Flat battery? We jump start it, then test it properly
Jump start from £60 with no call-out fee, anywhere in London, on the M25 and out on the wider motorway network. Then a load test on the battery and a charging check with the engine running, so you know whether the car will start again tomorrow morning.
- No call-out fee
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A battery call out from CityGrip Recovery is a jump start plus a diagnosis. We come to the vehicle anywhere in London, on the M25 and on the wider motorway network, get the engine running, then test the battery under load and check what the alternator is actually putting back with the engine running. Jump start is from £60, there is no call-out fee on anything, and the price is agreed on the phone before anyone sets off. If the battery is finished, we tell you the exact type and rating the car needs. If the real fault is the charging system, a bad earth or a parasitic drain, a new battery would never have fixed it. Call or WhatsApp 07960 200253, any hour.
What a battery call out includes, and what it deliberately is not
Four things happen when we get to you. We jump start the vehicle from a professional pack rather than from another car. We put the battery under a proper test instead of glancing at a resting voltage and guessing. We check the charging system with the engine running, at idle and with the electrical load switched on, because a battery that keeps going flat is very often an alternator problem wearing a battery costume. And we check the dull things that strand more drivers than anything else: a loose or corroded terminal, a hold-down clamp that has let the battery shake itself apart, and the main earth strap between the engine and the body.
This page is about batteries, charging systems and starting faults only. If the car needs to be moved to a garage, to your home or to a body shop, that is recovery and towing, which starts from £80 and is covered on our breakdown recovery and roadside assistance page. Keeping the two apart matters because the answer is different: a battery fault gets fixed where the car is standing, a mechanical or transmission fault gets loaded and taken somewhere.
One honest limit worth stating up front. We are not selling you a battery at the roadside, so we have nothing to gain by condemning one that is only discharged. A conductance tester needs the battery at roughly 70% state of charge or higher before its result means anything, so on a deeply flat battery the correct answer is often charge it properly first, then test it, rather than declaring it dead on the spot. If we cannot give you a straight answer at the kerbside, we say so.
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What the car is telling you before it will start
Flat batteries are the single biggest cause of breakdowns in this country. On the first Monday of 2025, 24% of all RAC breakdowns were flat batteries, and in an AA survey of 15,115 British drivers in October 2021, 39% said their most recent breakdown was a flat battery, ahead of an engine that would not start at 31% and a flat tyre at 29%. The noise the car makes when you turn the key narrows it down fast.
A slow, lazy crank that gets better on the second attempt is a battery low on charge, usually from short journeys. One single loud click and nothing else is normally the starter solenoid pulling in without turning the motor, or a heavy voltage drop through a corroded terminal or poor earth. Rapid machine-gun clicking means there is not enough current even to hold the solenoid in, which is a very flat battery or a bad connection. Bright dashboard lights but no crank at all points away from the battery and towards the immobiliser, the ignition switch, the park or neutral switch, or the starter itself. Nothing whatsoever, no dash lights and no central locking, is a completely dead battery, a disconnected terminal or a blown main fuse or fusible link. Starting then dying within a couple of seconds is usually immobiliser or fuel supply, not battery. Starting on a jump but being dead again the next morning means the battery will not hold charge, something is draining it, or the alternator is not charging. Starting fine but with the battery warning light staying on, or coming on while you drive, is a charging system fault: keep the electrical load low, do not switch the engine off, and get it looked at before you are stranded somewhere worse.
If you are sitting in the car right now, work through this before you phone anyone. Switch off the headlights, blower, heated screen and everything else. Open the bonnet if you can and try to twist each battery terminal by hand: if either moves, that alone can be the whole fault. On an automatic, put the selector firmly in P, then try N as well, because a worn inhibitor switch will refuse to crank in one and not the other. Try the spare key, which rules out a failed immobiliser transponder. Then call or WhatsApp 07960 200253 and describe the noise it makes. That one detail usually tells us what we will be dealing with before we arrive.
