I would like to have a battery backup for my sump pump in our basement, and many of the ones that have have found online are well over $1000. Can I make one myself for less?
In particular, is it feasible to plug a trickle charger into the wall, have it charging a marine (deep cycle) battery, hook up a power inverter to the battery and plug the pump into that? If I do it that way, then the biggest expense is the power inverter, but even then, I am well under $1000.
Could I use a UPS for a computer if it were big enough? I have seen answers to the negative without explanation. If I cannot, then why not?
You would be running on mains power, but you then need to change from that to the inverter, and if the pump is working from a starter, then even a small interruption during changeover will cause it to trip off.
You can get electronic switching modules that can cope with this, depends upon the current rating you require, won’t be cheap though.
Another way might be to run it solely from the inverter, use a tranformer and rectifier and maybe a regulator to supply the inverter from your mains supply, with blocking diodes to prevent discharge back into your battery, the transformer also needs blocking diodes inline to prevent the battery trying to discharge through it.
Your transformer will provide the mains isolation for the low voltage supply from the mains.
This is a method I have seen used regularly in emergency power supplies in critical areas such as keeping instrumentation running.
The pump has a float switch to activate it. It is a simple on/off switch that does not require any starter. Any power interruption would cause the pump to be off while it is without power, but come back on when power is restored because the switch is still on.
Your second description sounds like what I proposed, or have I misunderstood your design? The pump will be plugged into the DC->AC inverter which is attached to the battery which is on a constant AC->DC trickle charge from the mains. I assume that both the power inverter and the battery charger have the circuit protection components in place. When mains power is available, the power would be drawn from the battery connection and the charger would supply the power rather than the battery. When mains power is unavailable, the battery would supply the inverter.
The components that I am thinking of are simply equipment found at an automotive supply store, rather than getting into any need for special wiring…
You could get a standard 12 volt sump pump.
You can also get a marine type battery.
The trickle chargers are in the stores.
I don`t think you even need a switch-over device. The thing will always be charged and will run when the float (Set higher then the AC powered sump) is activated. The charger can continue to charge the battery as the pump runs.
Plug the charger into the outlet, hook up the battery and install pump. Sounds simple to me.
WAY under $1,000. More like $300.
I forgot to add, run the 12 volt pump in the same well as the AC 120 volt one. This way youll have redundant pumping and even if the power never goes out youll have the 12 volt one there if the AC unit goes south on you.
Sounds like whuckfistle’s drained out a basement or two. Redundancy is always good, especially in a basement, and running the pump directly off the battery equates to what the UPS folks call “fully on-line” or some such jargon, meaning the load is always running off battery power, rather than the typical PC UPS that’s “near line,” switching from line AC to the battery-powered inverter in a 60-Hz cycle or two.
As for extending the running time of a regular UPS, is there some reason the UPS’ internal battery couldn’t be removed and a larger battery connected? (I was pondering this very concept yesterday as I was replacing a tired battery in one of my UPSes)
The second description I gave would be more appropriate for fairly high levels of current, where the battery only supplies current for maybe half an hour, and perhaps several hundred watts of power are needed continuously.
A simple relay could easily do the trick here, energised when mains is present to supply mains direct to the motor with changover contacts.
Very easy and would only cost the price of a suitably insulated box, fuses and fuseholders, terminal strips, you could knock something up for less than $100.
While matching battery voltages is a good idea (!), there still could be problems with a larger battery being too high of a load on the charging mechanism and burning it out. You would have to run thru the circuit for the charging mechanism and compute if it can handle the battery you are putting in. Does it automatically limit the current drawn? If you aren’t careful you could run into the following: works great for a while until a longish outage discharges the battery a good deal, then on power back on-phhtt.
This is why I like my ancient UPS: Uses car batteries. Semi-old ones work great. But on the other hand it’s heavy and then some.
As to OP’s semi-question about using computer UPSes for pumps, the reasoning against it probably has to do with the momentary spike in power drawn during motor startup. Something $30 computer UPSes aren’t designed to handle. But if you want to spend a few hundred $s, there are “computer” UPSes that could no doubt handle it.
…which describes how to build a complete, self-charging, self-switching backup power system. Owing to the plug configuration and some references in the text, I believe this is a 240VAC Australian system (prices in Australian dollars, too).