drivesafe wrote:

If the voltage from the alternator was the same as it would have been from your battery charger, then the alternator would have been providing the same or more charge current.

drivesafe wrote:

I note how you posted up all the specific details except the most relevant one. You conveniently neglected to mention the voltages at which the battery was taking the different charges.

Robbie wrote:

From there there will be a further decline and within 3 or 4 minutes the battery will be receiving around 4 amps at 14.7v before decaying further to around 2 amps at this same voltage.

After its charge on my Ctek my battery is now at 98% SoC and I am now off to York.

 Land Rover - Turning Drivers into Mechanics Since 1948

Battery & Quiescent Current Drain Testing

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drivesafe wrote:

…and while you listed the alternator’s voltage, you didn’t list the battery chargers.

drivesafe wrote:

The alternator was not responsible for the current decreasing at the battery.

drivesafe wrote:

As the battery was near fully charged, the battery reduced it’s current draw from the alternator ( based on the voltage being applied to the battery at the time ).

I've covered the voltage part already and the ability of a battery to absorb current as its SoC increases is basic battery charging stuff, but we can look into it in more detail:

Perhaps the first thing to note is that rate of charge is a function of voltage and available amps until the point where the battery reaches its acceptance limit. Nothing new there then and we can see the familiar graph shape of amps over SoC. So lets look at the D4 system during and after start:

Click image to enlarge

Looks textbook to me and as I covered earlier at the early part of the cycle the alternator is behaving just like a regular alternator. But the BMS is still active and has all the information required to exceed normal charging limits. As a result it has ramped up the voltage to 15v, and maintains it whilst the amps decline. As the amps decline due to the acceptance rate falling the BMS drops the voltage to 14.8v or so as the curve flattens out to just below 20 amps. Other than the BMS being able to exceed the normal limits (which can still be used if the BMS fails) this is all normal and the rate of acceptance of around 18 amps is pretty much on the money for an AGM at 87~88% SoC. Again, all as I covered earlier on this thread. So if everything continued as per a normal alternator, without BMS intervention, the amps would remain at this level and slowly decline over the next few hours of driving as the SoC increased towards 100%:

Click image to enlarge

No wait, the volts are holding steady at around 14.8v but the amps are falling fast. In fact within 2 minutes the current is down to only 4 amps.

drivesafe wrote:

In all battery charging situations, in the final 20% of the charging cycle, the battery itself governs the current draw ( again based on the voltage being applied to the battery ), not the device supplying the charge.

So has the battery has been filled in seconds and is now approaching a high state of charge? Nope, the SoC has remained at around 87~88%. Is the alternator short of power? Nope, by design it can provide enough for all conditions and only 3 minutes ago it was kicking out over 50 amps. Does the alternator need more RPM? Nope, its capable of almost full output at idle and in this case the RPM and speed is increasing. Is the voltage regulator struggling with the change in RPM? Nope, it holds steady through numerous gear changes. Is the battery overheating? Nope, it is holding at around 24ËšC. So the conditions have not demonstrably changed, the physics and battery chemistry remain unaltered. Is it possible that JLR's technical papers are correct and that they can restrict the current available to the battery and choose to do so when the BMS parameters are met and the SoC is above 75%?

But what about this regenerative power, when does that start to kick in? Well, pretty early in the journey and the BMS sort of tests the water as soon as it can. Winding back a few seconds we can see throttle being applied yet both the amps and volts take a bit of a dip. The BMS is prioritising engine power delivery over alternator output. A few seconds later there is a brief period accelerating then coming off throttle where the conditions are met for prioritising electrical power (watts = volts x amps) and both are seen to rise:

Click image to enlarge

drivesafe wrote:

You could have a 1,000 amp battery charger, but if the voltage is not high enough then the battery will only draw a small amount of current.

