Posted by michel.capel (michel.capel@…>)
Rick,
I’m currently overhauling my DC system and just bought 4 new AGM
batteries and a new alternator, so I read a lot about this subject
lately.
The answer to your question (can I charge both batteries by
switching them parallel)is ‘it depends’.
You have batteries with totally different properties, so the main
principle is NEVER to switch then parallel. The worst battery of
them all becomes the norm if they are connected(weakest link).
However, when charging with your engine alternator with (I suppose)
it’s normal single step charge regulator, it is not a big problem to
charge them in parallel for a short while, because single step
regulators do not charge batteries to the full capacity anyway and
they don’t charge very efficiently either, but you need a lot of
charging time for that last 20% of charging and your batteries could
get hot and evaporate water. When both batteries have more or less
the same state of charge, it’s not a big deal.
If you have a three or four step regulator (e.g. on an intellent
inverter/charger, or on the engine), you should NOT put them in
parallel for charging with the ‘Both’ swith. Most modern yacht
chargers have two charging outputs that provide exactly what the
connected batteries need, with different charging characteristics on
each output. Your deep cycle batteries have very different needs
than your engine cranking battery, especially when the deep cycle
battery has been emptied 50% or more.
I’ll explain why. When an empty deep cycle battery is charged, the
first phase gives high amps on 13.8V (bulkcharge), after a certain
voltage is reached, the second phase gives decreasing amps with
14.4V (absorbsion charge)and the third phase a low amp maintenance
charge at 13.8 (float charge). The amperage changes are regulated by
either a time limit or a certain voltage reached by the battery.
You can imagine that your engine cranking battery which is never
discharged below maybe 85%, has a totally different response to the
charge provided, with a different timing need to switch to the next
charge phase.
What happens when you charge these batteries in parallel? Either you
boil the crank battery to death by overcharging in the bulk phase or
you never get a fully charged house battery in the absorbsion phase,
depending on where the charger takes its measurements from.
Even a separation diode block cannot solve this, only a charger with
two outputs. Fortunately my good old Freedom (Xantrex) 1500
inverter/charger has a seperate output for the cranking battery.
Hope this helps.
Michel Capel
F44#4 Alabama Queen
— In FreedomOwnersGroup@yahoogroups.com, “Rick Higgens”
<higgens@…> wrote:
Your comment brought a question to mind. When leaving the
batteries
charging onboard, do you need to charge the crank and the house
batteries separately or can the combiner switch be left on both? I
have a standard 12V crank battery and 6V golf cart batteries for
house.
Thanks…
Rick F30 #12 Nauti-Belle
— In FreedomOwnersGroup@yahoogroups.com, Thomas Wales <twales@>
wrote:
Al,
I, too, remove my batteries and store them in my
garage. When I bought the boat, the batteries had been left in
the
boat with the battery switch turned to “both” for over 2
years. Needless to say, they were DEAD. They looked like
fairly
new West Marine deep cycle’s so I brought them home and charged
them,
hoping for the best. That was over 3 years ago and they’re
still
going strong. This past weekend, I put the meter on them and
all 3
registered about 12.5 volts. So, I’ll charge 'em up before
re-installing them and get another year out of them. The thing
that
amazed me was their ability to re-charge and hold it after being
fully discharged for so long. I think I’ll maintain my current
practice, despite the fact that the batteries get heavier each
year…
TW
At 11:28 AM 4/16/2008, you wrote:
Those of us who pull our boats during the winter are usually
advised
to remove the batteries and trickle charge them at home
periodically
in order to prevent fatal self-discharge (or leave them aboard
if you
have access to AC current while on the hard). This week I had an
experience that calls into question this advice.
Early last fall, I replaced the battery in my car since it was
about 7
years old and I was concerned about it making it through the
winter.
I kept the old battery in my basement and ignored it. On
Monday, my
wife discovered that her battery was dead. I used jumper cables
to
hook up my old battery, self-discharging all of these months,
to my
wife’s car and it started on the third try. I used a voltage
meter
and discovered that the old battery still registered 12.6 volts.
I’m not sure that I’ll remove batteries from a boat again.
Al Lorman
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