Solar charging with BQ24074 + PowerBoost 500

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purpleturtle
 
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Solar charging with BQ24074 + PowerBoost 500

Post by purpleturtle »

Goal:
I want to re-create the charging circuitry for this type of solar USB charger:

Image

Background:
I had purchased the solar USB charger pictured above from an online retailer. I noticed it didn't seem to charge my USB battery bank at all, so I opened up the controller box out of curiosity and was surprised to find an utter lack of circuitry:

Image
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Following the traces, all it has is:

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Whereas a proper controller board should look like this (source: https://techobsessed.net/2017/09/baligh ... -teardown/):

Image

So I had a unit with real PV panels but a fake controller board. The seller at the online retailer has vanished and I was unable to obtain a refund for this product. So, I'll turn this into an educational opportunity to try and recreate the charging circuitry.

Plan:
There are 5 photovoltaic panels, each rated 6 volts, 5 watts, connected in parallel. Testing them with a multimeter confirmed this. From my research, I'll need:
- Solar charger https://www.adafruit.com/product/4755
- 4700uF 10V electrolytic capacitor https://www.adafruit.com/product/1589 (for the 4755)
- 5V boost converter https://www.adafruit.com/product/1903
- JST cable https://www.adafruit.com/product/4714 (to connect the 4755 and 1903)

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Questions:
- Will this setup work, or am I missing any components from the plan?
- I am not interested in having a LiOn/LiPo battery connected to the 4755, as I'll be charging a USB battery bank in any case. Will the 4755 function without a battery connected?

Thanks in advance for any assistance!

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adafruit_support_bill
 
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Re: Solar charging with BQ24074 + PowerBoost 500

Post by adafruit_support_bill »

That might sort-of work. But probably not very well. There are a number of issues:

* The solar charger is not serving any purpose in that circuit. With no battery, it will simply pass the solar power through to the Load outputs - minus some loss in the regulation circuit.
* Boosting the output to 5v in order to charge a battery bank is doubly inefficient. You will incur conversion losses through the PowerBoost boosting to 5v. Then the PowerBank will incur further losses regulating the 5v back down to 4.2v in order to charge the internal LiPo. Only to boost back up to 5v (with associated conversion losses there). It would be more efficient let the charger charge the battery and minimize the extra boost & regulation stages.
* Unless your panels have integral protection diodes, you will need to add some to protect possibly shaded panels in your array from reverse current.

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purpleturtle
 
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Re: Solar charging with BQ24074 + PowerBoost 500

Post by purpleturtle »

I see, thanks for the response! So then, new plan:

- Solar panels, each with bypass diodes, connected to 4755
- 4755 to battery case (via BATT port) holding an 18650 battery
- The 18650 will be removed when charged, and subsequently used in other devices

Though, that leaves me wondering how the original circuit (not the fake one) would've been implemented in the first place. Would it not incur the same losses in efficiency while charging a USB battery bank?

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adafruit_support_bill
 
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Re: Solar charging with BQ24074 + PowerBoost 500

Post by adafruit_support_bill »

No real-world power conversion process is 100% efficient. Boost converters, regulators and charge controllers all get warm in use. That heat is energy that is wasted in the conversion process. The batteries themselves also get warm both when charging and discharging. That heat is also waste energy from the electrochemical conversion processes inside the cell. What you need to consider when designing circuits like these is how much energy can you afford to lose.

If you are plugging your phone into an AC powered charger for a quick-charge, you probably don't care much about how much is wasted in the process. When dealing with a limited (and highly variable) source like a solar panel, you need to think through the energy requirements in a bit more detail. For something like an unattended solar-powered weather station that needs to run 24x7, you will want to have some generous safety factors to account for things like a stretch of cloudy days.

Mike has a nice tutorial on the subject here: https://learn.adafruit.com/energy-budgets

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