Help designing a low voltage disconnect reconnect for LifePo4 3.2v batteries running a Rasp Pico

Thread Starter

Ralfy

Joined Feb 23, 2024
4
Hi,
I have a solar led light system. It uses a 3.2v LifePo4 battery. The LEDs are long since dead but the solar panel continues to trickle charge the battery OK. I'm trying to reuse the solar charging and batteries to run a Raspberry Pico W to which I've added a temperature sensor. The outcome is to use Alexa to get the temperature outside in the garden.

I can do all of this but have a problem. The Pico draws about 50ma at 5v. I've connected the battery to the Pico via VSYS and all works well. Pico VSYS takes a voltage anywhere between 1.8v and 5.5v. I can solder and follow circuit diagrams (although currently at a basic level, but willing to learn more). What I can't do is design circuits, and I need one that will low voltage disconnect at 2.5v and reconnect at, say, 3.2v. It's likely over prolonged grey weather the whole thing will be off longer than it is on due to the small trickle charge from the solar panel. I just want to baby step learn some stuff.

I've seen other examples of lvd's but they tend to be for Li-ion batteries at 3.7v and not for a Pico W and I'm also not quite there to be able to translate values from those examples to the values I need.

Is anyone willing to help with a diagram and briefly describe what and why? I know it's asking a lot. Any guidance is, of course, very welcome.

Regards
 

Thread Starter

Ralfy

Joined Feb 23, 2024
4
I found a diagram for a low voltage disconnect and reconnect on REUK but it uses a 5v supply to a voltage regulator. I know I can buy circuit protection for each battery off of Ebay.

I think, on thinking about this, I need a 3.2v equivalent of a 12v solar charge controller that has a solar input, battery connection and load output. I can only find completed boards that turn on the load automatically at night, basically a night light. This is not what I need. And of course again all examples I find are for 3.7v Li-ion batteries and most talk about a high voltage input (9v+). I need to check what the solar panel I have actually outputs but I don't think it's much. Possibly at most a 6v panel. If only the UK would brighten, just for one day then I could measure it.

Anyone know of a charge controller circuit with low voltage disconnect and reconnect that would meet my needs please?
 

Sensacell

Joined Jun 19, 2012
3,449
After reading the word salad, I still don't understand the requirements?

Can you distill this down to a diagram and clear bullet list of functions?
 

Thread Starter

Ralfy

Joined Feb 23, 2024
4
Thanks for the reply. Yeah my brain goes off in tangents and then I get too wordy. Hope this summarises it.

* I have an old garden solar led string light product. When it's daytime the panel charges a 3.2v LiFePo4 AA battery. When it's night the circuit switches on the output (the leds). During the day the output is off. So with the existing board I can't connect a Rasp Pi Pico as it will only fire up at night.

* So I connected the Rasp Pi Pico directly the the battery. Problem is it drains the battery completely. There's no protection on the battery

* To protect the battery I thought of using a low voltage disconnect to disconnect at a low voltage and reconnect once the battery is charged to a higher voltage.

* The alternative is to replace the existing battery charging circuit with another. solar in, battery charge/discharge, load out. This would still need LVD to protect the battery. This is to bypass the existing charging circuit that has the day/night function.

** Problem. I keep finding circuits to support 3.7v Li-ion batteries and I can't extrapolate to 3.2v LiFePo04

Regards,

Craig
 

Irving

Joined Jan 30, 2016
3,890
You don't need any extra hardware. Use the PICO's ADC to measure battery volts every few minutes and when it gets down to 2.7v or so, put the PICO into deep sleep mode, where it will take about 1.3mA. Even the smallest LFP battery (AAA @ 200mAH) at 80% discharge should sustain this for 12 hours or more. Once in deep sleep wake every x hours to check battery volts and resume deep sleep if not charged.

How long does the Pico run on a fully charged battery? There area number of things you can do to reduce power consumption by turning clock speed down etc. One of the worst things you can do is have the Pico running at full tilt while waiting temperature requests. Turn the speed down when not servicing a request. Better still, go into deep sleep, then wake every 10min, read the temperature and use MQTT to send it to a 3rd-party MQTT broker from where Alexa can read it, then resume deep sleep. Outside temperature doesn't change fast enough to require continuous reads. I do this with my ESP32 based temperature monitor to a phone app. This reduces power consumption to <10% from before.
 

Thread Starter

Ralfy

Joined Feb 23, 2024
4
Irving,
That's a great reply. I did consider rather than realtime requests to send readings to a queue or my NAS. I knew the Pico could go into deep sleep. What I wasn't aware of was the fact it could wake itself again. I'll take a look at that as a solution. Thanks.

Regards,

Craig
 

Irving

Joined Jan 30, 2016
3,890
Irving,
That's a great reply. I did consider rather than realtime requests to send readings to a queue or my NAS. I knew the Pico could go into deep sleep. What I wasn't aware of was the fact it could wake itself again. I'll take a look at that as a solution. Thanks.

Regards,

Craig
You're welcome, and welcome to AAC.

This article my be useful to you.
 
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