Batteries and power systems — study guide
The concept's fragments, read in order.
Powering a build without starving it
A battery is not just a voltage printed on a label. It is a promise to deliver energy at a rate, for a while, without collapsing under the demand. Treat it as a plain number and a build either browns out mid-move or cooks a part; treat it as a source with limits and the whole electrical side falls into place.
Four questions decide whether a power source is right for a job. Does it supply enough push, the voltage the electronics expect? Does it hold enough energy to run for as long as you need? Can it deliver current fast enough at the peak moments, not just on average? And does it stay steady enough, or does its voltage sag when the demand spikes? Push, stored energy, delivery rate, and steadiness — a source can pass one and fail another, which is why a battery is chosen against all four.
The unit that answers the first question is the cell: a single electrochemical source with one nominal voltage set by its chemistry. A battery pack is cells wired together to reach a voltage and a runtime no single cell offers. Almost every real build runs on a pack, and the raw voltage it produces is rarely the clean, fixed rail the electronics want — so a regulator usually sits between the pack and the delicate parts, handing them a steady supply while the pack's own voltage drifts and droops behind it.
Chemistry sets a cell's voltage
A cell's voltage is not something you dial in; it falls out of the chemistry inside. The two electrode materials and the reaction between them fix how much potential difference one cell develops, and every cell of that chemistry lands near the same value. That value is the cell's nominal voltage, and it is the first thing you read off a battery.
The common chemistries cluster at a few nominal figures. A standard alkaline cell sits at about 1.5 V. A NiMH rechargeable cell, the kind that replaces alkalines in the same slots, is a little lower at about 1.2 V. A single lithium-ion or lithium-polymer cell — the flat pouches and cylinders behind most modern devices — sits much higher, at about 3.7 V. These are per-cell figures, and they are why a lithium build reaches a target voltage with far fewer cells than an alkaline one.
The word nominal is doing quiet work: it names a typical mid-discharge value, not a constant. A real cell reads higher than nominal when freshly charged and sags below it as it empties — a lithium cell, for instance, is typically near 4.2 V full and drifts down toward 3.0 V before you stop drawing from it. So the nominal number is the honest average to design around, and the actual terminal voltage is a moving thing you should expect to change as the pack runs down.
Capacity, energy, and how long it lasts
Voltage tells you how hard a battery pushes; capacity tells you how much it holds. Capacity is quoted in milliamp-hours (mAh), or amp-hours (Ah) for larger packs, where one amp-hour is a thousand milliamp-hours. Crucially, this is a measure of charge — an amount, not a rate. A 2000 mAh cell can pass two thousand milliamps for one hour, or five hundred milliamps for four hours; the stored charge is the same, and the current you draw sets how fast you spend it. like a fuel tank feeding an engine: the tank's size sets how much is stored, the throttle sets how fast it is drawn, and dividing the one by the other tells you how long before it runs dry
Because capacity ignores voltage, it does not by itself tell you the stored energy. For that you multiply: energy in watt-hours equals the nominal voltage times the capacity in amp-hours, Wh = V x Ah. Two packs can both be rated at the same amp-hours and still hold very different energy if one runs at a higher voltage — the higher-voltage pack carries proportionally more. Charge is what the mAh number reports; energy is what actually powers the work, and the voltage is the bridge between them.
Runtime then follows from how fast the load draws. To a first approximation, the time a pack lasts is its capacity divided by the current the load pulls: a 2000 mAh pack feeding a steady 1000 mA load runs for roughly two hours. It is only an approximation — a heavy draw empties a pack a little faster than the tidy division suggests, and the usable capacity shrinks under hard use — but capacity-over-current is the estimate you reach for first when sizing a battery to a job.
Building a pack: series and parallel
One cell rarely gives you both the voltage and the runtime a build needs, so you wire several into a pack. There are two ways to connect them, and each buys a different thing. Wired in series — end to end, the positive of one to the negative of the next — the cells' voltages add while the capacity stays that of a single cell. Three 3.7 V lithium cells in series make a pack of about 11.1 V that still holds one cell's worth of amp-hours. like batteries in a flashlight: stacked end to end they add their pushes into a bigger total voltage, while set side by side across the same two ends they share the load and last longer without pushing any harder
Wired in parallel — all the positives joined, all the negatives joined — the opposite happens: the voltage stays that of one cell, and the capacities add. Three 2000 mAh cells in parallel behave like one 3.7 V cell of 6000 mAh, lasting three times as long at the same push. Series stacks voltage; parallel stacks staying power; and each leaves the other quantity untouched.
