[dashoin]

Whew, what a thread! Literally saw my entire automobile ignition systems presentation here!

My 2 pice:

1. Initial charging systems used dynamos - DC generators. Starter motor was usually a DC series motor. (Why series is a different story)

2. Ammeters and voltmeters - I can only speculate that they were provided to see if the battery was being charged or not. Plausible reason ? Thats right - DC generators - remember charging current was being taken through the commutator and carbon brushes. These tend to deteriorate pretty soon under load conditions. To see a commutator and carbon brush arrangement, see your mixie motors and before you guys ask me, it is a universal series motor operating on AC. The commutator and brush arrangement would be a lot more complicated than this.

3. Voltage or battery capacity for starting? Well, remember dynamos were replaced by alternators we still get to see the DC series motor quite often. A characteristic of DC series motors is that they do not know speed control - bit like my scooter which does not go below 40. This makes them suitable for high torque applications like - you guessed it - starting motors. The speed of a series motor is dependent on the voltage supplied to it. Now once the voltage is applied to it, the motors tries to turn but does not as it has to overcome the inertia of the engine. It will then draw more power (read current) from the battery to try to rotate. Hope the point is clear now.

4. Charging is of 2 types - float and boost. Float or trickle charging is given just to equalise the battery voltage ie maintain the battery voltage same as the DC bus (14 v or 12 v, you guys can decide). Float charging capacity is approx 2% of the battery capacity. Boost or deep discharge charging is done when the battery is completely drained out. Charging capacity is approx 20 % of battery capacity. n case you have a drained battery, better remove it and get it charged. Yes, I know - plenty would have successfully driven their vehicles after oush starting and driving around on a discharged battery - but please spare a thought for the poor alternator - it cannot supply beyind a particular load.

5. Alternators - yes we have AVR`s which regulate the voltage and the output increase with the speed. Remember, it is not infinite - the output would increase only till the rated capacity is reached. If the other case would be true, then voila - we have solved india electricity shortage problem. My Bajaj would be the first power plant.


Whew.

A note about the ammeters and voltmeters. The stuff about shunts etc is correct but technology has progressed a lot after that. Ammeters are now connected to CT`s (Current transformers) for smaller ranges also. Running the power cables around is not a good idea. For the shunt, remember voltage drop in the wires would give you a wrong reading.

OK, now the suggestion. The system is very reliable nowadays. If you would like to check the system periodically, buy a digital multi-meter with a tong tester. You can check both the voltage and the current.

[vina]

Originally Posted by Sutripta

...
So what happens if one connects an alternator producing power to a battery which is completely discharged? What currents and voltages will we see at various points of the circuit? And why? (That after all is the focus of this thread. This is the scenario we will get if we have an almost dead battery, push start the vehicle, and then maintain something like ~2K rpm hoping to charge the battery.)

Let me guess - assuming the battery has a max charging current limit, either the alternator can provide that much current (after satisfying other demands like lighting) or not. If it can, then some sort of ammeter will have to monitor the battery current and automatically reduce the alternator voltage if the charging current starts to become unsafe. If alternator can provide less current than the max. charging current for the battery - life is simpler, the load on the alternator will make the voltage to drop on its own.

Originally Posted by dashoin

...

4. Charging is of 2 types - float and boost. Float or trickle charging is given just to equalise the battery voltage ie maintain the battery voltage same as the DC bus (14 v or 12 v, you guys can decide). Float charging capacity is approx 2% of the battery capacity. Boost or deep discharge charging is done when the battery is completely drained out. Charging capacity is approx 20 % of battery capacity. n case you have a drained battery, better remove it and get it charged. Yes, I know - plenty would have successfully driven their vehicles after oush starting and driving around on a discharged battery - but please spare a thought for the poor alternator - it cannot supply beyind a particular load.

Can you elaborate more on the boost charging part? Also what if the battery is partially drained out?

[Aroy]

Originally Posted by dashoin

Whew, what a thread! Literally saw my entire automobile ignition systems presentation here!

4. Charging is of 2 types - float and boost. Float or trickle charging is given just to equalise the battery voltage ie maintain the battery voltage same as the DC bus (14 v or 12 v, you guys can decide). Float charging capacity is approx 2% of the battery capacity. Boost or deep discharge charging is done when the battery is completely drained out. Charging capacity is approx 20 % of battery capacity. n case you have a drained battery, better remove it and get it charged. Yes, I know - plenty would have successfully driven their vehicles after oush starting and driving around on a discharged battery - but please spare a thought for the poor alternator - it cannot supply beyind a particular load.

