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Battery life and Buck/Boost or LDO....?

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rayyolanda

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Hi guys,
I have a requirement to power a circuit by battery for as long as possible and I need a few ideas.
The circuit is a PIC and an RF receiver module. The voltage is 5V, current draw is around 4mA and will peak at times to 10mA but only for a few seconds a few times each day.
The problem is what do I use for power that I hope will last for 1 year. Buck/Boost seems a possible solution but I don't have experience with them.....any wisdom will be warmly received...
Thanks
Ray
 
What is the selected power source?
What's PIC doing?
I assume the PIC does some kind of measurement, and this 10mA peak comes from using the RF module for a short burst.

Using a linear regulator is usually wasteful in an application that requires battery power and longevity.

Powering the device from a larger voltage battery (say 12v) and using a buck converter would work well as 4mA at 5v becomes about 2mA at 12v (assuming 75% efficiency).

If the PIC isn't doing anything important between measurements/transmissions, you could disable the peripherals and put it into sleep and wake up every time the RF module needs to be used.

Otherwise, keeping the PIC on with a constant 4mA of draw will drain most common batteries in less than a month. For example, running them off 3 AA alkaline cells (~2500mAh), the cells will be flat in less than 4 weeks.

So for longevity you either need to run them from large capacity batteries, higher voltage batteries with a Buck Converter, or incorporate sleep mode (perhaps also watchdog timer) into your code.
 
4 milliamps for a year is 35 amp hours. That would require 42 AA cells... or find a way to put things to sleep, like Demonic said.
The reason I'm talking about alkaline batteries is that these are the only batteries I know about that won't self discharge in a few months.
 
Thanks for the comments.
Yes the laws of physics can not be cheated.....42 cells is not an option. Higher voltage may help out. Putting the PIC to sleep is a bit problematic. It needs to wake up with received signal...which is there as noise and real data....so the PIC spends all of its time in the interrupt handler.
Demonic....the PIC is decoding Manchester code from the Rf module then sends a signal based on received data. I have another PIC encoding and sending the data. Everything works fine just this power issue. Maybe solar cells and nicads.? From those cheapo garden lights....hmmmm
Thanks
Ray
 
Do you control the transmission end? If so, send a preamble of a single frequency "tone" that lasts about 1 sec before commencing with the Manchester encoded data burst. The receiving PIC can spend 90% of its time asleep, wake up about twice per second, while awake it can sample the receiver output, detect the presence of the tone as opposed to noise, and if it finds the tone, stay awake to receive the rest of the data burst.

How long does it take to wake up your receiver? You may be able to shut it off, and wake it up in step with the pic, thereby saving more power.

btw- I found a 433.92Mhz receiver that only uses about 80uA when powered up, and it wakes up in ~0.5sec. I have an algorithm which can be coded for a PIC to discriminate periodic modulation vs. the no-signal receiver noise.
 
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The most important thing if you need to reduce power of an RF receiver is to find one that can turn on quickly. 0.5 seconds isn't particularly good as if you want to monitor a signal every second, the receiver needs to be on half the time.

There have been big improvement in the RF receivers recently, especially in turn-on times.

Other strategies include only asking for the data occasionally. You could even synchronise the receiver to regular signals from the transmitter.

The other strategy to consider is getting the remote unit to transmit its data regularly and no bothering with the receiver. 10 mA for a few seconds a day is so far below the current used at 4mA for the rest of the day, that if you can virtually eliminate the 4mA you can afford the 10mA for much longer than it is at the moment.
 
hi Mike / Diver
Mike, yes i control all data flow but the problem is that the transmitter also runs on battery and only wakes for transmit which is infrequent and not predictable. That sort of rules out preemptive decisions about when to transmit and receive.

Putting the receiver PIC to sleep does not help much anyway..it is the RF module that sucks the 3.4mA or so, the PIC is just 200uA. The PIC interrupt would have to be a Change on Interrupt as I am not/ can not use USART sync on the PIC...so I am using software UART and I tried COI RBint but found due to RF noise the PIC was in the Interrupt handler all the time.

Is there a way to squelch these RF modules? (RF is not really my game...I fumble a lot here).
Mike......433.92Mhz receiver that only uses about 80uA when powered up....sound interesting. Can you supply more details?

Turn on time is 30mS.?....lot faster than 0.5 seconds? So says the datasheet...
MO-RX3400-A ASK receiver module.

Thanks guys....I think I covered most questions....maybe I will just have to go with a plug pack instead of battery.
 
Hello there,


I think most of the issues have already been addressed fairly well, so i'll just add a few comments to the bunch here.

