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I have the following 12V car battery: Varta Silver Dynamic AGM 595901085 (12V/95Ah/850A), and the following car: Jaguar S-Type 2.7D.
In the technical cars manual it says this:
So in cold winter mornings, when I initially start the car, the alternator is charging the battery with with 15.5V.
Is this safe for the battery?
Varta recommends charging it with up to 14.8V (according to what I found on their website).
This seems a reasonably good charging voltage. You didn't specify what "cold winter morning" means, it could be anything from minus 40 degrees Celsius to plus 15 degrees Celsius so I can't comment on whether this is the perfect charging voltage for that temperature because I don't know the temperature.
AGM batteries are sensitive to charging with too high voltage and you can't add distilled water to AGM batteries, so you generally want the charging voltage doesn't cause electrolysis into hydrogen and oxygen in the battery acid/water electrolyte solution. The theory is that the valves of the AGM battery allow slight overpressure, and as long as the hydrogen and oxygen generation is below a certain low rate, the hydrogen and oxygen are being fully recombined back to water. However, the colder it is, the more voltage should you be using to charge the battery. I would say 15.5 volts is too high on temperatures where you don't need the engine heat to heat the cabin, but on winter 15.5 volts might be just fine.
Also, cars usually have a capacity factor of 5%, meaning 95% of the time they are unused and only 5% of the time is the battery being charged. Therefore, you don't need intelligent three-state charging that would first charge at constant current, then at absorption voltage and finally at float voltage, and you don't need two-state charging where the voltage is float voltage either. It's perfectly fine to use two-state charging where the voltage is the absorption voltage. Absorption voltage for AGM is usually 14.4 V - 15.0 V plus 0.09 volts for each degree below 25 degrees Celsius, so for temperatures between 13 - 19 degrees Celsius the optimal voltage would be 15.5 volts, but since cars have only two-state charging and not three-state charging, you should probably use slightly lower voltage, so I'd say it's perfectly fine to charge at 15.5 volts at 0 degrees Celsius. Also, the temperature here is the temperature of the battery, at minus 40 degrees Celsius once you start driving the car, the battery temperature heats up from engine heat so it's not minus 40 degrees Celsius forever. Also in cars you might have some other considerations such as short life of incandescent and halogen light bulbs if the voltage is too high, so you might not want to fully extend the voltage compensation to minus 40 degrees Celsius, but instead save your light bulbs by limiting the temperature compensation to some maximum value.
In cases where the battery is being charged 24/7, then you want float voltage of 13.5 - 13.8 volts plus 0.06 volts for each degree below 25 degrees Celsius. Usually batteries have float life between 5-10 years depending on the quality of the battery.
An ideal charging system would have an alternator with separate voltage sensing wire that is connected directly to the battery terminals so cable voltage drop doesn't affect the voltage measurement, and also an ideal charging system would have a temperature sensor directly at the battery to calibrate the charging voltage as battery heats up from driving. The separate sensing wire is sometimes seen in cars but I don't know of any car that would directly measure the battery temperature.
I have a Jag Xtype, 2.2D and it has the same charge controlling system.
Don’t worry about it - mine does get to 15.3V certain times in winter and rapidly comes down to 14.8V.
Now as the temperatures are rising it is staying below 14.7 - actually between 14.0 and 14.7V as it sorts itself out.
The charge controller will also reduce the alternator demand when full engine power is demanded so it controls the situation very finely.
As long as it is working properly and the connections are tight with a good battery then all will be well. If the battery is poor then that can cause many spurious faults to appear - none of which are real - our machines like a good battery.
