The title is pretty much self-explanatory. Using car batteries with the necessary additional equipment to make a functional and affordable, uninterrupted power supply.

Let's see what is there to offer by doing this DIY vs what the ready-made options have to offer.

Current market offerings:

The market offerings vary over a range of products from both cheap to expensive. But taking a look at some of the more professional options:

Schneider APC:

Schneider APC Inverters are advanced and dependable uninterruptible power supply (UPS) systems made by Schneider Electric.

These inverters use high-quality components and advanced technology to ensure they perform well and protect devices during power outages. They come in different models for various power needs, from homes to large businesses. They also have smart management software for easy monitoring and control of the UPS status and power settings.

Its quite easily a reputable option for our operation.

Eaton Tripp-lite:

Eaton Tripp-lite inverters, just like Schneider APC Inverters, are exceptional backup power systems that ensure uninterrupted power supply (UPS) for your valuable devices. While both brands offer reliable performance, they come with subtle differences to meet varying needs.

Similar to Schneider APC Inverters, Eaton Tripp-lite devices are built with advanced technology and high-quality materials, guaranteeing smooth operations during power outages.

Thanks to their reliability and versatility, they are a popular choice for anyone seeking dependable power backup solutions.

Cyberpower UPS:

What sets CyberPower UPSs apart is their smart features and advanced technology. They come with intelligent LCD displays, providing real-time information on power status, battery levels, and runtime. Users can easily customize settings to optimize UPS performance.

Moreover, CyberPower UPSs are energy-efficient and some models even work with smart home systems, allowing remote monitoring and control for added convenience.

Overall, these options, though being extremely feature rich, can cost a hefty amount. Not to be taken wrong, yes it does cost a fair bit to have a professional UPS setup that complies with safety regulations etc.

But it is possible to make a suitable setup that cuts the unnecessary corners to make an equally capable UPS system by ourselves.

Understanding our goal:

Since we have our baseline established, let's understand what our overall goal is:

What are we using to put this together?

  1. Off-the-shelf UPS from reputable brands like Tripp-lite or APC: Choosing a reliable UPS as the foundation ensures stable power supply and protects your connected devices effectively.
  2. Large batteries: Opt for deep-cycle batteries capable of providing extended power backup. Deep-cycle batteries can withstand multiple discharges and recharges without performance degradation, making them ideal for UPS applications.
  3. Heavy gauge wire (0-2 gauge) or bus bars: To ensure efficient and safe power transmission between the batteries and UPS, use heavy gauge wires or bus bars. These components handle high currents and minimize energy losses during power transfer.
  4. 150A fuses: Incorporate 150A fuses into the circuit to protect the UPS and connected devices from excessive currents and potential electrical hazards.
  5. Test equipment (DVM, Kill-a-watt): Utilize a Digital Voltmeter (DVM) to measure voltage levels accurately and ensure proper connections. A Kill-a-watt meter can be useful for monitoring power consumption and optimizing UPS efficiency.

Step 1: Identify your power capacity needs

Choosing the right UPS capacity for your needs is a crucial step in building a reliable power backup system. Here's a concise guide to help you calculate the appropriate UPS capacity:

  1. Assess your power requirements: Calculate the total VA load of your entire network system. Consider not only the current power needs but also anticipate future expansions and potential heavy-duty equipment additions.
  2. Multiply by 2 for headroom: Once you have the current VA load, multiply it by 2 to create a buffer for future growth. This way, you won't have to worry about upgrading the UPS system in the near future.
  3. Consider having at least an extra 25% headroom: If you have no immediate plans for expansion, still leave at least 25% headroom to accommodate unforeseen power spikes or increases in demand.
  4. Calculate with full load numbers not idle power numbers: When measuring power draw, ensure you use the full load numbers of the system, not just the current equipment in use. This accounts for worst-case scenarios where all devices are running simultaneously.
  5. Account for transience spikes: Adding the extra 25% to the total load to account for transience spikes or unexpected power surges that may occur. This ensures your UPS can handle sudden power demands without compromising performance.

Step 2: Calculate the Runtime battery requirements

To determine the right battery capacity for your UPS system, consider the desired runtime and battery voltage, using a simple water pipe analogy for clarity.

Think of electricity as water in a pipe, where Volts represent pipe size, and Amps signify water pressure. A smaller pipe (low Volts) needs higher pressure (high Amps) to move water, while a larger pipe (higher Volts) requires lower pressure (lower Amps) for the same flow rate.

Similarly, in your UPS system, battery voltage is like pipe size, and Amps represent electricity pressure. Batteries typically have lower voltage than 120v AC from the wall. Adjust Amps for the battery's voltage using the calculator.

