Can a Powerbank be used to start a car?

[LateralNw]

Hi all, this is my first ever post.

This is purely an academic question.  I have no intention of doing it.  However I have a "friend" who thinks it can work and I would like to know if they could be right.

I have a background in electronics so I do understand some of the answers that may pop up.  eg No.

I want to get technical about it.

Lets say it is a 20Amp powerbank with the sole purpose when built to charge phones and possibly run low current devices.

Consider this unit as the example, 20000mAh Power Bank, ROMOSS Sense 6 Plus USB C Portable Charger with PD 18W QC 3.0


I'm sure there are many learned friends out there that will know the reason it can or can't be done.

I don't want the device to be modified but I guess it could be pulled apart to access the batteries.

Look forward to your feedback and help.

Cheers.

[Benta]

Well, if it can supply 300...500 A current while delivering at least 8 V for a few seconds, perhaps.
(I don't think so. I've seen so many hare-brained attemps on this that I'm yawning over this one).

[ataradov]

There are car starter power bank-like devices, but even those only work when connected in parallel with a battery. The battery may not have enough juice to start the car, but it must have some charge in it. The power bank just adds enough to start the car.

There is absolutely nothing you can do with a simple USB power bank like this.

[LateralNw]

LOL
Thanks for the reply.

What about a trickle charge if PD was somehow enabled?
PD can supply 15-20 Volts but I am unsure if it could supply enough current.

I agree there is no way it can dump high current as I would expect the batteries will be a low discharge rate type.  Unlike say Radio controlled cars batteries.

[brucehoult]

There are car starter power bank-like devices, but even those only work when connected in parallel with a battery. The battery may not have enough juice to start the car, but it must have some charge in it. The power bank just adds enough to start the car.

There are two different kinds.

There are some with a battery inside, enough to start the car. I don't think they depend on the car battery itself at all, and you could even disconnect it, except that once the car is started you want its own batter to start charging.  I first saw these in Chicago in 2001, but they've probably around a lot longer. Everyone I knew in Chicago then and Moscow later had one in their car.

The much newer kind, maybe only in the last 5-10 years, doesn't have a battery. It's a giant capacitor. You charge it slowly from the "dead" battery for a few minutes, and then it releases the energy quickly to start the engine.

[Jwillis]

Your power bank is rated for 5V. An average car battery is 12V. An average car wouldn't crank over with 10V. So the amount of current available is irrelevant. You could have a million  Amps and that car still would not crank at 5V. 

[mawyatt]

Remember as a kid when neighbor was studying transistors in trade school. He got all excited about using a transistor when he discovered they had current gain of 100 or more and was going to revolutionize the world by use a couple D cells and transistor to replace the standard car lead acid battery

Best, 

[Benta]

The much newer kind, maybe only in the last 5-10 years, doesn't have a battery. It's a giant capacitor. You charge it slowly from the "dead" battery for a few minutes, and then it releases the energy quickly to start the engine.

Youtube much?

"Never give a sucker an even break."
W.C. Fields.

[Jwillis]

The much newer kind, maybe only in the last 5-10 years, doesn't have a battery. It's a giant capacitor. You charge it slowly from the "dead" battery for a few minutes, and then it releases the energy quickly to start the engine.

Youtube much?

"Never give a sucker an even break."
W.C. Fields.

Far to many people trying to get an education from the University of You Tube. 

[brucehoult]

The much newer kind, maybe only in the last 5-10 years, doesn't have a battery. It's a giant capacitor. You charge it slowly from the "dead" battery for a few minutes, and then it releases the energy quickly to start the engine.

Youtube much?

"Never give a sucker an even break."
W.C. Fields.

Far to many people trying to get an education from the University of You Tube.

What youtube?  These products are seen in reputable stores.

https://www.jaycar.co.nz/capacitor-based-12v-300a-jump-starter/p/MB3765

Possibly they are incorrectly described. I don't know. I haven't bought one and certainly haven't disassembled one. But the descriptions are very clear.

[Jwillis]

The much newer kind, maybe only in the last 5-10 years, doesn't have a battery. It's a giant capacitor. You charge it slowly from the "dead" battery for a few minutes, and then it releases the energy quickly to start the engine.

Youtube much?

"Never give a sucker an even break."
W.C. Fields.

Far to many people trying to get an education from the University of You Tube.

