DIY Backup with sealed lead acid battery

[Dannyx]

Re: DIY Backup with sealed lead acid battery

So I say periodically disconnect whatever is charging the battery (be it an adapter directly, or a purposely built circuit), then probe it via the resistor divider for X seconds/minutes to get a (relatively accurate?) reading. If the reading is high enough and not dropping too fast (which would indicate a faulty battery which is not keeping the charge), keep the battery disconnected from the charger. If it's below a certain threshold, throw it back on the charger.

In battery mode, I believe the battery should be disconnected to prevent over discharge which shortens the lifespan of SLAs....deep cycles batteries are too expensive for this purpose. Even if a boost converter would technically be able to continue powering the load even when the battery is below 10v, it's not a good idea to drain it to the absolute 0....

[Curious.George]

Re: DIY Backup with sealed lead acid battery

I was thinking of the resistor divider idea to "monitor" the battery's voltage and toggle the charging on and off, but we'd run into the same issue of the divider seeing the adapter voltage rather than the actual battery voltage, so I'm still bidding on the constant current LM317 idea. Correct me if I'm wrong in my assumption.

Whether you're driving the battery with a voltage source ("adapter voltage") or a current source, you still won't see the actual "battery voltage" -- even if you monitor on the battery's terminals. (i.e., a voltage source still has an output impedance that acts to limit the charge current)

The "less healthy" the battery, the more your voltage/current source will "pull" the battery in "their" direction.

Even if you "disconnect" the charger in an attempt to monitor the battery by itself, you'll still find surface charge will mislead your (high impedance!) attempt to probe the voltage (and, from that, determine state of charge).

[You'd have to "burp" the battery to remove that charge before you could see the true state of the battery]

[stj]

Re: DIY Backup with sealed lead acid battery

use a microcontroller to turn the psu on and off, and monitor the voltage.

[Dannyx]

Re: DIY Backup with sealed lead acid battery

I was thinking of the resistor divider idea to "monitor" the battery's voltage and toggle the charging on and off, but we'd run into the same issue of the divider seeing the adapter voltage rather than the actual battery voltage, so I'm still bidding on the constant current LM317 idea. Correct me if I'm wrong in my assumption.

[Curious.George]

Re: DIY Backup with sealed lead acid battery

Seconded. Maybe use a resistive divider that needs at LEAST 10VDC (or whatever the minimum acceptable battery voltage is). Use a voltage regulator chip to keep the 12V at a constant level for the DVR to use. For the resistive divider, fuse it at like 1A or 2A and connect that to the battery. Connect the coil terminals from the resistive divider to negative, and switch the power to the voltage regulator using that. Then when the power comes back on, the adapter kicks on and the relay restores power to the DVR.

You can use the 555/comparator to implement any desired amount of hysteresis just by feeding back a weighted output (that drives the relay coil) to the input.

[Dannyx]

Re: DIY Backup with sealed lead acid battery

Would this resistor divider idea be in conjunction with an op-amp or a 555 then ?

[TechGeek]

Re: DIY Backup with sealed lead acid battery

Note that the float voltage (as with the charging voltage) varies with temperature. So, a "constant" works best when temperature is also constant. E.g., at higher temperatures, a battery outgasses at a lower voltage (what's the tempco of your AC adapter?).



What limits the charge current? I.e., when battery has drooped to ~10V and mains power comes back on... Are you counting on the current limiting inherent in the AC adapter to do this? I.e., if you used a BEEFIER adapter, your charge current would be "less limited"?

The down side of float charging is they don't give you the fast charging rate that a normal charger would.



Deliberately open the battery lead when the voltage drops to a certain point (add plenty of hysteresis so it doesn't oscillate, there.) You can probably use something as small/cheap as a 555 (to exploit its high current output to drive a relay coil, directly)

Seconded. Maybe use a resistive divider that needs at LEAST 10VDC (or whatever the minimum acceptable battery voltage is). Use a voltage regulator chip to keep the 12V at a constant level for the DVR to use. For the resistive divider, fuse it at like 1A or 2A and connect that to the battery. Connect the coil terminals from the resistive divider to negative, and switch the power to the voltage regulator using that. Then when the power comes back on, the adapter kicks on and the relay restores power to the DVR.

