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treich
join:2006-12-12

treich

Member

Building an battery backup

Guys
I am just wondering if anybody could point to me how to build an good battery backup for an cable modem and Ethernet switch and an Poe that is 24w .5 amp?

cowboyro
Premium Member
join:2000-10-11
CT

cowboyro

Premium Member

KISS.
Buy a UPS and call it a day.

aurgathor
join:2002-12-01
Lynnwood, WA

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If you *really* want to build one, all you need are two 12V batteries and a 24V charger sized to handle the current draw while keeping the batteries "floating".
You may be able to get away with two 12V chargers, so you don't need to build one.

Getting a UPS should be much simpler and probably won't cost a whole lot more.

BlueMist
join:2011-01-24
Cookeville, TN

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What I did was visit a local charity, Habitat for Humanity, and picked up a couple of APC Back-UPS 500's for $10. each. True they did not have batteries in them but those were replaced quickly VIA the internet. They were new units (older style) that a computer store had removed the batteries from and then donated the UPS's to the charity for resale.

You might want to visit a couple of charities or even used computer stores and see what is available. Most used UPS units just need new battery or two and they are as good as new. Just be sure to replace all batteries in a unit at the same time to help eliminate problems later. Like a flashlight you don't want to mix old and new batteries if the unit takes more than one. Many times pawn shops have them for sale as well but watch out for the price as most of them want too much for a used UPS, charities are where I find them the cheapest as most of the time they have no idea what they are let alone what they cost retail.

Much easier than trying to build one, which I had done in the past using two deep cycle boat batteries and an external battery charger rated for 24 volts. It would go down to a low powered trickle charge after fully recharging the batteries. I also had to keep the batteries in a well ventilated area so no gas buildup could cause an explosion during a heavy recharge session. Much easier to use actual UPS units now that I am in an apartment.
TheMG
Premium Member
join:2007-09-04
Canada

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I agree, just get a UPS. Putting together a DC backup system is likely going to be much more expensive and complicated.

I've dumpster-dived a few UPSs in the past. Install new batteries and good as new.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

2 edits

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Here is an excellent off-the-shelf DC UPS module: »www.mini-box.com/OpenUPS

It supports both lead-acid and lithium-iron-phosphate batteries for backup. The OpenUPS2 module (»www.mini-box.com/OpenUPS2) can hold 3 Lithium Iron Phosphate batteries directly on the PCB.

Default is 12V output, so before use, please configure output voltage to 24V using Windows laptop. You can actually make it any custom PoE voltage upto 24V.

You can connect the DC input to AC-DC adapter or solar panel or any other type of DC power source that provides 10-30V range and enough current to power load and float charge the main backup battery.

You can also connect the DC input to a separate huge 12V lead-acid battery, with a float charger connected in parallel to that battery. That gives you backup for your backup battery.

P.S. You can also make DIY versions using DC-DC modules. DROK modules sold at Amazon are really cheap, but works reasonably well.
lutful

1 recommendation

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This is a response to a question from OP via PM.

- set the DC-UPS output to highest voltage you need which is usually 20V-24V for WISP PoE.

- use DC-DC step-down module(s) to make lower voltages from that output.
example: »www.amazon.com/DROK-Adju ··· 0BYTEHQO

- make DC connections using standard round plugs with 2-wire pigtail. You can cut-off cable from old AC adapters.

- find which wire goes to the inside of the barrel (measures zero ohm) and connect that to the positive screw terminals of UPS output or DC-DC output.