A flat battery and jump-starting

The car clicks, or does nothing, and the dashboard lights flicker. A neighbour has jump leads and is happy to help. This is the one job in this course where you connect two cars' electrics together, and Nissan's handbook for the LEAF opens its jump-start pages with the reason to slow down: "If done incorrectly, jump starting can lead to a 12-volt battery explosion, resulting in severe injury or death."¹ Done the way the handbook says, in a safe place, with a battery that is fit to be jumped, it gets you going in minutes. This lesson is about telling which of those you've got, and about the order the leads go on and come off.

The motorway clause matters here, because a jump start is never done there, as the next section explains. Nothing in this lesson asks you to connect leads for practice: the exercise is done on paper, from your handbook. It uses UK words with the US word once: bonnet (hood), jump leads (jumper cables), and earth for what American handbooks call ground.

Is it the battery?

Lesson 5, "The 12 V battery: what it does, and why short trips flatten it", gave the signs: a slow crank, dim lights, and this line from the AA, a motoring organisation that sells breakdown cover and repairs: "If the dash lights are flickering or don't come on, it's likely the battery is discharged."² It also gave the AA's three free checks before blaming the battery (fuel, a dead key fob, wheels jammed against the kerb). A jump start is the answer only to a flat 12 V battery. It won't fix an empty tank, and it cannot help a battery that's damaged.

A jump start, or a call?

A jump start is Yours, on conditions, and the conditions come in the next sections. Before those, two sources remind you it's a choice. The AA, which sells breakdown cover and repairs, is candid about what it would rather you did: "For your safety, and our peace of mind, we'd always prefer you to give us a call and leave the jump starting to our trained mechanics."³ That is a safety point and a sales point in one sentence, and the same page goes on to give its own steps. Toyota's handbook for the 2023 Corolla sold in the US offers the other route plainly: "You can also call your Toyota dealer or a qualified repair shop."⁴

What decides it, on this course's reading, is three things: where the car is, what state the battery is in, and whether you have the handbook's method. Where comes first.

Where a jump start is never done

On a motorway in Great Britain, the Highway Code's Rule 277 lists what to do after you have stopped, and it includes "DO NOT attempt repairs on your vehicle".⁷ That's advice (the rule says "you should"), not a MUST rule. No source read mentions jump starts by name there, so this is this course's reading: a jump start is a repair in the sense that matters, because it needs two cars parked close with their bonnets up and people standing between them, next to traffic moving at motorway speed. So on a motorway, a hard shoulder (in the US, the shoulder) or an emergency area, nobody jumps a car. The callout's steps apply instead, and lesson 17, "Motorways and fast roads: go left, the live lane, and the smart motorway question", teaches them in full.

On a US freeway, this course found no official source on jump-starting at the roadside. Its reading is the same: nobody jumps a car on a freeway shoulder, and the callout's US steps are the ones to follow there.

Anywhere else, this course's reading is that a jump start follows the same placement rules as a wheel change in lesson 19, "Changing a wheel safely", and the stopping place in lesson 16, "When the car stops: where, who stands where, and the call": off the road, hazard lights on if the car is causing an obstruction, and nobody standing between the cars and traffic. If the car can't be got off the road, that's Call someone. For Great Britain, Rule 276 (advice again) says "do not stand (or let anybody else stand) between your vehicle and oncoming traffic".⁷ A drive or a car park is the right kind of place. The verge of a busy road at night is not, and lesson 16 deals with that decision.

Why the last clamp goes where it does

The danger in a jump start is hydrogen. Nissan: "Explosive hydrogen gas is always present in the vicinity of the 12-volt battery."¹ Lesson 5 showed from HSE, Great Britain's workplace safety regulator, why: a battery gives off hydrogen and oxygen while it charges, and a spark can set them off. HSE's leaflet, written for workplaces in 2011, then says exactly when the risk peaks: "Many explosions happen when batteries are being connected or disconnected. The sparks produced when this is done incorrectly may cause the battery to explode, especially if it has just been charged."⁶

So the connection that matters most is the last one, made away from the gas. HSE, for the case where no maker's instructions are available, has the black lead go from the good battery "to a suitable unpainted metal part of the chassis or engine of the dead vehicle. The point of attachment should be at least 15 cm from the battery and away from fuel and brake lines."⁶ (15 cm is about 6 inches, this course's conversion.) Ford, Nissan and the AA (which sells breakdown cover and repairs) put the last clamp in the same kind of place, as the table below shows. None of the sources read says in so many words why it is the last clamp in particular that goes there. This course's reading of HSE's paragraphs is that the connection which ends the sequence is the one made at a distance, so that any spark it makes is made 15 cm or more from the gas, and not over the battery that has just been giving it off.

The same logic explains the other warnings. Toyota: "Do not allow the + and - clamps of the jumper cables to come into contact with each other", and don't let the other end of the positive lead, once one end is on, touch "any other parts or metal surfaces in the area, such as brackets or unpainted metal".⁴ The AA, which sells breakdown cover and repairs: rings, necklaces and watch straps off, and no metal objects touching the batteries, because they "could cause a spark and possibly make the battery explode".³ Nissan adds "remove rings, metal bands, or any other jewelry", wear eye protection, and "Do not lean over the 12-volt battery when jump starting."¹

The order, as the handbooks give it

The table gives the order from each source this course read, for a car whose negative terminal is earthed, which HSE says is usual.⁶ Read the rows for what they share, and then for where they differ.

Sources: ¹³⁴⁵⁶.

Two things stand out. Every source read puts the last clamp on metal away from the flat battery (Toyota's in its drawing, which shows a metal point on the engine), and every one runs positive to positive first. But they do not agree on which end of the red lead goes on first. Ford, Toyota and Nissan start at the flat battery. HSE describes the lead without saying which end goes first, and the AA's page gives both: its step 7 finishes "with the red lead from the other car", and its list for taking the leads off finishes at "the positive (+) terminal on the flat battery".³ Run backwards, the first puts the red lead on the working car first and the second puts it on the flat battery first. So there is no single "correct order" that you can carry from car to car. HSE's jump-starting section opens with the rule: "Whenever possible, follow the manufacturer's instructions for jump-starting a vehicle."⁶ The AA says the same of its own steps.³

The drawing below is Ford's four steps, with HSE's distance added.⁵⁶ It's a schematic, drawn by this course; it is not to scale, and your handbook's picture is the one to follow.

Some cars hide the battery, and the handbook gives jump points instead. Ford's hybrid Maverick "has two pins accessible from under the hood", and it's jumped the same way through them.⁵ Nissan's LEAF keeps its battery in reach but puts the last clamp on a body ground point shown in its drawing.¹ That is one more reason your own handbook comes before any table in a lesson: the places differ from car to car.

Portree, a Saturday morning

In Portree, a petrol hatchback on its owner's drive will not start: a click, then nothing, and the dash lights flicker. It's been standing for a fortnight. A neighbour offers his petrol estate and a set of leads. This course read no UK petrol car's handbook, so for the example, say her handbook gives the same four steps as Toyota's for the 2023 Corolla.

Is it the battery? Standing a fortnight and flickering lights both fit the AA's account of a flat battery.² She checks the fuel gauge first anyway.

Can this battery be jumped? She looks at it from where she stands, with the bonnet up, touching nothing: no crack, no swelling, no powder on the terminals, and on a mild morning a frozen battery isn't in question. The AA: "Never try to jump start a battery that looks damaged or is leaking".³ Both cars are 12 V; Ford's handbook puts it as "Connect batteries with only the same nominal voltage", and HSE says that if you are not sure, "do not proceed".⁵⁶ The leads have insulated clamps and no damage.

Where? Her drive, off the road. The neighbour parks close enough for the leads to reach, with the two cars not touching (Ford: "making sure the two vehicles do not touch").⁵ Handbrakes (parking brakes) on, ignitions off, rings and watches off.

The order, from her handbook. Positive clamp to her flat battery's positive terminal; the other end to the neighbour's positive; negative clamp to the neighbour's negative; and the last clamp on the metal point her handbook's drawing shows, well away from her battery. Nobody leans over either battery while the clamps go on.

Starting. Toyota's steps have the donor's engine run a little above idle for "approximately 5 minutes to recharge the battery of your vehicle" before trying the flat car.⁴ Then she starts her own car.

That leaves the leads coming off, which is where the sources part company.

Taking the leads off: where the sources disagree

Every source read takes the leads off in the reverse of the order they went on. They disagree about whether the engines are running while you do it.

Sources: ¹³⁴⁵⁶.

On the engines, the AA's page is consistent with itself; it's the other sources it does not match. Ford's and Toyota's steps never switch either engine off before the leads come off, which this course reads as both running; Nissan keeps the LEAF in READY; and HSE says outright to leave the dead car's engine running, and explains why: "the battery is still flat, so you will need to drive several miles before it has enough energy to restart the engine."⁶ The AA, which sells breakdown cover, switches both off and then restarts, and it gives a reason for its rule: removing the leads with the engines running "can cause serious damage to the cars' electronics".³

HSE and the AA each give a reason, neither answers the other, and this course won't invent a reconciliation. What is established is narrower: the leads come off in reverse order, the handbook for the car you are rescuing governs whether its engine runs, and the donor's handbook governs the donor. If your handbook is silent, this course's reading is that it's a question for the maker's dealer or a garage that works on that make, not a gap to fill with whichever rule you heard last.

After it starts

A jump starts the engine. It doesn't leave the battery charged: HSE says that afterwards "the battery is still flat",⁶ and lesson 5 explained why short drives don't fill it either. The advice after a jump is the same shape everywhere, with each source's own numbers: the AA, which sells breakdown cover, says "Charge the battery by driving normally (not in stop-start traffic) for at least 30 minutes";³ Nissan says keep the LEAF in READY for over 20 minutes; Toyota says have the car inspected at a dealer as soon as possible.¹⁴ If the car will not start again, the AA's advice is that "there's probably a more serious problem requiring professional help".³ Nissan recommends a visit to a certified LEAF dealer after a jump "to have the 12V battery tested".¹ A battery that needs a second jump soon after the first is one to have tested too, and lesson 5's signs are the ones to watch.

Electric cars and hybrids

Lesson 4, "Hybrids and electric cars: how they work", set up the two batteries. For a jump start, that has four consequences.

The small battery can be jumped; the big one cannot. NHTSA, the US road-safety regulator: "The high-voltage battery found in EVs and HEVs cannot be jumped." It adds: "However, in most cases, the 12-volt battery, which powers things like lighting and instrumentation, can be jumped".⁸

A jump doesn't fill the big battery. Nissan: "Jump starting does not charge the Li-ion battery. The Li-ion battery must be charged before the vehicle can be driven."¹ It also warns: "Do not attempt to perform a jump start on the 12-volt battery at the same time that the Li-ion battery is being charged."¹ Lesson 4's DC/DC converter is why the order matters on a LEAF too: Nissan warns that not following its steps "could result in damage to the DC/DC converter and cause personal injury".¹

An electric car shouldn't be the donor. Nissan: "LEAF cannot be used as a booster vehicle because it cannot supply enough power to start a gasoline engine." It goes on: "However, a gasoline engine vehicle can be used to jump start LEAF's 12-volt battery."¹ The AA, which sells breakdown cover and repairs, of the donor car in general: "avoid a hybrid or electric car as this could cause damage".³ Nissan's statement is about the LEAF, and the AA's is general advice; this course read no maker that says the reverse.

Nothing on the high-voltage side is touched. The jump points on these cars are 12 V points the handbook shows, and every clamp stays on them. Anything orange, anything blue on a 48 V mild hybrid (which may use blue for its wiring), or anything labelled high voltage, is Call someone. Lesson 4 gave the MOT manual's caveat that some imported cars may use another colour, so the handbook's labels and pictures are what tell you. Lesson 14, "High voltage: hybrid and EV safety, charging at home, fire and flood", is about why.

Push-starting

A push start isn't a jump start, but three handbooks say which cars can't have one. Ford: "Do not attempt to push-start an automatic transmission vehicle." Toyota, for the 2023 Corolla: "The engine cannot be started by push-starting." Nissan: "An EV cannot be push-started or tow-started."¹⁴⁵ This course doesn't teach push-starting for any car, as its own scope decision: it means people pushing a car on a road, which the placement rules above exist to avoid.

The conditions, in one place

A jump start is Yours, on conditions. On the sources read, and this course's reading of where, every condition has to hold:

• The handbook allows it and gives its method, and you follow that method's order and points.
• The battery is fit to be jumped: not frozen (Nissan: "It could explode"), not damaged or leaking (the AA, which sells breakdown cover and repairs).¹³
• The same voltage: Ford's "same nominal voltage", HSE's "do not proceed" if unsure, and Nissan's "The booster battery must be rated at 12 volt."¹⁵⁶
• The leads are fit: Ford's "adequately sized cables with insulated clamps", the AA's "Don't use jump leads that are damaged" and "Stop using the jump leads if they get hot".³⁵
• The donor is not an electric car or a hybrid (Nissan for the LEAF; the AA for both), unless its own handbook says it can be one.
• The place is off the road, and never a motorway, hard shoulder or emergency area (this course's reading of Rule 277).
• No flame, no smoking, no jewellery, nobody leaning over a battery, with eye protection as Nissan asks.

If any fails, it's Call someone. A booster pack changes the donor, not the conditions: Ford's handbook says to follow the pack maker's own instructions,⁵ the AA's page gives its own steps and defers to the car's handbook,³ and this course recommends no pack.

Battery acid on skin or in an eye is first aid, not car care. First Aid and CPR lesson 10, "Burns, cuts and small injuries", teaches chemical burns and a splash in the eye.

What people get wrong

"The last clamp goes on the flat battery's negative terminal." Every source read puts it on metal away from the battery; Nissan says "not to the 12-volt battery".

"An electric car can jump another car." Nissan says the LEAF can't, and the AA, which sells breakdown cover, advises against using a hybrid or electric donor. On the sources read, it can be jumped but shouldn't be the donor.

"A jump start charges an electric car." It wakes the 12 V system. The big battery still has to be charged before the car can be driven.

"Any car can be push-started." Not an automatic, on Ford's handbook; not the 2023 Corolla, on Toyota's; not an EV, on Nissan's.

"Switch the headlights on while you take the leads off, to protect the electrics." Ford says the opposite, for its car: the peak voltage could blow the bulbs.

"There's one right order for the leads." The sources agree on positive first and a last clamp away from the battery, and differ on the rest. The handbook for the car is the order.

Practice

For your plan

Add a jump page to your car file: the handbook's page reference, the connection order in its words, the last clamp's place (with your sketch), what it says about taking the leads off, the conditions it lists, and your own line on where you'd never do it. If your car is electric or a hybrid, add whether it can be a donor and what the main battery needs after a jump. Lesson 16 adds a breakdown page with the call-out numbers.

Connections

Lesson 5, "The 12 V battery: what it does, and why short trips flatten it", gave the hazards and the signs; this lesson has used both. Lesson 4 set up the two batteries in a hybrid or electric car. Lesson 14, "High voltage: hybrid and EV safety, charging at home, fire and flood", covers the big battery that no owner touches. Lesson 16, "When the car stops: where, who stands where, and the call", and lesson 17 on motorways teach where a car should stop and what to do there; this lesson applied lesson 16's stopping place to a jump start. Lesson 19, "Changing a wheel safely", has the placement rules this course applies to a jump start.

Go deeper

• Your own handbook, its jump-start pages. Every order in this lesson is one maker's for one car.
• HSE, Using electric storage batteries safely (INDG139), free, 10 pages, read here in full. Written for workplaces in 2011; its jump-starting section covers where the last lead goes and why connecting is the risky moment.
• Nissan, 2024 LEAF Owner's Manual, pages 6-10 to 6-13, read here in full. The jump-start section for that electric car, with the donor warning and the order for taking the leads off.
• Halderman and Ward, Automotive Technology: Principles, Diagnosis, and Service, 7th edition (Pearson, 2025), whose Section VII, "Electrical Systems", has a chapter titled "Automotive 12-Volt Batteries". A technician's textbook; this course read only its table of contents.

Sources

1. Nissan North America, 2024 LEAF Owner's Manual and Maintenance Information (US market), PDF. Read in full: pages 6-10 to 6-13 ("Jump starting" and "Push starting"), re-read 26 September 2026. A maker's handbook for its own car.
2. The AA (a motoring organisation that sells breakdown cover and repairs; not an official body), "What to do if your car has a flat battery", last updated 11 January 2024. Read: full text, re-read 26 September 2026. The page sells the AA's own battery service.
3. The AA, "How to jump start a car in 9 steps", last updated 26 June 2024. A motoring organisation, as above; the page sells breakdown cover and battery replacement. Read: full text, re-read 26 September 2026.
4. Toyota Motor Sales USA, 2023 Corolla Owner's Manual (OM02568U), US market, PDF. Read for this lesson: pp. 383 to 386 ("If the vehicle battery is discharged"), 26 September 2026. A maker's handbook for its own car.
5. Ford Motor Company, 2024 Maverick Owner's Manual, edition 202307, US and Canada, read from Internet Archive capture 20250505042038 (Ford's site refuses automated readers). Read for this lesson: printed pp. 284 to 287 ("Jump Starting the Vehicle"), 26 September 2026. A maker's handbook for its own car, sold as petrol or full hybrid.
6. Health and Safety Executive, Using electric storage batteries safely, INDG139(rev1), July 2011. Great Britain's workplace safety regulator, writing for workplaces. Read: full text; paragraphs 20 and 24 to 31 re-read 26 September 2026.
7. The Highway Code, "Breakdowns and incidents (275 to 287)", updated 14 September 2021. Great Britain. Read: Rules 276 and 277 in full, re-read 26 September 2026 through the GOV.UK content API. Both are advice (should), not MUST rules.
8. NHTSA, "Electric and Hybrid Vehicles", read from Internet Archive capture 20260924154857 (nhtsa.gov refuses automated readers; the live page is linked). Read: full text, re-read 26 September 2026 from the same capture.