. I don't know if it's recommended though, for these batteries. But it wouldn't be very useful for davinci's (cool name btw) project, I think.

TomWS

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Re: Outdoor sensor node (Relatively low cost)
« Reply #18 on: December 15, 2014, 01:34:36 PM »
What is the average power draw on your PIC nodes? Have you ever measured it or tried to estimate?
I did calculate it, measure it, and prove (through battery life) that the calculations were 'close enough'.  Precise calculations, as in this case, weren't feasible because it was DEFINITELY not a controlled environment  ;)

I designed those over 8 years ago so, sorry, I don't remember the specifications.

BTW, I was using Linx Technology radios in that system.  They worked pretty well, but I had to code the comms protocol myself and I'm glad that I don't have to do THAT again!

Later...


ColinR

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Re: Outdoor sensor node (Relatively low cost)
« Reply #19 on: December 15, 2014, 05:51:10 PM »

OTOH, I admit that multiplying the Ah capacity by the number of batteries is a little over-simplifying things, but as you pointed out, it provides 3x the energy so the Wh increases approx. 3-fold. It means around 1/3 of the amperage thus 3x longer service time, in general. The end result is the same from this standpoint, and that's what is important.

With three batteries, you either get three times the lifetime at the same voltage of one battery by putting them in parallel, or three times the voltage by putting them in series.

You can think of three 1.5V AA batteries in series as a 4.5V battery with the combined energy of the three. If you had a fixed power load, you would indeed have three times the capacity, at 1/3 the current, giving you 3x lifetime. If you had a fixed current load, your rate of power discharge would be 3x, and your lifetime would be the same as a single cell. If you have a fixed impedance load (like a resistor), you'd get three times the discharge current at three times the voltage, and your lifetime would actually be cut by 3x.

For something like a linear regulator, the power required for the load is the same, but you lose everything above the required voltage, meaning higher voltage actually reduces the effective capacity you have available. A 600mAh 3.7V LiPo is roughly as effective at powering a 3.3V device through a linear regulator as 600*9/3.7= 1459mAh at 9V.

C
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TomWS

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Re: Outdoor sensor node (Relatively low cost)
« Reply #20 on: December 15, 2014, 06:43:17 PM »
<...snip>

You can think of three 1.5V AA batteries in series as a 4.5V battery with the combined energy of the three. If you had a fixed power load, you would indeed have three times the capacity, at 1/3 the current, giving you 3x lifetime. If you had a fixed current load, your rate of power discharge would be 3x, and your lifetime would be the same as a single cell. If you have a fixed impedance load (like a resistor), you'd get three times the discharge current at three times the voltage, and your lifetime would actually be cut by 3x.
Correct, so far... although I'd never put batteries in parallel, especially LiPo! ;)
For something like a linear regulator, the power required for the load is the same, but you lose everything above the required voltage, meaning higher voltage actually reduces the effective capacity you have available. A 600mAh 3.7V LiPo is roughly as effective at powering a 3.3V device through a linear regulator as 600*9/3.7= 1459mAh at 9V.

C
uh... not quite.  The current load (not the power load), at the power source, is the same in both cases (3.7V LiPo or 9V alkaline) so a 600mAh battery in either case would last the same time.  Your own example demonstrated that, 3 batteries would last the same as a single cell with the same current load.

It's true, with the 9V alkaline, you would be throwing away a lot of wattage in the form of heat across the linear regulator (roughly 9/3.7 X) but we're talking about mAh rating of the battery.  A 600mAh 9V battery COULD deliver more power, if it wasn't thrown away in the regulator.  If it was a switching regulator, then the current load would be dramatically reduced in the 9V power source and a 600mAh 9V battery would last a lot longer (9/3.3) vs (3.7/3.3) if the efficiency of the switching regulator was the same for both voltages (not entirely realistic, but good enough for the example).

Tom

kobuki

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Re: Outdoor sensor node (Relatively low cost)
« Reply #21 on: December 15, 2014, 07:09:47 PM »
@ColinR

OK, but we're talking about Moteinos, not theoretical loads. We can assume the Moteinos are in the "fixed power load" category.

The LDOs are a different matter, now we're just looking at the devices as black boxes consuming power. But as I've also mentioned in this thread too, it's possible to remove the LDO from the Moteino and use AA cells directly and thus increasing power efficiency. There's a thread about it with recent posts too.

The rest is more adequately answered by TomWS than I would have written.

ColinR

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Re: Outdoor sensor node (Relatively low cost)
« Reply #22 on: December 15, 2014, 07:15:45 PM »
<...snip>

You can think of three 1.5V AA batteries in series as a 4.5V battery with the combined energy of the three. If you had a fixed power load, you would indeed have three times the capacity, at 1/3 the current, giving you 3x lifetime. If you had a fixed current load, your rate of power discharge would be 3x, and your lifetime would be the same as a single cell. If you have a fixed impedance load (like a resistor), you'd get three times the discharge current at three times the voltage, and your lifetime would actually be cut by 3x.
Correct, so far... although I'd never put batteries in parallel, especially LiPo! ;)
For something like a linear regulator, the power required for the load is the same, but you lose everything above the required voltage, meaning higher voltage actually reduces the effective capacity you have available. A 600mAh 3.7V LiPo is roughly as effective at powering a 3.3V device through a linear regulator as 600*9/3.7= 1459mAh at 9V.

