Homemade UPS

This article was originally published in 2010. In 2018, I'm no longer using this homemade UPS. The automatic battery charger and inverter broke several years ago, and I chose not to replace them. I tried to have the battery charger repaired locally, but they weren't able to fix it properly. The inverter had overheating issues.

If I were to build it again today, I would probably select different components, though I'm not sure what those would be.

I'm currently using an APC Smart-UPS and a whole-house diesel generator with an automatic transfer switch, so the generator starts and the house is switched over automatically when there's a power outage.

Do you need a UPS that can run your computer gear during all day power outages? I put together a homemade double conversion online UPS using off the shelf components, and now when the power goes out, my computer doesn’t skip a beat, even if the power is off for an hour or two… or all day.

The main components of the UPS are:

  • A pair of 6 volt golf cart batteries wired in series
  • An automatic battery charger with a DC UPS mode
  • A 300 watt sine wave inverter

With a double conversion online UPS, your equipment runs off of batteries and an inverter full time. When there is line power, an automatic charger supplies enough current to power the inverter and keep the batteries float charged. When line power is lost everything continues running normally, with the batteries handling the load. When line power is restored, the automatic charger supplies enough current to power the inverter and recharge the batteries.

This also has the side effect of providing really clean, isolated power to your computer equipment, assuming you’re using a sine wave inverter. Power is always converted to DC and back to AC before being used (double conversion).


To get an all day run time, you need some big batteries. Most golf cart batteries are at least 200 AH, and offer a good value at the expense of some convenience. You need to check the water levels in the batteries and add water to them periodically. Also, you should not use them indoors due to outgassing. Lead acid batteries outgas hydrogen as they charge, so they should be used outdoors and need to be well ventilated to prevent explosion.

If you’re willing to spend more money (perhaps twice as much), you could look into using sealed AGM batteries, which don’t normally outgas unless overcharged. These are often used indoors, though I think I’d still prefer keeping them outside if possible.

Here you can see the pair of golf cart batteries I’m using, inside a battery box. I’m using Water Miser Vent Caps on the batteries to help reduce battery watering and to make watering more convenient with flip-tops. They’re optional, though.

I’ve heard Trojan makes great golf cart batteries. I’m using Motolite golf cart batteries, which is what I could find locally, and they work fine. They’ve been in use 3+ years now and still going strong.


I’m also using a Power Pulse desulfator connected to the batteries. This is optional, but I think its really helped. Over time, sulphate will form on the lead plates in the batteries causing reduced capacity. The desulfator helps prevent and even reverse the sulfation. It’s not perfect and the batteries won’t last forever, but it helps.

Before I hooked the desulfator up to the golf cart batteries, I hooked it up to an old 37 AH SLA battery pack that had been in use for about 10 years and was starting to show substantial wear. I kept the Power Pulse desulfator on the battery pack along with a 500 mAH charger for a few weeks, putting it through a few charge/discharge cycles and it really helped improve the condition of the old battery pack. I wish I’d gotten a second Power Pulse desulfator to use on my car battery.

Battery Connections

The golf cart batteries are 6V so to get 12V for the UPS, a pair of them are wired in series. I’m using a 65A automotive fuse between the negative post of the first battery and the positive post of the second battery. It’s really important for safety to make sure everything is fused properly.

I’m using heavy gauge wire for the battery connections. On the batteries themselves, I’m using some thick yellow automotive wire (labeled with a marker +/-). For the inverter and charger connections, I’m also using heavy gauge (8mm2) wire, but it’s properly color coded (black & red). If I was doing it over again, I’d use the color coded wire for the battery connections, too.

The batteries are wired into an ordinary 50A 240V AC circuit breaker mounted on the outside of the battery box. Ideally, this would be a DC circuit breaker. AC and DC circuit breakers are designed a bit differently, and there are situations where using an AC circuit breaker with a DC load could be problematic. Since the batteries themselves are fused separately, I’m mainly using this as a disconnect switch for convenience. It seems to be working fine for me so far.

From the circuit breaker, connections are made to both the automatic battery charger mounted on the front of the battery box, and the sine wave inverter mounted on the side of the battery box underneath the circuit breaker.

Automatic Battery Charger

I’m using a Samlex SEC-1230A 12 Volt 30 Amp Automatic Battery Charger. I have the DIP switch settings set to “Battery with Load” so the charger effectively becomes a DC UPS. There’s a good manual that explains the settings, and it’s available from the Samlex website as a PDF.

The charger comes wired for 120V 60Hz input, but there’s an internal jumper that can be changed to make it 230V 50Hz. If you do that, you need to change the fuse as well. I made the change, and use it with a 220V 60Hz power source and so far I haven’t had any problems.