A note about the ammeters and voltmeters. The stuff about shunts etc is correct but technology has progressed a lot after that. Ammeters are now connected to CT`s (Current transformers) for smaller ranges also. Running the power cables around is not a good idea. For the shunt, remember voltage drop in the wires would give you a wrong reading.

OK, now the suggestion. The system is very reliable nowadays. If you would like to check the system periodically, buy a digital multi-meter with a tong tester. You can check both the voltage and the current.

4. Take an alternator of 60A, and assume that it can use 40A for battery charging, it will take only 2 hours to charge an 80AH battery. If the speed is low and the alternator can only "part" with 20A, it will take 4 hours. All that you are loosing is the fuel while charging the battery. The alternator is designed to deliver its power day in and day out, so no harm will come to it.

Unfortunately the CT (Current transformer) works only on AC not DC, similar is the case with a clip on meter.

Testing DC requires either direct connection, or a Hall effect transducer (extremely expensive)

Using shunt for current measurement you normally measure the voltage drop across the shunt and not the current flowing through the shunt, where ever the wires are long. A voltmeter depending on its FSD (full scale deflection) sensitivity will consume any where between 100 microamps to 5 milliamps, not much when detecting 5-30A. (before you protest, technically most of the meters are current meters, so a volt meter will be measuring current in the circuit which has a high value resistor in the path. You can easily calculate the resistor value - for 1ma FSD to measure 15V you need a value of 15/0.001= 15K, for a 0.1ma FSD the value is 150k). I have a 0.05mA FSD meter in my tool box, but it is delicate piece and would spoil in an automotive environment - too much vibration.

[dashoin]

Well, now we have something interesting going on.

A battery stores its charge as chemical energy. For more details, please look up lead-acid batteries. Boost charging basically applies higher voltage across the battery terminals to allow the battery to regain it original ionic state.

CT are used in AC circuits. However, clip on meters - digital type - can measure both AC and DC currents. Last time I checked, the price range was approx INR 2K. The prices have fallen down like anything.

Well, I agree with you. I was talking about placing the shunt in the engine bay close to the battery to avoid running the DC wires to the dash. In that case, you do get a voltage drop, right? We can add compensation circuits to take of the error but it would be too much of a hassle.

Alternators are manily sized to cater to the complete electrical system load (standard) and float charging. The major component is the vehicle electrical system load. For eg. 2 headlights of 55 W each would consume 110 / 12 = 9 A straightaway. Apart from the lights, you have the radiator fans and other loads also. Last time I checked, the fan itelf was drawing about 15 Amps. So for a 60 A alternator, we have already consumed 25 Amps. Now the tell tale lights, ignition system consumption etc would bring it up to 40 - 45 Amps. And if you have a powerful music system, well no margin left.
You could increase the charging rate by putting out all lights, music systems etc and get additional capacity. However, as I mentioned in my earlier post, have a heart and get the battery charged outside. Your poor alternator would thank you for it and you would have peace of mind with a fully charged battery

BTW, the alternators also come with carbon brushes, but carry a minimum amount of current.

[Sutripta]

Hi dashoin,
Nice to see a new face (well, a new handle) in this thread. However, confused about a few things you have said.

Originally Posted by dashoin

The commutator and brush arrangement would be a lot more complicated than this.
What is more complicated than what?

3. Voltage or battery capacity for starting?
CCA at a specific terminal voltage.
Well, remember dynamos were replaced by alternators we still get to see the DC series motor quite often. A characteristic of DC series motors is that they do not know speed control - bit like my scooter which does not go below 40. This makes them suitable for high torque applications like - you guessed it - starting motors. The speed of a series motor is dependent on the voltage supplied to it. Now once the voltage is applied to it, the motors tries to turn but does not as it has to overcome the inertia of the engine. It will then draw more power (read current) from the battery to try to rotate. Hope the point is clear now.
Agreed series wound motors have best stall torque.
Other than that totally confused as to what you are trying to say.