For example, a 20VA load at 120v needs around 0.15 Amps. With a 12v battery, Amps increase to about 1.5 amps (considering a power factor of 0.6).

The goal is to find the right voltage-amps balance to choose the appropriate battery capacity (Ah) for your UPS. By using this ratio and the battery calculator, select a reliable and efficient power backup solution meeting your specific needs.

In essence, determine the Amps your system consumes at 120v and use it to find a suitable battery with ample voltage.

Battery voltages are in the multiples of 12v, such as 12v, 24v, 48v etc. This is mostly in part due to the way these batteries are made.

Notes about efficiency:

Increasing the voltage in the inverter leads to more losses, resulting in wasted energy. Inverters commonly experience around 20% losses. The inverter's main task is to convert the battery's direct current (DC) into alternating current (AC) at 120v, enabling our computers to use it.

Interestingly, computers themselves operate on DC at voltages similar to batteries. Therefore, we essentially convert low voltage DC into AC and then back into low voltage DC using the computer's power supply. This process causes efficiency losses at each step. To simplify this process, a DC buck converter can be used to make it a one-step process, but it involves different voltage levels, which is beyond our current topic.

While running the system with 48v batteries is possible, it is more suitable for much larger systems. For our context, we will work with a 24-volt system.

Duty Cycles:

In the world of Uninterruptible Power Supplies (UPS), the "duty cycle" becomes a crucial factor to consider. The duty cycle simply indicates how long a UPS can operate continuously without problems. UPS devices have different duty cycles, which help us distinguish between regular units and heavy-duty ones.

Regular UPS units, like the ones used in homes or small offices, are designed for intermittent power backup needs. They work well for short outages and small loads. In contrast, manufacturers design heavy-duty UPS systems for industrial or critical applications, allowing them to provide continuous power for extended periods without encountering any issues.

Understanding the duty cycle is vital for choosing the right UPS to meet specific needs. It ensures correct usage, prolongs the UPS's lifespan, and prevents potential problems that may arise from exceeding its capacity.

For this application, carefully look for UPSs categorized as "Extended Runtime," as they will be the most suitable choice.

Step 3: We need a really good UPS

A UPS and an Inverter are not the same thing.

A UPS is specifically designed to manage the low current DC power adapters needed for running multiple devices together. On the contrary, other devices meant for single use often fail when handling loads below half their capacity, as I've observed when powering numerous small computers simultaneously.

With most of the key points out of the way, let's talk about the more supporting components in general.

Battery Tech

Currently, the types of batteries commonly available are:

Lead acid: The cheapest option. Same as the lead acid car batteries. 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 batteries, they will literally fail after 3 or 4 runs to 0, which is unacceptable for a reliable UPS system.

Sealed lead acid (SLA): 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 because when you charge a lead acid battery, some hydrogen gas is released, which can be explosive. Sealed Lead Acid batteries don't release any hydrogen, making them safer for closed spaces.

Lithium-ion: Lithium-ion batteries are the cornerstone of 21st century innovations. They are deep cycle, high capacity, lightweight, 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, providing efficient and long-lasting power backup.

Lithium Iron Phosphate (LiFePO4): The cream of the crop of batteries is LiFePO4. They have all the benefits of Lithium-ion but offer even more cycles before degradation and are even lighter weight. These batteries provide exceptional performance, making them a top choice for high-end UPS systems. However, their premium features come at a significantly higher cost, making them less suitable for budget-conscious UPS solutions.

Wiring and additional protections:

This part is crucial for the UPS system and should not be overlooked. We need thick wiring between the batteries because they can handle a lot of current. Calling them the lifelines of your system would be a sheer understatement.

The batteries must work together as a single unit. A fuse will be placed in line between the main feed line to the UPS and the batteries. There won't be any fuses between the batteries, so the wire itself acts as a backup fuse, and it must not fail.

While bus bars can be used instead of wires, they are not necessary for computer applications. Simple 2-gauge or larger wires are sufficient to ensure they won't overheat or fail.

Using an inline 150A fuse between the battery and the UPS has always been considered an industry standard, so feel free to add that.

Make sure to adhere to the right circuit to prevent shorting, remember when dealing with batteries we must be safe. Explosions and shocks from these devices can be fatal. Kindly deal with caution.


In conclusion, when making a DIY UPS system with car batteries, remember to consider your power needs and choose the right UPS capacity. Understand the battery requirements, like voltage and amps, to select the suitable battery capacity.

Opt for reliable battery types such as sealed lead-acid or lithium-ion for better performance. Also, ensure proper sizing of the wiring and fuses between the batteries to create a safe and effective UPS setup.

By following these steps, you can build a cost-effective and dependable power backup solution tailored to your requirements.

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!

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