Sine Wave Inverter

For the inverter, I’m using a Samlex PST-30S-12A 300 Watt Pure Sine Wave Inverter (12VDC-120VAC). I’ve used a modified sine wave inverter with my computer equipment in the past, and it mostly worked fine, but there was some buzzing on my phones. There’s no buzzing with the sine wave inverter.

Some of my gear uses 110V, some uses 220V, and some will use either. For the stuff that needs 220V, I use a small 100 watt step-up transformer. I also run my computer (an HP Laptop) off of 12 VDC with an HP KS474AA 90W Auto/Air/AC Adapter. Other laptops may have similar 12 VDC adapters available.

You’ll need to calculate your load requirements. If you’re using a desktop system instead of a laptop, you may need a larger inverter. My inverter is supplying power for a 22" LCD monitor and an assortment of peripherals: a pair of USB hard drives, speakers, DSL modem, router, switch, USB hub, ATA, and 3 phones.

I’ve got a power meter hooked up to the inverter output, and I’m using less than 100 watts. So for my use, a 300 watt inverter is plenty. The inverter could handle my laptop, too. I already had the 12 VDC laptop adapter, though, so I use it for maximum efficiency.


I keep the custom built battery box on an outside covered patio that gets plenty of ventilation and is conveniently located next to my office. There are plastic screens on the top and the sides of the box to let the batteries vent.


I’ve got a set of connectors mounted in the wall in utility boxes to keep things clean. The outdoor connectors (from left to right) are: antenna, ground, line power, UPS power inlet, and 12 VDC input posts.

There’s another set of connectors mounted on the other side of the wall in my office. The indoor connectors (from left to right) are: two 12 VDC outlets, two 110 VAC UPS outlets, line power inlet, ground, and antenna. The antenna connector isn’t used for the UPS.


The rough cost for the major components of the system was:

  • $250 Two 6 volt golf cart batteries
  • $225 Samlex SEC-1230A Battery Charger
  • $135 Samlex PST-30S-12A Sine Wave Inverter

I got the golf cart batteries locally, and ordered the charger and inverter from altEstore.

An alternative to all this, is to use a small off the shelf UPS for short power outages, and to simply take a break from the computer during longer power outages. I hear its nice to spend time outdoors. What do you do when the power goes out?

Please note: I’m not an electrician, but I did have an electrician do the wiring for this UPS system for me. There’s no guarantee any of this meets electrical code. Please consult an electrician and check your local electrical code if you want to undertake a project like this.

Update February 2012

I just replaced the golf cart batteries in my UPS with a 100AH 12V AGM battery, that I got from Pure Ener-G (the sponsor of the Solar Panels Philippines Forum). The golf cart batteries were still working, but they’ve been in use for around 5 years, so I thought it would be good to replace them now rather than have a surprise failure later.

With the old golf cart batteries, I made the mistake of not coating the battery terminals and lugs with some sort of sealant, and they became corroded.

I had a few power outages where the UPS failed. One of the battery lugs was so corroded that it crumbled apart when I tried to remove it. So that picture is after removing the old battery lug, with the remnants of the old lug laying on top of the battery. I know I shouldn’t have let it fall into disrepair, but I was planning on replacing the battery anyway, so I hoped it would keep going a little longer.

Here’s the new 100AH 12V AGM battery. I’ve got some dielectric silicone grease coming that I’ll use on the new battery.

I was planning on replacing the batteries with a 12V 200AH Motolite Solarmaster battery, but I went with this AGM battery instead. These are hard to find locally, so thankfully I was able to have it shipped to me from Manila (via Partas).

I replaced the automotive fuse I was using with a Blue Sea 100A DC fuse and fuse block, which you can see sticking out from the positive terminal on the left.

I also replaced the AC circuit breaker with a proper DC circuit breaker, an 80 amp Bussmann MRCB circuit breaker.

And I added a MidNite Solar battery meter, too.

I should also note that I’m using a Mini-ITX system with a 12VDC power supply with this UPS instead of a laptop now. The 250 watt M4-ATX DC power supply connects to my battery, bypassing the inverter. These are also used for Car PC systems. It runs my i7 2600K system without difficulty (using an Intel DH67CFB3 Mini-ITX motherboard with the integrated graphics from the 2600K).

So those were the UPS modifications I made. While I have less capacity with the 100AH AGM battery vs. the 200AH system with the golf cart batteries, I’ve also got a diesel generator now, so hopefully the UPS won’t be pushed too hard. And I won’t have to fill the golf cart batteries with water anymore.