4. Charging is of 2 types - float and boost. Float or trickle charging is given just to equalise the battery voltage
There are various charging regimes, certainly more than float and boost. Float charging and trickle charging are different. And equalising charge normally means something else.

n case you have a drained battery, better remove it and get it charged. Yes, I know - plenty would have successfully driven their vehicles after oush starting and driving around on a discharged battery - but please spare a thought for the poor alternator - it cannot supply beyind a particular load.
Agreed for lots of reasons. But pity for the alternator is not one of them!

5. Alternators - yes we have AVR`s which regulate the voltage and the output increase with the speed. Remember, it is not infinite - the output would increase only till the rated capacity is reached. If the other case would be true, then voila - we have solved india electricity shortage problem. My Bajaj would be the first power plant.
Can't get what you are trying to say.

Whew.

OK, now the suggestion. The system is very reliable nowadays. If you would like to check the system periodically, buy a digital multi-meter with a tong tester. You can check both the voltage and the current.
My personal experience with affordable DC clamp on meters has been very disappointing. And sometimes one has to observe the trend. Difficult to drive around with such a setup.

Originally Posted by vina

Let me guess

Guess???? Work it out!

Originally Posted by dashoin

Boost charging basically applies higher voltage across the battery terminals to allow the battery to regain it original ionic state.

During boost charge, what should be monitored and controlled?

Regards
Sutripta

[vina]

Originally Posted by Aroy

Testing DC requires either direct connection, or a Hall effect transducer (extremely expensive)

Even a Hall Effect sendor is a direct series connection. Also it isn't as expensive as people make it out to be - high current high voltage tolerant sensors with milliseconds of reaction time are available for less than $5 when bought in volume (a few hundred). When bought in several thousands the price will come down to less than $2

Originally Posted by Aroy

Using shunt for current measurement you normally measure the voltage drop across the shunt and not the current flowing through the shunt, where ever the wires are long. A voltmeter depending on its FSD (full scale deflection) sensitivity will consume any where between 100 microamps to 5 milliamps, not much when detecting 5-30A. (before you protest, technically most of the meters are current meters, so a volt meter will be measuring current in the circuit which has a high value resistor in the path. You can easily calculate the resistor value - for 1ma FSD to measure 15V you need a value of 15/0.001= 15K, for a 0.1ma FSD the value is 150k). I have a 0.05mA FSD meter in my tool box, but it is delicate piece and would spoil in an automotive environment - too much vibration.

All coil type (or thermal type) voltmeters or ammeters are actually current meters. Measurement is done by measuring the magentic field strength generated (and hence attaraction/force/torque and hence deflection against a spring) by a coil.

However practically every one of the digital voltmeters (and hence all ammeters - the ammeter measures voltage across a known shunt resistance and then does V/R) is an ADC - it measures the voltage directly by comparing it against a reference voltage. These can be extremely rugged and pretty cheap (again if bought in volume) - a chip with complete signal conditioning etc. costs less than a dollar. Complete boards cost less than $3 in volume.

Hall effect sensors are also effectively voltmeters - they meaure voltage generated by deflecting current by a known magentic field (Hall Effect)

Originally Posted by Sutripta

...


Guess???? Work it out!

dada, to work it out I need data. You already have the data and know the answer - so I think I'll limit myself to guessing till you give the answer

[DerAlte]

Originally Posted by dashoin

... However, clip on meters - digital type - can measure both AC and DC currents. ...

Clip on meters are basically a CT and meter in the same housing. If CTs work only on AC, how do they measure DC current?

Originally Posted by dashoin

... Alternators are manily sized to cater to the complete electrical system load (standard) and float charging. ...

By "complete electrical system load" did you mean Alternator rating = (Headlamp + winders + EL1 + EL2 + ... + ELn + margin)? [EL1..ELn -> Electrical Load 1..n, i.e. *all* the electrical loads in the car]

Not so. Alternator rating is always quite less than the sum. It is based on a 'duty cycle' (even the battery size is). Duty cycle here refers to which loads will (statistically) draw current simultaneously - not all of them do. This depends on a few factors:
* Day or night driving, and the ratio of day driving to night driving expected. Night here refers to non-availability of sunlight - even if 4PM for you is afternoon in India
* The ratio above comes from which region in the world the car will be driven in
* The average daily temperature in the region designed for
* Driving habits - how much a/c usage
* General city / highway driving ratio of that region

The trick is in creating a realistic duty cycle. And, of course, relying on effective cooling controlling the temp rise in the coils, while all the time keeping the coils as hot as practical without causing long term insulation deterioration.

