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LIPO SOLAR CHARGER LIMITS
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

LIPO SOLAR CHARGER LIMITS

by andyrob on Sun Oct 17, 2021 2:41 pm

Hi, I'm using the LiPo Solar charger to try and power an Arduino and two servos. With some additional voltage regulation and some big capacitors I can get the whole thing to work, but the voltage output from the module drops from about 4v to around 3v when a servo operates. I presume this indicates I'm trying to draw more than the rated 1.5A from the module? Overall I will be averaging a lot less than this, but am I damaging it? If there are any suggestions about how I might even out the power consumption (bigger capacitors only seem to get me so far), that would also be great.

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Re: LIPO SOLAR CHARGER LIMITS

by adafruit_support_bill on Sun Oct 17, 2021 2:57 pm

Which Solar LiPo charger are you using? And what servos?

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Mon Oct 18, 2021 12:48 pm

Hi, I'm using the bq24074 (4755) solar charger rated at 1.5A. I'm using a SAV-SW0230MG Savox water proof servo, and the other servo is a homemade hack combining the control circuit from a Power HD LW20 Waterproof High Torque Metal Gear Servo HD-LW-20MG and a worm-gear motor - this combination seems to create less load than the Savox servo. I'm running both the servos on a 5v regulated supply. I have a separate 7.5v regulator for the Arduino going to VIN; I've also tried a 5V regulated input directly to the 5V pin, with similar results. The servos are rated at 10Kg/cm for the Savox and 20Kg/cm for the Power HD, but that would be at a higher voltage (6.5V).

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Re: LIPO SOLAR CHARGER LIMITS

by adafruit_support_bill on Mon Oct 18, 2021 1:02 pm

The Savox will likely show peaks of around 1.5A at the start of a move since motor startup is equivalent to a stall condition until things start moving. But assuming that it actually does move, the average load should be lower.

Please post a diagram showing all of the components in your circuit.

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Mon Oct 18, 2021 1:07 pm

OK, I will do, but it's going to take a little while to draw it out (but lots of Adafruit products you'll be pleased to hear)

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Sat Oct 23, 2021 6:38 am

Here are a couple of diagrams (apologies for poor skills).

1. Block diagram of overall system:
Block Diagram.png
Block Diagram.png (27.42 KiB) Viewed 84 times


2. Here's the solar power part in more detail, with the servos:
Solar_schem.png
Solar_schem.png (112.74 KiB) Viewed 84 times


Apologies for poor labelling on capacitors - wrestling with Fritzing. I'll also post an oscilloscope trace later when I get some time.

Thanks for your time

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Re: LIPO SOLAR CHARGER LIMITS

by adafruit_support_bill on Sat Oct 23, 2021 6:49 am

I don't see any power sources in your diagram? What are the specs of the solar panel and battery you are using?

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Sat Oct 23, 2021 2:33 pm

OK sorry, the battery is a 4v, 13.4Ah LiPo cell (well obviously it's a number of balanced cells in a single package), and the Solar panel is the Adafruit 6v, 3.5W large solar panel (product ID 500). Currently I have been running all the tests just using the battery.

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Sat Oct 23, 2021 2:42 pm

The test program is sweeping the Savox server slowly, each sweep taking 800ms. The slow sweep increments every 4ms approximately, so you see a 240Hz signal overlayed on top of the larger voltage fluctuations. Every 10 sweeps the other servo is activated in it's own little 400ms window, but it takes around 6s to do a sweep (the motor is going through a high reduction worm drive).

Everything works fine for about 20 minutes or so and then crashes. I have a watchdog timer set up on the Arduino which sometimes is able to restart after the crash, sometimes not.

Here 's a couple of traces from the Picoscope.

The blue trace is the voltage at the output of the Solar charger module. The red trace is the 7.5v input to VIN on the Arduino.

20211023-0001.png
This is a high level trace showing the variation over 20s
20211023-0001.png (38.85 KiB) Viewed 72 times


This is a more detailed trace showing the shorter 800ms cycle:

20211023-0002.png
20211023-0002.png (27.1 KiB) Viewed 72 times

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Re: LIPO SOLAR CHARGER LIMITS

by adafruit_support_bill on Sat Oct 23, 2021 3:04 pm

the battery is a 4v, 13.4Ah LiPo

4v? The BQ24074 is intended for 3.7v nominal, 4.2v full charge LiPo chemistries. There are other variants of the BQ2407x chip intended for different battery chemistries.

There are a couple of boost converters in your circuit. A voltage boost comes with a corresponding drop in current - plus some loss in the conversion process. Assuming a 90% conversion efficiency, if your servo is generating peak demands of 1.5A @ 5v, you will need closer to 2.2A at 3.7v on the input to the boost converter. That would exceed the capabilities of the BQ24074 and cause it to reduce the output voltage (see section 9.3.4.1.3 of the datasheet). A large capacitor (e.g.4700uF) between 5v and GND mounted close to each of the servos might help fill in for the worst of the startup current.

The 7.5v boost converter is a bit wasteful, since the input to the Arduino is going to be immediately regulated back down to 5v. So you will incur conversion losses in both the boost and re-regulation. I'd replace that with another 5v boost converter and feed 5v power to the Arduino via the USB jack.

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Sun Oct 24, 2021 11:19 am

Looking on the packaging of the LiPo cell it says 3.7V, so I think that's just me being a bit loose with the specification - I wasn't aware there was more than one kind of LiPo cell.

I'll definitely try your suggestions with the capacitors, and I'll see if I can get better results with using 5V rather than VIN for powering the Arduino.

Thanks for looking at this.

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Re: LIPO SOLAR CHARGER LIMITS

by andyrob on Sun Oct 24, 2021 11:22 am

One last question, you suggested 5V via the USB. Is this different from 5V via the 5V power pin? It's a zero so it's going to get regulated down to 3.3V for most of the electronics.

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Re: LIPO SOLAR CHARGER LIMITS

by adafruit_support_bill on Sun Oct 24, 2021 11:30 am

That should work. Although it will bypass the polyfuse and the automatic power selection circuit. So take care not to short anything and don't plug anything into the VIN jack.

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Please be positive and constructive with your questions and comments.