How to choose a battery (capacity, starting current)?

Choosing a battery is often reduced to one number — capacity. In reality you have to match several parameters at once, otherwise the battery will work badly, age quickly or won’t even fit in the car. Below we explain, one by one, what really matters when choosing a battery and how not to overpay or buy one that fails at the first frost.

The good news is that all the data you need is on the label of your current battery and in the car’s documentation — nothing has to be guessed. The difficulty is rather in not skipping any of the parameters, because each one can cancel out the others.

The key parameters

• Capacity (Ah) — the energy reserve, matched to the car’s equipment
• Starting current (A) — the ability to start, especially important in frost
• Dimensions and case type — must fit the mounting spot
• Polarity — the layout of positive and negative (left/right)
• Type: flooded, EFB or AGM — depending on the car

Capacity (Ah) — how much energy the battery stores

Capacity given in amp-hours (Ah) is simply the energy reserve the battery has. The more current-consuming devices a car has — and modern cars have plenty: infotainment, heated seats, many control units, LED lighting — the greater the capacity needed to keep everything running stably even with the engine off.

The key rule is that the new battery’s capacity should match the factory one or be close to it. Too small a battery will constantly be drained “to the bottom”, which drastically shortens its life. Too large a one, in turn, may be undercharged by the alternator, because the charging system was designed for a different value — and it often simply doesn’t fit the factory tray. Sticking to a value close to the original is the safest choice.

Starting current (CCA) — why it matters more in winter

The starting current, usually marked as CCA (Cold Cranking Amps), tells you how hard the battery can “kick” at the moment of starting the engine. It’s this parameter that decides whether the car will start in a biting frost, when thickening oil offers more resistance and the cold battery chemistry gives up current reluctantly.

When choosing a battery, never go below the factory CCA value. A higher starting current than the original won’t hurt — the car draws only what it needs — and it gives a reserve for winter and for the battery ageing. Too low a CCA, on the other hand, is a straight road to the situation where on a January morning, instead of starting the engine, you hear only a muttering “click”.

It’s also worth knowing that every battery’s starting current naturally drops with age — a battery that had a solid reserve of power when new gives noticeably less of it after a few years. That’s why a small CCA “surplus” at purchase isn’t extravagance but a sensible buffer for the winters ahead, taken ever less easily.

Dimensions and mounting — the battery has to fit

Even ideal electrical parameters won’t help if the battery physically doesn’t fit the compartment. The battery tray has set dimensions, and the battery itself must be firmly securable — a loose battery means vibration, damage and a risk of short circuit. Pay attention to the length, width and height of the case and to the type of clamping bar or bracket, because different standards aren’t interchangeable.

The position and shape of the terminals also matter, as does whether the battery has a gas vent routed to the correct side — especially important in AGM batteries mounted in the cabin or under a seat. These are details easy to overlook in a shop catalogue, yet they decide whether the new battery can be mounted correctly and safely at all.

Polarity — plus and minus on the right side

Polarity defines which side of the battery has the positive terminal and which the negative. This isn’t a cosmetic detail: the factory cables have a limited length, so reversed polarity can mean the cable simply won’t reach the right terminal. Connecting “by force”, stretching the cables, risks damaging them and a short circuit. That’s why the terminal layout must match what your car requires. In practice there are two layouts — colloquially “left plus” and “right plus” — and it’s these, alongside the dimensions, that most often decide that a seemingly identical battery still doesn’t fit a particular model.

Battery type — flooded, EFB or AGM

This is the parameter easiest to forget, and it can be critical. Classic flooded lead-acid batteries work well in older cars without advanced electronics. Modern cars with a start-stop system, however, require an EFB or AGM battery, adapted to hundreds of charge and discharge cycles a day.

The rule here is clear: you can replace an EFB with a stronger AGM, but never the other way round — putting a plain flooded battery or even an EFB in place of a factory AGM in a car with an elaborate start-stop and energy recovery ends with the battery quickly “running into the ground”. The car was designed for a specific technology and it has to be respected.

In short: AGM is the most durable, sealed design, handling the greatest loads and deep discharges, which is why it goes into cars with elaborate electronics and start-stop with energy recovery. EFB is an intermediate solution — stronger than a plain battery, intended for simpler start-stop systems. The classic flooded battery remains a good, cheap choice where the car provides for it from the factory, that is in older and simpler models without advanced energy management.

How to tell the old battery needs replacing

Before you even get down to choosing, it’s worth making sure the battery really is due for replacement and not just discharged. The classic signs of a worn battery are a lazy, “grinding” start, dimming lights during starting, an ever more frequent need to hook up a charger, and trouble starting after a colder night.

A typical flooded lead-acid battery lasts usually 4 to 6 years. If yours already has that behind it and is starting to play up, it’s a natural moment for replacement — best planned, not an emergency in the middle of winter, in a car park, when the car won’t start. Replacing it ahead of time, calmly and with a properly chosen battery, is always cheaper and less stressful than rescuing yourself with “jump leads” in the morning.

Does the battery brand matter

The battery market is heavily consolidated — behind many brands stand a few of the same large manufacturers, so the differences are often smaller than the prices suggest. More important than the name itself is that the battery has the right parameters (Ah, CCA, type) and is fresh — a battery is a product that ages already on the shelf.

That’s why it’s worth checking the production date stamped on the case and avoiding units that have sat in a warehouse for a year or more. A sensibly chosen mid-range battery from a reputable manufacturer will usually serve longer than the cheapest “no-name” battery bought purely on price.

Remember to register the new battery

In many newer cars choosing the right battery isn’t the end — the new battery still has to be registered in the on-board computer so that the energy management system knows it’s dealing with a fresh cell. Without this, charging stays matched to the parameters of the old, worn battery, which shortens the new one’s life.

Frequently asked questions about choosing a battery

Can I fit a battery with a larger capacity than the factory one?

Within a small range and if it physically fits — yes, nothing bad will happen. The problem appears with a large difference: the alternator may not charge too big a battery to full, so it works undercharged and ages faster. It’s safer to stick to a capacity close to the original.

Can a higher starting current (CCA) do harm?

No. The car draws from the battery only as much current as it needs to start, so a higher CCA is simply a reserve of power and more comfort in winter. It’s the other way round that’s risky — too low a starting current in frost means starting problems.

Where do I get the factory battery’s parameters?

The simplest way is to read them off the label on the old battery — you’ll find the capacity in Ah and the starting current in A there. In addition it’s worth checking the owner’s manual or choosing the battery by VIN, which eliminates mistakes over type and polarity.

I have a start-stop car. Can I save money and buy a plain battery?

No. It’s a false economy that quickly backfires. The start-stop system stops and starts the engine even dozens of times during a single city trip, and each such cycle heavily loads the battery. A plain flooded battery isn’t adapted to this and will wear out in a few to a dozen months, and on top of that the car may switch off the start-stop function when it sees the wrong cell type. You have to fit an EFB or AGM in line with the manufacturer’s requirement.

Is a new battery fully charged straight away?

Usually yes, good batteries are sold charged and ready to fit. Bear in mind, though, that after fitting the battery “tops up” to full during normal driving. If the car mainly covers very short routes, the alternator may not manage to charge it — then a charger comes in handy now and then.

Too weak a battery won’t start the car in frost, and too big won’t fit — the choice is matching several parameters at once.

Don’t guess — we’ll choose for you

You don’t have to study catalogues or compare dozens of codes. Give us the make, model and year of the car, and we’ll select the right battery with the correct capacity, starting current, dimensions, polarity and type, then replace it with call-out — registering it in the car where required.