Current Cottonpickers experiments

Current stuff I'm playing with...
ForestBE
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Re: Current Cottonpickers experiments

Postby ForestBE » Sat Jun 22, 2013 8:30 pm

Dear Cottonpickers,

How is it going with your USB timer project?
I have been further reflecting on my Pi A build and your USB timer. To operate this in a proper way I would need to add an RTC to the PI to make sure it goes into sleep before the power is cut. I also found a suitable voltage converter that will raise the LiPo voltage to the required 5V. So I should be able to go with a standard LiPo (still not sure about an unbalanced LiPo pack).
Please could you be so kind and establish an email contact between Matthijs Cottignie (cottigniem) and me. As he is also from Belgium and got already your stuff it might be good to talk to him directly about his experience and his project.

With kind regards,

ForestBE


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cottonpickers
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Re: Current Cottonpickers experiments

Postby cottonpickers » Sun Jun 23, 2013 6:49 am

Forestbe: Well I've requested and paid the factory for the changes needed to make the 5v timers. I'm giving them a few more days before I ask for progress. I've got the additional USB parts needed here waiting for the. I'm estimating they will ship later this week and arrive here the following week. Of course I'll race to complete and test the first one. :) will update when I get shipping notice

You should consider using lipo / li-ion in parallel so they always remain balanced. Consider a protection circuit to prevent over charge/discharge/current and shorts. It's one reason I recommend the USB packs as they have that already built in.

You could look at a simple program timer that checks if the pi time has exceeded say 50min and if it has, shut it down. Match that with the USB timer set to cut power at 1hr and the pi should be in a nice state when cut. One idea anyway!

I can't give out emails I'm afraid but this board has a personal email facility - you could try leaving a message there?

ForestBE
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Re: Current Cottonpickers experiments

Postby ForestBE » Sun Jun 23, 2013 4:07 pm

Dear Cottonpickers,

Thanks a lot for your efforts on the timer - I am waiting patiently ;-)
I am using LiPos to fly a Quadcopter. That is why I am hesitant to use a LiPo pack without a separate balancer plug (a single LiPo cell does not need this) for charging packs in parallel (as they are never equal to each other). This is why I do not trust the USB LiPo packs at all.
I am thinking about running the Pi for some 12 hours (during daytime) and to put it into sleep before the timer cuts the power at sunset.
I have looked at member's section of the board and there is a part that says contact, but I could not see anything there - now I see the PM link (maybe I am getting old :shock: ).

Thanks a lot,

ForestBE

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cottonpickers
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Re: Current Cottonpickers experiments

Postby cottonpickers » Sun Jun 23, 2013 7:46 pm

ForestBE: Glad you found the PM link. Most of the packs are Li-ion rather than Li-Po and tend to be permanently paralleled to maintain the same voltage. But as you have experience I'll stop teaching you to "suck eggs!"

ForestBE
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Re: Current Cottonpickers experiments

Postby ForestBE » Mon Jun 24, 2013 6:31 pm

Dear Cottonpickers,

Sorry for coming back with further questions ...
You write that all your panels are equipped with a blocking diode.
This diode's function is to prevent from draining your charger and battery back into the panel?
If this is the case I gain much needed 0.7 V (by not using the diode built into the charging module).

Curious,

ForestBE

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Re: Current Cottonpickers experiments

Postby cottonpickers » Tue Jun 25, 2013 6:19 am

ForestBe: The diode is to prevent batteries from discharging. Without a diode my V5 V7 and V9 would have flat batteries overnight (or at least lose some of their charge) Most devices additionally have a diode, so for the 1.5amp model the diode is likely not required for the battery as the 'usb power packs' would almost certainly not drain whether with or without the panel diode because they have another one just inside their power pack.

Remember that the panels voltage is 'nominal' - In the same way that a "nominal" 1.2v NiMh battery starts life at about 1.4v and falls to about 1.1v as its used, a 5v panel actually is a little higher than that without the diode - Same with a 6v panel. You can add around 14% to the 'nominal' value to see what voltage they might typically be. Confusingly Solar will also 'drop its voltage to meet the need' so if you connected a 20v panel to a 1.2v battery and measured the voltage as it charged you would see it showing a little higher than 1.2v but not 15v or 20v for example. So for typical applications people pick a panel with more than enough voltage. Anything too high is then just 'wasted' unless an expensive power point tracking charger is used.

