Shore Power Charging - turn it OFF?
Greetings list,
I have been reading some posts on http://www.cruisersforum.com/.
The question I have has to do with leaving your shore charger on 24/7 when
tied up at the dock. In theory, the charger is supposed to completely turn
itself off when the final charge cycle is completed.
The reality is some(perhaps most) chargers do not actually shut off, but stay
in a 'float' mode, causing possible gassing to occur.
Also, if continually plugged in, the charger will kick into the regular bulk,
absorption, float cycle even if the bank is ever so slightly discharged for
lights, bilge pumps etc.
This becomes a problem since you are initiating a charge cycle when the bank
is at 99% capacity. I certainly am no battery expert, but I think everyone
agrees that there is a finite number of cycles a battery will provide before
death, so one would think limiting the number of cycles would be prudent.
There are charts for batteries that discuss the theoretical number of
charge/discharge cycles and the percentage of discharge for the expected
number of these same cycles - the idea being to select the optimum number vs.
% of drawdown - also factoring in the actual total amount of power for the
life cycle of the battery. This hopefully gives you a 'plan' for how much to
deplete the battery before a prudent charge cycle should be initiated.
My practice has been to just leave the shore charger on 24/7 when the boat is
in the slip. If I visit the boat dockside, any loads of lights, pumps. TV etc
are picked up by the charger/battery bank. Watching the Link 10 battery
monitor tells me how many amps I may use (which are few) and the shore charger
puts back the difference, then returns to 'float' mode.
So, back to the original query. Is this foolish battery management?
Some say it is better to shut off the battery charger after a complete
recharge, then let the battery bank deplete to the % that has been selected
before beginning a new recharge cycle. The batteries should last longer this
way.
Those that like things to be more ''automatic'' can even put a timer on the
batt charger circuit to shut it off after a set period of time.
I am pretty sure I have not seen this topic on this forum discussed in the
last few years.
Brian Shanafelt
Isobel K
Seattle
Brian Asks:
The question I have has to do with leaving your shore charger on 24/7 when
tied up at the dock. In theory, the charger is supposed to ......
The reality is, some chargers ...............
REPLY
The truth is even more complicated. Even so called "smart" chargers can
get fooled if you have enough load turned on in the boat.
The smart charger may not transition into FLOAT mode if there is enough
load turned on to exceed the nominal current needed to transition into
float. Typically this is 1% - 2% of whatever the charger max output is.
So what happens is this. Because the charger does not see the current
tapering down to the very low value, the regulator "thinks" the battery is
still absorbing a charge and maintains the voltage at maximum of 14.2V and
boils off the electrolyte. If its an AGM or Gel, it gets even worse.
If the charger was rated as a 100 amp charger, the current would have to
drop below 2 - 1 amps to trigger the transition to float.
One 20 watt Halogen bulb uses 20 / 14.2 = 1.41 amps. Two such light bulbs
definitely would be above the threshold of most chargers.
Some of the very best and smartest chargers will time out after 10 -12
hours regardless of current flowe and go to float for a few hours before
shutting off completely; but most would never shift out of absorption
mode and would therefore boil off the battery electrolyte.
So to answer your question; YES it is best to turn off the charger
completely after a certain interval.
Regarding cycling and battery life:
The life cycle curve is exponential. The less discharge you do before
recharging the longer the battery life compared to deeply discharging
before recharging. For example I have seen some deep cycle batteries
claim 500 cycles if discharged to 80% - 100% depth of discharge. But these
same batteries will be shown as having 3000 cycle life if you only
discharge 10% - 15%. It is not a linear relationship.
Adding a timer is not practical if you have a combination
inverter/charger. Especially if you have one of the very popular and
common Heart brand models. If you were to disconnect the AC input with
some kind of relay / contactor driven by a timer, the unit would
immediately stat to invert if the AC was turned off. Even if no load was
present the parasitic load current of the inverter itself would be a
considerable drain on the batery.
A practical solution has been suggested by Andina for long term
liveaboard application. It also applies to long term storage dockside. She
suggest having a small inexpensive battery which is sacrificed in
service while you are dockside. Your expensive house battery is fully
charged but left disconnected. While you are dockside let the automatic
charger feed the less expensive battery and it powers your lights and
other DC appliances. As long as the DC load does not exceed the capacity
of the charger this battery will not deplete, since the charger provides
the needed current. The inverter will not kick in unless dock AC power
goes out.
When you leave the dock to go cruising, switch over to your big house
bank. A standard 1-2-BOTH type selector switch is a convenient way to do
this and doesn't require tools.
Given the rising cost of lead acid batteries we will have to develop
better procedures to get the maximum life from them.
If your needs are more complicated, well there are additional $olutions,
but they co$t and take time to install.
It all depends on whether you are looking at $400 replacement cost or %3000.
regards
Arild
Brian Shanafelt bshanafelt@fastmail.net writes:
Some say it is better to shut off the battery charger after a complete
recharge, then let the battery bank deplete to the % that has been selected
before beginning a new recharge cycle. The batteries should last longer this
way.
Those that like things to be more ''automatic'' can even put a timer on the
batt charger circuit to shut it off after a set period of time.
A couple of comments:
Yes, I think that it's a bad idea to leave a large bank on float over the
winter. When I rebuilt Island Eagle, I put in four Concorde 8D AGM batteries.
Three years later, they would no longer hold a good charge. They were
connected to a Xantrex 4024 running in float mode for that entire time (the
boat did not leave the dock). I replaced those batteries with AGM L-16s and
at the same time started turning off the charger over the winter. I have a
friend who looks after the boat and when he sees that the voltage has dropped
to about 12.25 volts he switches the charger on for a few hours. The L-16s
are 2 years old now and seem perfect.
Now, one further bit of information: some chargers (including the Xantrex
4024) have a "Low Battery Transfer" charge mode. This essentially automates
what I describe above. Instead of running in a "float" mode, the charger will
allow the battery to discharge to a preset level, then it will initiate a
full charge cycle. It will then switch off and wait for the voltage to fall
to the preset again. This sounds like an excellent mode, but since I am not
on the boat in the winter (I'm actually 3000 miles away) and I use the boat
all summer, I have not had a chance to try it. However next spring I am going
to give it a try. I'm also going to install a voltmeter with a computer
interface so I can more easily monitor the charge/discharge cycles.
Scott Welch
FirstClass Product Manager
www.firstclass.com
"Things turn out best for people who make the best of the way things turn
out." - John Wooden
I've always left my smart charger on. 7 years now and the batteries are
still fine.
The PO of the boat had a "dumb" ferroresonant charger in there when I bought
the boat. He had it hooked up to a lamp timer, so it only ran X hours per
day. Just an idea.
Keith
"It has been said that man is a rational animal. All my life I have been
searching for evidence which could support this." - Bertrand Russell
----- Original Message -----
From: "Brian Shanafelt" bshanafelt@fastmail.net
The question I have has to do with leaving your shore charger on 24/7 when
tied up at the dock. In theory, the charger is supposed to completely
turn
itself off when the final charge cycle is completed.