Jump starter amps explained

The big numbers on a jump starter box are written to win the shelf, not to help you choose. The number that is hard to inflate is the battery size. Here is why it matters, and where to find it.

Why the big numbers can mislead

Peak amps are the highest current a unit can put out, for a very short moment. There is no single standard for how they are measured, so one maker can measure them over a different time window than another. Two units with the same number on the box can behave very differently when you turn the key.

The “will start engines up to” claims have the same problem. They are set by each maker, and competition pushes them up. A unit rated for a big engine on the box may be a stretch in real life, especially on a cold morning or with a very flat battery.

Starting an engine is not a moment. The starter motor keeps drawing power for as long as it takes to turn the engine over. That takes energy, and the energy comes from the battery.

Battery size: the number to look for

The battery size tells you how much energy the unit holds. It is printed on the jump starter itself, on the label or the case, usually as watt hours (Wh) and often as milliamp hours (mAh) too. Of those, the Wh figure is the one you can rely on being consistent.

The units, in plain terms

• mAh means milliamp hours. There are 1,000 milliamps (mA) in 1 amp (A), so 1,000mAh is 1Ah, and 20,000mAh is 20Ah. Most jump starters state their battery size in mAh.
• V means volts. It is how hard the battery pushes its current. A single cell is 3.7V or 3.2V, a battery pack depends on how many cells are in series, and vehicles run on 6V, 12V or 24V systems.
• Wh means watt hours. It is the energy the battery holds: the voltage × the amp hours. Because the voltage is part of it, you can compare Wh between any two units.

Why the advertised battery size can mislead

The battery size on the box is correct, but it can be misleading. Almost every jump starter states it as something like 20,000mAh, which is 20Ah. The part that is left off is the voltage it is quoted at: 3.7V per cell for LiPo, or 3.2V per cell for LiFePO4 (cells are explained below).

20,000mAh is easily read as 20Ah at 12V, which overstates the energy. Here are four examples using four 5,000mAh cells, two LiPo and two LiFePO4:

Battery	Cells	Capacity	Volts	Energy
LiPo (3.7V per cell)	4 × 5,000mAh, in parallel	20,000mAh (20Ah)	3.7V	20 × 3.7 = 74Wh
LiPo (3.7V per cell)	4 × 5,000mAh, in series	5,000mAh (5Ah) Box usually says 20,000mAh	14.8V	5 × 14.8 = 74Wh
LiFePO4 (3.2V per cell)	4 × 5,000mAh, in parallel	20,000mAh (20Ah)	3.2V	20 × 3.2 = 64Wh
LiFePO4 (3.2V per cell)	4 × 5,000mAh, in series	5,000mAh (5Ah) Box usually says 20,000mAh	12.8V	5 × 12.8 = 64Wh

Most jump starters are built in series, like the series rows above, but the box usually shows the cells added up, 20,000mAh, with no voltage. The missing voltage is what makes the number misleading.

Joined in parallel or in series, the same cells hold the same energy. LiPo and LiFePO4 differ because the voltage per cell is different, so the same 5,000mAh cells give 74Wh as LiPo and 64Wh as LiFePO4. That is why the Wh is the number to compare.

The Wh printed on the unit doesn’t have this problem. Two quick checks you can do with it:

• Wh ÷ stated Ah = the voltage the mAh was quoted at. For example 74Wh ÷ 20Ah (20,000mAh) = 3.7V, so the 20,000mAh is a cell-voltage figure.
• Wh ÷ the pack voltage = the battery size at that voltage. For example 74Wh ÷ 14.8V = 5Ah. The same battery is 20,000mAh at 3.7V and 5,000mAh at 14.8V.

Want to know how the lithium battery types differ? Learn about lithium batteries.

How battery size is worked out

• A jump starter battery is built from individual cells. Lithium polymer (LiPo) cells are 3.7V each. Lithium iron phosphate (LiFePO4) cells are 3.2V each.
• Most of them join the cells in series. Some join groups of cells in parallel first, and then join those groups in series.
• The voltage depends on how many cells are in series, usually 3, 4 or 7. Four LiPo cells in series make 3.7V × 4 = 14.8V. Four LiFePO4 cells in series make 3.2V × 4 = 12.8V.

The battery pack’s voltage is not the same as your vehicle’s voltage. A 14.8V pack is the battery inside the unit, and the unit is made to jump a 12V vehicle.

The 3.7V and 3.2V cell voltages are nominal, which means an average while the battery is in use. A fully charged battery is higher. A 4-cell LiPo runs up to about 16.8V (4.2V per cell), and a 4-cell LiFePO4 runs up to about 14.4V (3.6V per cell).

Series and parallel in plain terms

Start with one 5,000mAh cell at 3.7V. Now add three more of the same cell:

• In parallel, the capacity adds up and the voltage stays the same. You have a 20,000mAh battery at 3.7V.
• In series, the voltage adds up and the capacity stays the same. You have a 5,000mAh battery at 14.8V.