Real packs combine both. A configuration described as three-series, two-parallel wires cells into three series groups of two, reaching the series voltage and twice a single group's capacity at once. That is how you hit an arbitrary target: pick the series count for the voltage the electronics want, then the parallel count for the runtime the job demands. Sizing a pack is choosing those two numbers.
C-rating: how fast a pack can pour
Capacity says how much charge a pack holds; the C-rating says how fast it can safely give it back. The C-rate expresses current as a multiple of capacity, so it scales with the pack rather than being a fixed number of amps. A rate of 1C means a current that would empty the full capacity in one hour; 2C is twice that current, emptying it in half an hour; 0.5C is half, taking two. To turn a C-rate into actual amps you multiply it by the capacity — a 2000 mAh pack at 1C delivers 2000 mA, at 10C it delivers 20000 mA.
Every pack carries a maximum C-rate it can sustain without overheating or damage, and that ceiling is what decides whether it can feed a hungry load. Ask a pack for more current than its rating allows and it does not simply comply: it overheats, its voltage collapses, and its life shortens — with some chemistries the failure is dangerous. A physically small cell has a low capacity and, at any given C-rate, a correspondingly small current, so it cannot pour out huge amps no matter how the rest of the circuit is arranged.
This is the question that separates a battery that runs for a long time from one that can drive a demanding load. A pack sized only for runtime — high capacity, low C-rate — can still fall short the instant something draws a hard surge, such as motors under load. Matching the pack's C-rating to the load's peak current demand is a distinct decision from matching its capacity to the runtime, and both have to be right.
Sag under load, and the steady rail
A battery's voltage is not fixed while you use it. Measure the terminal voltage with nothing connected and it reads near its nominal value; start pulling current and it droops below that, further the harder you pull. This droop is voltage sag, and it happens because a real cell has a small internal resistance of its own: the current flowing through that internal resistance drops some voltage inside the battery, so less reaches the terminals. Light loads sag a pack a little; heavy loads sag it a lot. like the water pressure at a tap dropping when a second faucet is opened hard: the greater the sudden draw, the more the pressure falls below what the closed-tap gauge promised
Sag is why a pack can look fine at rest and still misbehave under load. A source sitting at its nominal voltage on the bench can dip below what the electronics need the moment a big current is drawn, and the dip is deepest exactly when the demand spikes. A pack with a higher C-rating and lower internal resistance sags less for the same current, which is another reason the delivery-rate question matters as much as the how-much-charge question.
Because a drooping, drifting supply is a poor thing to run sensitive electronics on, a build usually inserts a voltage regulator between the pack and those parts. The regulator takes the pack's wandering voltage and holds a steady output rail — a fixed supply the electronics can count on even as the battery sags and slowly empties. The details of how a regulator does that belong to their own topic; here it is enough to know the steady rail comes from putting one between the battery and the load.
The RC car's power system
For a small self-driving car, the battery is the single component the whole build leans on, and every part of this concept shows up in choosing it. The pack has to supply the voltage the motors and electronics expect, which fixes the chemistry and the series count; it has to hold enough energy to run for a useful session, which fixes the capacity and the parallel count. Get those two numbers wrong and the car either will not move or dies within minutes.
The delivery questions bite hardest here, because the motors are the heavy load — they draw a hard surge of current the instant the car accelerates or fights a slope. A pack sized only for a long runtime but with a modest C-rating sags under that surge, and when the voltage dips the electronics running the control code can brown out and reset at the worst possible moment. So the pack is chosen for its peak current as much as its capacity, and a regulator holds a steady rail for the sensors and the onboard computer while the pack's own voltage swings with the driving.
None of this touches how the motors are actually commanded — that is a separate layer of the build. What batteries and power systems give you is the foundation underneath it: a source sized to push hard enough, last long enough, deliver fast enough, and stay steady enough that everything above it can do its job. A robot is only ever as reliable as the power feeding it.