Float charging for batteries with constant 25w load?

[147Gm]

I've been searching google and this forum and haven't found a clear answer for this. I have what is essentially a large UPS for some critical devices, featuring 400ah of 12v AGM batteries and a 1000w PSW inverter that stays on 24/7 ready to supply the critical loads the instant utility power fails (a few times a week). The inverter draws about 25w continuously, so of course the batteries need to be charged regularly even if there are no outages just to keep up with this draw. I previously had this charging from a solar panel so it would reach float a few times a week on the sunny days, but I have repurposed that panel for different project and have used a simple 7 stage 10A automotive battery charger to charge this system a few hours each day.

What is the best strategy for battery health and longevity? Leave an automotive style float charger connected 24/7? Put a battery charger on a timer to run X times per day? Any suggestions for an AC to 12vdc charger?

 

[Hedges]

How about a supply that doesn't switch to "float" entirely, but rather delivers 2A at float voltage?
Maybe a CC/CV supply feeding through a diode?

 

[147Gm]

How about a supply that doesn't switch to "float" entirely, but rather delivers 2A at float voltage?
Maybe a CC/CV supply feeding through a diode?

I had wondered about that, like if I have a power supply that puts out 13.5vdc with 3A or 4A rating, the battery will just draw what it needs, usually about 2A, right?

 

[Hedges]

Except when inverter draws down battery. Have to address all use cases. Resistor and diode, maybe, if you don't want to get a supply with CC mode.
Does temperature vary? Should float voltage vary with it?

Proper voltage should give long life like 10 years, assuming your charging is correct.

Me? I'd plug in one of my Sunny Island with a 48V battery and let that handle it. (not quite on-line UPS, allows a couple cycles dropout.)

 

[147Gm]

temperature is quite stable, 24-28C. perhaps a 13.5vdc power supply for the idle load, and an automotive type charger that runs for several hours after an outage?

 

[Hedges]

Whatever supplies you standby load while maintaining proper float voltage.
But for recharging, you want to follow the proper profile for bulk, absorption, float. That is the key to long life.

This is the one I use for occasional charge of neglected batteries that aren't in a system; don't know how correct its profile is.

4 Amp Fully Automatic Microprocessor Controlled Battery Charger/Maintainer

Amazing deals on this 4 Amp 12V High Freq Battery Charger/Maintainer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Good PV chargers will let you set the exact voltage and absorption time you want. Of course, that 2A baseline draw is going to confuse it's detection of tail current during absorption.

 

[147Gm]

This is the one I use for occasional charge of neglected batteries that aren't in a system; don't know how correct its profile is.

4 Amp Fully Automatic Microprocessor Controlled Battery Charger/Maintainer

Amazing deals on this 4 Amp 12V High Freq Battery Charger/Maintainer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Is there any reason a smart charger like that couldn't handle the float stage 24/7 fine as well? When I tried leaving my (supposedly 'smart') automotive battery charger, it seemed as though it could never reach the absorption voltage it wanted (because of the 25w load?) so it stayed for many hours at ~14.3 before I stopped it, whereas I expected it would hit about 14.5 and then go to float at about 13.5. Would that make sense for a continuous 2A load to prevent a charger from reaching final absorption voltage and preventing it from going to float?

 

[Hedges]

Is there any reason a smart charger like that couldn't handle the float stage 24/7 fine as well? When I tried leaving my (supposedly 'smart') automotive battery charger, it seemed as though it could never reach the absorption voltage it wanted (because of the 25w load?) so it stayed for many hours at ~14.3 before I stopped it, whereas I expected it would hit about 14.5 and then go to float at about 13.5. Would that make sense for a continuous 2A load to prevent a charger from reaching final absorption voltage and preventing it from going to float?

It certainly might, since absorption voltage is supposed to be held until current has tailed off, then kept for a couple hours more, followed by decrease to float voltage. It isn't surprising that a charger would sit indefinitely at absorption voltage if it reached that at all. Or, drop to float and let battery discharge a while before recharging.

I could come up with some scheme involving a CV/CC supply and one or two diodes to support the load and let battery charge/float without being drawn down by idle consumption. That might be easier to design in a UPS with sensors for battery current separate from charger current.

I'm going by the technical manual for the battery I use:

http://sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

But for other sealed batteries I've seen different charging voltages for cyclical vs. standby use. Look up recommendations for your brand. There may be a particular continuous charging voltage that is considered acceptable rather than the 3-state charge I'm using.