What youtube?  These products are seen in reputable stores.

https://www.jaycar.co.nz/capacitor-based-12v-300a-jump-starter/p/MB3765

Possibly they are incorrectly described. I don't know. I haven't bought one and certainly haven't disassembled one. But the descriptions are very clear.

 

Sure. I've used those myself. But that's comparing apples with oranges. The Opening post is describing a 5V USB power pack not a 12V battery booster for cars.

[ataradov]

There are some with a battery inside, enough to start the car. I don't think they depend on the car battery itself at all

I have a battery-based very generic one, that is branded under 100 different names. And at least in the manual it says that the battery of any sort is needed. No idea if it tests for it.

From the feeling - I don't know how well it will start the car on its own. Even with a semi-dead battery it struggles.

But overall, I would say it is worth it, and it saved me during pandemic when the car was sitting for a long time.

I do worry about that huge battery pack sitting around that is exposed to high current draw. I was not aware of the capacitor ones, I will need to look into that.

[Benta]

What youtube?  These products are seen in reputable stores.

https://www.jaycar.co.nz/capacitor-based-12v-300a-jump-starter/p/MB3765

Possibly they are incorrectly described. I don't know. I haven't bought one and certainly haven't disassembled one. But the descriptions are very clear.

That thingy is a supercap bank that needs to be charged from another fully charged battery. In short: a replacement for jumper cables.
Your "magic revival of a flet battery" is not in there.
"Reputable stores?" More like "very selective customer information" stores.

Youtube much?

[brucehoult]

What youtube?  These products are seen in reputable stores.

https://www.jaycar.co.nz/capacitor-based-12v-300a-jump-starter/p/MB3765

Possibly they are incorrectly described. I don't know. I haven't bought one and certainly haven't disassembled one. But the descriptions are very clear.

That thingy is a supercap bank that needs to be charged from another fully charged battery. In short: a replacement for jumper cables.
Your "magic revival of a flet battery" is not in there.
"Reputable stores?" More like "very selective customer information" stores.

Youtube much?

They 100% definitely say they can start a car with low battery after charging it from the *same* battery.

Not from a 100% dead one, obviously. Just one that can't crank the engine. As a rough guide, I'd expect that if the battery can still run the dashboard lights, radio -- certainly if the headlights work -- then there is absolutely no reason why charging the device at 10A for 60 seconds can't then supply 200A for two or three seconds.

[brucehoult]

Sure. I've used those myself. But that's comparing apples with oranges. The Opening post is describing a 5V USB power pack not a 12V battery booster for cars.

Obviously.

[ejeffrey]

To answer the original question: no you cannot start a car with an unmodified standard 5V USB power bank regardless of size.  But the batteries inside it might be able to if set up properly.  That's essentially what the lithium ion jumpstart batteries are and they can and do supply the hundreds of amps needed to start a car, even with a completely dead battery.

[amyk]

Some of them definitely can, but I personally wouldn't want a lion battery with that much energy in my car.

[brucehoult]

Some of them definitely can, but I personally wouldn't want a lion battery with that much energy in my car.

300A for 3 seconds is 250 mAh, which is 20% or 25% of what a AAA holds. (Obviously you'd need 10 AAAs to get sufficient voltage)

[MrAl]

Hi all, this is my first ever post.

This is purely an academic question.  I have no intention of doing it.  However I have a "friend" who thinks it can work and I would like to know if they could be right.

I have a background in electronics so I do understand some of the answers that may pop up.  eg No.

I want to get technical about it.

Lets say it is a 20Amp powerbank with the sole purpose when built to charge phones and possibly run low current devices.

Consider this unit as the example, 20000mAh Power Bank, ROMOSS Sense 6 Plus USB C Portable Charger with PD 18W QC 3.0


I'm sure there are many learned friends out there that will know the reason it can or can't be done.

I don't want the device to be modified but I guess it could be pulled apart to access the batteries.

Look forward to your feedback and help.

Cheers.

Hi there,

There are a couple different scenarios to look into here.

First, it will not start the car if there is no regular car battery in the car.  The starter requires way too much current for those little power banks.
However it could start a car if the regular car battery is still in the car and the battery is not bad, just depleted somewhat.  If the battery reads below 8 volts it is probably bad, but it might perk up with a charger.  So we will have to assume the battery in the car is not bad.