C
uh... not quite.  The current load (not the power load), at the power source, is the same in both cases (3.7V LiPo or 9V alkaline) so a 600mAh battery in either case would last the same time.  Your own example demonstrated that, 3 batteries would last the same as a single cell with the same current load.

It's true, with the 9V alkaline, you would be throwing away a lot of wattage in the form of heat across the linear regulator (roughly 9/3.7 X) but we're talking about mAh rating of the battery.  A 600mAh 9V battery COULD deliver more power, if it wasn't thrown away in the regulator.  If it was a switching regulator, then the current load would be dramatically reduced in the 9V power source and a 600mAh 9V battery would last a lot longer (9/3.3) vs (3.7/3.3) if the efficiency of the switching regulator was the same for both voltages (not entirely realistic, but good enough for the example).

Tom

Yes, tis true. They hide the potential energy of the battery by reporting it in units that divide it out. The 600mAh LiPo has energy roughly proportional to 3.7 * 600mAh, while the 9V is 550mAh * 9V, so it's just how much you're throwing away that comes out as the ratio of voltages.

C
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TomWS

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Re: Outdoor sensor node (Relatively low cost)
« Reply #23 on: December 15, 2014, 07:28:05 PM »
<...snip>
Yes, tis true. They hide the potential energy of the battery by reporting it in units that divide it out. The 600mAh LiPo has energy roughly proportional to 3.7 * 600mAh, while the 9V is 550mAh * 9V, so it's just how much you're throwing away that comes out as the ratio of voltages.

C
To my mind, mAh rating is the most useful parameter there is.  If you know the average current load, you know how long any battery will last given its mAh rating.  I have a spreadsheet that, given the load model, tells me how many hours (or years, as the case may be) any of most commercially available batteries will last.

Tom

ColinR

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Re: Outdoor sensor node (Relatively low cost)
« Reply #24 on: December 15, 2014, 07:48:32 PM »
It really depends on what you're powering. Wh is plenty useful for comparison across power sources.
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ColinR

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Re: Outdoor sensor node (Relatively low cost)
« Reply #25 on: December 15, 2014, 07:52:47 PM »
@ColinR

OK, but we're talking about Moteinos, not theoretical loads. We can assume the Moteinos are in the "fixed power load" category.

With the LDO, the Moteino will take 3.3V * whatever current it needs. The power consumed is input voltage * whatever that current happens to be. To be most efficient, you want just enough voltage to satisfy the LDO regulator.

I'll eat the efficiency loss of that tiny LDO for the peace of mind and stability it offers.

C
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kobuki

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Re: Outdoor sensor node (Relatively low cost)
« Reply #26 on: December 15, 2014, 08:18:56 PM »
Well, yeah, you're right. And with all the battery-powered low power devices that's why the mAh rating is probably the most useful one. And LDOs are very cheap and simple components compared to small switching regulators.

davinci

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Re: Outdoor sensor node (Relatively low cost)
« Reply #27 on: December 16, 2014, 08:15:32 AM »
Found these Lithium-Ion batteries interesting: http://www.tadiranbat.com/index.php/tli-series-rechargeable

Datasheet: http://www.tadiranbat.com/pdf/TLI-1550A.pdf

Perhaps they could be charged with existing standard li-po solar charger boards?

Does not look like many stores are stocking them.. Any experiences with this specific battery or similar products?

TomWS

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Re: Outdoor sensor node (Relatively low cost)
« Reply #28 on: December 16, 2014, 06:42:12 PM »
I think the only thing I like about this battery is the voltage.  4.1V is perfect for 3.3V LDOs.  However, the battery voltage drops almost immediately and pretty linearly to about 2.7V and then falls off a cliff.  The worst of it is how it collapses pretty quickly at cold temperatures and, if you were using the RFM69HW, it wouldn't take too many transmits to get the voltage down below the LDO dropout voltage. 

I doubt that LiPo chargers would work with this battery, although I did NOT research that.  It's just that the charging protocol is different than LiPo.  The protocol is simple, but dependent on temperature and, IIRC, rate of discharge usage.

I'm also not impressed with the mAh rating. 

They make a grand claim about battery life, but then advise that voltage falls off much faster after about 3000 charge cycles.  At the rate this thing discharges, you'd have to charge it every day or more.

Net: These are my 'impressions', I did not do a thorough analysis and do not plan to, given these impressions.

Tom

davinci

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Re: Outdoor sensor node (Relatively low cost)
« Reply #29 on: December 18, 2014, 02:35:47 PM »
Tom, when you mention that the voltage drops more quickly after the 3000 charge cycles, would you calculate with one charge cycle a day for a Moteino running a sensor or two, transmitting once a minute?

If it is so, the voltage drops are first after 8 years of usage... (365/3000 = 8,21)