This is true, but it is you and not I that keeps introducing voltage control as the sole means to control current to the battery. But back to that battery charger piece; after we turned off the ignition just after the system was producing 14.8v at around 87~88% SoC but only 2 amps were being drawn through the battery terminals:

Click image to enlarge

If the SoC really has remained at around 88% and the charge rate prediction is at an amperage in double digits yet the amps at the terminal are down at just 2 amps, could we use a battery charger to double check that the battery really is capable of absorbing a higher current, even if the voltage is a bit lower? Well, that's what I did when I plugged in my Ctek:

Click image to enlarge

Hmmm, I would need a bigger battery charger to see how high the current could go and still be absorbed by the battery at 88% SoC. Indeed, my Ctek topped-out at 7.3 amps on the clamp meter and due to voltage drop only 13.2v made it to the terminals, even with the higher voltage mode selected. But even with the BCM taking 3.27 amps the current flowing into the battery was still doubled and demonstrated that it could take a higher current than the 2 amps the alternator was providing moments before. Of course, we already suspected it would as it had done so during the first few minutes after start in near identical conditions, plus the charge rate vs SoC charts predicted it.

drivesafe wrote:

So your post literally confirms what I have been saying, the alternator is NOT controlling the current directly, and can not do so without reducing the output voltage.

My posts have never confirmed this - the data is the data and none of this is revolutionary. If we wind back the clock a few years a number of us had to learn the new system rather quickly when the BMS issues started to appear. As covered earlier, back then I could only datalog all the parameters, spin them in Excel and compare them with the predictions as I could not see the sensed values at the BMS and BCM. I can now compare real-world data, sensed data and the predictions to spot the bad PCM, BCM, BMS, battery or alternator. Many a forum member has driven around with my voltage, current and battery temperature data loggers attached to their vehicles.

Click image to enlarge

drivesafe wrote:

And once again, if, according to you, lowering the voltage increases the current, and therefore increases the load on the motor, why is it that Land Rover, and many other makers of vehicles, are doing everything they can to get the vehicles to operate at lower voltages

For a given electrical load if you reduce the voltage the amperage has to increase. I feel I am repeating myself when I remind you that it is you that is claiming that reducing the voltage is the sole means by which power is being reduced. I do not recognise your statement that motor manufacturers are doing everything they can to reduce voltages and it remains un-attributed.

By contrast the desire to increase voltages to reduce the current is well published by SAE and major manufactures. Indeed, the SAE discussed an increased automobile standard voltage as early as 1988. If it was not for the march towards hybrid vehicles the 42v standard may have been adopted to power the increasingly energy-hungry electronic systems in modern vehicles. Stealing a quote from SAE the other arguments for a higher voltage included the reduction of weight in the wiring system, improved stability, and reduced voltage drop. With three times the voltage, thick conductors can be reduced to a third of the cross-section, and at the same time the relative voltage drop can also be reduced to a third. For the same cross-section, the relative voltage drop is now no more than one ninth. The voltage level resulting from these arguments was so close to three times the present voltage that 42 V became the choice for the second voltage level.

I will keep within the forum rules and I will not lower myself to your level, but I am sure you know that I found your manners and tone to be completely inappropriate for this forum as that is what you intended. Recently you have questioned my education, background, intellect and even called me irresponsible. You have pored scorn on some pretty basic physics and dug yourself some rather large holes on power equations, Ohms Law (have a quick look at my Quiescent Current Guide) and you even threw in an incandescent bulb as an example of a constant resistor so we had to cover non-Ohmic components too (see my Glowplug guide for another use of non-Ohmic filaments to increase resistance and lower current).

This is not a soulless large American forum full of internet heros. The UK is small and forum members / forum members meet each other. We drink and eat in each other's kitchens, we work on each other's cars. Quite a few members know who I am and what I do. I have even worked with some forum members in my day job and others know people I work with. Quite simply there is nowhere to hide.

That said, I have tried to help you like I would any other forum member. Indeed, it would be advantageous for you to know what the BMS system does to the voltage and current supplied to the battery as you sell dual battery controllers. It would be great if you understood how they work with the BMS and in what conditions they do not. If you have access to a standards portal I can recommend reading the supporting papers on the SAE database. I will not take offence if you utterly dismiss anything authored, contributed or originated by me. In the interim, as a free safety tip, please do not connect a tiny resistor to a high voltage. Resistors are sold with wattage limits and this is a function of volts x amps. Even if you managed to keep the amps really low the potential difference can be enough for it to overcome the insulation or just jump the gap.

A long post, but it is evidence based. DSO downloads, pass/fail waveforms and known-good CSV data is available to the technical gurus on request.

 Land Rover - Turning Drivers into Mechanics Since 1948

Battery & Quiescent Current Drain Testing

Diagnostics for:
Defender, FL2, D3, D4, Evoque, RRS & FFRR
A not-for-profit enterprise

 

Robbie wrote:

I won't delve into your theory that a regulated voltage has to decay if the battery can accept more amps than it is being provided with.