Using mini-solar panel to charge batteries

[sir Ene]

I'm building a project (remote measuring station), that will draw approximately 35mA for one second in 15 minute intervals (4 times per hour) continously. In the idle state, everything is going to be either in stand by mode or disconnected, thus drawing less than 1mA.

The problems I'm having are with power supply and storage. I intend to use either 2 or 3 batteries and a switcher voltage regulator to convert the voltage levels to 5V (all other components work at 5V). Now, I'm trying to charge these batteries with a mini solar panel, but don't know how to do things properly.

I guess if I use NiCd batteries I could charge them continously with a trickle charge drawn from the solar panel, but is that such a good idea, since the device might remain unatended for months to come. (I was told lead-acid batteries are better for this kind of usage, but they are not an option here due to size !.)

Basically I have been looking at this for a starting point:
**broken link removed**

The LM334 and LT1300 are not to be found in the local electronics shop (I found a MAX756 instead of LT1300 though). I'm unsure about a LM334 alternative though...the datasheet suggests a 10mA current, when the author of the schematics above mentiones much higher currents.


Ma solar panel specs:
9V with open contacts (6V nominal)
75mA max (60mA nominal)
_____________________________________

If you have any suggestions about how to do this properly (with durability and efficiency in mind), any links or other scematics, calculations or just simple comments, please be so kind and let me know .

Thanks

 

[sir Ene]

If any administrator reads this: I'm new to this forum and just now realized that my help request might be better placed in the "Electronic Projects Design/Ideas/Reviews" sub-forum. If possible please move it there. Thank you.

 

[Tarsil]

I'm building a project (remote measuring station), that will draw approximately 35mA for one second in 15 minute intervals (4 times per hour) continously. In the idle state, everything is going to be either in stand by mode or disconnected, thus drawing less than 1mA.

The problems I'm having are with power supply and storage. I intend to use either 2 or 3 batteries and a switcher voltage regulator to convert the voltage levels to 5V (all other components work at 5V). Now, I'm trying to charge these batteries with a mini solar panel, but don't know how to do things properly.

I guess if I use NiCd batteries I could charge them continously with a trickle charge drawn from the solar panel, but is that such a good idea, since the device might remain unatended for months to come. (I was told lead-acid batteries are better for this kind of usage, but they are not an option here due to size !.)

Basically I have been looking at this for a starting point:
**broken link removed**

The LM334 and LT1300 are not to be found in the local electronics shop (I found a MAX756 instead of LT1300 though). I'm unsure about a LM334 alternative though...the datasheet suggests a 10mA current, when the author of the schematics above mentiones much higher currents.


Ma solar panel specs:
9V with open contacts (6V nominal)
75mA max (60mA nominal)
_____________________________________

If you have any suggestions about how to do this properly (with durability and efficiency in mind), any links or other scematics, calculations or just simple comments, please be so kind and let me know .

Thanks

Scuze...I meen sorry ...u have >6V
Haven't read everything
Use the regulator above ....u can buy a 100uH/1A coil from **broken link removed**

 

[sir Ene]

Erm, yes... (6V) that's the rated voltage with NO load. It's a small solar panel (hence the "mini" part of the name) of 11cmx6cm, so those rated values are extremely optimistic...or maybe measured in a spaceship orbiting the sun.

So I'm still uncertain about the LM334 due to its current rating ...and I'm not really sure what good that 100uH/1A coil is going to do for me.

Sorry If I can't see the obvious, but I'd really apreciate some more in-depth help if at all possible.

Thank you for the effort Tarsil.

 

[JimB]

I think that we dont have enough information.
I think you need to do a few experiments with your solar cell and the NiCads.
Connect them together, use a diode so that the battery does not back-feed into the solar cell. Include a meter in the circuit and measure the current supplied to the cells under various light conditions, dark wet miserable rainy day, dull dry day, bright sunny day.
Once you have that information, consider if the NiCad will be overcharged and damaged on a long sunny spell in summer.
Also consider if the NiCads have enough capacity to run for a couple of weeks of dark dull days in winter.
As your solar cell is small, it may not have enough oomph (a technical term) to damage the NiCads with an overcharge.

JimB

 

[audioguru]

I think "oomph" is the force that lets smoke out of electonic components. Then they don't work any more.

 

[Tarsil]

Sorry ...that post was intended for "getting 5v for IC, best method" post. <lol>
The 100uH was for this SMPS: http://www.techlib.com/electronics/regulators.html

About your post:
If you charge continously the NiCad, the cels will have a shorter life, but I subscribe to the oomph thing so ...the bateries will last a reasonable amount of time if u use cells like 1Ah. At that curent won't get hot and the memory efect can be reduced if u, once in a while, discharge them at around 1V/ cell.
The rest is......tweaking <lol>

 

[sir Ene]

Finally I had some time to do some testing for this little esperiment of mine...

