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Drew, the battery actually supplies 100% of the power for everything and the alternator (not generator) charges the battery as necessary. Without a battery (not installed or dead), the generator will not provide output to run everything.
Again, if one wants to prolong, there are many RV forums that this is ongoing forever.
I think you misunderstood me, maybe it's a reading comprehension issue. When the engine is running the alternator is the source for all devices that require power including the battery charging system. I have substantial experience with modern alternator's operation. When setting up standalone ECU's and modern alternators all of the desired functionality has to be set up/programmed. My point was that increasing the voltage increases the efficiency of the system and that is why modern systems are running up to 16V. As the regulated voltage is increased the amperage can be decreased to net the same amount of required wattage.
Originally Posted by C8J
Andy too long, ...
Drew, the battery actually supplies 100% of the power for everything and the alternator (not generator) charges the battery as necessary. Without a battery (not installed or dead), the generator will not provide output to run everything.
Again, if one wants to prolong, there are many RV forums that this is ongoing forever.
Yes, it could be an a "warning to replace [the battery] soon" or it could indicate a malfunction in the charging system. To make a short story long:
Given the title of the thread, Charging Algorithm, and the subsequent statement that GM is holding the charging algorithm secret, I thought the question was what is the charging algorithm. Hence the response(s) regarding the description of the battery monitor process, the charging modes, the conditions leading to the mode selected, and the generator output voltage (which will be the voltage seen on the DIC) range. There is considerably more detail in the Service Manual than presented in the posts above. Granted, there is lot unexplained. The exact algorithm by which the battery state of charge is not explained in detail - it just says calibrates itself by measuring voltage when the car is off, then measures current flow into and out of the battery to keep a running estimate of the battery state of charge, then uses that as one factor in selection of the charging mode. How its estimated is not explained. And there are other conditions besides state of charge that are factored into the charging mode selection.
But apparently that was not the question. Rather it was, why, after all this time, I am seeing 14.9 volts when I never saw that before ("My battery is still the OEM from July 2020 delivery. Any chance it's giving me a warning to replace soon?). So, the question, in bold, is inviting speculation about what is going on with the battery, rather then whats going on with the charging system. So, here's a little speculation. And, its based on the assumption that the description of the charging modes in the manual is accurate -
What's going on with the battery that causes the generator output voltage to be 14.9 volts? My approach is to look at the charging system to determine what battery condition might cause it to produce the observed 14.9 volts.
There are only two charging modes that will result in a generator output voltage of 14.9 volts. One is the Battery Sulfation Mode. The Body Control Module (BCM) will enter this mode when the interpreted Generator output voltage is less than 13.2 V for 45 minutes. When this condition exists the BCM will enter Charge Mode for 2-3 minutes. The Charge Mode voltage can range from 13.9 to 15.5. Considering that most of the time in steady state, the BCM will select the Economy Mode which has a voltage range from 12.5 to 13.1, I would expect the Charge Mode to be entered for 2-3 minutes out of every 45 minutes of running. It does not mean that one would see 14.9 volts, but it does mean that one will see a voltage between 13.9 and 15.5 volts, every so often.
The other charging mode with that voltage range is the Charge Mode. The BCM will enter Charge Mode when ever one of the following conditions are met:
1) Windshield wipers are ON for more than 3s.
2) Climate Control Voltage Boost Mode Request is true, as sensed by the HVAC control module via serial data. High speed cooling fan, rear
defogger, and HVAC high speed blower operation can cause the BCM to enter the Charge Mode.
3)The estimated battery temperature is less than 0°C (32°F). 4) Battery State of Charge is less than 80%.
5) Vehicle speed is greater than 145km/h (90mph) 6) A current sensor malfunction exists. 7) System voltage is determined to be below 12.56V
When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9-15.5 V,
depending on the battery state of charge and estimated battery temperature.
