Hello, on an active building site i have a few stands with some monitoring sensors, rpis, cameras etc used to monitor the build process, prevent theft etc. There are occasions, when the mains electricity has to be turned off for safety.

In there have a small switching module (https://www.aliexpress.com/item/4000038641968.html), that switches between power in case of mains failure. Primarily it is using 12VDC power coming from power adapter powered by mains 230VAC, in case there is an electricity outage it switches to backup power from 12V traction lead acid battery. Lead acid battery is charged by simple "refresher" lead acid battery charger (one of the stands have also 18V 100W solar panel with charger controller for lead acid batteries). Its what i had home, so i used those.
This refresher chargers are not powerful enough to provide current to handle the load, and after some time throw out a fault and stop charging completely. So i cannot use it in a way where i power all directly off the battery.

The problem with the switching modules is, that they do not switch the power without interruption. There is a short, maybe 0.5 seconds delay, which causes all the electronics to reset.

--------- question starts here ------------

So the voltage is 12VDC. As a test i have added 25V 10000uf capacitor and 45V 20A schottky diode (its what i had home) "between" the module and the load. This is enough to keep the electronic to survive the short interruption. However to have this circuit to be somewhat complete, i would like to add a capacitor discharge resistor and probably a NTC thermistor to limit the inrush current when first charging the capacitor.

I do have a 5W 2000Ohm cement resistors at home, so i think those should be ok as discharge resistors. If not , what discharge Resistor should i use ?

Question is, what NTC thermistor (value, type, etc) should i use to limit the inrush current. A direct link to ali is ok, but i can find it myself.

Very simple picture of the circuit in attachement.

I dont want to spend too much additional money to buy a different chargers, modules, etc.
I do not want to directly connect the 12V power adapters to the battery.

Any other tips, connection shemes etc are welcome.

Thanks.

 

Any other tips, connection shemes etc are welcome.

Why not just use a UPS and avoid needing caps, diodes, surge limiters, etc?

 

So you have a system running on 12 volts powered from mains power, which also maintains a 12V standby battery. When power is interrupted the system switches over in about half a second, which causes everything to reset. You want to know whether a super capacitor can bridge that switch-over time. That all depends on what the load is and how much capacity the capacitor has. 10,000µF isn't much of a super cap. That's actually an old term, 10 Milli-Farads. Haven't heard (or used) that term in many years.

Here's a 1F 5V super-cap: https://www.mouser.com/ProductDetai...Cu9HefNWqpn3b3cwPheW83FuAC6L03SDDMZlAY8VzgA==
One Farad isn't likely going to be enough to hold very many circuits going. AND this is merely 5V, not the 12V you're looking for. For a bank of super-cap's that can measure up to 12 volts (or 18 volts) you're going to need a lot of them. And they're not cheap.

Here's a bigger one: https://www.mouser.com/ProductDetail/PowerStor-Eaton/HS1040-3R8157-R?qs=sGAEpiMZZMsCu9HefNWqpn3b3cwPheW8jtGws25UTkHI52iq2dZkKQ==
That one is 150F at 3.8V. It's "unit" cost is $11.56. You'd need four of them. That'd be over $40.00.

Just a guess, but maybe 150F is enough to run the electronics during switch-over.

Consider an alternative: A 12V battery charger that can also provide sufficient power. Not the solar charger. Powered from mains, the charger can maintain the battery voltage AND power the electronics during mains interruptions. That way the electronics are always on battery power regardless of mains power or the lack thereof. I'm assuming that power interruptions are not long and the 12V battery is sufficient to power all the electronics during that interrupt period.

Good luck with your project. If you want better help then we need to know a lot of things: Battery type, capacity (amp hour rating) how many amps all the electronics are drawing, and maybe even the switch type that switches from mains power to backup power. That way we can get a picture of how the system is performing. We also need to know what the "interrupt" period (length of time) is. You may be able to get by on a small 12V Lead Acid battery if the interruptions are short and the current draw on it is not too heavy. A heavier load would require a bigger battery. But all I can do is merely guess at what you might need. Without the details (home of the devil) I'm not able to give you any further advice.

I WILL say this: A 12V car battery is typically held to a float charge of 13.8V. I don't know if your electronics are going to be sensitive to voltages above 12V. And a cheap car battery charger might not regulate the voltage very well. I have a cheap 40 amp charger. I've seen voltages go up into the 15V range. So do the research and get the right equipment for the job.

 

Why not just use a UPS

Primarily it is using 12VDC power coming from power adapter powered by mains 230VAC, in case there is an electricity outage it switches to backup power from 12V traction lead acid battery.