I got my hands on some batteries; they're not NiCd as planned though, but NiMh. I googled a little an read that NyMh can apparently be charget by trickle current just like NiCd. The max current allowed for "constant" charging however is supposedly 1/100 of the capacity. I have 2500mAh batteries so...that gives me 25mA max.

Here comes the problem though...if the 25mA is truly a limit, than even my mini-solar panel produces too much current. Today in direct sunlight at its peak intensity (around 13:00) I measured 51mA into a 2 battery pack. The current dropped to around a safe 20mA when a light shadow was over the panel though.

Does anyone know if what I read is correct and the 1/100 capacity current limit has to be considered ? If so, I guess a proper current source will in fact be needed as i thought. Any ideas ?

Thanks

 

[Nigel Goodwin]

Here comes the problem though...if the 25mA is truly a limit, than even my mini-solar panel produces too much current. Today in direct sunlight at its peak intensity (around 13:00) I measured 51mA into a 2 battery pack. The current dropped to around a safe 20mA when a light shadow was over the panel though.

I don't see as you have a problem?, you just specified the battery can withstand a constant, permanent, 25mA charging current. Unless you have bright sunlight 24 hours per day?, the average charging current over any 24 hour period is almost certainly going to be under 25mA.

 

[audioguru]

For their Ni-MH cells:
1) Panasonic recommends a max continuous trickle charge of 0.05 times the capacity. So for 2500mA/hr cells it is 125mA max.
2) Energizer recommends a continuous trickle charge of 0.04 times the capacity. So for 2500mA/hr cells it is 100mA.

Your average current is much less.

 

[sir Ene]

I know that and I agree that the averagy current is not going to go over 25mA. However what I read about NiMh, doesn't say anything about an average current limit, but a MAX current limit. Maybe what I googled up was just some bad information, but....if 25mA is OK for average, then what is the absolute maximum I'm supposed to observe, given the fact that this is going to happen on a cyclic daily basis ? (It's still going to be 51mA for a couple of hours during clear days nevertheless.)

I guess it's not just like you calculate the average and use that..otherwise you could just charge your batteries with a high current for a couple of seconds and then be off with it. The average per 24h would still be pretty much low. (This does not apply to my situation, but is just some thinking to explain what I mean.)

 

[sir Ene]

---Audioguru---- Thanks for the info, I was just typing when you replied.

So the information I read was misleading. And I guess that solves all the issues regarding charging, no current source necessary, just a diode at the solar panel.

Thanks a lot guys.

 

[Nigel Goodwin]

I know that and I agree that the averagy current is not going to go over 25mA. However what I read about NiMh, doesn't say anything about an average current limit, but a MAX current limit)

No, you were quoting the maximum CONTINUOUS current (not a limit) - you can charge at MUCH higher figures (and usually would), but not continuously.

 

[sir Ene]

Nigel, I understood what you wrote and also what I read. I know about ways of charging a NiMH cell (continous - trickle, fast ---etc.). I know what I read was a max for continous charging (not the correct value, but still) - I made the mistake of not writing the "continous" between "MAX" and "current"; I thought it was clear. I know you can charge a cell with a much higher current if you charge it with a faster techinque.

If the 25mA figure was correct and my panel produced something over that, for let's say 8h per day and the cell would be full, that would probably not be considered "trickle charging" and bad for my cells in the long run, right ? The average would still be much lower than 25mA (due to the rest of 16h).

The question is solved though and this is just theoretical pondering.

 

[Nigel Goodwin]

If the 25mA figure was correct and my panel produced something over that, for let's say 8h per day and the cell would be full, that would probably not be considered "trickle charging" and bad for my cells in the long run, right ?

No - wrong!.

8h per day at 50mA will do no harm at all to your batteries, trickle charge figures are spread over a long time scale - probably more days than hours. If your panel provides 50mA in bright sunlight it's going to average considerably less then 25mA over 24 hours. For ambitious figures we'll imagine 8h per day at 50mA, another 8 hours at 25mA, and 8 hours of none (night time) - that averages to 25mA during the 24 hours.

Bear in mind the 25mA figure isn't just "trickle charging" it's trickle charging 24 hours per day, 7 days a week, 365 (and a quarter) days per year.

That panel will be fine, and I seriously doubt it'll average 25mA over a 24 hour period - you're worrying with no reason!.

 

[sir Ene]

OK, thank you for another explanation. I wasn't worrying anyways, as I said above it was just "theoretical pondering". I'm aware my figures are going to look much lower than that in the end.

Cheers mate.