Why go through all that rigmarole? Cost. You save $2 over half a million cars, that's a cool million $. That is the difference between a car manufacturer buying your alternator or not, since the design and sizing of the alternator is the responsibility of the alternator manufacturer, not the car manufacturer. Ditto for battery.

[SS-Traveller]

Originally Posted by DerAlte

Alternator rating is always quite less than the sum. It is based on a 'duty cycle' (even the battery size is). Duty cycle here refers to which loads will (statistically) draw current simultaneously - not all of them do.

Apart from the electrical load imposed by the starter (which obviously cannot use any alternator output to power itself ), the alternator rating is ALWAYS designed to be a few amps more than the maximum electrical load installed as OE in a car. The OE battery rating & specifications would be based on the needs of the starter motor only.

Please indicate ANY car where the OE electrical load is greater than OE alternator rating.

[Aroy]

Actually the Alternator has to cater to full OE load in the worst case scenario.
. Head lights, tail lights, hazard lights
. Wiper motor
. AC clutch
. Blower Fan
. Ignition circuit
. Horn
. OE music system
. Lastly battery charging current

I do not think any one of us has ended up with a flat battery, driving at night in rain with AC on, and of course listening to our favourite music on the ICE. It has happened to me in an old car with suspect "Dynamo" and a battery near the end of its life. But never in a car which is two years old. So the alternator is sized to take the full OE load at a moderate speed.

[DerAlte]

Originally Posted by SS-Traveller

... the alternator rating is ALWAYS designed to be a few amps more than the maximum electrical load ...
Please indicate ANY car where the OE electrical load is greater than OE alternator rating.

Originally Posted by Aroy

Actually the Alternator has to cater to full OE load in the worst case scenario. ...

Please indicate ANY car for which we HAVE REAL DATA that leads to you infer that? Are you sure data is so simple? Please don't go by "wipers motors are rated so many W, window winders are that much W, ICE is rated 40W, so on and so forth"

LOL Do you seriously think equipment manufacturers make money with simplistic empirical logic like that when doing design? That's what was called 'Russian' overdesign: add everything up, multiply by a constant (2-5x) to take care of overages, and lo and behold - one has a device that will never fail but weigh a significant %age of the equipment it goes into. I am sure @aroy should recognize what I am referring to.

That way you would find a limited invariant range of models available in the catalog and no R&D. One could design for 3 vehicle segments - small, medium and large - and be done with it. That's because within a range, the electrical loads are within +/- 5%.

It is possible to make alternators to work at 80degC average internal temperature or higher, not the ambient temperature we reckon with and think they should be kept around that. Design assumes peaks and troughs of current demand, including worst case, and the corresponding temperature profile. And handle much (over)load than you can imagine with a sizing you would say 'impossible'.

Of course if one puts in a SPL ICE which belts out 140db+ and pulls 4KW, one has to put a different alternator and batteries.

[dashoin]

Sutripta - I am a regular visitor - but a poor contributor.

To clarify my points,
1. I was just comparing the commutator of the mixie motor to the dynamo`s. With the amount of interest shown, I thought someone would like to get a look of the commutator!

2. The best method to control the speed of a DC series motor is by varying the voltage across the terminals. Normally, when a battery is discharged ( say 80%), it will indicate the proper voltage accross the terminals. The moment you load it, the voltage would drop. So if the battery is nearing a complete discharge the motor would get 12 V for a fraction of a second. The discharged battery would not be able to maintain the voltage and then motor would stall. I was just trying to corelate the symptoms with the actual reason.

3. Dada - all charging regimes ultimately boil down to these two. Trickle and equalising charging are same as float charging. ( OT - Trust me, I design electrical AC and DC power systems where requirements are more stringent than the electricals of a normal car)

4. Regarding charging of the battery from the alternator - apart from the fuel consumed, the alternator would have to run at full load. This is NOT desirable for any kind of equipment. It is akin to redlining your engine continuously. While electrical equipment can certainly be run at full load longer than mechanical equipment ( read IC engines), it would still face premature failure. The biggest culprit would be heat generated within the windings. Other things like voltage regulators etc are generally more robust.

5. Alternator and my scooter - this was just to remind everyone that the alternator has a limited capacity and can only supply electric power upto a certain point. (OT - I do wish it were the other way round and i could generate power from my scooter - lots of power cuts in Gurgaon).

