Using constant current buck converter with common ground

Projects General Guidance
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I'm planning to use a CC/CV buck converter to float charge 12V SLA battery. The issue is that pretty much all buck converters I was able to find on Aliexpress and ebay have the shunt for current sensing between the input and output ground. So technically non-isolated regulators don't share the same exact ground, but through a shunt.

If the load needs to draw something way higher (5-10A) than the buck module is designed for (~500mA max CC for charging the battery) the shunt causes large voltage drop if the external load's current passes through it.

However if I connect everything to the main circuit ground (before the buck, and thus before the shunt) the buck converter doesn't see what current is being drawn and will damage the battery, because there's no current regulation.

I would like to avoid complex battery charging circuits and I'm not able to isolate the battery completely, because I want the load to be able to be powered from Vcc (the same +12V input for the circuit and the buck converter), so the ground will need to be the same for the whole circuit.

What are my options in this case and is this the correct approach?

Buck converters, most of which I've got:
https://www.aliexpress.com/item/32800349586.html
https://www.aliexpress.com/item/4000470070083.html
https://www.ebay.com/itm/191978186172
https://www.ebay.com/itm/191902394789
https://www.aliexpress.com/item/32807827797.html

Qucik schematic:

Battery Regulator
Battery Regulator1200×663 13.1 KB

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It looks like things went wrong with the two links. They both point to this topic.
Can You verify that the charger current is 500 mA but Your need for battery current might be 5 - 10 Amps at the most.
If so, it sounds like changing the charger is needed.
Not using the setup You want I understand Your question.

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Your description doesn't make sense to me on several points:

Typical buck converters don't have a shunt in the ground.
You said constant current and charge an SLA, you charge an SLA with constant voltage.
What you described, as far as I can tell, is simply, you charge the battery with a constant 13V8 and connect the load across the battery.

Please provide a schematic of what you are proposing instead of a description, which is near to useless.

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It looks like things went wrong with the two links. They both point to this topic.

Please provide a schematic of what you are proposing instead of a description, which is near to useless.

All links seem to work for me and I've already included a schematic. I'll edit the post to show it by default.

Can You verify that the charger current is 500 mA but Your need for battery current might be 5 - 10 Amps at the most.
If so, it sounds like changing the charger is needed.

You said constant current and charge an SLA, you charge an SLA with constant voltage.
What you described, as far as I can tell, is simply, you charge the battery with a constant 13V8 and connect the load across the battery.

As far as I know it's recommended for pretty much all types of batteries to be charged with constant current roughly in the first half of the charge time, then hold a constant voltage until the current reaches the end charging current according to the battery spec.

My SLA is a 12V 4.5Ah which states < 1.35A initial charging current or < 500mA for long lifespan (and float charging at ~13.5V).

Typical buck converters don't have a shunt in the ground.

I don't have that much experience with buck/boost converters, but from what I can find online all have either THT shunt, long PCB trace or ~0.05-0.1 ohm SMD resistor as shunt accross the ground.

Not using the setup You want I understand Your question.

A better question would be how can I modify existing one or where can I find buck + boost converters with adjustable constant voltage and constant current capability that have non-isolated single ground plane? Or should I take a different approach to the whole idea?

What I'm trying to do is switch a load between the main VIN input and bypass all charging/battery circuit when there is power on VIN, and transfer to a powerfull buck/boost converter connected after the battery when the power is out. The charging buck converter ONLY charges the battery with 500mA when there is power and shuts off when there is no power.

Something like a bare-bones DIY UPS that needs to output regulated, stable 12V NOT directly from a battery (ranging 10-14.5V on typical SLA).

Hope that makes sense, even though my main concern was about the ground shunts on the buck converters.

5

Every battery chemistry has different requirements, there is little you can say about one chemistry that is applicable to another. For lead acid batteries you just need to put 2V2 across each cell, so 13V2 for a 6 cell ('12V') SLA battery. There is some margin to that, and it can be increased slightly for a faster charge when flat. There are also various schemes for varying the voltage during the charge. If you want simple stick to 2V2 per cell. If you want advanced details search for The Battery University or see the manufacturer's data for the battery. Any current limiting is to avoid damage to the charger, an SLA battery will take what it needs. I would expect any buck converter of any kind to have built in current limit to protect it from damage in the case of overload.

I've never seen a buck converter as you describe, not saying they don't exist...

You don't need a buck converter as you describe for the reasons given above.

Your schematic is incomplete, where, for example, does the input to the buck converter come from? I see nothing that suggests a problem with grounds.

What is the relay for?

What is Vcc in this context?

What is the purpose of the circuit?

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VCC will be AC-DC SMPS 12V 5-10A. It outpus exactly 12V so the battery can't be fully charged directly. I'll need a buck/boost converter to supply stable ~13.5V for float charging. However the type of buck converter I would need here might not work, as also noted here: ground - Is the '0V' on a non-isolated buck converter the same on the input as it is on the output? - Electrical Engineering Stack Exchange

Every converter I've had has short/OC protection, but that works only if the current is flowing between the output + and GND. If the grounds are common it doesn't regulate or protect from OC anymore and might get destroyed if there's no any external regulation or protection.

Transferring between VCC and battery. The switching is controlled most likely by a nano with a relay.

From my last reply:

I might end up redesigning everything in that case, unless I find suitable converters.

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You are over thinking it; what you are trying to do is simple, no relay, no switching, no problems with ground. You need a power supply with 13V2 output and enough current capability to supply the load. You put the power supply across the battery and leave it on. Next you need a buck converter with an output voltage that is correct for the load. You connect the buck converter input across the battery and the output to your load.

You should put a fuse in series with one of the connections to the battery because a lead acid battery can easily supply enough current to heat up wires and start a fire if there is a short circuit.

That's all you need.

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