What uses up a car's battery?
Discussion
The battery is charged by the engine when the car is being driven and if everything is working correctly and the car is being used normally will remain topped up and will need no intervention.
Accessories which draw electrical power can use the battery when the engine is off. The alarm / security system will draw a very small amount. Using the radio or interior lights with the engine off will consume battery but the size of the battery is such that it can all run for some hours before the battery state of charge will be too low to start the engine.
Side lights or interior lights left on for many hours I.e. overnight can flatten the battery to the point that it won’t start in the morning.
Accessories which draw electrical power can use the battery when the engine is off. The alarm / security system will draw a very small amount. Using the radio or interior lights with the engine off will consume battery but the size of the battery is such that it can all run for some hours before the battery state of charge will be too low to start the engine.
Side lights or interior lights left on for many hours I.e. overnight can flatten the battery to the point that it won’t start in the morning.
Technically the alternator charges the battery, if working properly it should put out over enough electric in order to keep the battery topped up as the cars electrical components will be drawing around 12v whilst the engine is running. A simple test is to get a volt meter and check the battery voltage with the engine off (should be over 12v) and when the engine is running it should be over 14v.
Typically over the winter your car battery gets a tougher time than normal due to the cold weather and potentially your driving habits changing. If you aren't using the car as often the battery is depleted by things like the clock & alarm. When in use you are more likely to do regular journeys where you have the headlights, wipers, heaters/ac , heated seats etc on max & you may find that you haven't fully replenished the power required to start the engine & run the accessories before switching the engine off. Do this too often and you will start to notice the engine taking a few more turns to start or the starter motor spin a little bit slower as these can be signs of a weak battery.
Typically over the winter your car battery gets a tougher time than normal due to the cold weather and potentially your driving habits changing. If you aren't using the car as often the battery is depleted by things like the clock & alarm. When in use you are more likely to do regular journeys where you have the headlights, wipers, heaters/ac , heated seats etc on max & you may find that you haven't fully replenished the power required to start the engine & run the accessories before switching the engine off. Do this too often and you will start to notice the engine taking a few more turns to start or the starter motor spin a little bit slower as these can be signs of a weak battery.
Main points.
- Under normal circumstances the battery has only one function - starting the car.
- As soon as your car is started it's running completely off electricity from the alternator, which is also topping up the battery again. Once running, a car doesn't really need a battery at all.
- Lead acid batteries (car batteries) are weird things. They are nothing like the battery in your mobile phone and behave completely differently.
- Car batteries hate being discharged. The easiest way to kill one is to charge it, flatten it, leave it flat for a month, re-charge it, flatten it, leave it etc.
- The key skills of a car battery are (a) to supply enough voltage to run the car's electronics through the start cycle, and (b) enough current (a BIG current - look at the thickness of your battery cables) to crank the engine on a cold start.
- A car battery likes to be fully charged at ALL times. Whenever it is less than fully charged it is deteriorating through a process called sulphation.
- Battery chargers are solely for emergency use. If your battery is getting knackered buy a new battery, not a charger.
- So-called battery tenders are different. They're permanently connected and keep the battery full at all times without overcharging it. Products like CTEK. (They are different from simple trickle-chargers which may overcharge the battery if left connected for too long.)
- When a battery is being overcharged it suffers damage. This is not the right expression but essentially overcharging boils off the acid.
- Cars with start/stop systems have a different, more fussy and more expensive type of lead acid battery called AGM (Absorbed Glass Mat). Lots of explanations in Wikipedia etc. They generally should be "matched" with the car's electonics whenever you buy a new one.
Why did I say 12.6 v is "normal healthy voltage" when a car's voltmeter will usually show between 13.7 and 14.7 volts with the engine running?
Essentially the alternator has to provide a slightly higher voltage in order to push electricity into the battery.
A car battery isn't really a 12 volt device. It should be a 12.6 volt device. Each lead acid cell gives 2.1v and there are six of them so it adds up to 12.6v. "12" is just a handy, round number. Yes, a car has 6 cells packaged into a single battery of cells in a black plastic case. (Use of the word battery for a single AA or AAA is not really correct, they are cells not batteries. The French word for battery is "pile", quite literally describing a vertical "pile" of cells, which is how batteries were first invented.)
As a battery ages and deteriorates its nominal voltage will decrease. For practical purposes any car battery which shows a voltage of less than 11.4v after being charged and then left to stand indoors overnight - is knackered.
Essentially the alternator has to provide a slightly higher voltage in order to push electricity into the battery.
A car battery isn't really a 12 volt device. It should be a 12.6 volt device. Each lead acid cell gives 2.1v and there are six of them so it adds up to 12.6v. "12" is just a handy, round number. Yes, a car has 6 cells packaged into a single battery of cells in a black plastic case. (Use of the word battery for a single AA or AAA is not really correct, they are cells not batteries. The French word for battery is "pile", quite literally describing a vertical "pile" of cells, which is how batteries were first invented.)
As a battery ages and deteriorates its nominal voltage will decrease. For practical purposes any car battery which shows a voltage of less than 11.4v after being charged and then left to stand indoors overnight - is knackered.
rockin said:
Why did I say 12.6 v is "normal healthy voltage" when a car's voltmeter will usually show between 13.7 and 14.7 volts with the engine running?