On the subject of converter (any type) vs LDO, dont let the LDO terminology fool you as it eats plenty of power unless the input voltage is close to the output voltage, and then it can possibly eat less than many buck circuits. What this means is that the choice of LDO vs Buck would depend on the average input and output levels as well as the efficiency of the buck. A very high efficienct buck would be hard to beat for example, but a more typical one would be beaten by the LDO if the level of input voltage does not go much above the output voltage. It's something that you have to look at when you decide what buck circuit to use. Notice also that im talking about a buck circuit for comparison here, that's because you probably dont want to use a boost circuit as they usually eat more power than a buck and you dont really have to resort to boosting a single cell up to 5v anyway. So the decision to go buck or LDO is going to be mostly based on input vs output voltage over most of the run time.
There is another factor which you mentioned however and that is that you dont seem to want to go into using a large number of batteries, even if the cost is lower than one big battery. That seems to immediately rule out using AA cells as you would need too many of them to get the full 1 year run time. In this case, you'll probably want to go into a larger battery with a buck circuit of reasonable efficiency, perhaps a synchronous buck. That would take you from 12v input to 5v output with reduced current from the 12v cell, and as you may already know when you use a large battery for a small current (relative to it's normal operating current) you end up with a practical increase in ampere hour capacity. That means if you buy a 15 Ahr battery you may be able to get 18Ahr out of it simply because you're running at a very low current (like 2ma or less). That means a single lead acid battery with a decent buck circuit might just do it.

Now the questions for you include...
1. Just how important is that one year spec, what would happen if it died in 9 months. Would someone die or would you just have to replace the battery?
2. Related to that question, do you have a year or two to test this or does it have to be a one time deal where you can not afford to say double the battery size if it didnt go the full year on the first try?
3. You have any sort of budget?
4. Is there any way of charging say at the 6 month period or something like that if you use two batteries instead of one?
5. What is the working environment like, as to operating temperature? This would affect the self discharge of any battery you consider for use and thus might demand a higher capacity battery.
6. Any chance you can use li-ion ?
 
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hi MrAl, thanks for your informative reply.

Answers to your questions....
1. No one will die.....6 months....9 months....I guess the real answer here is that it needs to be long enough for the user to forget the last time they changed batteries...
2. No time to test.....this is a one of......maybe a few more later...
3. Budget.....like everyone...as cheap as possible.
4. Two batteries....maybe....rechargeable batteries....maybe.....?
5. Environment is not too bad......5 to 35 deg celsius would be the extremes.
6. li-ion is a possibility...really this would just be a cost issue.

What do you have in mind...?

Thanks...Ray
 
Hi again,


Well, the cheapest is probably a lead acid with small buck circuit. If you can have one battery charging and the other in use, then switch to the other battery after 6 months that would probably work out pretty nice. You'd have to check the self discharge of lead acid over a years time to make sure it can go the distance if you cant swap out after 6 months, maybe oversize the battery to compensate.

Li-ion is a good technology and lighter in weight. Probably more expensive of course.

Perhaps a small additional circuit to notify you in case the battery starts to run down unexpectedly fast.
 
Hi again,


Well, the cheapest is probably a lead acid with small buck circuit. If you can have one battery charging and the other in use, then switch to the other battery after 6 months that would probably work out pretty nice. You'd have to check the self discharge of lead acid over a years time to make sure it can go the distance if you cant swap out after 6 months, maybe oversize the battery to compensate.

Li-ion is a good technology and lighter in weight. Probably more expensive of course.

Perhaps a small additional circuit to notify you in case the battery starts to run down unexpectedly fast.

LiFePO4s. longer cycle life and safe. we are replacing SLAs with LiFePO4s and saving money. it holds up 10W for 30 seconds from a $10 600mAHr LiFePO4 in place of a $40 SLA

even without such torturous loads an SLA would need to be replaced every 2-3yrs. the LiFePO4s will probably never have to be replaced.
 
Thanks again.
Lithium batteries are wonderful devices....but still probably too expensive for my app.
OK.....I am looking at lead acid with Buck.
I am also going to research a solar cell with a nicad and boost.......what do you think?
How well do solar cells work from indoor lighting or indoor daylight?
Ray
 
Thanks again.
Lithium batteries are wonderful devices....but still probably too expensive for my app.
OK.....I am looking at lead acid with Buck.
I am also going to research a solar cell with a nicad and boost.......what do you think?
How well do solar cells work from indoor lighting or indoor daylight?
Ray
+1 for LIFEPO4 cells, they can take a beating. Nice flat voltage curve over time, and low self discharge, but not quite as high of storage capacity like the nimh.

