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Car Battery UPS - The (new) Right Way

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:

  • Goal: Achieve extended power backup for 8 hours
  • Target Power heavy-duty network stack, several servers, and PoE devices
  • Purpose: Provide sufficient time for manual equipment shutdown or generator deployment during power outages, ideally something close to 8 hours.

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
  • Sealed lead acid
  • Lithium-ion
  • Lithium Iron Phosphate

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.

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