In theory, if the car can start within 4 seconds and the starter draws 600 amps, then the maximum charge would be 2400 ampere seconds, which is only 2/3 ampere hours or in decimal about 0.67 ampere hours.
The power bank shown there is said to be 20000mAh which is 20 ampere hours, so at first it may sound like there is no doubt that it could start the car if the power bank is connected to the battery somehow.  There is just a little more to it though, and that is the voltage.  The voltage output of the power bank without modifications or an extra QC circuit 'fooler' will be 5 volts, and to charge the battery it will have to be stepped up to around 14 volts, and due to inefficiencies, we could say it is 15 volts for now.  Since we have to step 5 volts up to 15 volts that means the current will step down from whatever the power bank can put out by a factor of 3, which means if it was rated for 4.5 amps out at 5 volts we could get roughly 1.5 amps at around 14 volts.  It may be a little less, but from experience I know you can charge a car battery with 1 amp as long as the battery is not too old and is not sitting at 0 volts.
To get from 5v to 14 volts, we would use a boost converter.  That would take an input of 5v and convert it to 14 volts.  That would then start to charge the car battery.  If the current is 1 amp and we know we need 0.67 ampere hours, that means it has to charge for at least two thirds of an hour, which is 40 minutes.  If the current comes up higher then that's even better, but there are some power banks that only put out 1.5 amps at 5 volts.  That would mean that we would only get about 1/2 ampere out of it, which is very small for a car battery although it may still charge if it is not too old.  It will take at least twice as long though, like 80 minutes, in theory.

So that's about the size of it as theory would predict.  The only other possibility then is to use a QC fooler circuit that allows you to set the output voltage higher, but you may not be able to get the right output voltage for charging without the power bank shutting down.  That means it would depend highly on the power bank being used.  Using a boost converter with adjustable output though means we can adjust the voltage (and thus the current) and that means the power bank should not shut down.  It may be necessary to add a small series resistor though in series with the output of the boost converter (or power bank).

So theory says it can be done, all that would be needed to do is a few experiments.  When I worked with a solar panel and car battery I found that it kept the battery charged very well even with a current of 1 amp for part of the day.  In the case of charging with a power bank, I would opt for one that can allow us to put out at least that 1 amp at 14 volts using a boost circuit.  To get that, it would have to put out 5 volts at 3 amps minimum, which means a charge time of about 40 minutes.  If we do not let it go for the full time and try to start the car say after 20 minutes, the car may actually start, but if the car does not start then we have to start all over again and wait that 40 minutes anyway, and since the first try we stopped at 20 minutes, that would mean we would have spent an hour trying to start the car instead of 40 minutes.

[radiolistener]

Vehicle starter requires at least 200-300 Amps in summer and up to 600-800 Amps in winter.
The power bank that you show is unable to provide such current.

You're needs to search power bank which can provide at least 12*300 = 3600 Watt power (3600000 mW)

[Terry Bites]

Nah, but you can jumpsart your motor with an 18V drill battery.

True.

[Jwillis]

Car batteries are rated with CCA (Cold Cranking Amps) at 0^oC.A small 12V car battery is around 300 - 400CCA. Conversion to AH is CCA/7.25=AH. So a 300 CCA battery would have A little over 41AH at 12V or 492Wh. A Lead acid battery is considered to have 0% capacity at 11.59V. This means that a load will drop the voltage to 0V simply because theirs not enough amps.
To fully charge a 12V battery you need at least 13 -14 volts. But its possible to partially charge at 12V.

That power pack is 5V at 20A or 100Wh. You would have to use a boost converter to get 12V. So 100Wh divided by 12 give you the Ah of you pack at 12V. So your pack at 12V only has 8.3Ah. That's not including the losses of the converter.

8Ah is 20% of the lead acid 41Ah battery capacity. That would raise the capacity of a 300CCA car battery 20% Or to 11.76V. That a gain in voltage of 170mV. This is not taking into account any losses from converter and a Lithium battery voltage drops out fast after 80% discharge.


 

[BeBuLamar]

The car's alternator generates more than enough electricity for the car when the engine is running. The battery is only needed to start the engine and if the car can be stated with the little power bank why do they put such a large battery in the car?

[shapirus]

They 100% definitely say they can start a car with low battery after charging it from the *same* battery.

This is legit. Supercap based jump starters definitely work.

The amount of energy required to start an engine is very -- surprisingly -- low. Even a half-dead car battery contains more than enough of it. But the problem is that this energy must be released in a very short time, and this is what a depleted or degraded lead-acid car battery is not capable of. A sufficiently large supercap bank, OTOH, can do it, and with a proper charging circuit it can be charged from any low-power source, including a powerbank, that would never be able to start an engine on its own.