There is one other use for the diode.... For Apple charging voltage is really important, so you can deliberately put a (possibly unneeded) diode in to reduce the voltage a litte. On my panels a diode is needed on the 5v panels to ensure the voltage is correct (accepted by the iPad/iPod/iPhone) otherwise it will reject the panel as a source of power. For the iPad Solar Smart Cover I actually use a lower voltage panel without the diode, OR a 5v with a diode.

Finally (hope this doesn't confuse you) there are standard rectifier diodes and also Schottky (low voltage drop diodes) - I use a combination of these depending on the panel / use and whether I need to drop the voltage a little as well. So, to your question! You could remove the diode if you dont need it, or better still just put a small wire across it so that the electricity chooses the route without the diode but allows you to take the wire away to reinstate the diode if you need it later. If you have a v5 or v7 it has a 6v panel and a schottky diode (to maximise the solar / charge) with a drop of about 0.2v if you have a 1.5amp panel it has a 5v panel and a standard rectifier diode to allow one port to be apple compatible with a drop of around .7v

At some point I plan to write a $2 Kindle boook on 'practical small scale solar' to help describe this type of thing more clearly with diagrams. Hope for now this helps.

ForestBE
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Re: Current Cottonpickers experiments

Postby ForestBE » Tue Jun 25, 2013 7:28 pm

Dear Cottonpickers,

The charger and power distribution set up I want to use (and its chip specifications really look good and very suitable for this task) have a note that says: "We've tried 5V panels and they don't work. 5.5V panels sometimes work, 6V is really the best!".
However, your 5V panel delivers far more mAs than theirs (530 mA vs your 1500 mA). From reading this board I understand that you solder a diode on the panel itself. Your panel is 10 cells (10x 0.5 V makes the 5V) theirs (from Voltaic) is 12x 0.5 V and gives about 6V output. But their charging/load-sharing board has a blocking diode at its entry and thus looses 0.7 V there.
I was then thinking if your panel uses a Schottky diode (having only a 0.2 V drop) this would already get me in the range of the other set up (well still 0.5 V difference) but these are not exact numbers anyway.
Ideally I would love to have your panel with 12 cells and ending up with slightly more than 5V.

Dreaming,

ForestBE

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Re: Current Cottonpickers experiments

Postby cottonpickers » Tue Jun 25, 2013 8:50 pm

ForestBE: My V9 are 6Volt (920ma) but I really wanted to maximise the current - hence the 1500ma panels at 5V. If I went for 12 cells and 6V the current would have to drop by 20% so they'd be around 1200ma. As I'm targeting the USB market I went with 5V on these. I could get these made up at 6V but would need a number of people to commit to purchasing (otherwise I'd get stuck with them ;-) If the board/charger you plan to use works from a PC USB port (which typically supply 500-700ma) then my 1500ma 5V should certainly work. Remember you can bypass the schotky very easily. Even placing some tin foil over the diode would work as it would conduct the current around/over the diode and is easily removable. If you are looking at charging lipo, don't forget to check my Li-ion chargers with display? These use the LT4054 chip (CC/CV) and operate in a nice wide 4v-6v range.

ForestBE
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Re: Current Cottonpickers experiments

Postby ForestBE » Wed Jun 26, 2013 6:21 pm

Dear Cottonpickers,

Thanks a lot for sharing your design considerations.
The charge/load-balancing set-up (built around a MCP73871 chip) I want to use is somewhat more complex than a LT4045 set-up (but this one does not need a blocking diode!). The MCP73871 is supposed to work with USB input too - a Schottky diode in the panel should do the trick.
I am not looking for a charging only solution - but for one that charges AND drives a Pi at the same time in a flexible manner (this is why I ended up with a MCP73871 based design).
I know how to handle a soldering iron thus tin foils are not really an option ;-)
So once your USB timer is ready I will place an order for the timer and the panel.

With many thanks,

ForestBE

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cottonpickers
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Re: Current Cottonpickers experiments

Postby cottonpickers » Thu Jun 27, 2013 7:19 pm

Funny you should mention the MCP73871 as I've been looking at that too. I use one of their 4.35v chips and have taken a look at this as an option for li-ion charging. I'd be interested how you make out with it. Will you set it to 4.2v or go for 4.1v for longer battery life? It should certainly be fine at 5v solar level. LOL on the tinfoil!! -I'm so sorry about that comment! but its so hard to tell what level people are at :oops:


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