Is it the battery or the alternator? The voltage sequence we run
The test order matters more than any single reading. First, resting voltage with the ignition off and everything switched off: a healthy 12V battery reads around 12.6V at rest. A reading well under that means it is only partly charged, and any test done in that state is unreliable. Second, what the voltage does during cranking, which tells us far more than the resting figure because it shows whether the battery can actually deliver current rather than just hold a number. Third, voltage at idle with the engine running, where a working charging system should sit in the 13.7V to 14.7V band, typically the mid-14s. Fourth, the same reading with headlights, blower and heated rear screen all on, to see whether the alternator holds its output when it is asked to work.
Modern smart charging complicates the picture and is worth understanding before you panic at a low number. Variable-voltage alternators are managed by the ECU to cut drag on the engine and improve economy, so once the battery sensor reports the battery is full, charging voltage is deliberately reduced and can drop to around 12.5V at idle. That is designed behaviour, not a fault. What matters is whether the system raises voltage again on demand, under load and on the overrun. Persistently high readings above the normal band are the more damaging fault: an overcharging alternator boils the electrolyte out of a battery and can destroy a new one in a matter of weeks.
A failing diode pack inside the alternator is the classic reason a battery keeps dying with no obvious drain. It charges badly, and a shorted diode also lets current leak back out of the battery overnight, so the car sits there discharging itself through its own alternator. It is also why fitting a new battery to a car with a bad alternator is money thrown away: the new one will be flat within weeks. And a point that surprises people, the alternator is not a battery charger. It is sized to replace what starting used and to run the car's electrical load, not to bulk-charge a battery that has been run right down. Leaning on it to do that repeatedly leaves the battery chronically undercharged and takes years off its life.
Why the battery keeps going flat
The usual suspects are age, short journeys that never put back what starting took out, cold weather, a parasitic drain, a failing alternator, a poor earth, and an interior, glovebox or boot light that never goes out. Our article on why a car battery keeps dying covers each of those at consumer level. What matters here is how we tell them apart at the roadside and what each one means for the bill, because the same symptom can be a £60 jump start or an auto-electrical job.
Parasitic drain is the one most often measured badly. There is no single correct milliamp figure: acceptable draw is calculated from the battery itself, by dividing reserve capacity in minutes by four, or the amp hour rating by 2.4. A battery with 100 minutes of reserve capacity should therefore sit under roughly 25 milliamps once everything has gone to sleep. That last part is the trap. Control modules shut down at very different rates and some take hours, with one documented case still awake six and a half hours after the car was switched off, so a reading taken two minutes after you lock the doors is meaningless and will send you chasing a fault that is not there. The genuine culprits are usually aftermarket: alarms and trackers, dash cams hard-wired to a permanent live, aftermarket stereos and amplifiers, faulty boot or glovebox switches, and that shorted alternator diode.
Private hire and delivery drivers eat batteries for a specific reason. The pattern is engine off, ignition on, an app running, two phones charging, the heater or lights going while you wait between jobs, then a five or ten minute drive. The alternator never catches up, so the battery lives in a permanent state of partial charge and ages fast. If you have a dash cam or tracker hard-wired in, make sure the kit has a voltage cut-off that disconnects it before the battery is too low to start the car, because that single component is the difference between parking overnight and needing a jump start. Cars that sit unused are the mirror image: a modern car with an alarm, telematics and keyless entry can be too flat to start after a couple of weeks, sooner if the battery is already tired, which is where a maintenance charger or a solar maintainer earns its money. And the reason so many batteries die on the first properly cold morning is simple: cold thickens the oil so the engine needs more current to turn, at exactly the moment the battery can deliver least. The battery that was marginal in October is the one that fails in December.
Stop-start cars: AGM, EFB and battery coding
A stop-start car works its battery in a way an older car never did. It restarts the engine many times per journey, it runs the electrics while the engine is off, and it is deliberately kept at a partial state of charge so there is headroom to absorb regenerative charging when you lift off or brake. VARTA is blunt about what happens when you ignore that: a conventional flooded starter battery is not designed for that level of strain and cannot handle the frequent starts, partial charge states and repeated cycling. An EFB battery delivers around twice the number of charging cycles of a conventional starter battery, tested to EN 50342-1 and EN 50342-6, and AGM sits above that again.
