How Batteries Store Energy (and Why It Is All DC)

Part 22 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Pop open a fully charged phone battery and you will not find electricity inside it. There is no reservoir of electrons waiting to spill out. What you have instead is chemistry held under tension: two materials that badly want to react with each other, kept apart, so the only way they can do it is by pushing electrons through your circuit first. That is how batteries store energy. They do not store electricity at all. They store the chemical potential to make it, on demand, the instant you close the switch.

Understanding that one idea explains almost everything else about a battery, from why every cell on Earth outputs direct current (DC) to the two numbers printed on every grid-scale storage datasheet in Malaysia.

How batteries store energy: chemistry under tension

A battery stores energy in chemical bonds, then converts it to electricity through a controlled reaction when you connect a load. The unit that does this is the electrochemical cell, and it is simpler than it sounds.

Take two materials with different appetites for electrons. One gives them up readily; the other grabs them eagerly. Put them in direct contact and they react at once, releasing their energy as useless heat, the way a fire does. A battery stops that. It separates the two materials and connects them only by two paths: an internal one that lets charged atoms (ions) drift across, and an external one, your wire, that lets electrons flow.

Now the reaction can only proceed if electrons travel the long way round, out through your circuit, doing work on the way, before rejoining their atoms on the far side. Ions shuffle across inside to keep the books balanced; electrons do the heavy lifting outside. The reaction still runs downhill toward the same low-energy state a fire would reach, but you have forced it to pay a toll, and that toll is the electrical work that lights your torch or spins your motor. Charging simply pushes the reaction back uphill, restacking the chemistry for next time.

Why is every battery DC?

Every battery is DC because the chemistry only runs one way. Electrons leave the negative terminal, cross your circuit, and enter the positive terminal, steadily, in a single direction, for as long as the reaction has fuel. There is no mechanism in a cell to make that flow cycle fifty times a second. Direct current is not a design choice engineers bolted on; it falls straight out of the fact that a chemical reaction has a downhill direction.

That is a problem, because the TNB grid and nearly everything it powers runs on alternating current. (If you want the full story of why the grid alternates while your phone does not, we cover it in AC vs DC: why the grid alternates and your phone does not.) So every battery that has to join the AC world needs a translator. Discharging, an inverter chops the battery's steady DC into a 50 Hz AC wave the grid accepts. Charging, a rectifier does the reverse, turning AC into the DC the chemistry needs to run backwards. A grid-scale battery energy storage system (BESS) is, electrically, a big DC tank with a power-electronics gateway standing between it and the network. Those conversions are not free, which matters for the numbers below.

What do the two ratings on a battery datasheet mean?

Every storage system carries two independent ratings: energy in kilowatt-hours (kWh) and power in kilowatts (kW). They measure different things, and confusing them is the single most common mistake operators make when sizing storage.

Energy is how much the battery holds. Power is how fast it can move that energy in or out. The cleanest way to picture it is a water tank: the kWh rating is the size of the tank, and the kW rating is the width of the pipe. A big tank with a thin pipe holds a lot but trickles it out slowly. A small tank with a fat pipe empties in a rush. The two are set by different parts of the design and do not track each other, which is exactly the distinction we draw in power vs energy: the difference between kW and kWh.

The ratio of the two gives you the C-rate, the plain-language shorthand for how hard a battery works. A 1C battery charges or discharges its full capacity in one hour. 0.5C takes two hours; 2C empties in thirty minutes. So a system rated 400 kWh of energy and 100 kW of power runs for four hours flat out, which is a 0.25C battery. That is not a coincidence: Malaysia's MyBeST grid projects are each 100 MW / 400 MWh, four-hour batteries built to cover the evening demand stretch.

What is round-trip efficiency, and why does it cost you?

Round-trip efficiency is the fraction of energy you get back out of a battery compared with what you put in, and the gap is energy you paid for and never see again. Store 100 kWh and you might recover 90. The rest is lost as heat in the cells and, more so, in the inverter and rectifier doing the DC-to-AC translation on both trips.

Modern lithium-ion grid systems land at roughly 85 to 95% round-trip efficiency; most AC-coupled installations (the battery and grid each sitting behind their own inverter, so you pay the conversion loss twice) sit between 88 and 94%, and the US National Renewable Energy Laboratory uses 85% as its utility-scale planning benchmark (NREL / sustainability-directory summary). The practical consequence: you buy the kWh the battery loses. If you charge at night to discharge by day, the arithmetic only works when the price gap, or the demand charge you avoid, more than covers that lost tenth.

How long does a battery last?

A battery wears out by cycling, not just by ageing, and how hard you run it decides how fast. Every full charge and discharge is a cycle, and each one degrades the chemistry a little through heat, mechanical stress on the electrodes, and the slow growth of an insulating film inside the cell. Capacity fades until the battery is retired, usually at 70 to 80% of its original figure.

The numbers are workable. A lithium iron phosphate (LFP) cell, the chemistry most grid systems now use, benchmarks around 6,000 cycles at 80% depth of discharge (draining it to 20% full each time), and up to 8,000 cycles down to 70% state of health (retired once it holds only 70% of its original capacity) if run gently (NextG Power). But speed punishes you: push the same cell at 2C and it can reach 80% health in only 2,000 to 3,000 cycles, versus 5,000 to 6,000 at a relaxed 0.5C. Sizing a battery bigger than you strictly need, so it never has to sprint, is often cheaper over its life than buying a smaller one and hammering it.

Why is storage suddenly everywhere?

Storage is spreading because it got cheap, fast. The volume-weighted average lithium-ion battery pack price fell 20% in 2024 to a record $115 per kWh, the steepest annual drop since 2017, driven by cell overcapacity, cheaper metals, and the shift to LFP (BloombergNEF, December 2024). A technology that was a science project a decade ago now pencils out on a spreadsheet.

Malaysia is moving with it. Under the National Energy Transition Roadmap, which targets 70% renewable capacity by 2050, the Energy Commission launched the MyBeST programme in November 2024: four grid-connected projects of 100 MW / 400 MWh each, 1,600 MWh in total, targeted for 2027, with four of twenty-eight applicants shortlisted at the end of 2025 (Energy-Storage.news). TNB's Santong BESS, also 100 MW / 400 MWh, launched in May 2026 as Malaysia's first battery connected to the national grid (The Star).

Where does a battery pay off in a Malaysian building?

The clearest case today is peak shaving against your maximum demand charge. Under TNB's RP4 tariff, medium-voltage commercial and industrial sites pay RM89.27 to RM97.06 per kW every month, set by their single worst 30-minute demand spike. A battery that discharges through that spike lets the meter see a lower peak, and you are billed on the shaved figure for the whole month. The battery's kW rating has to be big enough to cover the spike, and its kWh has to hold out for the length of it, which is exactly why those two ratings matter. We work the full economics, including how to size it against your load profile, in battery energy storage and peak shaving.

None of it changes the fundamental, though. A battery is still just two materials that want to swap electrons, kept apart, made to do their trading through your wires. Everything else, the inverter, the C-rate, the round-trip loss, the demand-charge maths, is bookkeeping on top of that one quiet reaction.

This is Part 22 of 23 in Cobler's Electricity Fundamentals series. Previous: Rectifiers and Inverters: Electricity's Translators. Next: Power Factor Correction: What Capacitor Banks Do.

Sizing storage or peak-shaving against your TNB maximum demand? CobiNeural's Max Demand KPI and Plan & Verify module build the before-and-after load data that tells you how big a battery you actually need. Book a demo and we will walk your load profile with you.