looking for a 12V battery UPS design+code using arduino

[nudist]

I am trying to make an egg incubator. This is also my first ever electronics project. It cannot be mains powered but only 12V. So I also need a UPS circuit which keeps the 12V lead-acid battery charged and powers the system. my primary incubator circuit is mostly done, so looking for any existing designs+code I can use to add a UPS.
The batteries would be 40Ah to 150Ah to get even 10 hours uptime from the battery. The UPS only needs keep the 12V lead-acid battery safely charged and durable, DOES NOT need to generate mains voltage (250V). My circuit should directly take power from that battery. The circuit will consume 12V4A to run the heater in the machine.

Any suggestions or links would be great help.
Thanks.

[Ian.M]

No Arduino required.  All you need to do is float charge the battery, using a CC/CV PSU, that can supply >4A at between 13.5V and 13.8V, and is protected against loss of mains supply while connected to the battery.

An easy way to do that is to use a CC/CV (nonisolated) DC-DC buck converter module plus a second hand 19V 4.7A (90W) or higher laptop PSU to power it.
This module appears suitable: https://www.amazon.com/Adjustable-Converter-Step-Down-0-8-28V-Inverters/dp/B07QB6577N
you can find similar on Ebay, Alibaba, etc.  Its important to choose one rated for at least double the output current you need as Chinese power ratings aren't trustworthy and you need it to be reliable.

Salvage the 12V rail Schottky rectifier from a dead desktop PC PSU (or buy an 8A or 10A, 20V or 30V Schottky diode) to use between the laptop PSU output and the module input to protect against loss of mains supply.  Keep the diode on the heatsink it was fitted to.  Set the module output voltage to be appropriate to float your battery (it depends on exact Lead Acid battery technology and ambient temperature - best to check with your battery manufacturer), then set the short-circuit current to be over 4A but under 10% of the nominal battery capacity in AH.  The higher you set the current, the faster the battery will recharge after a power outage.  The product of the current and the voltage shouldn't exceed 80% of the laptop PSU's wattage. 

Finally you can hook it up to the battery, and use it.

You'll probably also want to add a low battery alarm circuit so you've got enough warning you need to find an other source of power if the power cut is longer than expected.

If you are using a flooded cell Lead Acid battery with removable cell caps, so the electrolyte can be topped up, it *MAY* be worth considering putting the charging circuit under control of an Arduino, so it can track the charge state of the battery and after a power cut, boost the voltage for more rapid recharging, cutting it back to the float voltage as soon as the battery is charged.

[innkeeper]

Here is a good explanation on how to do it.

https://electronics.stackexchange.com/questions/71813/will-a-trickle-charge-lead-acid-battery-system-last

the concept is really pretty simple
1. maintain a regulated voltage of 13.7v to the led acid cell (reg1), too low and the battery will die and early death, and to high, it will overcharge and will end up boiling off the electrolyte. (take into account the voltage drop across the diode)
2. set the resistance to meet your trickle charge current expectations.

the diodes in the schematic will take care of switching the load over to the battery when the supply voltage at the load doips lower than the voltage from the led acid cell. the diodes also prevent back-feeding current into the regulators when power is lost.

though there are limits to this design. how much current are you needing?

[Ed.Kloonk]

../
though there are limits to this design. how much current are you needing?

From the OP...

The circuit will consume 12V4A to run the heater in the machine.