Battery float voltage for DC UPS?

D

DellAxim

Gawd
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Feb 14, 2003
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I happen to have a 12v, 22AH SLA battery sitting around, and it's just going to go bad if I don't use it for something. I want to make a 12v DC UPS to power my router.

The concept is very simple, I charge the battery with a DC-DC power supply powered by a wall wart which allows me to adjust the voltage and current.
At the same time, there is a 12v buck/boost regulator to keep the output at exactly 12v, even when the battery is fully charged or mostly dead.

From what I read the float voltage of a SLA type battery is 13.5-13.8v. Why can't I just set the charger at 13.0v, just a hair over fully charged? If I do this the battery does not drain until the power shuts off. Is there a reason I need to up the voltage to 13.8v? Seems like the battery would last longer floating at a lower voltage, am I wrong?
 
Chart

It really depends on many factors, including the room temperature and battery construction.

IMO, while your goal is noble, you are risking a home-wide disaster. Just buy a small UPS.
 
Grebuloner said:
Chart

It really depends on many factors, including the room temperature and battery construction.

IMO, while your goal is noble, you are risking a home-wide disaster. Just buy a small UPS.
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I'm not a moron. I have worked with electronics since I was a small child. The only thing I am not familiar with is lead acid charging voltage, because lead acid batteries are obsolete. (Thus a large battery I have no use for) My home is powered by a solar system I designed and installed, currently running from a 25kw 48v lithium battery I built myself at this moment as the sun rises. That is among the safer things I do. There's also a 550v DC circuit...and I built my own generator from scratch.

There is nothing "disasters" that could happen here, lead acid batteries and 12 volts are extremely safe. No, I'm not buying a UPS, I already have all these parts lying around because that's the sort of thing I do. I also have a giant metal box I burn fires in, which might be dangerous if I was a child, but is perfectly normal heat source for competent adults.

I am aware this is probably the wrong forum to ask this question, as you demonstrate.
 
DellAxim said:
I'm not a moron. I have worked with electronics since I was a small child. The only thing I am not familiar with is lead acid charging voltage, because lead acid batteries are obsolete. (Thus a large battery I have no use for) My home is powered by a solar system I designed and installed, currently running from a 25kw 48v lithium battery I built myself at this moment as the sun rises. That is among the safer things I do. There's also a 550v DC circuit...and I built my own generator from scratch.

There is nothing "disasters" that could happen here, lead acid batteries and 12 volts are extremely safe. No, I'm not buying a UPS, I already have all these parts lying around because that's the sort of thing I do. I also have a giant metal box I burn fires in, which might be dangerous if I was a child, but is perfectly normal heat source for competent adults.

I am aware this is probably the wrong forum to ask this question, as you demonstrate.
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You should have placed all those accomplishments in your sig, that way no one would have offended you by offering a solution that is beneath you.
 
DellAxim said:
The concept is very simple, I charge the battery with a DC-DC power supply powered by a wall wart which allows me to adjust the voltage and current.
At the same time, there is a 12v buck/boost regulator to keep the output at exactly 12v, even when the battery is fully charged or mostly dead.
Click to expand...
How are you keeping the wall-wart DC-DC power supply from constantly powering the buck/boost?
 
Is there a reason you want to trickle charge to lower than recommended voltage?
Also your router that accepts 12V may very well work fine directly off the of lead acid battery, since it will have various regulators internally to regulate 12v down to 3.3v, 1.8v, 1.2v, etc. Those regulators usually have a voltage range for their inputs. However to confirm this 100% you would need the datasheets.
 
michalrz said:
How are you keeping the wall-wart DC-DC power supply from constantly powering the buck/boost?
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It won't. The guy needs a dedicated UPS or a power switching board with a purpose built SLA battery charger.

If you just slap 13.8v across the battery, and apply the load to the same circuit, it's going to cause erratic problems. Whichever source has the higher voltage potential is what the load is going to draw from, which is going to be the power supply more often than not.
 
GiGaBiTe said:
It won't. The guy needs a dedicated UPS or a power switching board with a purpose built SLA battery charger.

If you just slap 13.8v across the battery, and apply the load to the same circuit, it's going to cause erratic problems. Whichever source has the higher voltage potential is what the load is going to draw from, which is going to be the power supply more often than not.
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Just want to point out that in battery backup setup at steady state in normal operation you WANT all of the power to be coming from the power supply. Otherwise that means you are discharging your battery even though you are plugged in.
 
serpretetsky said:
Just want to point out that in battery backup setup at steady state in normal operation you WANT all of the power to be coming from the power supply. Otherwise that means you are discharging your battery even though you are plugged in.
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Except for the problem if you tie the battery and the power supply together on the same power bus as the load, there's no control over charging the battery. It's always going to be random as to what the battery gets. And when the power does fail, the battery will try to back feed the power supply.

And there are situations where you DON'T want to have all of the power coming from the source supply, as exists in double conversion UPS units. The powered device never sees the mains, which gives maximum protection. The battery can be cut in circuit to supplement what may be deficient mains, like in the case of a brownout or frequent power cuts.
 
You need to separate the battery and charging system from the buck/boost except when the power goes out.
This can be done with a diodes(low power) or a mosfet(high power) for near instantaneous switch over.
Use a capacitor to smooth out the transition and the output after the buck/boost circuit never notices the difference.
 
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