Both are the same energy: 3.7V × 20Ah = 14.8V × 5Ah = 74Wh. The mAh number looks four times bigger in the first one, but it is the same battery power.

A bigger battery gives a unit more to work with on a large engine, in the cold, or for several starts in a row. It is not the whole picture, because the unit also has to be able to deliver that energy fast enough, so match it to what you are starting as well. But if a maker doesn’t print the battery size, ask why.

Always compare the Wh. Use it as your starting guide, then look at the cables and the build quality.

Peak amps, constant amps and CCA

Peak amps

Peak amps are the highest current the jump starter delivers at the very start. It is not regulated, so each maker decides how to measure and state it. In our experience some peak amp ratings are measured over milliseconds and others over 1 to 2 seconds. A very high peak figure may be for a much shorter moment than a lower figure from another maker. The peak is what gets a stationary engine turning.

Constant amps

Constant amps are the current the unit keeps delivering after the peak settles down. The peak gets the engine turning, and the constant amps keep it turning. Keeping it turning usually takes less power than getting it moving, so this is the part that carries the engine through until it fires. It is also not regulated, and not every maker states it, so it is worth looking for.

Cold cranking amps (CCA)

You may also see CCA, which is a battery rating, not a jump starter rating. It is the current a battery can deliver for 30 seconds at -18°C while keeping enough voltage to run a starter motor. The important part is the time window and the temperature, both stated in the rating.

A jump starter’s peak amps and a battery’s CCA are not the same measurement, so you can’t compare one directly with the other. For any rating, ask “for how long, and at what temperature?”

Cable size and build quality

A jump starter has to get its energy to the starter motor through its cables and clamps, and that is where many units fall short.

Starting an engine pulls a lot of current through the cables. Cables that are too thin resist it and heat up. That wastes power, can overheat the cables, and means less reaches the engine. A unit that can crank an engine without cooking its cables has thick cables, good clamps and solid connections.

Build quality matters for the same reason. The internal wiring, the connectors, the clamps and the protection electronics all affect how much of the battery’s energy actually reaches the engine, and how long the unit lasts.

What to look at:

• How thick the cables are. Thicker cables handle more current.
• The size and grip of the clamps, and how the cables connect to the unit.
• Whether the maker shows the unit cranking a real engine, not just quoting numbers.
• The warranty, which tells you how much the maker trusts the build.

Why lithium jump starters took off, and why we grade them differently

A big reason lithium jump starters have become so popular is that modern petrol and diesel engines generally start very easily. Most start quickly, so a compact unit that delivers a strong burst is often all it takes.

That is why a recreational or occasional-use jump starter can seem to do the job at first. The trouble comes with heavier workloads, or an engine that takes a little longer to start. A smaller unit with an inferior build tends to fail there: the short burst is used up, and a small battery, thin cables or a lighter build can’t keep cranking.

A product built for the use it is recommended for is much more likely to sustain heavier workloads. Our commercial and heavy-duty range is designed to crank for much longer, with a bigger battery, stronger build quality and heavier cable sizing. That is why we grade jump starters into groups, and why a unit made for a lighter group isn’t a substitute for a heavier one.

What to check before you buy

1. Battery size (Wh). Find it on the unit or in the specs, and compare in Wh.
2. Cable size and build quality. Thick cables and solid clamps and connections let the unit use its battery without overheating.
3. What it is made to start. Treat the maker’s engine size claim as a guide, not a guarantee.
4. Voltage. A 12V unit is for 12V systems. If you have 24V vehicles, you need a unit rated for 24V. Some units do both.
5. Petrol or diesel. A diesel engine usually needs more than a petrol engine of the same size, because it runs a higher compression ratio.
6. Your largest vehicle. If you start more than one vehicle, choose for the largest.

What decides how much power you need

The power needed to start an engine depends on the engine itself, and on the day. Bigger engines need more. Diesels usually need more than petrol, because a diesel engine has a higher compression ratio. Cold weather makes engines harder to turn over and batteries weaker. A very flat battery is harder to jump than one that is only a little low.

The groups run from small vehicles and equipment up to mining and heavy earthmoving. Each group is the biggest we rate a unit for, so a unit in a higher group also covers the groups below it. We show the battery size on every product page.

Common questions

Is a higher peak amp number always better?

No. A higher number only helps if you know how it is measured, and many are overrated. Look at the battery size and what the unit is made to start.

Where do I find the battery size?

On the label or case of the jump starter, normally shown in Wh and often in mAh as well. On our product pages it is listed with the specifications.

Can a smaller unit start a bigger vehicle?

Sometimes it will, for example on a warm engine with a battery that is only a little low. But it isn’t what the unit is rated for, so we don’t promise it, and it won’t work every time. Choose the group that matches your largest vehicle.

Does voltage matter?

Yes. Use a unit rated for your system’s voltage. If you have both 12V and 24V vehicles, look for a unit that covers both.