Bob Petruska

Your chances of hurting the battery are probably very low. Do you realize how many AGM batteries are charged with normal chargers and still work for many years?

ex-quattro PETE

Yeah, i know i did not hurt it. I just probably did not charge it fully since agm typically calls for slightly higher voltage. Next time...

Redd

Pete-
Not necessarily. AGM settings and wet lead settings overlap, depending on the manufacturer and the specific chemistry and construction. Assuming it is s stock Bosch battery, you'd need to check with Bosch to find out the range of correct settings for that battery.
Sometimes that's on the web, sometimes you have to ask them for it.

I suspect that there is a "power control computer" in the Q5, or some kind of similar "abnormal" electrical boss, because I've seen mine putting out something like 12.4~14.4 with no apparent rhyme or reason, regardless of my speed, whether I've just started, and what loads are turned on. Of course, Audi of America will put on the clueless expression usually found at Radio Shack and tell you "You need to ask your dealer's service manager about that." And the service manager probably still enjoys getting a good belly laugh on hearing that news. Yet again. (They may know if it has one, but they'll have no idea what it is up to.)

ex-quattro PETE

Thanks, Redd.

My charger claims to put out 14.5V for wet cell and 14.8V for AGM. So based on your readings, even 14.5V was probably sufficient. Of course, you are supposed to compensate for ambient temps as well. Some chargers do.

As far as owners manual, it just states not to exceed 14.8V when charging. Granted, this has nothing to do with what Bosch may have recommended. Audi probably just wants to make sure voltage isnt too high in order to protect sensitive electronics in the vehicle.

Bob Petruska

Yes the Q5 power control module will charge the battery at different voltages depending upon what load is being used.

You can charge the Q5 battery just fine from the front under hood jump start terminals.

I use this charger and it eliminates any guesswork for any type of battery. It does an outstanding job in auto mode.

ex-quattro PETE

You still have to manually select the "snowflake" mode if you want to use the higher charging voltage meant for AGM batteries.

Bob Petruska

That is true if you want to charge the AGM battery to 100% for some reason (storage), otherwise "Normal" mode will charge at 14.4V and give you 90% charge, preventing possible
thermal runaway and damaging the AGM battery. I don't charge the battery to 100% as I'm not storing or using the charger as a battery minder. I charge to 90% and then let driving the vehicle
charge it very slowly to 100%.

Also an AGM battery should have the snowflake icon (according to Ctek) on the battery to indicate you should use snowflake mode, otherwise normal mode is more forgiving. You should always contact the AGM battery manufacturer if in doubt what mode to use.

ex-quattro PETE

Thanks. I was finally able to take a look at mine. It is a VARTA AGM, 92 Ah.

Redd

Ctek make good chargers, but I would *not* be happy to charge an AGM battery to 90%. AGM batteries have a quirk, they prefer to be charged to 100% all the time, or their lives are shortened significantly. So adding a low-power solar charger that is always feeding them and always keeping them topped up when the car is always sucking them down (i.e. with the entry and antitheft systems) can be a good idea.

I'd also be wary of using the front "emergency" connections to the battery for long-term charging. There is probably a 0.2-0.3 volt drop in the cabling from there to the battery, so that your carefully engineered Ctek charger may think it is feeding the battery 14.8 volts, but the battery will only be getting 14.5, and again, AGMs don't like that.

I have a dedicated (and fused at 30A on both sides) battery cable running from the battery posts, past the spare tire, then up through the floor plate and past the rear seat. I *know* that at that power rating, the cabling I have is nearly zero voltage drop.

And this assumes that Varta is using "standard" AGM voltages--even though the optimum charging voltages for AGMs may vary by several tenths, and ideally you want a charger that is matched to the battery. I've no idea how well the Ctek matches what Varta wants, but these "inconsequential" differences of 0.2-0.3 volts can make the difference between a 3 year battery, and a 5 year battery. Or more. (In deep cycle use, the difference can be 3 years /vs/ 8 years.)

Can't do much about thermal compensation, which is also a bigger issue than you might think. But a good charger will at least measure "ambient" temperature (it should have a temp sensor to mount on the battery) and adjust for that, in the assumption that the battery is in the same "ambience".(G)

Bob Petruska

I was saying that I charge my battery to 90% with the Ctek charger, then charge it 100% by driving the car. The Q5 voltage regulation system applies a better (slower) controlled charge than the Ctek for that last 10%. The Ctek charging current at the final 14.4 or 14.7V point is soooo small that there is negligible voltage drop from the Q5 front charging points to the battery. And even if there was the Q5 will charge the battery to 100% when you drive it for a short distance.

I know that you are obsessed for some reason with batteries and I respect that. If one spends $40-50K on a Q5 what is so wrong in buying a new battery for less than $200 every couple of years? The Q5 at many points in its charging cycle stops charging the battery at a low 12.5 - 12.7 volts. This doesn't cause the battery to not charge properly or shorten it's life.

The 14.7V is a max voltage applied to the charging battery, but it's not the immediate starting voltage and it's physics. A discharge battery has such a high internal resistance, and the charger can only put out so much current, thus V=IR controls that max initial voltage that can be applied to the dead battery. It goes from 0 to 14.7V. The remaining Float (trickle) charge starting at about 80 -80% of charge is much less than the 14.7V to prevent any overheating. Thus there isn't any magic constant 14.7V charging voltage, it ramps up from 0 and not to exceed 14.7V.