6. Regarding the tong tester - you seriously intend to drive around with that thing? The charging systems are more reliable nowadays and do not need to be checked so frequently. Checking the battery health is more important. The biggest indicator of a battery not getting adequately charged is the change in the specific gravity of the battery electrolyte. For a fully charged battery, it would be in the range of 1210 ~ 1230. Anything below 1180 means that the battery is not getting charged. This can be checked vary easily by any battery shop - just a 5 minute job.


DerAlte :

The sizing of the alternator depends on many things including derating at higher temperatures. I do agree that not many manufacturers give over-rated equipment. It all depends from manufacturer to manufacturer. case in point - I was able to run a weak battery in an Ikon and charge it during a drive of about 250 kms but the same did not happen with the Esteem. OT, the loads which are classified as intermittent - wipers, turn indicators and brake lights - may not be factored in the sizing of the alternator. In case of their operation, it may be possible that battery charging may be temporarily suspended. During trials it may be possible to tweak the designs but we still need to define the intermittent loads in this case or the duty cycle as you pointed out. Additionally, the car manufacturer has to specify the output required from the alternator to the supplier along with the duty cycle. With a higher duty cycle, it is possible that the alternator may be suzed even higher than the continuous rating one. Reason - temeprature. The ultimate responsibility of a good design with correctly sized components rests with the car manufactrer.

OT, you save $2 per car and you would be a hero! From what I know, even 10 cents makes a huge profit for a company over the lifecycle of the car!

[vina]

Originally Posted by dashoin

...

2. The best method to control the speed of a DC series motor is by varying the voltage across the terminals. Normally, when a battery is discharged ( say 80%), it will indicate the proper voltage accross the terminals. The moment you load it, the voltage would drop. So if the battery is nearing a complete discharge the motor would get 12 V for a fraction of a second. The discharged battery would not be able to maintain the voltage and then motor would stall. I was just trying to corelate the symptoms with the actual reason.

Call it my gnorance, but I didn't know a different way of controlling the speed of a DC motor (under a given load).

The point dada as well as others made already was that starters usually are designed to work from voltages much lower than 12V (because they draw such huge currents; I guess someone mentioned 6V earlier)- at least that's what I understood from the discussions.

Originally Posted by dashoin

4. Regarding charging of the battery from the alternator - apart from the fuel consumed, the alternator would have to run at full load. This is NOT desirable for any kind of equipment. It is akin to redlining your engine continuously. While electrical equipment can certainly be run at full load longer than mechanical equipment ( read IC engines), it would still face premature failure. The biggest culprit would be heat generated within the windings. Other things like voltage regulators etc are generally more robust.

How much heat do you think will be generated? Motors and Alternators give energy conversion efficiencies in excess of 95% at full load - in fact better efficiencies are obtained at full rated load. An Alternator designed for 1kW (Alternator specs being discused here have less than 70A rated o/p at less than 15V - that's about a kW) will thus generate less than 50W in heat - this is nothing, the alternator may get a little warm (a few degrees above ambient) but probably not even enough to burn your hand.

Going by your logic household ceiling fans wouldn't last much if run at full load, they are not even built to automotive specs. And yet they do - visit the nearby courthouse to see a demo.

In fact from both theory and experience I would worry more about the solid state regulator.

Originally Posted by dashoin

...

OT, you save $2 per car and you would be a hero! From what I know, even 10 cents makes a huge profit for a company over the lifecycle of the car!

While all manufacturers will try to save as much money as possible, let's put things in perspective. $2 is not even the cost of the soft drinks the dealer offers a family looking for a test drive. $0.10 is less than the cost of dusting the car once.

While these numbers are large for cheaper devices with razor thin margins, for things like cars if I were the production manager, I would save this kind of money by forcing the supplier to supply the same part cheaper - rather than selecting a cheaper part in the first place at the cost of specs.

[Aroy]

Most alternators are designed to run at 100% duty cycle over extended period (otherwise we would have to stop the power generators, every day or so!). Electrically there is nothing that will go wrong. What deteriorates with speed and time are the mechanical parts - bearings, slip ring & brushes etc. Automotive alternators have an inbuilt fan for cooling, plus the fact that at full power the vehicle will be moving quite fast, so it will rarely over heat to the extent of incapacitating it.

5% loss at 1kW may not be much, but when you go to mega watts, it is a major heat source, which has to be evacuated. All sorts of cooling strategies are used in power alternators, but the thing to note is that there is cooling.