The replacement rule is not a preference. An AGM battery may only be replaced with another AGM. An EFB car may be upgraded to AGM, but an AGM car must never be downgraded to EFB or to flooded. Fitting the wrong type causes deficiencies in the automatic stop-start system, failures of comfort functions and reduced service life, which is why a cheap battery on an AGM car turns into a second bill within a year or two.
Then there is registration, also called battery coding or BEM. It is mandatory on Audi, Seat, Skoda, Volkswagen and Porsche, and on BMW and Mini models fitted with an intelligent battery sensor from 2008. Mercedes, Toyota, Renault, Peugeot, Fiat, Honda and Volvo generally self-learn. Ford, Hyundai, Kia, Nissan, Vauxhall and Smart treat it as optional or recommended. Skip it where it is required and you get on-board computer error messages, comfort functions that work briefly then stop, a charging profile matched to a battery that is no longer in the car, and on BMW, if the battery type was changed, a battery that can be overcharged. BEM codes are manufacturer and battery specific and are not issued for aftermarket batteries. This is the real answer to why stop-start stopped working after a new battery: nine times out of ten it is coding, not a faulty battery. Budget for the other resets too, because disconnection wipes them: the radio or navigation code, one-touch windows and sunroof, the steering angle sensor, throttle and idle relearn, the clock and service reminder, and on some cars the tyre pressure monitoring system.
Jump starting a modern car safely, and when we will not do it
Method matters more than muscle. The red lead runs positive to positive. The black lead runs from the donor's negative terminal to a good earth point on the casualty vehicle, a solid metal part of the engine, never to the flat battery's own negative terminal, because a discharged battery gives off hydrogen and the last thing you want is a spark on top of it. Leads come off in reverse order. Do not crank for more than about five seconds at a time, or you will overheat the starter and turn one problem into two. The real risks with cheap leads and a stranger's car are reverse polarity, voltage transients hitting sensitive modules, undersized leads that get hot, and a donor engine being revved into a dead battery. A professional pack controls the connection and the current instead of hoping for the best.
Half the awkward jobs are about where the battery lives. Plenty of BMW, Mercedes, Vauxhall, Renault and Peugeot models put it in the boot, under a seat or under the floor, and provide remote jump posts under the bonnet for exactly this situation. Use those posts. Clamping onto the wrong lug can blow a fusible link and turn a £60 jump start into an auto-electrical repair. If the battery is completely dead you may not be able to open the car at all: electronic door handles, keyless entry and the electric boot release all stop working, and on some models an electric handbrake will not release. There is an emergency key blade inside the fob and usually a hidden lock behind a cover on the driver's door, and that is normally the way in.
There are batteries we will not jump, and we would rather tell you on the phone. A battery that is cracked, leaking, visibly swollen, badly corroded or frozen is a replacement, not a boost, and attempting it is genuinely dangerous. Hybrids and electric cars do have a 12V battery that goes flat and it can usually be jumped from an external low-voltage supply, but a hybrid or EV must never be used as the donor vehicle: Tesla's owner's manuals state plainly that the car cannot be used to jump start another vehicle because damage can result, and the same principle applies across the segment. Vans, minibuses and motorhomes bring their own issues: much higher cranking demand, dual battery and leisure battery setups where the starting battery may be perfectly healthy and the leisure side is the problem, and larger vehicles running a 24V system, where a 12V pack simply will not do the job. Motorbikes, scooters and other small-capacity batteries need the opposite treatment again, because the clamps and current settings that suit a diesel van will cook them.
What actually puts the charge back after a jump start
Once the engine is running, drive for at least 30 minutes in normal conditions before you switch off, and try to avoid heavy stop-start traffic. Idling does charge the battery, because the alternator is still turning, but the varying engine speed of real driving does it far more effectively. Kwik Fit suggests roughly five to ten minutes of idling or gentle revving before you pull away, and puts running charge voltage at about 14.4V to 14.8V. If you drive two miles to the shops and switch off, you will have taken more out at start-up than you put back.
