SparkFun Forums

[prosys]

I have been looking for a circuit to do the following ...

I have a very low power PIC application that basically logs data every few minutes in a very low power mode, doing so for months at a time with occasional wireless downloads of the logged data when a fair amount of power is required for a few minutes, the box will be outside and I'd really like to supplement a Li-Poly battery with a Solar charger to keep the battery topped up when possible, running from the battery as the Vcc for the circuit.

Challenges I'm trying to fix are ..
- finding a small enough, yet powerful enough solar cell, I think the SFE PRT-07845 should do the job nicely.
- Finding a Li-Poly charger that specifies the Minimum input voltage and current required for it to start charging a 2000mAh Li-poly battery(SFE PRT-00340)
- Figuring out if I need a voltage step up stage before the charge IC to get the charger circuit to work properly from the Solar cell ?

Anyone have any experience with this kind of thing...

[bigglez]

Greetings (No First name Supplied),

I'd really like to supplement a Li-Poly battery with a Solar charger to keep the battery topped up when possible, running from the battery as the Vcc for the circuit.

- finding a small enough, yet powerful enough solar cell, I think the SFE PRT-07845 should do the job nicely.

Why? What criteria brought you to select this one?
Solar panels have much higher internal impedance (resistance)
than any other practical voltage source (battery, DC power
supply with a transformer, etc.).

The panel you have selected has 4.5V open circuit and 100mA
short circuit. So a First-order model is a 4.5V voltage generator
in series with a 45 ohm resistor.

- Finding a Li-Poly charger that specifies the Minimum input voltage and current required for it to start charging a 2000mAh Li-poly battery(SFE PRT-00340)

Most (if not all) battery chargers are a series pass element
and rely on the bulk DC input to be higher than the battery
at end of charge. This is seldom the case with a single
element solar panel.

To overcome this problem with a conventional charger circuit
would require a much higher panel voltage than the battery
(to allow charging under partial sun conditions).

- Figuring out if I need a voltage step up stage before the charge IC to get the charger circuit to work properly from the Solar cell ?

Any converter will consume some of the available power,
although high-efficiency converters are possible with
careful component selection. This loss might make a second
or larger voltage panel the better choice.

A correct charger for your battery should consider the
chemistry and follow the battery maker's charging
recipe. This will involve a precision voltage monitor
circuit and current regulator. To protect the battery
you should also include an under-cut circuit to remove
the load if the battery is depleted and no solar power
is available for recharge.

It's doubtful that a single IC exists to do all this for you, but
a good circuit could be fashioned from several blocks
(or from straight analogue circuit design).

Do you have a handle on how much sunlight is available
and what the energy consumption of your load will be?
That would be a good starting point.

Comments Welcome!

[prosys]

Bigglez,

Thanks for the detailed reply, to answer some of your questions...(I think I saw most of the issues you describe, but being a digital guy, this level of analogue stuff has me thinking harder than I'm used too )

The criteria for the Solar cell was essentially physical size and voltage, the 100mA short circuit capacity seemed pretty hopeful in terms of the demands of mu circuit... detailed below..., the box must be small and unobtrusive.

The circuits average current consumption is of the order of a few hundreds of microAmps at 3.3volts (PIC and sensors in sleep mode), though the circuit varies its load quite considerably over a 1 hour period as follows...

Wakes every 10 seconds to interrogate the sensors (pressure sensor) during which time current consumption will rise to perhaps 5 milliamps for a about 0.01 seconds before going back to sleep mode

About every 30 minutes the circuit wakes and decides whether to power up a wireless link (SFE: WRL-00151) to see if a base station is in range and if so downloads the latest data logged to the base station, this process is the highest current draw ..
- if a base station is not detected this lasts only 1 second at 30milliamps
- if a base station is detected, the download can last minutes at 30milliamps
- only one download per day will be allowed/required.
- logged data will only be collected on one or two days at the weekend, limiting downloads to a maximum of 2 per week.

During the rest of the devices life, the device is sitting indoors for up to one week with no power to the solar cell, but on weekends is lilkely to be fully exposed during all daylight hours to direct outdoor sunlight (best case California, worst case England ).

I'm building a logger for my gliding club, they normally have a volunteer person logging all aircraft off the ground and back down, its error prone and at rental rates of $100+ per hour, missing a few minutes (or an entire flight) can have serious consequences for the solvency of a non-profit club...)

I am hoping to get months worth of life out of the battery even if there is insufficient charge to keep it topped up, but am hoping that in the best circumstances the device can be completely hands off, never needing to be removed for charging, managing to maintain the battery with whatever daylight it is exposed to.

I fully expect to use a suitable Lithium polymer batter charger IC for the job of getting the charge cycle right for the battery.But I don't have the analogue skills to adequately answer my questions about how I satisfy that IC with whatever power I get from the Solar Cell, though as yo say i pretty much arrived at the requirement for a (very efficient) voltage step up circuit.