UPS is not a bad idea. But since the system in current use already has a 12v source for the electronics, the size of which "source" we don't yet know, a battery could be maintained on that 12V battery - IF the electronics can tolerate a 13.8 float voltage. UPS is not as efficient in some cases, and the cost is even higher than four 150F super-caps of about $46.24 (the ones I linked to). It's my opinion (and no one is obligated to accept my opinion) is that use of a decent charger would likely do the job and just run the electronics off the battery at all times. Regardless of mains power, the battery is always there and there are no brief interruptions during switching. Yes, an UPS has either very fast switching OR it's powered (at mains voltage) but uses the same sort of backup I'm talking about. The UPS powers electronics from its battery and it maintains the battery voltage at all times during mains power availability. When mains drops out, the UPS remains uninterrupted.

 

The shottky diode is fine for the discharge side of the supercap, which you will need 3 of the five volt ones in series. For the charging side just use a 2 amp diode and a 100 ohm resistor. Certainly it will take a few minutes to charge the supercap, but that should be OK. I am thinking the failures do not happen in quick succession. One very simple circuit to charge, and then a diode circuit to deliver the charge.

 

Why not just use a UPS and avoid needing caps, diodes, surge limiters, etc?

There are total of 6 of those stands, some have just single camera, some have multiple cameras, rpi, nvr, etc. They are far away from each other (50+ meters). They can be separately disconnected from mains power, as its needed for safety when working, or when some heavy machines need to go through and the temporary cables are moved, so backing up one central point is also out of option.
So i would need a separate UPS for each of those stands.As mentioned before, the system i have now, was pretty much all done with stuff i had already home. Buying 6 UPS is not economically viable, especially as this whole system is temporary, as when building is finished, it all would be dismantled and eventually some things like cameras, batteries etc repurposed on the finished building.

 

There are total of 6 of those stands, some have just single camera, some have multiple cameras, rpi, nvr, etc. They are far away from each other (50+ meters). They can be separately disconnected from mains power, as its needed for safety when working, or when some heavy machines need to go through and the temporary cables are moved, so backing up one central point is also out of option.

It would have been helpful if you had stated that in your original post. The bottom line is that you're going to have to make 6 of whatever you're planning. Batteries, capacitors, diodes, switching circuits, recharging circuits. In the end, I think you'll find 6 UPS's easier, cheaper, and more robust.

 

But since the system in current use already has a 12v source for the electronics, the size of which "source" we don't yet know, a battery could be maintained on that 12V battery - IF the electronics can tolerate a 13.8 float voltage. UPS is not as efficient in some cases, and the cost is even higher than four 150F super-caps of about $46.24 (the ones I linked to). It's my opinion (and no one is obligated to accept my opinion) is that use of a decent charger would likely do the job and just run the electronics off the battery at all times. Regardless of mains power, the battery is always there and there are no brief interruptions during switching. Yes, an UPS has either very fast switching OR it's powered (at mains voltage) but uses the same sort of backup I'm talking about. The UPS powers electronics from its battery and it maintains the battery voltage at all times during mains power availability. When mains drops out, the UPS remains uninterrupted.

UPS is not a bad idea.

Please see my previous post/reply to another forum member. I would need 6 UPS, one for each stand, that is something i am not willing to pay, especially as it is a temporary system, until the building is finished and some components reused.

But since the system in current use already has a 12v source for the electronics, the size of which "source" we don't yet know, a battery could be maintained on that 12V battery - IF the electronics can tolerate a 13.8 float voltage.

Each stand has different AC-DC 12VDC adaptor, depending on what i had home and what is connected there. For example a stand where there is only one camera (expected load is 300-500mA) have 12V 1A psu, stands where there are 2 cameras and wifi AP have 3A and 5A psus, and the stand where there are 2 cameras, wifi AP, rpi (rpi have its own 2x18650 ups), and NVR, have 10A psu (this one i know i cannot cover with just caps, here maybe a small ups is a good option) and similar....

IF the electronics can tolerate a 13.8 float voltage.

Each "LOAD", has and auto step up-down voltage regulator board in front, so even when its connected via battery, the devices connected receive stable 12V.

It's my opinion (and no one is obligated to accept my opinion) is that use of a decent charger would likely do the job and just run the electronics off the battery at all times. Regardless of mains power, the battery is always there and there are no brief interruptions during switching.

I have tried this method, of directly connected to 12V battery, an keep the chargers there. However I tried multiple chargers i had at home, even bought some new. Some chargers are able to keep the battery charged and handle the load , but they reset after mains out (you have to physically push a button so they start charging). Some chargers just are not able to charge the batteries fast enough. The chargers i have connected now are rated at 4A . They were able to keep the batteries charged for a few weeks, but eventually they shown fault and stopped charging entirely, thus the batteries started to discharge.Majority of the chargers i was able to check out, had a specific warning that they should not be used as power supplies. I think they have some protection against battery, that cannot be fully charged, as it is drained permanently.
The only charger i was able to keep everything running was a powerful ctek i use when working on cars, that has even a mode for car battery replacement (it keeps the car electronics under power while you exchange the battery), but that one costs 150 eur piece, which is not economically viable for this system.

