Using a battery backup with 18650 packs?

So I've been busy away at finally getting me some laptop batteries and testing away already. My big question is if I was to buy a battery backup like the ones used for computers could i just remove the batteries inside and maybe modify it some andreplace them with batteries of the same voltage size?
I have a 3d printer and would like to have backup to backup power supply that could last a few hours and maybe power other things if I need to so I guess I'd be making my own little powerwall in a way and I was thinking of going this route since if I wanted more power I could always just tie in more batteries to the system. I was just wondering before I went and bought an old power supply if these would be safe enough for use with something like lithium batteries is all?


Btw to add the power supply i was considering is like a used Apc Battery backup on eBay or something.

Welcome!

In theory, yes, you can replace the batteries in some old UPS. However, there are some strings attached. For example, these devices aren't designed to charge lithium batteries but lead acid instead. You can't charge lithium batteries in a safe and reasonable way with a charger that is made for lead acid. That means you can use lithium batteries in an UPS, but you can't charge them in there just so. You would have to charge them externally.

What you want is probably not a UPS but a inverter with the option to choose whether you want to loop through mains voltage to its output or use the battery input to do DC-AC conversion. This is basically the same thing as an UPS but a more convenient package overall. Basically an inverter with a UPS option.

Ok cool thanks for the reply any types of inverters you could suggest that might be cheap-ish?

Agree with DarkRaven. Also, you want to go server grade UPS as apposed to residential grade. The reasoning is in the quality of the unit. The Server grade units can run continuous in Battery mode for several hours under standard condition without issues. And basically no limit with proper cooling.
However, residential units are only designed to run for 15mins max, even under light duty. I have a UPS 650 unit I used with Hurricane Irma knocked us outta power for a week. After 3 days of 3-4hr usage each day the transformer started making a lot of vibration noise. So much so, I had to close the door to the room the unit was in as I couldn't hear the tv (19" lcd). I'm sure the transformer is ok, but the FETs might have had their limit to life.
Also, the Server grade units are easier to adjust the settings. Some APC Powerchute versions allow you to change the Float and Top Voltages. This comes in handy to basically turn OFF the charging as you can set the Float/Top voltage to 40V (as most of these units are 48V rated) or lower. And this will essentially keep the UPS from trying to charge the batteries. Then you have a dedicated charger for the batteries.
Side Note: It is "possible" to charge the lithium with the UPS. However, you would need to have a proper 48V DC input charger to put in between the UPS and batteries to handle the charging. But, this is not advisable as you can't be charging the batteries and using them at the same time. This would only work if you were charging them from the grid. Which, kinda defeats the premise of going all battery.

Addendum: DarkRaven I think was saying you are turning off the "Uninterruptible" portion of the UPS and using it in "Inverter Mode" only. These units are far cheaper and better quality than just a plain jane inverter. Especially from eBay.

Here's a basic search: https://www.ebay.com/sch/i.html?_fr...ment+-5ah+-18ah+-22ah+-kit+-Cartridge&_sop=15
You can find other types cheaper. You could search for the "sua2000" instead and getter 2kWh units. Or do a search for "smart ups (2000, 3000, 5000)" and see what you get. You might wanna add the restrictions for the batteries and such I have in the previous search, though.

Its also worth mentioning that on some UPS you can set the charging voltages. If you can change the float and bulk you could potentially use it for charging too... And switch that on/off with an ATS... Though may not be optimal.

Most people only use the Inverter function inside.

I just bought one of these: http://www.aimscorp.net/1200-Watt-Pure-Sine-Inverter-with-Built-in-Cord.html

Have not hooked it up and used it yet, just got the cables made up the other day.

So I'm kinda looking now at some type of cheap solar charger I don't really care for mppt since I want cheap but I want to see if I just put a load on the solar input with the right voltages instead of a panel let's say could that be used to charge some batteries and then just hook the batteries to an inverter? Of course a inverter charger all in one would be nice but I see those range from $200+. So any thoughts with this I might just try this with a cheap solar charger well a good cheap solar charger.

Could you trick a grid tie inverter with one?
To use off grid.







Lazyman342So I'm kinda looking now at some type of cheap solar charger I don't really care for mppt since I want cheap but I want to see if I just put a load on the solar input with the right voltages instead of a panel let's say could that be used to charge some batteries and then just hook the batteries to an inverter? Of course a inverter charger all in one would be nice but I see those range from $200+. So any thoughts with this I might just try this with a cheap solar charger well a good cheap solar charger.

The only way I can think of that you could trick a grid-tie to turn on without mains, is to have a low power UPS hooked up and it runs and the grid-tie senses the output from the UPS. The only problem is that the grid-tie outputs on the same line that it senses on. So you won't be able to use the grid-tie 100% and no load on the UPS. The UPS will probably beep like a mad man laying on the horn the entire time.
I think the UPS would end up burning up after awhile, too, because it's putting out a load as well. Oh, also, the UPS and the grid-tie would have to share the same battery as they would share commons.