So, if the voltage on the DIC is now reading 14.9 or higher consistently, it would appear that Charge Mode is being selected consistently, rather then the Economy Mode which maintains the battery at 80% charge, and shows voltages between 12.5 and 13.1. What would cause that? Ruling out conditions 1,2,3, 5, because they are not issues with the battery, leaves condition 4 (state of charge less than 80%), or 6) current sensor malfunction, or 7) system voltage less than 12.56v. Note that in the Economy Mode, the indicated voltage is between 12.5 and 13.1 - so if it drops to 12.5 in Economy Mode that would trigger the Charge Mode. Its not clear to me if that (dropping to 12.5 volts in Economy Mode) would indicate a battery issue, but it could, I suppose.
Not profound, nor do I claim it to be so, its a possible malfunction in the current sensor, or for whatever reason, the car is consistently estimating the battery state of charge to be less than 80%. That could indeed indicate that there is an issue with the battery that is preventing it from charging to 80%. This is where more detail on the algorithm that estimates the state of charge would be useful, but we don.t have it. So, all we know is that either the battery actually is not charging to 80%, or the the car incorrectly thinks the battery is not charging to 80%.
So, yeah, it could be a warning that the battery is aging, or it could be a warning that there is an issue with the current sensor or the process used to estimate the state of charge of the battery.
Today the systems are more complicated and have much more functionality since they are digitally controlled. PWM controlled alternators have been in use since the early 2000's. The voltage is also digitally controlled to improve the efficiency of the system. The voltage ceiling is set higher so the alternator can be throttled, and the devices can still draw the same number of watts. The alternators' primary function is to supply the devices with power and the battery is just one of the devices.
It begs the question of the purpose of showing voltage on the DIC and the operator is supposed to interpret it, doesn't it? A sudden change in the way it reads, for example, consistently showing 14.9 volts or higher, when it did not do so in the past, might mean something or it might not. But that voltage reading on the DIC, when the car is running, is not clue at all about the state of charge of the battery. The system no longer needs to put out a nominal voltage and live with the current that the battery plus the system absorbs. Now it can measure the current in and out of the battery, and adjust the voltage to achieve a desired current flow - it can adapt to changes in the battery.
The Owner's Manual gives no clue, not one, about the normal range for the displayed voltage, nor any indication about how to interpret a change in the voltage. The Service Manual indicates that
"it is normal for the voltmeter gauge on the instrument cluster or the system voltage display in the driver information center to fluctuate or change. This does not indicate a malfunction. Depending on the battery state of charge and the vehicle electrical load, these values may be anywhere from 12.5V to 15.5V." So, anything in that range is "normal" . I think in reality, we are left with relying on MIL (CEL) to indicate if there is an actual issue with the charging system or the battery. But, I do agree that if one has been paying attention to the voltage for a considerable period of time, and there is sudden change, as the OP indicated, now showing a high generator output voltage like 14.9 volts, it is an indication that something change. And what changed could very well be the battery age.
I preferred the ammeters of yesteryear, that I have on my older cars. Then I know for sure if the battery is charging or discharging. It does not tell me the state of charge of the battery, but it does offer a clue about whether its discharging, or perhaps overcharging. Computer regulated voltage does not do that - other than the clue it may offer about charging mode if one feels like obtaining and reading the Service Manual.
Why were ammeters replaced with voltmeters anyway. I did not do an exhaustive search about that, but the two examples found suggest it was a cost saving measure. Early implementations required all of the current to flow to, if not through, the meter. So a heavy wire that could carry all of the current in and out of the battery needed to be run to the meter in the dash. A shunt (a resistor basically) could be used so that not all of that current flowed through the meter mechanism. But that basically means all of the current into and out of the battery must flow through a resistor. But now, in the C8 and probably other cars, the "current sensor" is a tiny module between the negative battery post and the negative battery cable. I do not know how it works - it might still use a shunt, or it might use a "hall effect" sensor. Either way, it is measuring current, and could be (could have been) used to provide a digital signal to gauge on the dash to read current, rather than voltage. But, for whatever reason, GM chose to put voltage up there. One could try to use it to measure the "open circuit" voltage by putting the ignition into the accessory mode. Open Circuit voltage being defined as the voltage of the battery when its not connected to anything (or at minimal current draw). To estimate the state of charge from that, the battery has to have been sitting for several hours. The problem with that is that in the accessory mode, the majority of the cars systems are fully awake, so the draw on the battery is much higher than the 10 ma, so its not really the open circuit voltage. Still, it its too low, it would indicate a problem.