 

It would have been helpful if you had stated that in your original post. The bottom line is that you're going to have to make 6 of whatever you're planning. Batteries, capacitors, diodes, switching circuits, recharging circuits. In the end, I think you'll find 6 UPS's easier, cheaper, and more robust.

Yeah i mentioned there are multiple stands, but didnt specify the number. The fact is, that i didnt have to really buy that much for the system to work like it is. 12v lead acid batteries i already had, ups switching modules were like 4 eur a piece, 12v lead battery chargers i was able to get free from a friend (damaged packaging items from store), and all the small bits and pieces like caps, diodes, cables etc i already have at home (yeah i still payed for them, but in that was in the past )

I know the UPSs are the "perfect solution".

But they start at like 80 eur a piece for a smallest. And i would not be able to repurpose them. What do i do with 6 small UPS, that cannot be really used for anything useful. They would be small for anything like for workstation pc, 3d printer, networking rack, heat pump etc. And you dont need to have a ups for TV, radio etc.. Anyway the whole building is planned to have a short-term 3 phase ups + solar.

 

The shottky diode is fine for the discharge side of the supercap, which you will need 3 of the five volt ones in series.

the shottky diode there is mainly to prevent the capacitor to send power back to the circuit (switching module). I will see what supercaps i can procure.

For the charging side just use a 2 amp diode and a 100 ohm resistor.

So instead of the NTC to prevent the inrush current, i should use another diode and resistor, the resistor there is for current limiting i assume ? What is the diode there for ?

 

There was and error on my part on location of the NTC thermistor on the scheme, obviously it has to be limiting current to the capacitor, not to the load.

 

the shottky diode there is mainly to prevent the capacitor to send power back to the circuit (switching module). I will see what supercaps i can procure.

So instead of the NTC to prevent the inrush current, i should use another diode and resistor, the resistor there is for current limiting i assume ? What is the diode there for ?

The diode in the charging loop is to avoid the situation of an unpowered or even failed, power supply drawing current from the battery. The terminal impedance of a non-energized power supply is not always a very high value. So the diode prevents that from being a problem.

 

I did not realize that the load is different for the various "towers". So one size may not fit every installation. And I agree that a commercial UPS would not be a great choice. Much too expensive and probably not a great fit, and difficult to adapt to the exact requirements. And probably a bit too large. An actual power supply sized to power the whole assembly could work, though.AND iit could also charge the battery as I described. Lots of companies make 12 volt power supplies, and any adjustable supply can be set to provide a correct float charge voltage for a particular battery. And when the power failsthe backup can take over as soon as the diode switches on. That happens in microseconds.

 

Please see my previous post/reply to another forum member. I would need 6 UPS, one for each stand, that is something i am not willing to pay, especially as it is a temporary system, until the building is finished and some components reused.


Each stand has different AC-DC 12VDC adaptor, depending on what i had home and what is connected there. For example a stand where there is only one camera (expected load is 300-500mA) have 12V 1A psu, stands where there are 2 cameras and wifi AP have 3A and 5A psus, and the stand where there are 2 cameras, wifi AP, rpi (rpi have its own 2x18650 ups), and NVR, have 10A psu (this one i know i cannot cover with just caps, here maybe a small ups is a good option) and similar....

Each "LOAD", has and auto step up-down voltage regulator board in front, so even when its connected via battery, the devices connected receive stable 12V.

I have tried this method, of directly connected to 12V battery, an keep the chargers there. However I tried multiple chargers i had at home, even bought some new. Some chargers are able to keep the battery charged and handle the load , but they reset after mains out (you have to physically push a button so they start charging). Some chargers just are not able to charge the batteries fast enough. The chargers i have connected now are rated at 4A . They were able to keep the batteries charged for a few weeks, but eventually they shown fault and stopped charging entirely, thus the batteries started to discharge.Majority of the chargers i was able to check out, had a specific warning that they should not be used as power supplies. I think they have some protection against battery, that cannot be fully charged, as it is drained permanently.
The only charger i was able to keep everything running was a powerful ctek i use when working on cars, that has even a mode for car battery replacement (it keeps the car electronics under power while you exchange the battery), but that one costs 150 eur piece, which is not economically viable for this system.

First, entries, after a member comments, that clarify things does not require a scolding. Before I knew more details my comments were based on what I gleaned from your earlier comments. With greater understanding of what we're working with we can tailor our comments accordingly. There's no need to remind us we had it wrong prior to your clarification. I, too, thought a single UPS was going to be used to maintain all sensors (cameras now that we know).

Now: I am operating (for several years now) a 12V brick (wall wart) to maintain 13.6V float charge on an old car battery. It's output is 2 amps. I have a car radio hooked to the battery. No diodes either. Just the power brick. BUT I modified the power brick to either deliver a regulated 12V via a switch, or 13.6V via the same switch and a resistor. I play the radio a lot. The brick is powerful enough to play the radio at modest volume but when turned up it can sound pretty crappy. Hence, the car battery (which I had laying around). The resulting system works great. Never a problem, always a charged battery, and great sound. And if I over tax the battery, draw more than 2 amps, over night the charger I made out of my brick, recovers the battery nicely. And one advantage this has - it will play regardless of whether mains is available or not. Just if the battery is depleted then I'm out of luck. And it can take a week for the battery to recover after a deep discharge. But if we're talking a few seconds interruption or even a few hours, the radio will play on. The Energizer Bunny has nothing on me.

 

First, entries, after a member comments, that clarify things does not require a scolding. Before I knew more details my comments were based on what I gleaned from your earlier comments. With greater understanding of what we're working with we can tailor our comments accordingly. There's no need to remind us we had it wrong prior to your clarification. I, too, thought a single UPS was going to be used to maintain all sensors (cameras now that we know).

No scolding, reminding etc was on my mind, I just simply pointed to my previous posts to another member for more details, to not reply to you with the same. I see that in "business" jargon that could be translated in that, but truly it was not my message at all.

Now: I am operating (for several years now) a 12V brick (wall wart) to maintain 13.6V float charge on an old car battery. It's output is 2 amps. I have a car radio hooked to the battery. No diodes either. Just the power brick. BUT I modified the power brick to either deliver a regulated 12V via a switch, or 13.6V via the same switch and a resistor. I play the radio a lot. The brick is powerful enough to play the radio at modest volume but when turned up it can sound pretty crappy. Hence, the car battery (which I had laying around). The resulting system works great. Never a problem, always a charged battery, and great sound. And if I over tax the battery, draw more than 2 amps, over night the charger I made out of my brick, recovers the battery nicely. And one advantage this has - it will play regardless of whether mains is available or not. Just if the battery is depleted then I'm out of luck. And it can take a week for the battery to recover after a deep discharge. But if we're talking a few seconds interruption or even a few hours, the radio will play on. The Energizer Bunny has nothing on me.

Is it save to have the power adapter directly connected to the battery? All resources I have found are basically saying: "it should not be done, but you probably will get away with it". Especially if there is some fault of the battery, and the power adapter will just supply the maximum current it can, its probably not very safe. Espcecially as one of this adapters is 10A, that can start a fire no problem by overheating etc. I do have dc circuit breaker on each battery, solar, load etc, but they are there more in case od battery short circuit.... I am not comfortable to just straight up connect 12v adapter to the battery.

 

"D" is correct about the direct connection to a "power adapter". Thosemay be different from a battery charger circuit, which is made fora direct connection to the battery. My additional concern is the voltage applied, if the charger/adaptter is constantly active. I have seen an alarm system ruin batteries in just a few months. So the float charge voltage matters.

 

This is part of the board. Just above the word "Transformer" you see an additional resistor, some wires, a pot and a switch. Those few components near that transistor are what regulates the output. Up on the right side is the DC outputs, + & - . There are some caps and some coils. One Toride. Since DC can't flow through a cap, and there are none for it to flow through other than those coils, the only internal diode blocking anything from coming back is center, left. It's a 3 lead device on a heat sink. If you can show me where there's a danger of back-feeding then please do. I don't see any issues. And yes, I have the whole circuit board diagram if you feel it's needed.

 

Here's the whole board. It's time for dinner and M*A*S*H.

 

OK, tracing the positive side of the output back to the transformer I get to diodes. So that means no hidden path to draw current from the transformer when the device is not active. So this one is OK to connect directly. Where a problem can arise is if there is an over-voltage protection circuit that has an active reference powered by thhe main source, and when the main source switches off, the OVP decides that the battery is an overvoltage and it shorts out the battery. some power supplies have that, many do not. That is why some lab supplies have warnings about not using them to charge batteries.
But this one is safe in that aspect.

 

As I said, there is an additional 4K7 resistor in series with a 5K pot (10 turn). The SPDT switch is there so I can simply return the circuit to a regulated 12V output. But with the additional resistors in parallel with the voltage divider circuit and the TL431, I have it adjusted to 13.6 float volts. I've been checking the battery for years and have yet to see any depletion of water. AND this battery is a used battery out of a van. I don't know how old the battery was when I got it but it's been going just fine for the some seven years I've had it in service. Of course I don't know what its capacity is, it probably can't start a car any longer, but for the radio, it works beautifully.

I might mention there is one additional component I have neglected to mention - an in-line 5A fuse. The radio has an additional 10A fuse (will have to double check the fuse values).