DIY uninterruptible power supply

As an example I will give the requirements for my own UPS.

• Power: my UPS has to power a router, server, wifi, and some networking equipment. Measured average power consumption is 50 W. Peak is under 100 W.
• Run time: 4 hours should cover most outages.
• Charge time: I'm not in a hurry to have everything charged and ready again, so I don't really care.
• System voltage: 12 V, since I already have some 12 V parts I want to use.

• If you can find an inverter with built-in battery charger (and maybe even a transfer switch), that's worth the extra money. This will enormously simplify the build.
• Personally, I'd rather buy used professional equipment than cheap new products. There is not much that can wear out on an inverter, especially on professional units.
• When I was searching for inverters, used 24V inverters were usually cheaper than 12V ones, but this might be a temporal or local phenomenon.
• The cheapest, unbranded inverters are usually built down to a price, rather than quality. Those often times do not deliver what they promise. If you must use one of these, buy considerably higher power than you need.
• When in doubt, look for chunky metal box with cooling ribs and a fan.

My inverter is a Victron Energy Phoenix 12/500. I've had it for over 20 years, and it's been in several other projects before. The inverter runs on 12 V, can deliver 500 W continuous, and 1400 W peak. It has a cooling fan, all sorts of protections, it's durable, and (almost) idiot proof.

If I were to buy an inverter today, I would again choose a (used) professional model, but with a bit more power so I could also run some power tools. If possible, I would pick one with built-in battery charger and transfer switch.

There are several different subtypes of lead-acid batteries, designed for different applications. Not all types are suitable for daily use in an UPS.

Starter batteries (car batteries) are designed to deliver a short, high-current burst (for starting an engine), but not for deep discharge. In a UPS they will work a few times, then wear out quickly.

Small sealed lead-acid (SLA) bricks are cheap and compact, and often used in alarms and emergency lights. Those are fine for occasional backup, but they wear out very fast with daily cycling.

Absorbent Glass Mat (AGM) is sealed type of lead-acid that's more robust than the small SLA bricks, and often used in commercial UPS units. A good choice if your UPS is used occasionally (a few times per year), but they won't last as long as deep-cycle types under daily load shedding.

Deep-cycle flooded lead-acid are inexpensive and durable if properly maintained. Can handle daily cycling and last for years, but they need to be topped up with distilled water and kept in a ventilated space.

GEL batteries are sealed and maintenance-free. They tolerate deep discharge well and are best suited for modest power draw over a long period of time. Often marketed as household, traction, or solar batteries. These are among the best types for daily use in an UPS.

The perfect battery for your UPS (in terms of voltage and capacity) might not exist, but it's perfectly fine to combine multiple batteries into a battery pack. Just make sure all the batteries are identical (same brand, model, capacity, and age), because the whole pack will only perform as well as the weakest battery, and mixing and different types leads to imbalance, heat build-up, and premature failure.

Increase voltage (series connection): Connecting batteries in series adds their voltage. For example, two 12 V batteries in series make one 24 V battery pack. The capacity (Ah) stays the same: 2 × 12 V, 50 Ah in series = 24 V, 50 Ah

Increase capacity (parallel connection): Connecting batteries in parallel adds their capacity. For example, two 50 Ah batteries in parallel make one 100 Ah battery pack. The voltage stays the same: 2 × 12 V, 50 Ah in parallel = 12 V, 100 Ah

Combine series and parallel: With four 12 V, 50 Ah batteries you can make one 24 V, 100 Ah battery pack by wiring two sets in series, then putting those sets in parallel.

Wiring up multiple batteries is explained in more detail, and with photos in this tutorial.

The service life of all types of batteries depends on several factors. Most important are charging and discharging rate, and temperature. The values used below describe GEL and similar sealed lead-acid batteries.

Discharge current: GEL batteries are most suitable for modest discharge rates. A safe maximum is around 0.2 C meaning 20% of the battery capacity (in Ah). For example, a 100 Ah battery should ideally not be discharged at more than 20 A. Higher currents are possible, but shorten the lifespan.

Charge current: A good rule of thumb is to charge at about 0.1 C. For a 100 Ah battery, that means ~10 A. Anything between 0.07 C and 0.15 C is acceptable. Charging outside this range shortens the lifespan.

Depth of discharge (DoD): The deeper you discharge a battery, the fewer cycles it will last. To maximise life, try to keep the battery above 50% charge, ideally above 80%. A larger capacity battery helps: it lowers the relative discharge per cycle, reduces current stress, and allows faster charging.

Temperature: Heat drastically reduce lifespan. At 25 °C, lead-acid batteries can achieve their rated lifespan. For every 10 °C above that, the expected service life is cut roughly in half. So keep your batteries in a cool, ventilated space (basement, cellar, garage, or shaded shed). You probably don't want them inside your living room anyway.

All these factors (and a few more) come together when calculating the required battery capacity. To make life easier I've put together an UPS battery calculator that does the maths for you. Just fill in the values you know, and it will give you a minimum battery capacity.

The most interesting value to tweak is the depth of discharge (DoD). A smaller battery will be pushed harder: it has to deliver higher currents relative to its size, and it will be cycled deeper. Both of these cause more wear. A larger battery pack, on the other hand, runs "loafing along" at lower C-rates and shallower cycles, which usually means a much longer service life.

If the UPS is only for emergency backup, deep cycling is quite common, usually combined with AGM batteries. In that case, a DoD of up to 50 % (AGM) or even 80 % (flooded deep-cycle) may be acceptable. But if the UPS is used daily, for example during load-shedding, GEL batteries with a conservative DoD of about 20 % are often the better choice.

A bit of over-sizing is always a good idea, since battery capacity will degrade over time, and you may want to power more appliances from the UPS in the future.