Yes, it could be an a "warning to replace [the battery] soon" or it could indicate a malfunction in the charging system. To make a short story long:..........
Originally Posted by C8J
Andy too long, .............
Sure it is. That's why I put the short answer to your question right up front - yes it could be a warning that your battery is aging. The rest is just why I think that. For anyone that's interested.
There is no way of telling what the battery is actually receiving. The voltage that is shown is sourced from the ECU.
I assume you mean the Engine Control Module, or ECM. How do you know where the voltage on the DIC is sourced? The DIC display is labeled "battery voltage". But voltage is measured in lots of places, which can be seen by looking at the summaries of scan tool information in the Service Manual. I have looked for a specific statement regarding what the DIC display source is, in the Service Manual, but have not found it. The Battery Monitor Module does measure voltage. And it is provided to the ECM. And the ECM apparently also measures voltage because there is DTC code that can be set if the there is a discrepancy between the voltage provided by the Battery Monitor Module and the voltage measured by the ECM (5 volts which seem large). But no specific statement about the source of the DIC Battery Voltage (as labeled) display. Even if it came from the ECM, it could still be the voltage measured by the Battery Monitor Module, since that is provided to the ECM.
"The Battery Monitor Module communicates to the BCM via LIN. The BCM shares this information with the ECM. The purpose of the battery
sensor module is to transmit battery information that the BCM/ECM can use to make decisions regarding stop/start, battery saver mode, and load
shedding."
P058D
An absolute difference of 5.00V or more is detected between the 12V system reference voltage measured by the ECM, and 12V
battery voltage measured by the B110 Battery Monitor Module for 32 out of 40 samples.
The electronic controls are labeled differently from one mfg'er to another. ECU (unit) and ECM (module) are synonymous. Traditionally the voltage is listed as battery voltage which is the standard for the regulated or system voltage. It's possible that there are wires that run from the battery terms directly to the ECM for this measurement. The general point that I was trying to make is that a higher voltage does not necessarily mean the battery is being over charged.
The electronic controls are labeled differently from one mfg'er to another. ECU (unit) and ECM (module) are synonymous. Traditionally the voltage is listed as battery voltage which is the standard for the regulated or system voltage. It's possible that there are wires that run from the battery terms directly to the ECM for this measurement. The general point that I was trying to make is that a higher voltage does not necessarily mean the battery is being over charged.
Agree that the higher voltage does not mean the battery is being overcharged. Based on the description of the charging system in the Service Manual, a voltage of 14.9 means that the charging system is in the charge mode, since that's the only mode (other an Sulfation Mode that puts the car in Charge Mode) that that has a voltage range that includes 14.9 volts. But that does not mean its "overcharging". It means it charging though.
The information collected by the Battery Monitor Module (which is located at the negative battery terminal, between the negative battery post and the negative battery cable) is provided to the BCM by a Local Internet Network (LIN) according to the manual. That in turn is shared with the ECM. Its digital data shared via the network(s) in the car. Its not stated as to how the ECM measures the "12V system reference voltage" which it compares with the battery voltage, but the ECM would have a B+ source directly to it, so it likely uses that and compares it with the voltage provided to it by the Battery Monitor Module via the network.
