Version of LiPo Solar Charger (390) at ~7.4v?

by topherwhite on Mon Mar 18, 2013 5:28 am

We a very specific question about the USB/DC/SolarLithiumIon/PolymerCharger (#390). This product is based on the MCP73871 controller, which is only suitable for charging 3.7v batteries. Does Adafruit plan (hopefully in the near future) to release a version of this product based on the MCP73862 controller (or any other controller) capable of charging 7.4v batteries (and supporting a load around that voltage)? If not, could anyone suggest where we might find another charge controller that might be able to accomplish this? The voltage of the solar input is flexible, but would likely be ~12v.

As the operating voltage of our required system is ~5v, the efficiency loss in using ~3.7v charge controller has proven to be too much.

Quick note: we are aware of the multi-3.7v-cell charger system proposed by Adafruit here: http://bit.ly/142VyE9 . Unfortunately, the 3PDT switch system (manually or electronically controlled) is not feasible for the system we must build.

Many thanks in advance for any help and advice you can provide.
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by adafruit_support_bill on Mon Mar 18, 2013 5:46 am

We do not have any plans currently to build a 7.4v charger. It is possible to build the multi-cell charger system without the 3PDT switch. But you need to use 2 power supplies so that the 2 chargers do not share the same ground.
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by topherwhite on Mon Mar 18, 2013 4:00 pm

Thanks very much for your response. Given your suggestion--implemented using two solar panels grounded separately (each charging 3.7v battery though a separate LiPoly charger)--should we expect to see issues if the batteries were not always equally charged? Thanks again!
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by john444 on Mon Mar 18, 2013 9:34 pm

Hi Topher,

topherwhite wrote:should we expect to see issues if the batteries were not always equally charged?
Not if you use the "LOAD" connections on the 390.
Keep in mind that, on average, you will not be able to pull more Amp-Hours
from the LiPos than you put into them from the solar-panels.

A couple of assumptions here:
The solar-panels are the same size/capacity and have the same solar exposure.
The LiPos are the same model/size/capacity and about the same age.
The 390s are set to the same charge-rate.
You are pulling the same current from each LiPo (because they are in series)

Ideally, you would adjust the load to match the output of the solar-panels.
The output of the panels will be much less in the winter or on overcast days
compared to the panel's rating.
(The 'rating' is based on - solar noon, June 22, on a bright, cool, cloudless day).
In other words, expect less than the maximum rated output from any solar-panel.

topherwhite wrote:should we expect to see issues if the batteries were not always equally charged?
If the panels can completely re-charge the LiPos on a regular basis,
then the LiPos will be at the same charge.

Good Luck, John
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by topherwhite on Mon Mar 18, 2013 9:57 pm

Thanks very very much for the detailed answers!

I'll try to implement this ASAP and report back here when there are some results.

If you're interested, I thought I'd provide a quick note (based on your comments) to give you some context for the project: we're building a system that we need let to operate autonomously for indefinite periods--powered daily by solar panels--sitting in [only ever] indirect sunlight in the canopy of trees, where the efficiency of the solar panels is horrendous ( < 4% with the panels we're currently using). Thus, efficiency of the internal circuitry is of the utmost importance.

We're inexperienced, for sure, but passionate and determined to make it work! Thanks again for your help.
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by john444 on Mon Mar 18, 2013 10:41 pm

Topher,

I am interested. What are you doing in the canopy?
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Re: Version of LiPo Solar Charger (390) at ~7.4v?

by topherwhite on Mon Mar 18, 2013 11:13 pm

Hi John,

Cool! The project is called Rainforest Connection ( http://rfcx.org/ ). We're an early stage non-profit startup in San Francisco -- using old discarded cell-phones to monitor the sound of threatened areas in the tropical rainforest, identifying poaching as it occurs, and sending real-time alerts to allow for actual intervention (ie. catching the perpetrators). It works, but the next iteration of the hardware will (ideally) be able to operate entirely autonomously for months at a time (at least), by generating (on average over time) more energy than it consumes. That'd be easy in direct sunlight, but we really won't have it. Luckily, at equatorial latitudes, the seasonal differences (in terms of solar intensity and daylight hours) are minimal. No doubt, though, managing/minimizing power consumption is the single biggest engineering challenge (further complicated by warm, humid conditions).

Anyway, that's a very quick snapshot of the project. Let me know if you would like to know any more. We really appreciate your help!
-Topher
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