Essentially the alternator has to provide a slightly higher voltage in order to push electricity into the battery.
A car battery isn't really a 12 volt device. It should be a 12.6 volt device. Each lead acid cell gives 2.1v and there are six of them so it adds up to 12.6v. "12" is just a handy, round number. Yes, a car has 6 cells packaged into a single battery of cells in a black plastic case. (Use of the word battery for a single AA or AAA is not really correct, they are cells not batteries. The French word for battery is "pile", quite literally describing a vertical "pile" of cells, which is how batteries were first invented.)
As a battery ages and deteriorates its nominal voltage will decrease. For practical purposes any car battery which shows a voltage of less than 11.4v after being charged and then left to stand indoors overnight - is knackered.
I bought a cigar lighter to USB plug recently. Similar to this https://tinyurl.com/yxzure87Essentially the alternator has to provide a slightly higher voltage in order to push electricity into the battery.
A car battery isn't really a 12 volt device. It should be a 12.6 volt device. Each lead acid cell gives 2.1v and there are six of them so it adds up to 12.6v. "12" is just a handy, round number. Yes, a car has 6 cells packaged into a single battery of cells in a black plastic case. (Use of the word battery for a single AA or AAA is not really correct, they are cells not batteries. The French word for battery is "pile", quite literally describing a vertical "pile" of cells, which is how batteries were first invented.)
As a battery ages and deteriorates its nominal voltage will decrease. For practical purposes any car battery which shows a voltage of less than 11.4v after being charged and then left to stand indoors overnight - is knackered.
what should the readings be with a healthy battery and alternator?
rockin said:
Main points.
What a clear and informative pose, in accessible style.- Under normal circumstances the battery has only one function - starting the car.
- As soon as your car is started it's running completely off electricity from the alternator, which is also topping up the battery again. Once running, a car doesn't really need a battery at all.
- Lead acid batteries (car batteries) are weird things. They are nothing like the battery in your mobile phone and behave completely differently.
- Car batteries hate being discharged. The easiest way to kill one is to charge it, flatten it, leave it flat for a month, re-charge it, flatten it, leave it etc.
- The key skills of a car battery are (a) to supply enough voltage to run the car's electronics through the start cycle, and (b) enough current (a BIG current - look at the thickness of your battery cables) to crank the engine on a cold start.
- A car battery likes to be fully charged at ALL times. Whenever it is less than fully charged it is deteriorating through a process called sulphation.
- Battery chargers are solely for emergency use. If your battery is getting knackered buy a new battery, not a charger.
- So-called battery tenders are different. They're permanently connected and keep the battery full at all times without overcharging it. Products like CTEK. (They are different from simple trickle-chargers which may overcharge the battery if left connected for too long.)
- When a battery is being overcharged it suffers damage. This is not the right expression but essentially overcharging boils off the acid.
- Cars with start/stop systems have a different, more fussy and more expensive type of lead acid battery called AGM (Absorbed Glass Mat). Lots of explanations in Wikipedia etc. They generally should be "matched" with the car's electonics whenever you buy a new one.
The Mad Monk said:
I bought a cigar lighter to USB plug recently. what should the readings be with a healthy battery and alternator?
That device shows 12.8v. Possible explanations include,- The car has recently been running and the battery, having just stopped charging, hasn’t yet settled all the way back to nominal voltage,
- The battery may be above normal room temperature. Battery voltage will be higher if the temperature is higher, speeding up the chemical reactions inside the battery,
- The device showing the voltage may not be 100% accurate.
rockin said:
The Mad Monk said:
I bought a cigar lighter to USB plug recently. what should the readings be with a healthy battery and alternator?
That device shows 12.8v. Possible explanations include,- The car has recently been running and the battery, having just stopped charging, hasn’t yet settled all the way back to nominal voltage,
- The battery may be above normal room temperature. Battery voltage will be higher if the temperature is higher, speeding up the chemical reactions inside the battery,
- The device showing the voltage may not be 100% accurate.
What readings should I expect to see if I have just turned, or engine running, and when it's been running a while?
Anything between about,
11.4v (engine off, knackered battery)
12.6v (engine off, good battery)
14.7v (engine running and alternator charging battery at full tilt.)
I've never sat and watched an accurate voltmeter (not the one on the dash!) in a modern car while it's being driven but I suspect some will have voltage wandering all over the place within the normal range,
https://www.powerelectronics.com/content/article/2...
11.4v (engine off, knackered battery)
12.6v (engine off, good battery)
14.7v (engine running and alternator charging battery at full tilt.)
I've never sat and watched an accurate voltmeter (not the one on the dash!) in a modern car while it's being driven but I suspect some will have voltage wandering all over the place within the normal range,
- Some cars can shut off the alternator when it's not needed in order to maximise economy and emissions.
- Some cars favour use of the alternator during deceleration, acting effectively as a "mini-hybrid" with a small amount of regenerative braking,
- In town, cars with start/stop will be constantly juggling management of the battery and charging circuit,
- Voltage will swing around during start/stop under any circumstances and particularly if things like heated seats are in use.
https://www.powerelectronics.com/content/article/2...
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff