I haven't noticed this, so now I'll have to hook it up to one of my 12V auto batteries tonight to satisfy my curiosity.

 

In cold/extreme cold temps (below freezing, below zero, etc) 15 volts would barely be in the absorption stage, 14+ would practically be a float charge. It takes more voltage to overcome the internal resistance.

That's why I suggested the link to the battery faq and a voltmeter, and help get a handle on how various types of chargers operate and that it realistically takes a lot longer to charge up a battery than most people think. The tables usually show an hour or two on top of that once they reach a full charge.

 

I understand that as I work with this equipment daily. it takes hugely longer than most folks realize to bring batteries back up. My question was have you actually seen a charger that produces 15 volts unless it has an equalizer setting or has temperature compensation built in. For example, a Xantrex will go all the way to just under 16 volts in freezing weather as it modules based on battery temperature. Of course, that raises an entirely different issue as most DC equipment is limited to 16 VDC (module boards, electronics, etc.) and may smoke out as that limit is approached (ask me how I know). Then there were the old Trace inverters, which as I recall, went into a two hour equalization every time they were turned on, but that may prove to shorten life more than sulfation and could damage equipment. Battery charging is just tricky business.

The converters in RVs are not capable of running a higher voltage during absorption, but it also provides protection for onboard systems admittedly at the expense of battery life, at least in theory. Would folks benefit from better converters i.e. more expensive that temperature compensate? Maybe, who knows for sure. Even with the fancy charger ability, I haven't seen much difference in battery life, but it is fun to watch the meter move up and down.

Just my thoughts,

Steve

 

Yes - the standard "dumb" automobile chargers that have been available for years are just about perfect for a standard large auto start battery as near as is possible.
I don't believe there is any temperature compensation as such, it just takes longer when it's cold, but they are too "hot" as far as end point charging voltage in warm weather.

But the charging voltage times and current, track the published tables very closely for recommended battery charging profile. At 6 amp setting the classic Shumacher tops out at 16.25 volts in cold weather and negligible current. This is right at an "equalization" charge for flooded lead acid "maintenance free" batteries. On the 2 amp setting it will top out around 15 volts; if we look at the temperature adjusted tables in cold weather it's the equivalent of around 14.2 volts driving down the highway on a sunny day. In extreme cold, say -20F it would appear the 2 amp setting might be equivalent to a float charge. So the charger folks really know their stuff.

The problem of course, is "dumb" chargers aren't regulated and must be shut down when the battery is fully charged. They will boil a battery dry, at least in warm weather. It sort of seems like the entry level "smart" chargers though, are somewhat crippled as far as reaching the higher equalization voltages.

 

My thought about those kind of voltages for RVers is to be careful not to gas AGM batteries and be sure to pull the batteries cables before powering up the charger. The 16-volt limit on boards and electronics is real and they go to heaven in a heartbeat when their limit or a little lower is reached. For RV converters which temperature compensate, they have a setting for AGM batteries and the rest of the RV converters never go high enough to cause a problem. I have never seen anything approaching 16-volts on automotive chargers on my meter, but if it is out there, take care.