Solar powered project, multiple questions - Page 1
Forum section: electronics / beginners

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 08:35:58 pm
content: Hello friends, this is my first post hereI am trying to make a little solar-powered display device. I have several things I would like it to do. I would like it to have a battery pack that gets charged by a fixed solar panel during the daytime. I would like to use the batteries to power an ARM microcontroller board (likethis) and a little LCD screen for displaying text (likethis one here). I would like the run both of these 24/7, but turn off the backlight during the day. Ideally this device would last a very long time in a (dry) outdoor environment. I've been investigating battery technologies, and I believe that a NiCd battery pack is my best bet, based on the fact that it can withstand more than a thousand charge cycles and doesn't mind being trickle charged, along with being safe (even if not great for the environment). I'm pretty sure I have most of the details figured out and under control, but I have a couple problems.First, I would like to run the battery pack (4 AAA NiCd cells) in series, since the LCD screen requires close to 5 volts. However, I want to prevent the voltage from going over 5.2V or so to avoid damaging the electronics, but still allow it to fall to 4.8 or so. That is to say, I want to keep the supply voltage within the range of 4.8 to 5.2 volts. If the voltage is too high, I want to regulate it down to an acceptable voltage. If the voltage is too low, I would like the device to power itself off. I have looked everywhere for a design to accomplish this but have had no luck. Using something like a linear regulator would be great but I want it to simply allow voltage through if it's below the target voltage, instead of just dropping out.My second question also relates to the batteries. I would like to have a very simple charging circuit that is capable of charging the batteries in parallel from the solar cell. A solar panel with a high enough voltage to charge the pack in series would be too large and also risks causing cell imbalances over hundreds of cycles. I have found that with schottky diodes on each connection to the cells it prevents shorts across the cells, but I still need a way to prevent overcharging the cells. I was thinking that if I had a zener diode of the cell voltage I want driving a transistor to connect and disconnect the charger from each cell that I could easily float charge the cells as needed, but this doesn't seem to work. I need to figure this out, but yet again I've given up finding information on this.Any ideas? Thanks!

author: Paul Price
author_rank: Super Contributor
timestamp: January 22, 2015, 09:06:13 pm
content: Use a Li-Ion battery instead, use a boost convertor to raise the voltage to the levels you need directly from solar cells connected in parallel..A Microchip MCP1624 would work very well with low power solar solar cells. It can raise a very small voltage to an exact voltage to float charge a Li-Ion cell and the MCP current output would not ever cause the Li-Ion battery to be damaged by excess charge current. In this way the Li-Ion battery is protected from overvoltage and overcharge. I would suggest using one or more Li-Ion batteries in parallel of the type used in cellphones or Smart Phones. They are cheap, and very easy to buy and their flat geometry and light weight are attractive advantages over Ni-Cd. The Li-Ion battery can then directly feed a LDO regulator to power your MCU circuitry if your MCU runs at 3 to 3.3V.If you need 5V for your MCU you can then use a second MCP1624 fed directly from the battery to convert the 3.6 to 4.2V of the Li-Ion battery to a steady 5V.

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 09:11:59 pm
content: Do you have an idea of how efficient stepping up from 3.7V to 5V would be? I'm a little worried that it would eat too much power.Thanks

author: Paul Price
author_rank: Super Contributor
timestamp: January 22, 2015, 09:13:26 pm
content: The MCP1624 datasheet shows efficiency >90%..take a look at the performance charts in the datasheet.

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 09:22:50 pm
content: All right, thanks. This definitely seems like the simplest design, and 90% is a lot better than I was expecting. My only main concern is using Li-Ion cells since their cycle life is not great. I'm also still so curious to know whether the parallel charging thing could be made to workCan anyone think of a way to accomplish it?

author: Paul Price
author_rank: Super Contributor
timestamp: January 22, 2015, 09:37:37 pm
content: If a Li-Ion battery is charged slowly and to a conservative float voltage and your MCU is careful to not over-discharge the battery, the number of charge cycles should be comparable if not better than Ni-Cd. Be aware that Ni-Cd cells also suffer from "memory effect" charge problems. Li-Ion batteries live longest when slowly charged and slowly discharged, both at a fraction of their C capacity. Multiple cells will give the longest service life if your current demands approach the C capacity of a single cell.New Li-Ion cells can easily be paralleled if they are identical, new and first charged/discharged to close to the same voltage. Paralleled cells then function just like a single cell. Paralleled cells connected in series are almost always used inside laptop batteries.How often have you had to replace the Li-Ion battery in your cellphone?

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 09:52:56 pm
content: All right, I concede, sounds like I won't have any problems using Li-Ion cells. Thanks for the informationHow would you recommend charging a Li-Ion cell from a small 1W solar panel likethis one? Most of the time its output voltage will be below the voltage of the cell so I'm guessing I'm going to need some sort of boost regulator, but most of the ones I've found are for larger panels and are massive overkill.Thanks again

author: Paul Price
author_rank: Super Contributor
timestamp: January 22, 2015, 09:57:18 pm
content: That is exactly what I've been saying in my replies above, use a boost converter.The MCP1624 starts up and works well with under 1 volt feeding it.

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 09:58:49 pm
content: All right. Should I put some sort of Li-Ion charge controller after it, or is there a way to combine the two?I mean, I don't want to just push a fixed voltage into the battery right?

author: Paul Price
author_rank: Super Contributor
timestamp: January 22, 2015, 10:01:18 pm
content: The MCP1624 prevents over-current charging and its output is regulated and so exactly sets the float charge voltage to optimally charge your Li-Ion battery.

author: andermi
author_rank: Newbie
timestamp: January 22, 2015, 10:04:13 pm
content: Ah, okay. I didn't realize that the 1624 actually functions as a charging circuit as well. I think I get it nowThanks again!