That would be fine if I were to remove the batteries and charge them individually and put them back. Charging batteries in series always leads to one of the batteries charge faster and the total would show , say, 19v while one battery is charged 3,2v the other 4.2v. That happens even if all batteries are made by the same factory.

That's a complexity you just don't get with Lead-Acid.

 

You can use any type of battery, rechargeable or non-rechargeable.
Just remember to top up the rechargeable battery once every 6 months and every time you experience a power brownout or blackout.

That would be the most str8 forward but I like the set-up-once-and-forget kind of design.

 

That's a complexity you just don't get with Lead-Acid.

But I already got batteries. What am I going to do with them? Plus, I do not know any lead acid 20v battery.

 

But I already got batteries. What am I going to do with them? Plus, I do not know any lead acid 20v battery.

https://www.yuasa.com/uk/industrial-batteries/fire-security/np-series?nominal_voltage=1201

 

Nothing wrong with it, but I am just perplexed why anyone would buy an expensive backup battery the only use a quarter of its capacity.

I am not sure why you say "1/4 of its capacity." I use 5 x 18650 to get 20v . not 12v. I mentioned in my post that the voltage was just an example. My main question is whether it works or not,

 

That would be fine if I were to remove the batteries and charge them individually and put them back. Charging batteries in series always leads to one of the batteries charge faster and the total would show , say, 19v while one battery is charged 3,2v the other 4.2v. That happens even if all batteries are made by the same factory.

Batteries from the same factory (same model #), and especially same batch would never be that bad. You should charge them all up in parallel before putting the string together anyway. They will be just fine after that if you stick to seconds long discharges.

I have some 25 year old 18650's from a pack, and they charge in series just fine even after all that time. No imbalance.

 

What I have deduced is that the three computers with external power supplies are portable laptop computers. AND the present batteries are in the individual battery packs, one pack per computer.
SO I SUGGEST a quite different scheme: Use a DC backup battery system that can run in parallel with the external power supplies as a "hot backup" scheme. The battery backup can be set to half a volt less than the power supplies provide, and easily take over when the mains fail. Three of the large six volt wet-cell batteries could run the computers for a week, if they had to. Keep the present battery packs in place .

 

What I have deduced is that the three computers with external power supplies are portable laptop computers. AND the present batteries are in the individual battery packs, one pack per computer.
SO I SUGGEST a quite different scheme: Use a DC backup battery system that can run in parallel with the external power supplies as a "hot backup" scheme. The battery backup can be set to half a volt less than the power supplies provide, and easily take over when the mains fail. Three of the large six volt wet-cell batteries could run the computers for a week, if they had to. Keep the present battery packs in place .

That would be my preferred solution too, but I'd use a float charger, and I'd prefer VRLA over flooded. You can get 200Ah/6V which would last most power outages!

 

Yes, I neglected to mention that float chargers were also required. I just plain forgot to include that detail.

That would be my preferred solution too, but I'd use a float charger, and I'd prefer VRLA over flooded. You can get 200Ah/6V which would last most power outages!