Accumulators and Batteries: Technologies and Capacity

An accumulator is a device that stores chemical energy and returns it as electricity.

Capacitors and inductors typically store small amounts of energy over short intervals. If we want to power a system for a longer time, we need electrochemical storage.

This is where the electrical accumulator (or battery) comes in.

Unlike a capacitor, which separates charge and stores energy in an electric field, a battery stores it through electrochemical reactions. In accumulators, these reactions can be reversed during recharging.

Cell, Primary Battery, Secondary Battery, and Accumulator

Although we commonly mix up the terms, it is useful to distinguish them:

• Cell: Basic electrochemical unit, either rechargeable or not.
• Primary battery: In common usage, a non-rechargeable cell, like an alkaline one.
• Accumulator or secondary cell: A cell whose reaction is reversible within limits and can be recharged.
• Battery: An assembly of one or more cells connected and managed as a unit. It can be primary or rechargeable, although in everyday speech we usually associate it with the latter.

Key Quantities

When we buy a battery, we look at two numbers. If we don’t understand them, we will waste our money.

Nominal Voltage ()

This is the average working voltage. But be careful, this is not fixed.

• In many lithium-ion cells with a nominal voltage of 3.6 or 3.7 V, the common upper limit is 4.2 V, but the values depend on the chemistry and the manufacturer.
• Designing a circuit assuming the battery always delivers the same voltage is a mistake.

Capacity

It is measured in ampere-hours (Ah) or, for small devices, milliampere-hours (mAh).

It’s a simple multiplication. A 10 Ah battery could theoretically deliver:

• 1 Ampere for 10 hours.
• 10 Amperes for 1 hour.
• 20 Amperes for 30 minutes.

However, the capacity in Ah alone doesn’t tell the whole story. To compare batteries of different voltages, it’s better to also look at the total stored energy in Wh:

A 10Ah battery at 12V is not the same as a 10Ah battery at 3.7V.

Discharge Rate (C)

This parameter is important in drones, electric vehicles, and model aircraft. It indicates how fast we can drain the tank without breaking it.

• 1C: Means you can discharge it in 1 hour.
• 10C: Means you can discharge it 10 times faster (in 6 minutes).

A 2000 mAh cell with a 30C rating would have a stated current of 60 A. Check if the manufacturer is talking about continuous or peak current, for how long, and under what thermal conditions.

Common Technologies

Many chemistries exist, such as NiMH, NiCd, or sodium-ion. Here we will focus on two very widespread families.

Lead-Acid

These are the batteries in cars and large solar installations.

• Chemistry: Lead plates submerged in sulfuric acid.
• Advantages: Low cost, high capacity to deliver current, and a well-established recycling chain.
• Disadvantages: They are very heavy and have a short lifespan if deeply discharged.
• Voltage per cell: 2V (6 are placed in series for 12V).

Lithium-ion

They are in your phone, laptop, and electric car.

• Chemistry: Movement of lithium ions between cathode and anode.
• Advantages: Very lightweight, high energy density, no memory effect.
• Disadvantages: Expensive and delicate.
• Voltage per cell: Nominal 3.6V - 3.7V.

Battery Configuration

Like resistors, we can connect batteries to sum their properties:

• In Series: We add Voltage (V). The capacity (Ah) stays the same.

• Example: Two 12 V, 100 Ah batteries in series form a 24 V, 100 Ah system.

• In Parallel: We add Capacity (Ah). The voltage (V) stays the same.

• Example: Two 12 V, 100 Ah batteries in parallel ideally form a 12 V, 200 Ah system.