New deep-cycle batteries are great, but they're expensive. You can save quite a bit with used batteries, if you know what to look for, but it will always be a bit of a gamble.

• Touring coach batteries are often replaced on a fixed schedule, regardless of condition, which means you might find lightly used ones for cheap.
• Marine household batteries are generally cheaper than batteries marketed for RV or camper use, even though they're often very similar inside.
• If you're buying more than one battery, stick to identical units (same brand, model, capacity, and age). A mixed pack will lead to disappointment.

When buying used batteries, here are some thing to pay extra attention to.

• No cracks, leaks, or bulging cases.
• Ideally less than 5 years old, check date code if possible.
• The resting voltage should be around 12.6–12.8 V when fully charged. (Don't forget to bring your multimeter.)
• Under moderate load, such as car headlights, the voltage should stay above 12.0 V.
• All batteries in your set should test about the same.

When I bought the batteries for my UPS, price was an important factor. This build is meant as prototype, rather than for daily use. I had bought the batteries before I did any research on battery chemistry, capacity, charging and discharging rate, etc, but I happened to have made a good choice.

The batteries are two used marine traction batteries. I bought them for €45 each, local pick-up. They're Exide Maxxima 900, 12 V, 50 Ah Spiral Cell GEL batteries. I have wired the batteries in parallel, making a pack of 12 V, 100 Ah, to match the inverter voltage. DoD is slightly under 20%, also more by chance than by design, but I'm happy with the resulting value.

If I were to buy batteries for daily use, I would probably get bigger ones. I found a batch of used 12 V, 230 Ah touring car batteries online for €75 each.

Buying tips are similar to those for the inverter. I'd rather buy a used good brand battery charger, than a cheap new one. Not much can wear out on a battery charger. The better ones come in a chunky metal box with a fan.

I already had a battery charger I wanted to use for the UPS. Unfortunately it only supported starter batteries, so I went looking for another charger. This was before I had realised I had GEL batteries, so I spent quite a while looking for an AGM battery charger. The only used one I could find was 15 A, of a brand I'd never heard of, for €25 + shipping. The charging current is not ideal, but falls within the acceptable range of the battery. When it turned out my battery was GEL, I was lucky the charger supported both kinds. But had I realised this earlier, I would have chosen a different charger.

If I were to buy a battery charger again, I would have bought a (used) known-brand battery charger, with a charging current closer to the ideal 0.1 C.

• Make sure it can handle your peak load current, not just the average.
• A good unit includes a switch-back delay (usually around 30 seconds).
• If you go for a cheap unit, over-size it, buy one rated for more current than you actually need.

I ended up building my own automatic transfer switch, and that's what I'm using here. It's basically a relay in a box, and I've fully documented the design. Feel free to copy it if you'd like.

I wish I could give a simple buying recommendation, but the truth is that professional ATS units (even used ones) are very expensive, and the cheap ones are of questionable quality. So unless you find a decent mid-range model, DIY may actually be the best option.

• Good battery monitors use a shunt resistor and coulomb counting (measuring all current going in and out). These give you a much better picture of actual charge and runtime than one that only monitors voltage.
• A voltage-only battery monitor is still better than having no monitor at all.
• Having a low-voltage alarm can help you prevent over-draw. Some battery monitors have this built-in. Some inverters also come with a low-voltage cut-off.

The one I use is a used Victron Energy BMV-501, which I picked up for €25 + shipping. It was the only affordable unit I could find, but it does the job well.

Wiring up the UPS is not too different from wiring mains power, only the wires are somewhat thicker. The minimum required wire cross-section is dependent on the amount of current flowing through the wire. The higher the current, the thicker the wire should be. You can use the table below to get the minimum wire size.

The biggest current runs between batteries and inverter. Take my UPS for example: $\mathrm{Current}=\frac{\mathrm{Power}}{\mathrm{Voltage}\times \mathrm{Inverter\; efficiency}}=\frac{500W}{12V\times 0.80}=52A$, Looking this up in the table gives 10 mm², which matches the inverter's user manual. Use the same size wire between your batteries, if you have more than one, and wiring up the battery monitor shunt resistor.

A smaller, but still significant, current runs between charger and batteries. The wires that came with it are usually fine.

Wiring the battery monitor requires just signal wires, thin wire will do the job.

Keep high-current wiring short. Every bit of wire has resistance, resulting in voltage drop and heat. For an efficient system you will want to minimise this.

Use stranded wire, not solid core. Stranded wire is more flexible, resists vibration, and makes better connections in screw terminals.

A few safety tips.

• Double check polarity (positive/negative) before connecting. Positive is red, negative is black.
• Err on the safe side: if in doubt, use thicker wiring. You don't lose anything by over-sizing, except maybe a few Euros.
• Tighten everything properly, especially the thick, stiff battery wiring. These need to be mechanically fastened as well as electrically.
• Have a clean cable path, no running wires over contacts or sharp edges, and no chafing wires.
• Don't forget strain relief for cables going in and out of the UPS.
• When in doubt, look it up, ask someone with experience, and apply common sense.

My inverter came with 10 mm² (8 AWG) wires already attached. That's way overkill for my power draw, but it's good quality wiring, so I reused the extra length for battery wiring. The battery charger had 2.5 mm² (14 AWG) wiring attached, which is fine. For the battery monitor I used an old UTP (Ethernet) cable. Battery clamps came from the hardware store and are quick release types, which is handy when moving stuff around. The most channanging bit was attaching the thick battery cabling to the battery monitor shunt resistor. For this I used crimping lugs and some big pliers.

The battery charger, inverter, and battery monitor were already fused, so there was no need to add any more myself.

If I were to build a box, I'd probably use scrap plywood. I'd leave plenty of space around the top for ventilation, and make a hinged lid for easy access and taking batteries out.