How to run hundreds of phones while being struck by suicide drones

Problem

In Ukraine suicide drones strike critical infrastructure regularly.
Some outages are caused directly by the attacks - destroyed power plants, substations, transformers, power lines, some are scheduled in advance - power grid wasn’t designed to work while some of its power plants and transformers are destroyed and are under repair.
Cold season doesn’t help with outages either, load is increased greatly because people stay home longer, use electric heating which is just pure energy → heat transformer and it’s very consuming.
There even was an official program to replace your old lightbulb for a new LED one for free which is beneficial for both parties. Neat.

Solution

General necessities such as water pumps in apartment buildings, elevators and corridor lights are sometimes backed up if people voted to spend money on a local generator in the building.
Nowadays, on the fourth year of shutdowns, almost every internet provider has implemented some sort of a solution, either batteries for RJ45/copper connections (~12 hours of uptime) or FTTH (Fiber to the Home) which can work even longer, 1-2 days, because with this technology one central node can service thousands of apartments over passive fiber without the need for intermediate switches.
Small businesses often use generators to stay open during an outage, so it gets very noisy during shutdowns.
Fun fact: Large grocery stores and apartment buildings use big generators, they are the size of a car and way quieter than small generators.
Just like a motorcycle engine is way louder than a car’s.
(below is a photo I made)

People backup necessities inside their apartments with power stations (Bluetti and Ecoflow being most popular).
Prices on power stations go up 20-60% during shutdowns which is just market rules.
Routers could be powered with just a power bank + USB C → DC12 cable which became very popular in the recent years.

Our case

We need to power up x1 hp proliant dl380p g8; x1 hp proliant dl360p g8; x20 google pixel 6.
We measured that this whole setup draws around 500 watts.
Given that averaged scheduled shutdown lasts up to 5 hours, we calculated that we’d need around 2.5kWh battery capacity.

Next came the question of which battery/solution to buy.
From the Ecoflow brand there is a DELTA 2 Max costing ~$1500 and rated at 2kWh capacity, it’s not enough for us, and the next one is DELTA Pro for 3.6kWh priced at astonishing ~$2300.
Everything on the ready-to-go market feels extremely overpriced.

So we decided to build our own battery solution. Spoiler - it cost us $600 and it has better specs than $2300 Ecoflow.
At this point building your own battery systems became almost a national folk craft for tech-savvy Ukrainians who want to save or get more worth for their money.

For building your own gear you have to buy individual components like battery cells, protection board for them, inverter to 220V AC, charger, etc and assemble/connect it yourself.
It’s challenging because if you do something wrong you can destroy the equipment, burn your residence or harm yourself or your neighbours.

First, there is the choice of battery cell chemistry. Most common are Lithium-based family chemistries: Li-Ion, Li-NCA, Li-NMC, LiFePO4. I chose LiFePO4, as most Ukrainians do, because it’s the most optimal choice for price, performance and safety.
Safety is an important factor because if something goes wrong, like when the battery is overcharged, other battery types can explode or burn like thermite while LiFePO4 just swells and/or vents excessive gasses through special safety valves.
These gasses are not healthy but at least you won’t burn your apartment down.
The voltage of each cell is 3.45V so you need to connect several in series (in my case 4, for 12-13V).
The first thing I bought was 4 Envision LiFePO4 315Ah cells, which gives me a raw total capacity of 4.3kWh.
I found a local seller who bought several pallets of them (several hundred cells) directly from a Chinese factory.
I was also lucky that he had one spare BMS board (battery management system), which he bought for his own use and didn’t plan to sell, but sold me nevertheless saving me some time purchasing it separately. And I’ve paid for it with USDT which felt like the future 😎

The role of a BMS is to be in between the battery cells and the load, and to control that battery is not being overcharged/overdischarged, protect it from overcurrent/short-circuit and also balance voltage between individual cells during the charging process.
Different cells, even from one batch, are not perfectly identical, they can slightly differ in internal resistance which will lead to uneven charge and the loss of some capacity, so BMS will transfer some of the power from one cell to another to equalize the charge.

Next thing you need is some way to convert 12V DC battery voltage to 230V AC mains voltage to power common devices like servers.
You can independently buy an inverter to produce AC mains voltage and a high-current battery charger to charge the battery when power is on, but this is rather a jerry-rigged setup.
Fun fact: most of the battery chargers sold on Aliexpress and Ukrainian markets are made out of power supplies from old servers.

So those who want it to be more reliable buy so-called solar-inverters.
It’s a device designed to work with solar panels, converting their power into AC, mixing it with mains power or funneling it into battery if consumption is not enough.
Crucially, it can work without solar panels connected and contains a built-in battery charger using an AC power.
So, many of such hand-made builds utilize them as simple UPSes.
You can connect the whole apartment to such a battery system using cable with plugs on both ends, though it’s important to remember to switch off the main breaker beforehand, otherwise the inverter will fry when electricity is turned back on.

The proper way to do it is a device called “Automatic transfer switch” which, as the name suggests, will automatically switch inputs depending on which one has power.
Automatic transfer switch gear for one of our other farms:

At this point there’re only a few items left to buy, like power cables for the battery and various consumables, and we can begin the assembly of the whole setup.

Assembly of the battery is pretty straightforward, you stack battery cells side by side, alternating polarity because the negative terminal of one cell must be connected to the positive of the next one, placing fiberglass sheets in between as an insulator. Then you thoroughly fix them together with the duct tape.

After that BMS board is strategically duct-taped near the negative terminal of the last cell and connected to it, along with all balancing wires (it’s that braid of red wires), they are connected to the positive and the negative terminals of the battery and to the positive terminal of each cell individually.
As it happens I was assembling it while the electricity was out:)

Now the battery is assembled and the BMS board needs to be configured. There’s an Android app for that, it connects to the BMS via Bluetooth and allows you to configure many in-depth parameters of the battery.
It has a good default preset for LiFePO4 batteries, I only needed to adjust several parameters.

After that I’ve slapped it together on the table to test that everything works before final installation, also to stress-test it under max load to check if connections don’t heat up.

Discovered that BMS’s short circuit protection was falsely triggered under medium-high load, adjusted it in the settings. And that I forgot to tighten one of the connections, it was heating up.
Finally I brought everything to the site and installed it in its place.

Because our main activity is running a phone farm, we have some spare phones, so one of them was sacrificed as a Bluetooth terminal to show battery’s stats:)

And just like that, the backup battery had its trial by fire just four hours in.

Our startup

We now continue to work as before, getting social media engagement for us and our clients, applying to YC 2026 and other VCs.

We have a total of 60 Google Pixels across 3 locations in Kyiv.
It’s a very weird feeling that your business is under constant threat of physical destruction and that you have to fight with war’s consequences to serve your clients.

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Authors: Nicholas and my cofounder Ihor