You don't say what your location is, but the Westinghouse 4 pack of AA's or 18500,s are $10 at Walmart in the Landscape/solar light section. If you are lucky, you can currently find them clearance priced at $5.

Here are some inexpensive mini solar panels worth looking at. I have been using the 1/2W ones in a couple of projects, could really use some of the 1W ones during the winter months here. You might want to pick up one or two extra, because the Chinese quality control can be a little sketchy.

Unless you have the panels on a window sill facing South, and in direct sunlight, there won't be any charging going on, period. Consider that even in the window sill, the solar transmittance of the glass would at best be 80%, and likely much less (50-60%) for soft low-e coating.
 
Thanks nickelflippr.
Yes, I was a bit doubtful of using a solar cell indoors and expecting a good charge rate.
Walmart might be a little bit too far away.......my location is north of Sydney, Australia. I guess I would use a north facing window sill ....lol

I am going to do some experiments with changing supply voltage on the receiver...low voltage and hopefully it sees a signal...then increase voltage for the real data. Sort of a wake up signal. No idea if this will work, but it might save power.

I think I will try a 9 or 12 volt battery with a MCP1703T-5002 regulator and see how long it last, I can always redesign later if I have to I just did not want to test in the field this way.

Thanks again....Ray
 
Hello again,


The sun energy density near the surface of the earth is around 1300 watts per square meter. That's 1300 watts concentrated into one square meter which is roughly a square with sides about 39 inches each. Now take a typical indoor lamp which is 100 watts, which is spread out almost 360 degrees, and figure out how much power can possibly reach a solar cell say 10 feet away from the source noting that the intensity reduces as the inverse square of the distance. Luckily the app only draws 4ma so there may be a chance of that working if the solar cells are big enough. Small calculators can work off of small solar cells but they probably draw less than 1 ma.
 
Thanks nickelflippr.
Yes, I was a bit doubtful of using a solar cell indoors and expecting a good charge rate.
Walmart might be a little bit too far away.......my location is north of Sydney, Australia. I guess I would use a north facing window sill ....lol

I am going to do some experiments with changing supply voltage on the receiver...low voltage and hopefully it sees a signal...then increase voltage for the real data. Sort of a wake up signal. No idea if this will work, but it might save power.

I think I will try a 9 or 12 volt battery with a MCP1703T-5002 regulator and see how long it last, I can always redesign later if I have to I just did not want to test in the field this way.

Thanks again....Ray
I will trade my South facing for a North facing, Down Under window right now! The window sill is a good "relative" test area for solar. Like your RF circuit, have found an always on external LM339 comparator circuit (even at 1.5ma), to have been a real waste of power. A timer based wake-up schedule from a micro is in the works.

Per MrAl's post: I was thinking ambient light for some reason, so I just put a 60W incandescent :eek: in the drafting lamp. At 2ft. away from the previously mentioned 1/2W mini panel(6V O.C. and 100ma shunted, full sun 45Deg NLAT) readings taken were around 4.5V, and 2.5 to 3ma. So one or two 1W mini panels could be useful for inside testing, under the lamp.
 
Hi,
It must be cold over there...it was cool here yesterday..24 deg celsius....about 80 deg Far., but it is summer.......strange weather...

OK......I am going to box this thing up and just use battery at this stage....then wait for the complaint about battery life.....this will give me time to develop something else.

I have been looking at a RF detector idea which will "wake up" the receiver when required. Send a burst of RF....wake the receiver.....send real data....receive......back to sleep. Here is an interesting link....pg 8

**broken link removed**

Thanks guys, the discussion is very helpful.......Ray
 
Hello again,


The sun energy density near the surface of the earth is around 1300 watts per square meter. That's 1300 watts concentrated into one square meter which is roughly a square with sides about 39 inches each. Now take a typical indoor lamp which is 100 watts, which is spread out almost 360 degrees, and figure out how much power can possibly reach a solar cell say 10 feet away from the source noting that the intensity reduces as the inverse square of the distance. Luckily the app only draws 4ma so there may be a chance of that working if the solar cells are big enough. Small calculators can work off of small solar cells but they probably draw less than 1 ma.

LOL you forgot that of the 100W of consumption, only 15W gets out as light. at a foot away you get maybe 1% of that to the cell array that is only 10% efficient if you are lucky.... end result: 15mW
 
You could also consider using some of the new NiMH slow self-discharge cells (often labeled as "pre-charged"). They only self-discharge about 10% a year and are relatively low-cost compared to Li ion.
 
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