[Curious.George]

Re: DIY Backup with sealed lead acid battery

There is no need to be fancy charging a lead acid battery.

Constant 13.5V float charge - for cells rated for such usage (most are, but check) - will be just fine.

Note that the float voltage (as with the charging voltage) varies with temperature. So, a "constant" works best when temperature is also constant. E.g., at higher temperatures, a battery outgasses at a lower voltage (what's the tempco of your AC adapter?).

I added a lead acid battery to the unit itself, connected to the 12V of the hard disk drive which is permanently connected to the AC adapter input. Then I modified the AC adapter to tweak the output voltage to 13.5V so this would keep the lead acid cell topped off.

What limits the charge current? I.e., when battery has drooped to ~10V and mains power comes back on... Are you counting on the current limiting inherent in the AC adapter to do this? I.e., if you used a BEEFIER adapter, your charge current would be "less limited"?

The down side of float charging is they don't give you the fast charging rate that a normal charger would.

Only disadvantage right now is that if the voltage drops to the point where the HDD stalls (approx 9.5V) then power comes back on, the unit won't automatically re-mount the disk...I need to figure out how to get this to work. But, that's pretty unlikely, because once the battery drops below 10V, it usually only has a minute or so of runtime left before the whole unit resets.

Deliberately open the battery lead when the voltage drops to a certain point (add plenty of hysteresis so it doesn't oscillate, there.) You can probably use something as small/cheap as a 555 (to exploit its high current output to drive a relay coil, directly)

[stj]

Re: DIY Backup with sealed lead acid battery

same concept, use a simple buck or boost convertor to get a constant voltage until the battery is depleted.

go look at julien's video's
https://www.youtube.com/user/julius256/videos

he has been building convertors on a plank of wood with an arduino to drive it!!

[Dannyx]

Re: DIY Backup with sealed lead acid battery

What if I went the opposite way: up to 24v ?

EDIT: ok, I wrote that before I even thought about it - stupid idea considering I'd have to step it down to 12v again, which would lower the overall efficiency. I reckon for maximum efficiency there should be as little conversion as possible.

[stj]

Re: DIY Backup with sealed lead acid battery

you could run 3 6v battery's in series followed by a simple arduino-based buck convertor.
then you can have 12v throughout the life of the batteries.

[Dannyx]

Re: DIY Backup with sealed lead acid battery

Not sure about their low cost. They don't seem cheap, to me at least....I'm not sure how the devices (cameras, DVR, etc) would behave when the voltage of the battery begins to drop and it's somewhere in the "sweet-spot" threshold between the device actually running at its lowest acceptable input voltage (say 10v) and not running at all. It could cause some quick undesired transitions between the off and on states, the effects of which I don't know...

[tom66]

Re: DIY Backup with sealed lead acid battery

Well, I suppose it depends on what kind of lifetime you are expecting.

I have seen alarm and fire panels use designs just like mine, constant float charge, and they get 7-8 years out of a battery.

Most lead acid cells have less than 10 years life. But lead acid batteries are also highly recyclable and low cost.

[Dannyx]

Re: DIY Backup with sealed lead acid battery

There is no need to be fancy charging a lead acid battery.

Constant 13.5V float charge - for cells rated for such usage (most are, but check) - will be just fine.

I just installed a CCTV system for my house. Two cameras plus NVR box from Amazon. I added a lead acid battery to the unit itself, connected to the 12V of the hard disk drive which is permanently connected to the AC adapter input. Then I modified the AC adapter to tweak the output voltage to 13.5V so this would keep the lead acid cell topped off.

No fuss needed. As soon as the power cuts out the battery automatically takes over because it's in parallel. And it back-feeds the cameras through the power lead. A tiny 12V, 1.2Ah battery gives the unit an approximately 1 hour runtime, perfectly sufficient for running through short black outs (which the majority of power cuts are.) I can confirm that the HDD will operate perfectly correctly up to around 15 volts DC for those curious.

Only disadvantage right now is that if the voltage drops to the point where the HDD stalls (approx 9.5V) then power comes back on, the unit won't automatically re-mount the disk...I need to figure out how to get this to work. But, that's pretty unlikely, because once the battery drops below 10V, it usually only has a minute or so of runtime left before the whole unit resets.