[DerAlte]

Originally Posted by dashoin

... OT, the loads which are classified as intermittent - wipers, turn indicators and brake lights - may not be factored in the sizing of the alternator. In case of their operation, it may be . ...

They all are reckoned with - each component's actual current drawn profile. No assumptions. Nothing is too small to ignore.

And, "possible that battery charging may be temporarily suspended"? How???

Originally Posted by dashoin

... the car manufacturer has to specify the output required from the alternator to the supplier ... Reason - temeprature. The ultimate responsibility of a good design with correctly sized components rests with the car manufactrer. ...

LOL No. The responsibility rests with the parts vendors, not the car manufacturer - who is ill-equipped to handle that. Car manufacturers do not specify output required and duty cycle - vendors make proposals explaining everything, graphs and all. You have to remember that there are multiple vendors vying with each other to get the order of each component / sub-system.

Temperature? You are looking at it from the wrong end, baba. It is by manipulating the temperature profile and matching it with material properties (of the materials that go in) that one gets a design that is cheaper yet reliable, pound for pound for the same application. This type of design is 'closed-loop', FEM based design - design>test>modify_design, not a once through simple design. Only after that does the vendor make a proposal.

Of course, having done that many times, the vendors do have a library where the simplest dimensions are region, class of car and duty-cycle. Don't assume that Peugeot 105 cannot be a rugged rural car and a Jag is a hardy beast.

Originally Posted by vina

... While all manufacturers will try to save as much money as possible, let's put things in perspective. $2 is not even the cost of the soft drinks the dealer offers a family looking for a test drive. $0.10 is less than the cost of dusting the car once.

While these numbers are large for cheaper devices with razor thin margins, for things like cars if I were the production manager, I would save this kind of money by forcing the supplier to supply the same part cheaper - rather than selecting a cheaper part in the first place at the cost of specs.

Arrey Vina bhaiya, please don't expect to land a job in the automotive sector with that logic. The Production Manager looks at scheduling only, not at the cost of components and sub-assemblies.

And no one forces anyone to reduce cost - it is a common shared objective between car manufacturers and parts vendors. That's the difference between profit and loss. That's an eco-system, where everyone takes care of their own responsibility, and gracefully withdraws from the competition if it doesn't work out for them.

Originally Posted by Aroy

Most alternators are designed to run at 100% duty cycle over extended period (otherwise we would have to stop the power generators, every day or so!) ... All sorts of cooling strategies are used in power alternators, but the thing to note is that there is cooling.

Sir, ALL alternators run with a 100% duty cycle, being mechanically coupled.

And you are right about the cooling part. Just that this is not a post-design consideration, it is part of the design cycle.

[Sutripta]

Lots of interesting discussions. Though (IMO) off topic. Maybe we can discuss those in other threads.

Regarding alternator sizing: Some questions
If one had to write one or two sentences explaining the requirements of an alternator for any particular vehicle, what would it be?
I would think it is more a matter of philosophy, more than anything else. So hard data, esp. across segments (eg. econoboxes vs luxury sedans) would be interesting and illuminating.
With more and more safety related items being electrically powered (I mean electricity doing work, not signal processing/ computing) like eps, and a litigation happy society, is there a change in this philosophy? Esp. given the fact that the modern car gives no indication to the driver.

OT. - Wonder why electrics of British bikes, and Mr. JL had the reputation they did?

Originally Posted by dashoin

3. Dada - all charging regimes ultimately boil down to these two. Trickle and equalising charging are same as float charging. ( OT - Trust me, I design electrical AC and DC power systems where requirements are more stringent than the electricals of a normal car)

Sorry. Disagree. Strongly.

6. Regarding the tong tester - you seriously intend to drive around with that thing? The charging systems are more reliable nowadays and do not need to be checked so frequently. Checking the battery health is more important. The biggest indicator of a battery not getting adequately charged is the change in the specific gravity of the battery electrolyte. For a fully charged battery, it would be in the range of 1210 ~ 1230. Anything below 1180 means that the battery is not getting charged. This can be checked vary easily by any battery shop - just a 5 minute job.

Sp. Gr. is an indicator for SoC. If it is not upto the mark, then what do you do? And as you rightly pointed out, you can't go about driving with a tong tester. (How are you going to see its reading)! So what do you do?

Regards
Sutripta