The concept of a car battery UPS is elementary. However, like most things, there are very advanced concepts at play that need to be taken care of.

Batteries are extremely dangerous and volatile. They need to be treated well and with respect when building any system that relies on them.

My actual office network rack with custom UPS

Why do you want a car battery UPS system?

The simple answer is: cost!

So, anyone who has looked into UPS systems for their computers, network equipment or servers, will know just how expensive these things are.

In this write-up, I will be using exclusively Ah and VA as units. There are many manufactures using all different kinds of units, and it all gets confusing. I find these 2 units are useful and listed more often than others on spec sheets.

Current market offerings

APC and Tripp-lite are the current go-to's

It just is what it is, pretty much every single network rack in America is going to have either an APC or a Tripp-lite

That is because they simply do the job really well. I am not going to talk about the other brands much. Honestly, I would not touch them with a 10' pole. They are just too unreliable, and I have seen too many failures.

Maintaining a baseline

I am going to come at this from a professional's perspective. Where we have money to work with and need very reliable systems that adhere to building codes and are acceptable to other professional standards.

There are ways to cut corners and save a bit of money; however, this is not the goal.

The goal here is to build a system that, while saving money, is worthy of a production environment.

The goal:

Create a UPS system that can power a heavy-duty network stack, a few servers and some PoE devices for an extended amount of time. Shooting for 8 hours. This will allow ample time to manually shut down equipment or deploy a generator.

Let's build a car battery UPS

Components required:

Determine size of UPS

This is the obvious first step. How much power do you actually need? More than that, what are your plans for the future?

I like to calculate the current VA load of the entire network system, then multiply it by 2 and then try to build a system for that. This way, we do not have to worry about adding some heavy-duty gear down the road.

Of course, if you have zero plans for expansion, then you can cut back, but still, I would leave at a minimum 25% headroom.

Also remember, when taking measurements of power draw, ensure you are using the full load numbers of the system. Do not simply take calculations based on what equipment is being used at the moment.

So, determining the maximum draw is easy, just look up the specs for all devices, add it up and then add 25% for transience spikes or any random spikes. That is the problem with a UPS, if something decides to go rogue, it will bring the whole system down, you cannot use multiple building circuits for a single UPS like you can with a bare system.

Determine desired runtime

Once you have the load determined, you now need to figure out how long you want the UPS to hold up the system.

This gets a little tricky, as battery voltage comes into play, and we are still trying to determine what UPS and batteries to buy,

I use this calculator

It helps to determine the requirements; however, you will have to understand the following:

Volts and amps... if we think of electricity as water then Volts can be thought of as the size of a water pipe, and Amps can be thought of as the pressure in the pipe.

So, if we need to move let's say 10 gallons of water an hour, if we use a small pipe (Volts), then we will have to have a higher pressure (Amps). The inverse is also true, we can reduce the Amps if we increase the Voltage.

With relation to our UPS system. Batteries are always at a lower voltage than 120v AC that comes out of your wall. So, while using the above calculator, you need to adjust the Amps for what will be being taken from the battery. An example, a 20VA load with a conservative Power factor of .6 at 120v is only around .15 Amps

However, at a 12v battery the current will be 10 times that, or 1.5 Amps.

Now this is a basic way to think about it and there is a lot more going on, but these concepts can be applied to create the goal of this article the car battery ups

The goal is to determine how many amps at how many battery volts your load requires, so we can buy a battery system with the appropriate Ah (amp hours)

Bottom line, figure out how many Amps your load takes at 120v and write it down, so we can see what voltage battery system is the best for this use.

Efficiency and battery voltages

There are 3 main voltage levels of battery systems. 12v and multiples of 12 so, 24, and then we skip to 48v

This is because 12v batteries are like the standard, I am not certain why, but that is what it is.


So, the more you need to step up the voltage, the more loss there is in the inverter. I know we like in the space age and all, but it is still very common for 20% losses in inverters. Of course, an inverter changes the battery's DC into AV at the same time increasing the voltage to 120v, so our computers can run off it.

Ironically computers themselves run off DC at voltages close to that of the batteries, we are actually turning low voltage DC into AC, then the computer power supplies are turning it back into low voltage DC. There are efficiency losses at every step of a not inconsiderable amount. But there is nothing we can do about that; it is what society has chosen to do.

