A Larger Battery For An Old UPS
Consumer computer uninterruptable power supplies (UPS) used to be dirt cheap. If you lost power, these would provide a few minutes of electricity to your computer and monitor; just long enough to save your work and shutdown cleanly. Laptops are much more popular these days and they provide their own batteries with substantially longer run times. I got my APC branded 500VA UPS years ago. After the internal battery wore out, I tucked it away in the garage and forgot all about it.
During a recent power outage, I remembered the old UPS in the garage. I don't have a use for it for a computer but what about other uses? Most of the power outages in my area last no more than a few hours. I don't need to keep the refrigerator running; but if I had power for a few lights, that would be handy. All of my interior bulbs are LED so they all draw under 10 watts each. A UPS that could run even one lamp for several hours would be nice to have. So that's what this small project is all about - replacing the tiny old battery in the UPS with a larger one.
The old UPS battery was dead so I had to get a new battery just to get it up and running. What should I get? It's all about tradeoffs in size, weights, capacity and performance. The original battery was a mere 7.5 Ah and was small enough to fit inside the plastic case. Anything larger would require me to house the battery outside the existing housing. Lead is heavy and while this thing does not have to be portable, I also don't want it to be so large that it takes up too much space. In the end, I opted for a Duracell deep-cycle 35 Ah battery. Specifically, this is model DURDC12-35J. Deep cycle means I can draw down the battery more than I could a regular SLA (Sealed Lead Acid) battery in times of need, without damaging the battery.
The UPS consists of a battery and the electronics. The electronics contains the transformer and charging circuitry to feed the battery and the inverter to convert battery DC to 120 VAC when needed.
The only change I made to the circuitry was the removal of the small buzzer. This buzzer sounds every few seconds when the UPS is in discharge mode. For computer user, this is a useful reminder to save all the work and shut down. But when powering a light during an outage, this incessant beeping is a real nuisance. I used a pair of pliers to gently twist the buzzer until it snapped off.
The overall construction is fairly simple. I used 3/4 inch thick MDF to create a rectangular box to house the battery. The panels are held with just wood glue on butt joints.
I used 12 gauge silicone wire to extend the wiring between the charger to the battery. A quick disconnect in the cable adds safety during testing and final assembly.
Cutouts in the top of the box and the bottom of the charger allow the wires to pass through. There's extra space in the box above the battery and in the old battery charger for the excess wires to sit. This is important because I want extra slack in the wiring to make the final connection easier.
Contoured finger holds on two sides help with gripping the unit. Sometimes it's the little touches that are nice to have!
A 1/4 inch roundover is applied all around. MDF is just sawdust with a paper backing. The roundover reduces the chance of unsightly dings on the edges and corners.
Green spray paint seals and finishes the box. Why green? Because I already had it from an old project.
Finally the charger locks onto the box via 2 screws.
Here's the finished unit ready to use.