"The Battery Monitor Module communicates to the BCM via LIN. The BCM shares this information with the ECM. The purpose of the battery
sensor module is to transmit battery information that the BCM/ECM can use to make decisions regarding stop/start, battery saver mode, and load
shedding."
Why were ammeters replaced with voltmeters anyway. I did not do an exhaustive search about that, but the two examples found suggest it was a cost saving measure. Early implementations required all of the current to flow to, if not through, the meter. So a heavy wire that could carry all of the current in and out of the battery needed to be run to the meter in the dash. A shunt (a resistor basically) could be used so that not all of that current flowed through the meter mechanism. But that basically means all of the current into and out of the battery must flow through a resistor. But now, in the C8 and probably other cars, the "current sensor" is a tiny module between the negative battery post and the negative battery cable. I do not know how it works - it might still use a shunt, or it might use a "hall effect" sensor. Either way, it is measuring current, and could be (could have been) used to provide a digital signal to gauge on the dash to read current, rather than voltage. But, for whatever reason, GM chose to put voltage up there. One could try to use it to measure the "open circuit" voltage by putting the ignition into the accessory mode. Open Circuit voltage being defined as the voltage of the battery when its not connected to anything (or at minimal current draw). To estimate the state of charge from that, the battery has to have been sitting for several hours. The problem with that is that in the accessory mode, the majority of the cars systems are fully awake, so the draw on the battery is much higher than the 10 ma, so its not really the open circuit voltage. Still, it its too low, it would indicate a problem.
(Traditional) Ammeters require, as you guessed, a series resistance within the line (and that normally only exists at one point within the system, whereas you can measure voltage nearly anywhere)... while not a huge loss of power given the measuring accuracy needed here (typically several Watts), it's still an unnecessary loss. There are a few ways to measure DC current, but a Hall effect sensor is going to be the most logical here (they're quite inexpensive and rugged). A voltage measurement can be done in parallel and wastes very little power (in the range of tens of microWatts).
As surmised, ammeters were replaced with voltmeters because they provide a more useful data point as to the health of the charging system... the amount of instantaneous current draw doesn't say much other than the alternator is providing power, whereas voltage can provide some stronger clues that something might be amiss (like a failing battery cell that is dragging the system down). Cars have used alternators (rather than dynamos/generators) for the last 50+ years (anyone recall the last passenger vehicle to NOT use an alternator? Google shows the Chrysler Valiant as the first to use an alternator in 1960...), so the measuring system has followed suit.
Given that the system already measures current and voltage and already passes it around on the network digitally, it becomes a software development cost to add an ammeter.
The car already has selectable info tiles - one of the options could have been an ammeter based on the current being measured by the Battery Monitor Module.
Of course, software is not free either, and every feature developed requires testing so it would not be free to add it. But once done, there would be no additional recurring hardware cost.
Just sayin. This would not be a high priority item on my list, and I would normally not select it as one of my info tiles even if it were available. But if I suspected a battery or charging system issue, or just out of plain curiosity, I might look at it once in a while.
Below is a page from the 2021 Service manual describing the charging operation. Also the service manual notes that the battery Ah rated is for 20 hours. Thus if you have a 70Ah battery that is at 100% capacity you can discharge it at 3.5A for 20h. This time will slowly degrade with age as the H2SO4 fails to complete return to solution on charging.
Below is a page from the 2021 Service manual describing the charging operation. Also the service manual notes that the battery Ah rated is for 20 hours. Thus if you have a 70Ah battery that is at 100% capacity you can discharge it at 3.5A for 20h. This time will slowly degrade with age as the H2SO4 fails to complete return to solution on charging.
I do not see where what you posted indicates the battery amp hour capacity. But regardless, AH rating is based on a battery that is charged at 100%, and is the product of a given amperage and the time it takes to fully discharge the battery - that is - to essentially 0 or near 0 state of charge. In reality, the C8 does not charge the battery to 100% but rather has a target state of charge of 80% - so typically when the car is driven and then turned off, the battery state of charge will be 80%. And, I don't know what others want to do, but I would not want to allow my battery to fully discharge. So, using AH rating to estimate how much run down time you have for a given current might not give a satisfactory answer.