What works is to keep the change as minimal as possible. This is why large systems are exclusively 48v where smaller ones are 24v, sometimes even 12v.

This comes at a cost, 48v inverters, and battery systems are just more expensive.

So, load will determine whether it is ok to use a 24v or 48v system. As well as budget.

Let's assume a 24v system

I am just going to price/ part out a 24v system, as if you need a 48v system, chances are you have enough money to just buy everything off the shelf. However, everything here applies to all systems, 12v, 24v, & 48v

Selecting a UPS

This is critical and what most people ignore.

We need a UPS, not an inverter, not anything else on the market that is marketed for this task. A UPS is specifically designed to work with the many low current DC power adaptors that we will require to run.

Other devices on the market are meant to run singular devices and I have personally seen them fail when attempting to run loads at less than half their rating, but coming from many small computers at once.

Duty Cycle

In industry every device has what is called a "duty cycle" it refers to the amount of time the device can be run continually. This is often the only difference between a consumer device and an industrial one.

A lawn mower is a good example. A lawn mower designed for consumer use will expect just that, once or maybe twice a week use for a few hours. It will be able to do that well. However, if you were to put it in a professional law cutters garage, it would fail. It is simply not designed for a "full duty cycle" that the professional would be using it for.

The same is true for a UPS inverter, just slightly different. They designed the inverter to work with the capacity of the stock battery. If you attempt to run it for longer than the stock battery can, it can very well catch on fire or just fail.

This is the biggest problem with the guides I have seen for this project. They totally ignore the duty cycle of the device they are using.

To solve this, we need a UPS that is rated for "extended runtime" These are designed to allow the user to daisy-chain batteries and create insane amounts of runtime.

The cheapest and best units I have found are the following

Once you have found one of those units that suits your power requirements, we can move onto capacity.

Time for batteries

Currently, the types of batteries commonly available are:

Lead acid

The cheapest option. Good ol' lead acid car battery's. However, for a UPS we absolutely need Deep Cycle cells. This will allow them to drain to almost 0 and then be recharged. If you try this with normal lead acid, they will literally fail after 3 or 4 runs to 0, that is unacceptable

Sealed lead acid

SLA batteries are what comes with UPS systems. They are the same as normal lead acid, but they are sealed and always deep cycle. The sealed part is important, and not. Basically, when you charge a lead acid battery, some hydrogen has is released. Hydrogen gas... like that is used in hydrogen bombs. It is extremely explosive. So Sealed Lead Acid do not release any and thus are better for closed spaces.

However, non-sealed are not so dangerous because Hydrogen is so light, if you have any ventilation at all the Hydrogen will be quickly vented. It is not like Methane that is heavier than air and will pool on the bottom of the space.


Lithium-ion, the start of the 21st century. They are actually so incredible, the man who invented them won a Nobel Prize.

They are all deep cycle, they are high capacity, light weight and can do many more cycles before degradation than lead acid.

If you have the money, there is really no contest, Lithium-ion is the superior technology.

Lithium Iron Phosphate

The crem de la crem of battery's currently is LiFePO4

They have all the benefits of Lithium-ion but offer even more cycles before degradation and are even lighter weight.

They will cost you nothing less than an arm and a leg, and a kidney, maybe even a cornea. So they will probably not be the choice for someone looking to save some money on a UPS system.

Wiring/ fusing

This is a very critical component you cannot skimp out on. It will be the veins of your system.

We need, between the batteries, very heavy gauge wiring. This is because batteries have unlimited current potential.

We need the batteries to act as a contiguous unit. The fuse will be in line between the main feed line to the ups and the batteries, between the batteries there will be no fuses, and thus the wire will be a sudo fuse, and we need it to not fail

We can use bus bars, instead of wire. However, for computer applications they are overkill. Simple 2 gauge or large wire is more than sufficient to make sure the wires will not get hot or fail.

The industry standard is one 150A fuse on the main positive line between the battery circuit and the UPS.


That is really all there is to consider. I spared exact brands and details because you need to figure that out on your own. Remember, this system can kill you and those around you quickly. Batteries are not to be messed with if you do not have the experience to draw many of these conclusions yourself.

This was kind of a brain dump; however, it has been the culmination of over 2 years of trial and error, and research.

If you have any questions, just post them below!

Copyright © 2024 Meadowlark Marsh LLC.
Website/ Hosting by Meadowlark IT