[RoGeorge]

Can a Powerbank be used to start a car?
...
This is purely an academic question.

Yes it can.

Assuming enough energy is available in that powerbank, and all that energy can be stored then eventually pushed at a high enough rate (at high enough power), then yes, it can do whatever the job is, including starting a car.  There are Li rechargeable power packs already for this purpose alone, to start a car that has a dead/discharged/not-powerful-e
nough-in-cold-weather battery.

[Siwastaja]

Lets say it is a 20Amp powerbank with the sole purpose when built to charge phones and possibly run low current devices.

Consider this unit as the example, 20000mAh Power Bank, ROMOSS Sense 6 Plus USB C Portable Charger with PD 18W QC 3.0

A big discrepancy, first you say 20Amp then 18W, which, at 5V, is 3.6A.

Can you do it? Of course, most stupid ideas are possible. Just wire enough powerbanks in series and parallel. For example, three in series for 15V, 3.6A. Then hundred of these (so 300 total) in parallel for 15V, 360A max. Or oh well, use 600 powerbanks for 15V, 720A for smaller risk of cascading overcurrent detection and shutdown of the units. I'd suggest adding small value balancing resistors, one per each 3 powerbanks in series, such that they drop e.g. 0.5V at 3.6A.

Do a Youtube video of it.

[radiolistener]

PD 18W means that it provides 1.5 Amps at 12V. Since it has USB-A connectors it probably don't support voltage above 12V through PD protocol.

So, in order to get for at least 200 Amps, you're needs to connect 200/1.5 = 134 power banks in parallel (include PD triggers for 12V) in order to use it for a car starter

Also be careful, because if you use PD triggers for 12V, it may be damaged with a car battery which voltage can be up to 13-14V.

If you use USB-C port, you can use 15V PD triggers, but for 18W it can provide just 1.2 Amps at max. So, for 200 Amps you will needs to connect 200/1.2 = 167 power banks in parallel. But be careful, 15V may damage some electronics on your car. Most of all it will be ok, because it was designed for 14.4V and even a little more, but who knows...

[rteodor]

Had one of these starters and it was useful once when due to long standing in cold the car battery was at 4V.
It was also useful to somebody I've borrowed it. For a few weeks he used it almost daily until he replaced his car battery, but he never returned the starter.

I assume that the battery is specially built for the purpose. It has small capacity 55Wh but it is a lot bigger in size than a "domestic" 20Ah/70Wh power bank and a bit lighter. Also it has very thick wires and a 70A relay for protection. That relay is only active for a short period of time after boost button is pressed.

[brucehoult]

Lets say it is a 20Amp powerbank with the sole purpose when built to charge phones and possibly run low current devices.

Consider this unit as the example, 20000mAh Power Bank, ROMOSS Sense 6 Plus USB C Portable Charger with PD 18W QC 3.0

A big discrepancy, first you say 20Amp then 18W, which, at 5V, is 3.6A.

Can you do it? Of course, most stupid ideas are possible. Just wire enough powerbanks in series and parallel. For example, three in series for 15V, 3.6A. Then hundred of these (so 300 total) in parallel for 15V, 360A max. Or oh well, use 600 powerbanks for 15V, 720A for smaller risk of cascading overcurrent detection and shutdown of the units. I'd suggest adding small value balancing resistors, one per each 3 powerbanks in series, such that they drop e.g. 0.5V at 3.6A.

There is no need to use 300 or 600 of the described power banks. There is plenty of energy in just one of them, it merely has to be put into a suitable form i.e. 12V and a few hundred amps for a few seconds.

That is precisely the job the NZ$55 (US$35) device I gave a URL for is designed to do. One of its options is to charge it for 15 min from a USB power source, such as the given power bank. The other option is to charge it from the car battery itself, if it is at low voltage but not completely dead.

https://www.jaycar.co.nz/capacitor-based-12v-300a-jump-starter/p/MB3765

You just need the one power bank and one supercapacitor car starter device.

[sparkydog]

If you use USB-C port, you can use 15V PD triggers, but for 18W it can provide just 1.2 Amps at max. So, for 200 Amps you will needs to connect 200/1.2 = 167 power banks in parallel. But be careful, 15V may damage some electronics on your car. Most of all it will be ok, because it was designed for 14.4V and even a little more, but who knows...

