MG Hybrid Battery Diagnostics
Before you worry about the expensive battery, let us check the cheap one. A great many MG hybrid and EV battery warnings turn out to be the ordinary twelve volt battery, and owners are quoted for the wrong thing. We measure both and give you a real number.
Message us about your MG
Send the model and the warning. You will get a straight answer, not a scare.
or call 01744 808586
WHERE TO START
MG hybrid battery faults: check the cheap battery first
Two batteries, and the small one causes most of the alarm.
Every electrified MG has two batteries, and they are almost nothing like each other. The one owners think about is the high voltage pack that moves the car. The one that actually causes most of the trouble is the ordinary twelve volt battery under the bonnet, the same sort of thing that has been in cars for decades.
That small battery runs the electronics that wake the car up, close the contactors and let the high voltage system come online. When it is weak, the car can behave as though the expensive battery has failed. It may refuse to go into ready mode, throw warnings across several systems at once, or simply do nothing at all despite a healthy main pack.
Owners in that position are frequently quoted for high voltage work when a battery costing a small fraction of it would have solved everything. That is why we test both before saying anything, and why the first question we ask is not about the hybrid system. See MG battery fitting and EV battery health check.
HOW WE WORK
What an MG battery diagnosis involves
Three things that stop you paying for the wrong battery.
Both batteries, every time
Testing the high voltage pack while ignoring the twelve volt one is how owners get quoted for the wrong repair. We measure both as standard.
A number, not an opinion
Battery health is measurable. You get an actual capacity figure you can use when selling, buying or deciding whether to keep the car.
Told straight either way
If the pack is genuinely degraded you will hear that plainly. If it is not, you will hear that too, which is the more common outcome.
mg hybrid battery
SYMPTOMS
Every MG battery symptom, and what it usually turns out to be
Eight complaints, and how often the expensive battery is innocent.
This is what MG hybrid and electric owners actually arrive with, and what each turns out to be once both batteries have been measured rather than assumed.
| What the car does | What it usually is | What it means |
|---|---|---|
| Will not go into ready mode at all | The twelve volt battery, not the high voltage pack | The commonest of the lot and the cheapest. A full main battery is no use if the small one cannot wake the car. See MG battery fitting |
| Several warnings appear together | A weak twelve volt battery, or one poor earth | Multiple unrelated warnings almost always share a single cause. Chasing each separately is how a modest bill becomes a large one |
| Range has dropped noticeably | Cold weather, in the great majority of cases | Every EV loses usable range in winter and recovers fully in spring. If it has persisted through warm months, that is worth measuring. See EV battery health check |
| Charging has become slower | Ambient temperature, the charger itself, or a state of charge limit | Charging slows deliberately as the pack fills and in cold conditions. That is protection working rather than a fault |
| Hybrid system warning on an HS | A logged fault that needs reading rather than guessing | Frequently far less serious than the word hybrid suggests, and sometimes the twelve volt battery again. See car diagnostics |
| Flat if the car stands a week or two | Something not going to sleep, or a twelve volt battery at end of life | Measurable rather than a matter of opinion, and worth doing properly before buying anything. See battery drain diagnosis |
| Warning after a jump start or a battery change | Systems needing to relearn, or the wrong battery specification fitted | Very common and usually resolved without parts. Fitting the wrong type is easy to do because it fits and works otherwise |
| Genuine high voltage fault warning | A fault in the pack, contactors or management system | This one does need proper attention. Do not attempt to charge it and ring 01744 808586 before the car is moved |
WORTH KNOWING
A dead electric car is usually a dead ordinary battery
It is the most counter-intuitive thing about owning an electric or hybrid car. You can have a main battery three quarters full and a car that will not do anything at all, because the small twelve volt battery that wakes everything up has gone flat.
It happens most often on cars that stand for a fortnight or more, and owners understandably fear the worst. In reality it is the same battery, the same failure and roughly the same cost as on any petrol car. Before you accept a quote involving the high voltage system, have the small one tested. See MG battery fitting or ring 01744 808586.
THE PROCESS
How we check an MG battery properly
Six steps, and the expensive battery is not the first one.
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Which car, and which system
MG spans mild hybrid, plug-in hybrid and full electric, and they are genuinely different. What is worth checking depends entirely on which you have.
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The twelve volt battery, load tested
Not a voltage reading taken at rest, which tells you very little. A proper load test, because this is the single most common cause of everything below.
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Full system scan across all modules
Battery management, charging and vehicle control, not just one of them. Warnings in this area regularly originate somewhere unexpected.
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State of health measured
A real capacity figure for the high voltage pack rather than an estimate from the range display, which reflects driving and weather as much as the battery.
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Charging behaviour checked
How the car actually takes a charge, rather than what the app says. Slow charging is frequently temperature or the charger rather than the car.
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A written figure and an honest verdict
What the pack is actually at, what we found, and a real price if anything needs doing. Ring 01744 808586 or see repair costs.
BATTERY HEALTH
What state of health means, and why the range readout is not it
A measurement rather than a prediction, and why it matters when you sell.
A state of health figure is the single most useful thing an MG owner can have, and hardly anybody has one. It expresses how much of the pack original capacity remains as a percentage, measured from the battery management system rather than estimated from how far the car says it will go.
That distinction matters. The range readout reflects recent driving, temperature, load and how you have been using the car. It is a prediction, and a pessimistic one in January. State of health is a measurement, and it does not care what the weather has been doing.
It is worth having for three reasons. If you are selling, it answers the question every buyer now asks. If you are buying, it tells you what you are actually getting. And if you are deciding whether to keep the car, it turns a worry into a number you can make a decision with. See EV battery health check.
