Sealed and AGM battery outgassing hydrogen

[Delmar]

One of the main advantages of LiFePO4 is not outgassing. The failure state of an older AGM battery is outgassing hydrogen (I had a 220Ah 48V bank fail and started outgassing). That alone would prevent me from wanting to mix the two.

Just FYI (also not trying to hijack this thread) - All the UPSs I've ever seen have sealed Lead Acid batteries, NOT AGMs. That it, they are flooded lead acid, but sealed just like the "maintenance free" batteries for cars. AGMs generally don't off-gas Hydrogen, as the Hydrogen is absorbed by the glass matts. However, everything I heard when I was putting in the AGM batteries in our solar was to expect some small amount of hydrogen.

There may be some UPSs with AGMs. I just haven't seen them. If you pull the plastic top off of the little 12V batteries you'll see 6 little rubber caps over the flooded cells.

Diverted from the This guy thinks outside the box adding lead to a LiFePO bank thread

I was very surprised to hear this as virtually all UPSs have AGM batteries, and are located inside homes & offices. I have three AGMs under my desk right now.

My favorite Harbor Freight sealed battery also states AGM construction. Will have to open the cover and inspect.

 

[Q-Dog]

The UPS under my desk has a 7 or 9 amp hour at 12 volts battery in it. The small battery backup I built for my house has 200 amp hours at 24 volts, or 45 times more capacity. Also, the UPS trickle charges.

 

[Delmar]

I have an office Belkin 12V UPS, and built an APC 120V UPS for my Moms TV. All have Harbor Freight 35AH batteries sharing the living space.

 

[Luthj]

All lead acid batteries produce hydrogen and oxygen during charging. Sealed batteries, especially AGM batteries contain these gasses, and typically don't vent under normal operation. Most quality sealed batteries have a catalyst located near the top (typically in the caps or similar), which speeds the recombination into water. Better quality batteries have a high pressure case which only vents near 1psi, allowing for better containment even at higher charge votages.


Assuming the battery is not being charged outside is specified voltage range, it will not outgas any appreciable amount. The exception to this is during battery failure, which normally can only occur during end of life (not common), or significant damage.

The primary failure mode leading to outgassing is if a single cell shorts itself out. This results in the battery being down 1 cell. So when the battery is charged at the normal voltage, each cell is actually above the design voltage. This is equivalent to an extreme overvoltage charge, or an equalize cycle for a flooded battery. The resulting gas buildup often cant be contained by the sealed battery shell/catalyst, and will vent to atmosphere.

High end AGM batteries (lifeline etc) have a more robust separate which reduces the risk of short down. AGM batteries in general are more resistant to this issue than flooded typed (either sealed or open case design).


UPS are designed to not charge at high enough voltages to vent the batteries. This is damaging to sealed batteries, and is avoided at all costs. sealed (though not always AGM) batteries are used extensively in fire alarm system panel backup power, small UPS, and security systems. The only issues I have seen are due to failed power supplies overcharging the battery.

 

[Delmar]

Assuming the battery is not being charged outside is specified voltage range, it will not outgas any appreciable amount.

That is reassuring as the AH capacity is magnitudes more than the original UPS battery, so should charge at a magnitudes lower rate and minimize overvoltage.

Edit: I could be getting confused by the terminology. Are there flooded batteries that are also sealed?

 

[Luthj]

FYI many UPS are not designed for anything but occasional backup usage. For this reason they often don't use a proper charge profile for deep cycling, often never exceeding 14V. In fact many are single voltage only, which is essentially a float voltage.

For those modifying a UPS for other uses, make careful note of voltage, and what logic (if any) it uses.

 

[Horsefly]

Here's a pic of three batteries pulled from three APC UPS boxes sold for home use. Under those little rubber caps is a liquid electrolyte. Which is to say, these appear to be flooded lead acid, not AGM. Maybe higher end UPSs use AGMs? I dunno.

Edit to add: This is after I pulled off the sealed plastic top that appeared to be spot-glued to cover the rubber lids on the cells.

 

[Horsefly]

Edit: I could be getting confused by the terminology. Are there flooded batteries that are also sealed?

Yes. That's what I was talking about with "maintenance free" car batteries. They are sealed, "valve regulated" flooded lead acid batteries.

 

[Delmar]

My 12V Belkin floats at 13.6V and charges at only 3A. My office will draw the 35AH battery down to maybe 12.6V while charging with everything on (lights, fan, phones, etc). I need to test the two 24V APCs.

All the UPSs I have purchased were supplied with what appears as AGM batteries.

 

[Luthj]

AGM are always valve regulated. Flooded batteries can be valve regulated. Neither valve regulated type should vent in its intended application.

Automotive VRLA batteries (not AGM) do sometimes gradually loose water view the vent, that is due to them being cheap crap typically. Good VRLA don't typically need maintenance.

 

[Delmar]

The small battery backup I built for my house has 200 amp hours at 24 volts, or 45 times more capacity.

My primary system I built for my house has 170 amp hours at 48 volts (six 8V batteries) for 8KWH and back-feeds my breaker panel. The UPSs keep everything running during nuisance trips, or until I get the golf cart connected. The solar panels extend the run time during a hurricane, or in this case an ice storm. In Houston.