Hello

We have a startup project includes some wifi modules. These wifi modules working on 250mA. The module is isolated with an industrial box and it contains a wifi module circuit board, an SLA (selaed acid battery) battery and a relay. The modules will work as 7/24, they must be alive all the day and night, outdoor. We are not good at power electronics, so any useful recommendation here can work for us I guess, I wish.

Here the conditions:

1) We need 4.75V input minimum, the board has a 3.3V regulator, so we want to use 6V battery to feed the circuit board
2) We assume solar panel will get sunlight for 8 hours max in a day. So the battery must be recharged as much as fast but without damaging the battery
3) There is writing 20 hours on the SLA batteries mostly. It is optimal recharging duration as far as I researched but it is too long for recharging conditions. Also there is C/3 rule (capacity / 3) for the maximum constant current while charging the battery without damage, as far as I researched again.
4) The circuit board wasting 250mA and it will mean discharging the battery about 4A in 16 hours without recharging, according to the 24 hours - 8 hours sunlight (max) calculation (please see previous note).

Here the questions:

1) What should be minimum battery capacity for the life cycle without any shutdown? 4A, 6A, maybe 12A but it is so big, meaning the box will be very big for only that, it is a bad thing I think.
2) What is healthy charging time for the batteries? I mean the battery you will suggest as capacity
3) What kind of solar panel (also how powerfull do I need, how many Watts?) should be used?
4) Should I use a solar panel charger (please recommend a widely accessible one, also must be cost effective) or design my own circuit (please share a schema if possible)
5) Can I use a solar panel both for charging and feeding the circuit board if getting enough sunlight (maybe also means can effect recharging duration), otherwise feed with battery. Atleast using the solar energy directly if the battery is full but still good solar energy available, can be very useful. We just can't deal the what kind of circuit schematic since we are not good at power electronics, and dont want to damage the batteries, also maybe dangerious like explosion, I dont know, forgive me. I have any experience about it, all I know is using the voltage regulators when I need

The topic is a bit urgent. Awaiting your helps.
Best regards

 

Every module has own battery and solar panel by the way

 

Here is an idea to kick around Im doing it with one battery and an led lamp drawing 200 ma
for 12 hrs with an 8 hour charge time.

 

Don't use lithium ion batteries, stick with SLA.
Consult the SLA manufacturer's data sheet for maximum charging current.
Temperature compensate the battery charging voltage for best performance and max battery life, particularly if temperatures are extreme.
You'll have a very hard time finding gear for 6 volt batteries off the shelf; 12 volt systems are common.
If you want any sort of reasonable performance from your solar array you need a maximum power point tracking converter. Anything else will simply "collapse" the PV array voltage to the battery voltage, which typically will waste about a third of the power you could get.

gotta go, may return

 

You should consult a company that specializes in supplying PV power systems for this sort of application. Sizing the PV array and battery is not terribly difficult, but there are a lot of factors to consider. You assume 8 hours of sun per day. That is generally an unsafe assumption. What happens if it rains for three days? Will the array be operated reasonably close to Earth's equator? If not the angle of the sun relative to the array must be considered.

If you need 4.75 volts minimum input to a 3.3 volt regulator, you may have made a bad choice for the regulator. Is it a high efficiency switching regulator and is it well designed with proper attention to EMI and RFI management so it won't interfere with the RF signal?

 

Thanks for the answers. I realized 6V solar charging systems are not common after I designed the all things. I dont need already since I need only stable 3.3v and 5v for the relay. I choosed LM1117 / AMS1117 3.3 fixed regulator, I m open the suggestions if any about this.

This system will used in summer season only. I really dont know what can be done if it rains for 3 days. Probably manual charging operation will be required.

@N11778 schematic looking good except using lithium ion batteries, but I agree about consulting a PV specialized company this.

Yet, I will wait for other suggestions / solutions here, every information is valuable.

Thank you