COST
What it costs, and why it usually saves you a great deal
A fixed price for a real measurement.
The check is a fixed amount, quoted before you commit. On these cars it is the best value work we do, because the gap between what owners fear and what is usually wrong is enormous.
A twelve volt battery is ordinary money. A high voltage pack is not. Being told which of the two you are dealing with, with a measurement behind it, is worth having before you accept anybody quote. We would far rather charge you for an hour and send you away with a working car than sell you something large.
Fixed price, told up front
Not billed by the hour and not open ended, so you know what you are committing to before the car comes in.
The cheap answer is the common one
A twelve volt battery, a poor earth or a relearn after a jump start account for a great deal of what arrives here as a hybrid fault.
A capacity figure you can keep
Useful when selling, buying or deciding whether to keep the car. See EV battery health check for what is included.
Genuine pack faults told straight
If it is the high voltage system, you get that plainly along with what is realistic. No softening it and no pretending otherwise.
Spreading a larger bill
There is Payment Assist where a job is worth doing, on a soft search rather than a full application.
Everything else in the same visit
Tyres, brakes, servicing and the MOT, because an electric car still needs all of it. See MG diagnostics.
ALSO HERE
What else we do while the MG is in
An electric car still needs everything a petrol one does.
- EV battery health check — A measured capacity figure rather than an estimate from the range display. See EV battery health check.
- Twelve volt battery supply and fitting — The right specification for an electrified car, which is not the same as any battery that fits. See MG battery fitting.
- Battery drain diagnosis — For a car flat after standing, measured properly rather than guessed at. See battery drain diagnosis.
- MOT on site — Electric cars are tested here like any other, minus the emissions. See MOT.
- Servicing — Brakes, tyres, filters and fluids on schedule. An EV still wears all of it. See car servicing.
- Wider MG faults — Warning lights, engine and electrical faults across the range. See MG diagnostics.
Owner questions
MG battery questions, answered
Fifteen questions MG hybrid and EV owners actually ask us. If yours is not here, ring 01744 808586.
Very probably not. On an electrified car the usual cause is the ordinary twelve volt battery, which wakes the electronics and lets the high voltage system come online.
A main pack three quarters full is no use if the small battery cannot start that process. Have it load tested before accepting any quote involving the hybrid system. See MG battery fitting.
By measuring it rather than reading the range display. A state of health check reports the percentage of original capacity remaining, taken from the battery management system.
That is a measurement rather than a prediction, and it does not shift with the weather or how you have been driving. See EV battery health check.
Because it is a prediction based on recent driving, temperature and load rather than a statement about the battery. It is deliberately cautious, and in winter it is very cautious.
That is why owners panic in January and relax in May with nothing having changed. State of health does not move with the seasons. See EV battery health check.
That is normal on every electric car and it recovers completely when the weather warms. Cold reduces both usable capacity and efficiency at the same time.
If the drop has persisted through the summer, that is a different matter and worth measuring properly rather than worrying about. Ring 01744 808586 or see EV battery health check.
Both, as standard, because testing one and guessing at the other is exactly how owners get quoted for the wrong repair.
A weak twelve volt battery produces warnings that look like high voltage faults, and it is a small fraction of the cost. See MG battery fitting and car diagnostics.
That looks alarming and is usually one cause rather than several. A weak twelve volt battery or a single poor earth will upset multiple systems at the same time.
Treating each warning separately is how a modest job becomes an expensive sequence. A full scan shows the pattern immediately. See MG diagnostics.
Often nothing. Charging slows deliberately as the pack fills and in cold conditions, and the charger itself accounts for a good share of complaints.
If it has genuinely changed in like for like conditions, that is worth reading. Tell us where you charge and at what rate. See EV battery health check.
On an electrified car that is usually the twelve volt battery, either at end of life or being drained by something staying awake.
Both are measurable rather than matters of opinion, and worth establishing before buying anything. See battery drain diagnosis.
A fixed amount quoted before you commit, and on these cars it is the best value work we do given the gap between what owners fear and what is usually wrong.
It regularly ends with an ordinary battery rather than anything large. For genuine pack work there is Payment Assist. See repair costs.
Yes, and it is becoming the first thing buyers ask. A measured capacity figure answers the question that otherwise knocks money off the price.
It works the same way if you are buying. Ask the seller for one, or bring the car here before you commit. See EV battery health check.
Yes. Brakes, tyres, suspension, filters and fluids all wear regardless of what drives the wheels, and the schedule still needs keeping to protect the warranty.
Brakes in particular need attention on electrified cars, because regenerative braking means the discs are used less and can corrode. See car servicing.
Yes, the test bay is on this site. Electric cars are tested exactly like any other, minus the emissions element of the test.
Everything else applies as normal, including lights, brakes, tyres and any illuminated warning lamp. See MOT.
Very common. Systems can need to relearn after a voltage interruption, and the warning is frequently nothing more than that.
It is also worth checking that the right battery specification was fitted if one was changed, because the wrong type fits and works otherwise. See MG battery fitting.
It can be, and we will not pretend otherwise. What we will do is establish whether that is actually what you have before anyone starts talking about figures.
A great deal of what arrives as a suspected pack fault is not one. If yours genuinely is, you get an honest picture rather than a soft one. Ring 01744 808586 or contact us.
The model, whether it is mild hybrid, plug-in or fully electric, and exactly what the car is doing. Those three route the whole conversation.
A photograph of the warning helps, and so does knowing whether the car has stood recently or been jump started. See car diagnostics.