The 2020 C8 service manual looks similar to what you posted and gives an example of run down time vs current draw for low current draws, defined as the time in minutes it takes to discharge the battery from 80% state of charge to 50% state of charge. I like it because I would not want to discharge my battery below 50% - but I suppose that's opinion. The example is for a battery that has 110 minutes of reserve capacity (RC), and a cold cranking amp (CCA) capability of 690 amps. The amp hour (AH) rating is not stated, but lets look at the RC. RC is the number of minutes a battery can sustain a 25-amp load before the voltage drops below 10.5 volts, whereas amp-hour (Ah) rating indicates the total energy a battery can deliver over time.
The C8 OEM battery has the same RC, 110 minutes, as the example in the Service Manual, but a CCA of 730, a little higher than the example from the 2020 service manual. And it has an AH rating of 70 amp-hours.
Having the same reserve capacities, I think the example in the Service Manual is going to be in the ball park with respect to what will actually happen. And, table attached gives the time it takes to discharge the battery to 50% capacity , starting with an 80% charge. I think this is much better way to estimate how quickly the C8 battery will drop because 1) it starts with 80% which is exactly the target state of charge of the battery when running - the C8 does not charge it to 100%, and 2) I think a 50% state of charge is as low as I would like to go - I don't want to fully discharge the battery.
So the results in this table show that a parasitic draw of 25 ma (.025amps) will draw the battery down from 80% to 50% in 33 days. And, the inverse relationship between current draw and rundown time holds pretty constant for these low current draw. Thus, doubling the draw to 50 ma will draw down the battery in 16.5 days, exactly half. If the current draw increases 10 times, from 25 ma to 250 ma, the draw down time goes down from 33 days to 3.3 days.
From the table it can be inferred that the inverse relationship between current (I) in amps, and time (T) in hours is
(I*T) = 19.8 amp-hours or T = (19.8/I)
where T = time in hours and I = current in amps
This 19.8 amp-hours is NOT the amp-hour rating of the battery, but it is the apparent usable capacity when starting with an 80% charged battery and not allowing it to go below 50% capacity, based on this chart. And, perhaps with very high current draws this relationship might vary, as noted in a previous post, but for these low currents it holds pretty well. And it is an example, in the ball park - because this example battery has the same RC as the battery in the C8 (110 minutes).
A while back I measured the parasitic draw for the C8 at 10 ma after its all settled down. It wakes up from time to time so the draw is a bit more. But at 10 ma, and using the formula derived from the table, it would take 1980 hours or 82.5 days or 2.7 months, for the battery to discharge to 50% from 80%.
Attached is the page that notes that the battery Ah rating is based on a 20 hour rate. The discussion regarding the discharge rate vs time was just showing the relationship between the two. I also noted that this was for a fully charged battery. I agree with pretty much everything you discuss. I haven't come across where the 2021 service manual say that the target charge is 80%. Still looking. Note that the page I posted earlier shows the 6 modes of operation. The Charge Mode occurs as shown in right column. One case is when the battery is state of charge is less than 80% which seems to indicate that it will charge to battery above 80% at least in this case. Also the OEM battery in my 2024 is 730CCA and 10Ah. The 2021 manual has the exact same table Current Drain vs Days as you posted above.
There is no advantage to charging a lead acid battery to only 80%. If one does not routinely fully charge a lead acid battery, sulfation will start occurring which will result in early battery failure. The sulfites need to be completely drive off the anode and cathode.