Be clear that a jump start is a temporary measure, not a repair. A deeply discharged battery is properly recovered by a good multi-stage charger over several hours, which pushes it through bulk, absorption and float stages and reverses some of the sulphation that sitting flat causes. Repeatedly asking the alternator to do that work leaves the battery permanently undercharged and shortens its life. If the car starts on a jump but is dead again by morning, driving it around more is not the answer: either the battery will not hold charge, something is draining it, or it is not being charged in the first place.
One piece of common advice needs a warning attached. Leaving the engine running unattended while it charges is against the rules of the road, not just unwise. Highway Code Rule 123 says you must not leave a parked vehicle unattended with the engine running, and must not leave a vehicle engine running unnecessarily while stationary on a public road, citing the Road Vehicles (Construction and Use) Regulations 1986, regulations 98 and 107. Idling while stationary in traffic, or while a fault is genuinely being diagnosed, is permitted. It is also the classic way to lose a theft claim, because most policies will not entertain a car that was handed to a thief with the keys in it and the engine ticking over.
How a battery is tested, and how to know it is genuinely finished
There are two different questions and they get muddled constantly. State of charge is how full the battery is right now. State of health is how much of its original capacity it still has. A workshop conductance tester injects a small alternating signal at around 80 to 90Hz and estimates cold cranking amps rather than measuring them, needs the battery at roughly 70% state of charge or higher to be meaningful, and cannot measure capacity at all. The older carbon pile load test applies half the rated CCA for 15 seconds at 10C and passes if voltage stays above 9.6V. Both are useless on a battery that is nearly flat, which is why an operator who tests a dead-flat battery and immediately sells you a new one is guessing at best.
Reading the label saves money when you do come to replace it. Ah is capacity, CCA is the cold cranking current rated at -18C, and the type number tells you case size, terminal layout and post position. The catch is that CCA is defined differently by different standards, all at -18C: SAE requires 30 seconds at not below 7.2V, the IEC method uses 60 seconds at not below 8.4V, and the DIN method uses 30 seconds then a further 150 seconds at 6V. Figures quoted to different standards are not directly comparable, so a bigger number on the sticker is not automatically a better battery. Match the standard as well as the figure, and never fit less CCA than the manufacturer specifies.
The end-of-life signs are consistent: age beyond three to five years, which is the typical life of a car battery, repeated flattening, slow cranking on cold mornings, a case that is bulging or distorted, heavy corrosion on the terminals, and measured CCA well below the rated figure once the battery has actually been charged. What kills batteries early in the UK is not only cold. Sustained under-bonnet heat is at least as hard on them, along with chronic undercharging from short trips, repeated deep discharges, and vibration from a loose or missing hold-down clamp shaking the plates apart.
What it costs, and the legal points worth knowing
Jump start starts at £60. There is no call-out fee on anything we do, and the price is agreed on the phone before anyone sets off, so there is no meter running and no surprise at the roadside. The battery test and the charging system check are part of that call out rather than an extra line on the bill. If the car cannot be made to run, or if what we find means it should not be driven, the next step is recovery and towing from £80, and you get that figure before we start rather than after. For context on the rest of our pricing: fuel delivery is £70 to £200 including a set amount of fuel, wrong fuel draining is quoted on the call, mobile tyre fitting is from £140 on ordinary roads and £250 to £750 on motorways, and non-fault accident recovery costs the claimant nothing, handled through our sister service at citygripclaims.co.uk.
Two legal points come up constantly. The MOT does test the battery, under section 4.13 of the DVSA inspection manual: a battery that is insecure but not likely to fall from its carrier is a Minor defect, one that is insecure and likely to fall or cause a short circuit is a Major, and a leaking battery is a Major. If the battery is leaking, the tester must refuse to test the vehicle at all. The check applies to electric and hybrid vehicles as well. Second, the old battery is not ordinary rubbish. Under the Waste Batteries and Accumulators Regulations 2009 it is prohibited to dispose of waste automotive batteries in landfill or by incineration, and producers must collect waste automotive batteries free of charge, within a reasonable time, from a final holder on request, and must publish how to make that request. So never bin it, and never leave it rolling around in the boot.