That's exactly what I was thinking and even did do it at one point to power a car headunit at my former shop....*nostalgia*. I also modified a laptop power brick to drop its volts down to 14-ish volts (since I also threw a diode in there which dropped it down some more).

Of course, this would be totally doable here as well, but I fear it may lower the life of the battery than if I were to disconnect it entirely, hence the original idea.

[tom66]

Re: DIY Backup with sealed lead acid battery

There is no need to be fancy charging a lead acid battery.

Constant 13.5V float charge - for cells rated for such usage (most are, but check) - will be just fine.

I just installed a CCTV system for my house. Two cameras plus NVR box from Amazon. I added a lead acid battery to the unit itself, connected to the 12V of the hard disk drive which is permanently connected to the AC adapter input. Then I modified the AC adapter to tweak the output voltage to 13.5V so this would keep the lead acid cell topped off.

No fuss needed. As soon as the power cuts out the battery automatically takes over because it's in parallel. And it back-feeds the cameras through the power lead. A tiny 12V, 1.2Ah battery gives the unit an approximately 1 hour runtime, perfectly sufficient for running through short black outs (which the majority of power cuts are.) I can confirm that the HDD will operate perfectly correctly up to around 15 volts DC for those curious.

Only disadvantage right now is that if the voltage drops to the point where the HDD stalls (approx 9.5V) then power comes back on, the unit won't automatically re-mount the disk...I need to figure out how to get this to work. But, that's pretty unlikely, because once the battery drops below 10V, it usually only has a minute or so of runtime left before the whole unit resets.

[budm]

Re: DIY Backup with sealed lead acid battery

Some battery maintainer circuits:
https://www.youtube.com/watch?v=qxyVqjCuR80
https://circuitdigest.com/electronic...2v-sla-battery

[Curious.George]

Re: DIY Backup with sealed lead acid battery

You are making this more complicated than it needs to be why ?

Use a battery charger controller that is responsible for the charging of the battery use one that charges at a high rate then powers down as battery voltage hits 13.8 to 14.5 volts then goes to stand by

Or, buy a battery maintainer -- that already does this sort of thing.

If you're looking around for ideas for MCU projects, look for something that truly adds value, instead of replaces something COTS!

[Dannyx]

Re: DIY Backup with sealed lead acid battery

why not use a car battery, and have a circuit that disconnects it with a relay for 99% of the time once it's charged.
no need to waste power with 24/7 charging of a cell that has no load.

Exactly what I was planning on doing, but to do that we need SOME sort of monitoring and rather than messing with discrete stuff which is prone to large errors, I thought I'd use a micro.

Come to think of it, efficiency is another aspect I haven't taken into account: the alarm system (which let's face it is not even designed yet) would be a small consumer. I was also thinking of having it run a DVR and some cameras....straight off the battery Haven't tried it yet, but the "classic" approach in this industry is to have a proper stand-alone UPS plugged into the mains, the SMPS for the cameras and DVR plugged into that, so when power goes, it would essentially go

DC from battery>AC by inverter inside UPS>DC by adapter for DVR > DVR. I'm no expert but I'm wondering (outside of cost), is this more efficient than just going:

DC from battery> Dc input of DVR ? Just a thought.

Again: charge battery when mains is available to a certain level, disconnect it and forget about it, toggle battery over to the DVR when power goes out (simple relay job).

[sam_sam_sam]

Re: DIY Backup with sealed lead acid battery

My building hasn't experienced a power outage in ages TBH, so the battery would spend most of its life "idling", which may or may not be detrimental to its health and it might fail just as I need it and SLAs are relatively expensive (by my standards at least), so maximum battery life would be important, that's why I thought I'd "smarten" the thing up a bit by including a uController of sorts.

You are making this more complicated than it needs to be why ?

Use a battery charger controller that is responsible for the charging of the battery use one that charges at a high rate then powers down as battery voltage hits 13.8 to 14.5 volts then goes to stand by


Have your controller take care of this part

All you need to do is once the battery is completely charged turn the charger of then once a day check the battery voltage and check it the stand still charge if it is below 12.5 then turn the charger back on for a certain amount of time and recheck the next day if no charge is needed then skip that day

[stj]

Re: DIY Backup with sealed lead acid battery

why not use a car battery, and have a circuit that disconnects it with a relay for 99% of the time once it's charged.
no need to waste power with 24/7 charging of a cell that has no load.