Attached is the page that notes that the battery Ah rating is based on a 20 hour rate. The discussion regarding the discharge rate vs time was just showing the relationship between the two. I also noted that this was for a fully charged battery. I agree with pretty much everything you discuss. I haven't come across where the 2021 service manual say that the target charge is 80%. Still looking. Note that the page I posted earlier shows the 6 modes of operation. The Charge Mode occurs as shown in right column. One case is when the battery is state of charge is less than 80% which seems to indicate that it will charge to battery above 80% at least in this case. Also the OEM battery in my 2024 is 730CCA and 10Ah. The 2021 manual has the exact same table Current Drain vs Days as you posted above.
Thanks for posting that second page. It does not say its a 20 amp hour battery, though. It says Amp Hours is measured over a 20 hour period. So, a battery that is 75 AH that is 100% charged will be capable of supplying 3.75 amps for 20 hours before discharging. They do that over a specified time interval because the actual realized amp hours will vary somewhat at very high currents. The 20 hour period is not unique to GM, its an industry standard for amp hour specifications, according to what I found.
According to GM, in this tech link article here https://gm-techlink.com/wp-content/u...4-A-092023.pdf
your 2024 Corvette should have a battery that has 730 cold cranking amps, 70 amp hours, and 110 minutes of reserve capacity.
As for the 80% state of charge, that comes from the description of the Economy Charging Mode that states that the target state of charge is 80% and the conditions that select that mode. I don't know if the version of the manual you have included enough detail to determine that, but what happens is in steady state conditions the Economy Mode is the one that gets selected. The Economy Charging Mode includes this statement in its description: "The BCM will enter Fuel Economy Mode when the estimated battery temperature is at least 0°C (32°F) but less than or equal to 80°C (176°F), the calculated battery current is less than 15A and greater than 8A, and the battery state-of-charge is greater than or equal to 80%. Its targeted generator output voltage is the open circuit voltage of the battery and can be between 12.5–13.1 V. When fuel economy mode is active, the generator is not charging, only maintaining open circuit battery voltage. The BCM will exit this mode and enter Charge Mode when any of the conditions described above are present." Yes there is a Charge Mode that will provide a higher charging voltage - one of the conditions is that the state of charge is below 80%. There are no modes or conditions for charging it to 100%.
Using the info in the 2020 version of the manual, I made a graphical depiction of the charging modes, showing the resulting minimum and maximum charging voltage ranges for each mode, and listing the conditions that cause that charging mode to be selected.
There is no advantage to charging a lead acid battery to only 80%. If one does not routinely fully charge a lead acid battery, sulfation will start occurring which will result in early battery failure. The sulfites need to be completely drive off the anode and cathode.
GM thinks there is a fuel economy advantage. But that's why it has a Sulfation Charging Mode that gets invoked 3 minutes out of every 45 minutes of run time. Apparently it does not need to be charged to 100 percent, it just needs that higher charging voltage (and resulting current) every so often. I didn't make those modes up - nor is it my intent to defend them. Just saying what it says.
Some corrections. My first statement was "Attached is the page that notes that the battery Ah rating is based on a 20 hour rate." I did incorrectly post my 2024 battery as 10Ah where it is 70Ah.
The Economy mode statement as you noted occur when the battery when I draw is <15A and state of charge is greater than or equal to 80%. It doesn't say here it is charging it to 80%. It is actually floating the battery. If any of the charge mode conditions are met it goes into charge mode. One condition is state of charge is less than 80%. Sulfation is real. Not routinely fully charging a lead acid battery will kill it. The bar charts are charge mode vs battery voltage not state of charge.
Although this video is not directly related to the subject it is great to understand why it's NOT advisable to continually charge the common Li-Ion battery, LiFePo4 (Lithium Iron Phosphate) to 100% for optimum battery life. It's a bit complicated and related to EV's but Jason Fenske (Engineering Explained) does his usual great job. He has several videos on the subject.
Since like most on the forum my background is mechanical ICE cars and now with an E-Ray (including a 12-volt Li-Ion vs Lead Acid starter etc battery) and my wife's PHEV BMW SUV need to gain an understanding of this new technology.
