Joe,

Your car battery does not "explode" (or more appropriately "get damaged")
because it is not very deeply discharged between cycles.  The battery
voltage quickly reaches 14.4v (because that is what car regulators are set
at) and then maintains 14.4v by varying the current depending on the
electrical loads.  The regulator leaves bulk charge mode quickly (if it
even ever begins) and then reduces current.

Besides, exploding isn't really the issue.  I believe that the issue is
excessive electrolysis and localized boiling despite overall low bulk
battery temperature.  In a properly maintained wet-cell the owner would
ensure that this lost water was made up for.  If it were to happen more
often than normal or predicted by the manufacturer, then even an attentive
owner might not detect a low level before damage was done.  In the case of
AGM and gel-cells this water CANNOT be replaced and the effects are
perminent.

Yes a good regulator can be set to reduce output (ironically this seems to
be in contradiction to the idea of having a higher output alternator to
charge faster) and yes it can include a temperature detection.
Unfortunately proximity of the batteries to the regulator can make
temperature monitoring a challenge.

Ultimately, my original point was that while there are wide-ish margins
associated with electrical system design, simply getting a high output
alternator can cause more problems than they solve and the disadvantages
need to be considered.  Getting AGM or gell cells are popular but not
without their disadvantages as well.

In a sort of analogous way, I'm currently working on a speaker project.  In
selecting components, I'm finding speakers with options of between 50w and
90w (my load - the analog of batteries).  Then I realize I'm only
driving them with a 20w OEM amp (my source - the analog of a low output
alternator).  Do the new speakers have added advantages besides higher
power rating - yes.  Will they be louder for the same volume setting -
maybe but that is a function of their efficiency not their power rating.
Similary, I could replace the amp but the OEM speakers would not accept the
added power.  Does this mean that the OEM speakers will automatically be
blown - no because at low and even reasonable volume I'm not over powering
the original rating.  Although, the high power amp does create the ability
and increased chance of accidentally overpowering the speakers.  Does it
mean that even the OEM speakers will be loader (for the same volume
setting) - probably, but then I'll turn the volume down to prevent damaging
my ears and ultimately the speakers.

Josh


On Oct 17, 2017 10:35 AM, "Della Barba, Joe via CnC-List" <
cnc-list@cnc-list.com> wrote:



Batteries have an acceptance curve. You can see a plot of amps absorbed at
a given discharge state and charge voltage that assume that the capacity of
the charging source is infinite. The idea that the batteries absorb
whatever the alternator can produce is a common one, even though it is
wrong. For one example, my car has a 120 amp alternator and a 50 AH wet
cell battery. The battery does not explode every time I start the car J

It is commonly stated that wet cells charge at 0.25C max and gel types are
0.5C max. This is largely due to the chemistry, the wet cell batteries
**restrict
themselves** to 0.25C. If you are charging the batteries too hard, there
are a few ways to fix it. One is a temperature sensor on the battery. If
excess charging heats the battery, the voltage will back off. Another is a
sophisticated regulator that can control amp output as well as voltage.
Back in the day we used this feature to save belts and drag on the engine
more so than for battery issues. The third way is adjusting the voltage
setpoints. In my experience too much charging is the last problem you’ll
ever have on a sailboat, but say you did. Your 220 amp golf cart wet cell
bank is my some miracle accepting 140 amps instead of the desired 55 amps
and somehow the temp sensor is not backing it off or you don’t have a temp
sensor. Just do a test with low batteries and reduce the set point on the
regulator to see the desired 55 amp charge current. By far the bigger issue
is set points too high for the float mode, so you leave the dock with full
batteries and end up wrecking/boiling them with float voltage way too high.

The slow speed charging rate is also very important to sailboats. Figure
out your pulley ratios and look at the output curves of various
alternators. You may find the 180 amp version puts out no more than the 120
amp version at low engine speeds and you may find out no alternator is
going to get you much running at idle at anchor. I went through a few
alternators to settle on a Balmar 60 amp unit on my boat. It puts out a
fair amount more than the 120 amp 10si it replaced at low and medium RPMs,
which is not a thing for powerboats but very much is for us. Those of us
with Atomic 4s suffer from small engine pulleys that make it hard to get
any decent output at low RPMs.



Joe

Coquina



* none of this applies to Lithium batteries. You can blow them up real good
without a specialized battery management system



*From:* CnC-List [mailto:cnc-list-boun...@cnc-list.com
<cnc-list-boun...@cnc-list.com>] *On Behalf Of *Josh Muckley via CnC-List
*Sent:* Monday, October 16, 2017 9:32 PM

*To:* C&C List <cnc-list@cnc-list.com>
*Cc:* Josh Muckley <muckl...@gmail.com>
*Subject:* Re: Stus-List New Engine, now what size alternator



Joe, the damage I was considering would be caused by exceeding the charge
rate for the bank size.  The conventional wisdom is that typical lead acid
batteries should not be charged at a rate greater than 25% of their
capacity.  A 100 AHr battery would be limited to 25 amps.  Isn't it
possible to exceed the charge rate with a good regulator?  Particularly
applicable when the battery is more deeply discharged.



 All of the 3 and 4 stage regulators I've ever seen will ramp up to max
amps and hold there until ~14.6 volts is reached (80% full - end of bulk
charge) at which time the voltage will be held constant at ~14.6v as the
amps are reduced.  Once the amps lower to ~2amps the regulator shifts to
float mode and lowers the voltage to ~13.3v and holds it indefinitely.



As an example lets say that you are using a 100 amp alternator and
regulator to charge a 100AHr battery bank that has been discharged to 50%
capacity.  Isn't it likely that the charge current will ramp to 100 amps?
Or at least greater than 25 amps?  What other regulator function would
prevent this from happening?  I suppose battery temperature could input to
the alternator so as to reduce charge current.  Though, I'm not sure that
battery temperature responds quickly enough to prevent early over current
damage, only overcurrent damage as a result of longer term charging current
which has been applied long enough to raise temperature to the threshold.



Josh



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