I am building a weather station that wirelessly transmits data to a host system. The station is in a remote area that does not have any power sources, therefore, i wanted the system to run off of a single 3.7v li-ion battery (powering a 3.3v arduino pro mini at 8MHz). I also wanted to implement a solar panel to charge the battery during the day. Simplicity and cost are of major consern (as i may be building a lot of these for various areas). I know that there exist single chip solutions to battery charging, however, i am unsure that hey are appropriate for a system in which a battery is in-place and being charged and used at the same time. I know this is not a new idea. Everyones cell phones and iPods do this. I was looking at the MAX1555 (i think is the P/N). Is this the solution? Thanks.
Yes you can charge the battery at the same time you are using it with that IC. I'm using the same IC (charger board from SParkfun) for my project and am able to do both.
How well it does it is a different story. You'll only really be charging the battery if the charge current (from the MAX1555) is greater than the current being drawn by your project. You'll need to know the current draw on your project and the power output of the solar cells to see if it's practical. I'm sure there is a lot more to it but that's the easiest way to look at it.
Thats what i thought. So as long as i have a solar panel with a current rating greater than the draw of the system, the system will run and the battery will charge, with the remaining current. Once the sun goes down, the MAX1555 will recognize that the source charging voltage from the solar panle is gone, or too low to be useful, and the battery will take over and continue to power the system until the sun returns. I was looking at this also:
It seems to be the same type of device, however it is capable of higher charging currents (up to 1A).
Im thinking that an ideal setup would be a 6 volt solar panel outputting ~350ma as the input source to the charging chip which is charging a single 3.7V Li-ion battery and running the system with a maximum current draw < 200ma. Also, the system would sleep most of the time, thus average current draw would be significantly less and more current would be available for charging.
Does this sound correct? Or am i on the road to kill some batteries over time? Battery life is also a major consern for me and I was thinking that i should include a Solit State Relay to cut off the solar power to allow the battery to discharge at regular intervals. Or is that overkill?
Thanks again for the input.
Does this sound correct? Or am i on the road to kill some batteries over time? Battery life is also a major consern for me and I was thinking that i should include a Solit State Relay to cut off the solar power to allow the battery to discharge at regular intervals. Or is that overkill?
These kinds of questions are hard to nail down without having a specific design in front of oneself with links to the datasheets for the solar array, charger, battery cell and load specifications.
Solid state relays are used only for AC voltage control not DC. Either a relay or transistor switch would be required. That aside most solar arrays only utilize a series diode to prevent reverse current flow and just let the load decide how much current to try and draw from the array.
Li-ion batteries have several safety requirements, most are handled by a purpose designed Li-ion charger controller. However one you didn't mention was protection of the cell from being over discharged by the attached load, which will destroy the cell rather quickly. One needs to monitor the cell's terminal voltage and remove all loads to prevent the voltage from dropping below around 3vdc/cell.
Lefty
Has anyone seen or used this device? It seems like an all in one solution to single cell li-ion battery monitoring/charging AND system powering.
Check out page 10.
I agree with Lefty, there are lots of variables to consider. As for the battery I would look for one with overcharge and discharge protection built in to prevent battery damage. You'll also need to do some rough calculations on how long you can run the system without sunlight. If you're drawing 200 mA and your using a 1000mAh battery then the battery will only last 5 hours.
I have the BQ chip in a drawer somewhere but I haven't used it yet but for your application it might be a little overkill. I know the MAX chip has built in protection so you don't have to worry about the battery discharging back though the IC.
My major consern with a charging system in which the battery being charged is directly connected to the system (which in my example is continuiously drawing current) is that how will the MAX know the difference in system current demands and battery current demands? If the system is always drawing say 200ma and the source is always providing 300ma, then 100ma will be available for charging the battery, however, the max1555 will sense at least 200ma of draw even with a full battery and never register "charged", right? Is this point where one must rely on the internal battery protection circuit?
The BQ24071 is designed to monitor input, system output and battery seperatly and switch to battery power only when the input is insufficient to suply the system. Its also only about $.60 more than a MAX1555.
Here is a VERY interesting read on this exact topic.
http://www.power-eetimes.com/en/how-to-design-a-li-ion-battery-charger-to-get-maximum-power-from-a-solar-panel.html?cmp_id=71&news_id=222900632
You're right that was a very interesting article, looks like they're describing exactly what you are trying to do.
I made a circuit that did basically what you are looking for I think. I found my design at instructables at http://www.instructables.com/id/Solarize-your-backpack-and-power-all-your-gizmos/. Anyway, the circuit only charges the batteries if they aren't full and doesn't overdrain them. You obviously wouldn't need the backpack, but you might find the circuit useful.