According to Wikipedia: "depending on operating conditions, the vehicle electrical system voltage today can vary between 6.5 and 16 V, with a varying degree of ripple superimposed on this value." From people designing electronics for automotive use, I've heard that such need to be designed to withstand even higher voltage spikes (at least 30V). I would therefore suspect 15V isn't going to be an issue (I'm not warranting that, though!). Alternatively, you could do the sensible thing and disconnect the battery from the car while charging it.

[MrAl]

Hello again,

I see two different interpretations of the original question in this thread, and each one has been answered in a different way because they are two different questions.

[1] The first one is can a car be started SOLELY with a little power bank without there being the original car battery still in the car.
[2] The second is can a car be JUMP started with a little power bank with the original car battery still in the car.

The answer to #1 is NO because the power bank cannot put out enough energy fast enough.
The answer to #2 is YES but it takes time to charge the original car battery with the jump pack first, and it could take as long as an hour, but only if the battery is still good enough to take a charge from a lower current source.

For #2 there are little "power banks" that can work faster, but they are made for jump starting not for just charging phones.  They are hit and miss though because the car has to be able to start fast enough before the jumper pack disconnects from the battery automatically.  These type can put out 16 volts so they can charge the car battery much faster than a regular power bank made for charging just a phone.  Some of them have several Li-ion batteries (or similar) inside so they hold a lot of energy and can deliver it faster than a phone charger.

[shapirus]

[2] The second is can a car be JUMP started with a little power bank with the original car battery still in the car.

The answer to #2 is YES but it takes time to charge the original car battery with the jump pack first, and it could take as long as an

It depends on the definition of "little".

Assuming a fully depleted car battery of, roughly, say, 800 Wh, and not even such a small powerbank of, say, 60 Wh, it becomes clear that such a powerbank does not have a sufficient amount of energy to charge the car battery to the level at which it can output the current required to start the engine sustained during a few seconds.

[Siwastaja]

withstand even higher voltage spikes (at least 30V). I would therefore suspect 15V isn't going to be an issue

Of course it's not going to cause any issue because it's the normal continuous voltage on the car's 12V system during engine run condition. 12V is just a nominal rating. Alternators adjust their output voltages based on temperature, as fully charging the lead acid battery in cold conditions (e.g. -20degC) requires voltage of 15...16V (e.g. the magical 14.4V number won't cut it) in order to not sulfate and self-destruct in matter of months, which is exactly why it's normal to have 15V on the system, and quite obviously everything connected to the car needs to deal with it. Additionally, devices choose to cope with spikes of various durations and voltages based on how robust they need to be, but 15-16V would be the very minimum baseline, as it's both normal and continuous.

[radiolistener]

Alternators adjust their output voltages based on temperature, as fully charging the lead acid battery in cold conditions (e.g. -20degC) requires voltage of 15...16V (e.g. the magical 14.4V number won't cut it) in order to not sulfate and self-destruct in matter of months, which is exactly why it's normal to have 15V on the system, and quite obviously everything connected to the car needs to deal with it.

It depends on the car. On Toyota Corolla I didn't seen alternator voltage above 14.4V. But on another car type I was seen 15V and more... I'm not sure if these 15V is due to broken/failed alternator voltage regulator, or just by design.

Usually alternator has voltage regulator which can control alternator output voltage. It control it by change current through special alternator coils. Sometimes this regulator is failed and alternator can push high voltage on the battery. In such case it leads to a faster death of the battery due to overcharge. The bad thing is that all works ok, and you can even don't suspect that alternator regulator is failed, you can replace battery, it it also will be dead soon. This is why it is important to check alternator voltage in case if your battery is dead too quickly.

But there is also modern Ca/Ca batteries, which may require higher charge voltage than 14.4V, so it's not easy question.

Yes, 15V looks ok, for all vehicle electronics, because it should survive even higher voltage pulses. But who knows, may be some cars don't like when voltage is 15V and more...

[Siwastaja]

It depends on the car. On Toyota Corolla I didn't seen alternator voltage above 14.4V.

You did not measure when really cold, or the alternator regulator circuit was broken (or car improperly designed, but I don't think so for Corolla). 14.4V is about right for summer weather. With cold winters, and I don't even mean Alaska/Siberia tier cold, if you always only charge to 14.4V, the battery never really gets fully charged and in a few months it starts to sulfate and eventually fails to cold start. On the other hand, if you pick say 15.5V which is about right for extreme cold, then you'd overcharge during summer. Lead acid charging really needs temperature compensation (even rudimentary, for example the alternator circuitry of course only measures alternator temperature, there's no sensor at the battery), no way around it (except in lab conditions).