If you are broken down rather than sitting on a driveway, get the safety right before you get the battery right. Hazard lights on, and stop in a place of relative safety. On an ordinary road, put a warning triangle on the road at least 45 metres (147 feet) behind the vehicle on the same side, and never use one on a motorway. On a motorway, switch on the hazards, get out by the door furthest from traffic, make your passengers do the same, and wait behind the barrier rather than in the car. Use the free emergency telephone or call National Highways on 0300 123 5000; an emergency area is set back from the carriageway and gives better protection than the hard shoulder. Then call or WhatsApp us on 07960 200253 and tell us the marker post or junction number.
Frequently asked questions
How much does a jump start cost in London?
Jump start starts at £60 with CityGrip Recovery, with no call-out fee. The price is agreed on the phone before anyone sets off, so you know the figure in advance. That price includes testing the battery and checking the charging system with the engine running, not just getting you started. If the car turns out to need moving, recovery and towing starts from £80 and you are told that before we begin.
Who can I call to jump start my car?
Call or WhatsApp CityGrip Recovery on 07960 200253, any hour of the day or night. We are based at 68 Ruckholt Road, Leyton, London E10 5NP and cover all London postcodes, the M25 and the wider motorway network. Tell us the noise the car makes when you turn the key, the make and model, and where the vehicle is parked, and we will quote you a price on that call.
Do I need a new battery, or will a charge fix it?
It depends on why it went flat. If the battery is simply discharged after short journeys or a light left on, a proper multi-stage charge over several hours will usually recover it. If it has repeatedly gone flat, cranks slowly on cold mornings, is more than three to five years old, or tests well below its rated cold cranking amps once charged, it is at the end of its life. A test taken on a dead-flat battery is unreliable, which is why we charge first and test after where it matters.
Do I need a new battery after a jump start?
Not necessarily. A jump start proves the starter and engine will turn, not that the battery is finished. If the car then runs, charges properly and starts normally the next morning, the battery was just discharged. If it is dead again by morning, then either the battery will not hold charge, something is draining it while the car sits, or the alternator is not charging it. Those three are told apart by measurement, not by guesswork.
How long does a car battery last in the UK?
Typically three to five years. Cold gets the blame, but sustained heat under the bonnet is just as damaging, and chronic undercharging from short journeys is what finishes most of them early. Repeated deep discharges and vibration from a loose hold-down clamp also shorten life considerably. If your battery is past four years and has already gone flat once or twice, treat the next cold snap as borrowed time.
Will a jump start damage my car's electronics?
Done properly, no. Done badly, it can. The genuine risks are reverse polarity, voltage transients reaching sensitive control modules, undersized leads overheating, and cranking for too long. That is why the black lead goes to a solid earth point on the engine rather than the flat battery's negative terminal, why you do not crank for more than about five seconds at a time, and why a controlled professional jump pack is safer than clamping onto a stranger's running engine with cheap leads.
Can you jump start an automatic car?
Yes. Jump starting an automatic is exactly the same procedure as a manual. What you cannot do with an automatic is bump or push start it, because there is no mechanical link from the wheels to turn the engine. If an automatic will not crank but has bright dashboard lights, try the selector firmly in P and then in N, because a worn inhibitor switch will often crank in one position but not the other.
Can you jump start a car in the rain?
Yes. A 12V system is not going to electrocute anyone, and rain does not stop a jump start. The sensible precautions are keeping the clamps and lead ends out of standing water, making sure the connections are clean and dry enough to grip properly, and being careful where you stand if the car is on a live carriageway. Safety on the road is the real risk in wet weather, not the electricity.
Can you jump start a hybrid or electric car?
Usually yes. Hybrids and EVs still have a conventional 12V battery for the electronics, and when it goes flat the car will not wake up at all. That 12V battery can normally be jumped from an external low-voltage supply, using the manufacturer's designated jump points. What you must never do is use a hybrid or electric car as the donor to jump another vehicle. Tesla's manuals state directly that the car cannot be used to jump start another vehicle because damage can result.
Why does my battery keep dying after a jump start?