I have not found a suitable IC/Datasheet that adequately explains the consequences of repeated partial charging cycles of a battery, or the behavior of the IC if it is supplied with too little power to adequately do its job... I guess experimentation is the only solution here.. but batteries are not cheap

And I don't know why my post appeared in the PCB Design section, it should have been in the circuit ideas section... I guess I fat fingered it.

Peter

[bigglez]

Greetings Peter,

The circuits average current consumption is of the order of a few hundreds of microAmps at 3.3volts (PIC and sensors in sleep mode), though the circuit varies its load quite considerably over a 1 hour period as follows...

Thanks for the detailed description, sounds like a nice
project to tackle. You probably have enough data
to construct a timeline of the power demands, and
use these to find the energy drawn from the battery,
the predicted solar recharge energy, and account
for losses in the converter and regulator. This R&D
work will bring you to a high confidence on the
first prototype!

I have not found a suitable IC/Datasheet that adequately explains the consequences of repeated partial charging cycles of a battery, or the behavior of the IC if it is supplied with too little power to adequately do its job... I guess experimentation is the only solution here.. but batteries are not cheap :-)

Perhaps a Li-Po or Li-Ion cell is not required. It might
be possible to use a single NiCad or NiMH cell with
a battery booster IC. These cells are cheaper and
may only require replacement after a long service life.

I'd take a look at the major analogue IC houses
(ADI, Linear Tech, Maxim/Dallas, NSM, TI, etc.)
as they have produced several "single and dual
cell" PSU controllers for the phone and PDA
markets.

Quite possibly the solar panel will run the logger
while in flight, and the battery is there as a back
up and to carry the system through hangar storage.

One other thought I had was adding a separate
RTC IC (with it's own 32kHz Xtal and button cell
back up).

This would complicate your design a little but also
provide a reliable un-interruptable timebase and
a battery-backed small storage area for data. I'm
thinking of the Maxim/Dallas DS130xfamily of
RTCs, but other vendors have them too.

Comment Welcome

[prosys]

Peter...
Thanks again,..
[quote="bigglez"]You probably have enough data
to construct a timeline of the power demands, and
use these to find the energy drawn from the battery,
the predicted solar recharge energy, and account
for losses in the converter and regulator. This R&D
work will bring you to a high confidence on the
first prototype![/quote]
Yup, I just need to sit down and work out the details there, been estimating numbers to this point, though I've a reasonable degree of confidence that its achievable.

[quote="bigglez"]Perhaps a Li-Po or Li-Ion cell is not required. It might
be possible to use a single NiCad or NiMH cell with
a battery booster IC. These cells are cheaper and
may only require replacement after a long service life.[/quote]
Hmmm... I wanted a high voltage single cell battery that is light, high capacity, and has low self discharge characteristics, that pointed me at Lithium-polymer and most of the docs I read indicate that they are also not susceptible to the memory effect of bad charging practices that Nickel (cd or mh) based batteries suffer from, they also have a pretty wide thermal range too... important for sitting in the California heat duriung the day or the -30F temps at 18000ft (or the English hangar in winter). for hours on end.
(I fly here in California for the high temp testing, but my dad flys in England where the lower scale temps are available for 'soak' testing ...)

[quote="bigglez"]I'd take a look at the major analogue IC houses
(ADI, Linear Tech, Maxim/Dallas, NSM, TI, etc.)
as they have produced several "single and dual
cell" PSU controllers for the phone and PDA
markets.[/quote]
Yup you read my mind, that what I'm looking at, unfortunately most (all) of the nice small charger IC's are designed for use in portable devices where the expectation is that they be charged from a USB port which provides a nice fat 5V supply... I need to get there from the solar cell so I'm back to my voltage pump, maxim makes plenty of claims about their IC line for this stuff, I'll take a closer look. I may actually use the processor to decide whether the cell is producing enough power and then turn on the charge circuit interactively rather than just leave it connected permanently and hope it does the right thing at low power levels, need to check whether I have enough analogue/digital ports to support that, but that could remove any bad behavior exhibited by the charge IC at lower than ideal power levels.

[quote="bigglez"]One other thought I had was adding a separate
RTC IC (with it's own 32kHz Xtal and button cell
back up).[/quote]
Actually the PIC U;m using supplies an RTC function with the addition of a 32khz crystal, and I'm using a fairly large serial EEPROM to hold the logged data just in case I need to hold months worth of data before I can contact a base station, it makes my code a little more compicated but I like a challenge. My savior here would be the base station, if I can ever get a link to it from the aircraft device (I expect to do this in the hangar over night), it can re-sync times and not rely totally on the RTC function, the important thing is flight elapsed times, not so much time of day or date (though reasonable accuracy would be great), I need to again figure out if the PIC RTC is accurate enough over months to support this without a re-sync or needing a dedicated device.

Thanks for the comments, its great to discuss this with another mind, its helping me answer my own questions....wait till I talk to you about the bigger project I'm working on that allows me to procrastinate on this one...

Peter