[radiolistener]

Yes, I measured it in summer. I'm not sure if it makes difference depends on temperature, needs to check it.

[shapirus]

Yes, I measured it in summer. I'm not sure if it makes difference depends on temperature, needs to check it.

It does. I once saw 14.85 V on the battery terminals (KIA Sportage) shortly after the engine was started, and that was in the winter, and it was quite cold. In the summer I never saw anything higher than 14.4 V.

However that winter measurement was made with a dubious DMM. Yes it could potentially be THAT off :). I will need to verify it some time.

[MrAl]

[2] The second is can a car be JUMP started with a little power bank with the original car battery still in the car.

The answer to #2 is YES but it takes time to charge the original car battery with the jump pack first, and it could take as long as an

It depends on the definition of "little".

Assuming a fully depleted car battery of, roughly, say, 800 Wh, and not even such a small powerbank of, say, 60 Wh, it becomes clear that such a powerbank does not have a sufficient amount of energy to charge the car battery to the level at which it can output the current required to start the engine sustained during a few seconds.

I had gone all through that in a previous post.

[MrAl]

Alternators adjust their output voltages based on temperature, as fully charging the lead acid battery in cold conditions (e.g. -20degC) requires voltage of 15...16V (e.g. the magical 14.4V number won't cut it) in order to not sulfate and self-destruct in matter of months, which is exactly why it's normal to have 15V on the system, and quite obviously everything connected to the car needs to deal with it.

It depends on the car. On Toyota Corolla I didn't seen alternator voltage above 14.4V. But on another car type I was seen 15V and more... I'm not sure if these 15V is due to broken/failed alternator voltage regulator, or just by design.

Usually alternator has voltage regulator which can control alternator output voltage. It control it by change current through special alternator coils. Sometimes this regulator is failed and alternator can push high voltage on the battery. In such case it leads to a faster death of the battery due to overcharge. The bad thing is that all works ok, and you can even don't suspect that alternator regulator is failed, you can replace battery, it it also will be dead soon. This is why it is important to check alternator voltage in case if your battery is dead too quickly.

But there is also modern Ca/Ca batteries, which may require higher charge voltage than 14.4V, so it's not easy question.

Yes, 15V looks ok, for all vehicle electronics, because it should survive even higher voltage pulses. But who knows, may be some cars don't like when voltage is 15V and more...

Hi,

It is very interesting that you brought this up because I had talked about this on a forum dedicated to automobiles.  The battery voltages on different cars can be VERY different, which does not make sense at all.

On a Hyundai I once had, the voltage was a max of 14 volts, which never charged the battery enough.  I had to bring the battery into the house to charge on a battery charger about once every three months or so.
Also on a Volvo, max 14 volts.
On a Ford Mustang, the max voltage was about 14.2 if I remember right.
On a Buick, the voltage can reach up to 15 volts max, then slowly come down as the battery charges.

From my experience, the lower voltages can only work if the car is driven every day, and it may be questionable at that.
The 15 volt max works the best, because it charges the battery better and also brings the voltage down once the battery gets charged to some degree.  That's the best.

Now one more car, an older Toyota, with max voltage around 13.65 volts.  That's nuts, but I will test that more in the future because the guy just got a new battery so maybe it charges better with a new battery.  If it does not ever go higher than that, then I do not expect the battery to last long unless he charges it manually from time to time.  I don't have much data on that one yet though so I will be measuring the voltage from time to time also to see if the 'resting' voltage drops more and more.  If it keeps dropping, that's a sign that it is not charging enough.

I have not figured out why these voltages should be that different, and nobody seems to really know there are just guesses out there.
There is one thing that sounds like a 'fact' though, and that is the type of battery.  There are the older types and the newer types.  The older types are used in older cars and I think they charge better with lower voltages like 14 volts, while the newer ones are for newer cars that can charge better.  That seems to be a fact because the different battery types are recommended for the different cars, and the newer type chemistries are not recommended for older cars.  This I found out by looking up batteries for different cars.  What I do not know is how much of a difference this makes, as the best info I can find says that there is about a 0.1 volt difference between the two, but I don't know how accurate that figure is.

If someone knows more about this variation in automobile max charging voltages now's the time to speak up.