Three causes, and they need separating. The battery may no longer hold charge, in which case it is at the end of its life. Something may be draining it after you lock up, which is a parasitic drain from an aftermarket alarm, tracker, hard-wired dash cam, faulty boot switch or a shorted alternator diode. Or the alternator may not be charging it in the first place, which shows up as low or erratic voltage with the engine running and load applied. Repeated jump starting fixes none of the three.
Why won't my car start even with a brand new battery?
Because the battery was not the fault. A new battery will not fix a failed starter motor, an immobiliser that does not recognise the key, a corroded terminal or a broken engine earth strap, a fuel supply problem or a blown fusible link. On some cars it will not even fix the original symptom if the alternator is overcharging or undercharging, because the new battery will be ruined within weeks. And on VAG, BMW and Mini models needing registration, a new battery fitted without coding will misbehave from the start.
Does a car battery charge while idling?
Yes, but not efficiently. The alternator turns whenever the engine runs, so idling does put charge back, and roughly five to ten minutes of idling or light revving before driving off helps after a jump start. The varying engine speed of normal driving charges far more effectively, with running voltage typically around 14.4V to 14.8V. Sitting in traffic with the lights, heater and heated screen on can consume as much as the alternator is producing.
How long should I drive after a jump start?
At least 30 minutes in normal driving conditions, avoiding heavy stop-start traffic, before you switch the engine off. Treat that as a holding measure rather than a cure. A battery that has been run right down is properly recovered with a quality multi-stage charger over several hours. Relying on the alternator to bulk-charge a deeply discharged battery leaves it permanently undercharged and takes life off it every time.
Why did stop-start stop working after I changed the battery?
Almost always because the new battery was not registered to the car. Coding is mandatory on Audi, Seat, Skoda, Volkswagen and Porsche, and on BMW and Mini models with an intelligent battery sensor from 2008. Without it, the car keeps charging to the old battery's profile, stop-start refuses to engage, comfort functions can fail or work only briefly, and error messages appear. On BMW, if the battery type was changed without registration, the battery can be overcharged. It is a coding job, not a faulty battery.
Can I fit an ordinary battery to a stop-start car?
No, and it is a false economy. Stop-start cars need EFB or AGM batteries because they restart constantly, run electrics with the engine off, and are deliberately held at a partial state of charge to accept regenerative charging. A conventional flooded battery is not built for that cycling and fails early. An EFB car can be upgraded to AGM, but an AGM car must only ever be replaced with AGM. Downgrading causes stop-start faults, comfort function failures and reduced service life.
How do I tell whether it is the battery or the alternator?
By taking readings in order. A healthy 12V battery sits around 12.6V at rest with everything switched off. With the engine running, a working charging system should be in the 13.7V to 14.7V band, and it should hold up with the headlights, blower and heated rear screen on. If resting voltage is low but charging voltage is fine, it is the battery. If charging voltage is low, erratic or too high, it is the alternator or its regulator. Note that modern smart charging deliberately drops to about 12.5V at idle when the battery is full, which is normal, not a fault.
Where is my battery if it isn't under the bonnet?
Many BMW, Mercedes, Vauxhall, Renault and Peugeot models put the battery in the boot, under a seat or under the floor. Those cars provide remote jump posts under the bonnet, usually a red positive post under a plastic cover plus a marked earth point, and those are the connections to use. Clamping onto the wrong point can blow a fusible link and turn a straightforward jump start into an auto-electrical repair.
My car won't start on the motorway. What should I do?
Safety first, battery second. Get into a place of relative safety if the car will still move, put the hazard warning lights on, and leave the vehicle by the door furthest from traffic, making sure passengers do the same. Wait behind the barrier, not in the car, and never use a warning triangle on a motorway. Use the free emergency telephone or call National Highways on 0300 123 5000, then call us on 07960 200253 with the marker post or junction number.
Updated August 2026. Lines open 24 hours. Call 07960 200253.
Flat battery? Call or WhatsApp 07960 200253
Jump start from £60, no call-out fee, price agreed on the phone before anyone sets off. We start the car, test the battery under load and check the charging system, so you are not guessing about tomorrow morning. 24 hours a day across London, the M25 and the wider motorway network.