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Subject: TWL: Battery combiner versus echo charger versus diode isolator ( long )
From: Arild Jensen (Arild.Jensen@XXX.XXX)
Date: Mon Apr 02 2001 - 18:07:56 EDT
This past week-end several comments made regarding combiners and echo
chargers prompted the following discourse:
Whenever the topic of charging multiple batteries come up, one key issue is
usually ignored.
The relative size of the banks involved!
In nearly every case, the discussion revolves around making one single
charging source serve multiple uses.
This ignores the differences in battery type, size and duty cycle.
Most multiple battery charging situations involve several starter batteries
and one single house bank.
If the start battery is in good shape and it is dedicated to a single
engine, why would it require frequent recharging in between each engine
run?
There are no parasitic loads that drain the start battery when the engine is
off. At least in a good design there shouldn't be any loads.
A healthy battery will store a charge for a couple of months and still have
enough power to crank the engine.
A properly maintained engine will normally fire after less than three
complete revolutions of the crankshaft.
Failure to do so is normally an indication that some maintenance work is
required.
I'm discounting sever cold weather at -20 below zero since most boats do not
operate under such conditions.
Given the huge difference in duty cycle of a start battery and a house
battery, it is obvious that each battery type requires very different
charging routines.
A start battery typically gets depleted by about 5% of its capacity every
start. The engine alternator quickly replaces this amount even before the
engine warms up.
A house bank is typically depleted by anywhere from 30 -50% of its amp hour
capacity before any charging takes place.
This charging often takes place without any relevance to when the main
propulsion engine is needed to move the boat.
In many, if not most boats, the house bank is anywhere from four to ten
times as big as the start battery, with a correspondingly larger charge
current requirement.
Deep cycle batteries have a very different charge curve requirement, partly
dictated by the plate construction, partly by the fact that deep cycle
batteries are often AGM or gel while start batteries are normally flooded
cells.
Diode isolators produce a voltage drop of 0.7 volts across the diode
junction. Provided you feed the house bank first, this drop helps prevent
excess voltage on the second ( usually start ) battery. This drop is often
cited as causing an under charge condition. While true, it may not really
matter since the battery will most likely be fully charged the next time the
engine runs and recharged from the alternator. Diode isolators limit current
by virtue of creating waste heat and more voltage drop at higher current
levels.
Echo chargers are a better choice in that they do not waste energy or
produce voltage drops. Echo chargers limit the charging current to 15 amps.
For charging a smaller batttery with limited acceptance rate, this prevents
damaging heat build-up. Echo chargers do not shut off when the smaller
battery is fully charged. Echo chargers are voltage followers, that means
they output the same voltage that is being fed to the main ( big ) house
bank.
During most of the bulk charge stage, the terminal voltage of the battery
is less than the gassing point, so no damage occurs.
As soon as the voltage reaches a high (13.8 - 14.1V) set point, the 3
stage charger changes to absorption mode.
In this mode, the voltage is held constant ( 14.1V ) and the charge current
tapers off. The mode is typically terminated when the current drops below a
set value (Usuall;y 1 - 2% of battery capacity ) or a time limit is
reached.
If a start battery was already fully charged from a recent engine run, this
period of absorption charge mode represents a situation wherein the start
battery is subjected to a gassing voltage. This is not good for any
battery but especially not for sealed no-maintenance battery types.
If the house battery is connected to ongoing loads that exceed the
threshold for low current limit, the absorption mode may not be terminated,
or at the very least be extended much longer than necessary. This means
the start battery is stressed for that much longer.
Once the absorption mode is terminated, a 3 stage charger will then go to
float mode. Depending on model characteristics, this may continue
indefinitely.
If the charger is not temperature compensated, this float stage may still
be above the gassing voltage of that particular battery, depending on how
warm it is.
AGM batteries should only be floated for about an hour according to some
manufacturers. Then the charger should shut off completely.
An echo charger will reduce the chance of overcharging by virtue of
limiting the charging curreent, which can produce internal heat in battery.
The Echo charger; being a voltage follower, will not prevent gassing if the
main battery keeps the regulator in the absorption stage and hence a
higher voltage ( 14.1 V) for a long time.
A simple battery combiner merely creates a path between two or more
batteries when ever a voltage over a certain amount ( > 12.7 Volts ) is
present.
A Pathmaker is one brand of battery combiner which has the added feature of
adjustable voltage thresholds. With it you can set the Disconnect V to a
voltage which is below the gassing point of a warm battery. You can also
set the reconnect voltage and the over voltage disconnect threshold. (the
product has three separate adjustment pots)
This last would limit the damage in the event of a charger regulator
failure that produced voltages in excess of 15 volts.
Battery combiners are intended for charging applications. Unfortunately,
many people assume that if it is good enough for charging, it must also be
good for discharging - including cranking an engine. NOT SO!!
From my observation, this is the most common reason as to why battery
combiners fail in service. The contacts get burned or overheated.
Another source of heat is the energized coil. Since the contactor uses
current to energize the relay coil, there will be heat buildup.
Installations which subject a combiner to continuous charging voltages may
cause over heating of the coil, and subsequent failure or the coil winding.
This last situation is most likely to occur when you have a boat connected
to shorepower on a continuous basis so that a trickle charge is always
present on all batteries. When the vessel is underway, the alternator
provides the charging voltage and again the coil is energized.
Some of the low cost brands may not have a continuously rated coil. After a
few weeks of continuous tropical service, that coil could be come very
hot. ( no data available )
DO NOT USE COMBINERS AS EMERGENCY BYPASS SWITCH!
Multi bank charging works best when all batteries are approximately the same
size and have approximately equal charging time requirerments.
Batteries with very different duty cycle and usage patterns should not be
charged from a single common charge source.
Use separate and individual chargers for each application, or have a manual
selector switch so only one battery at a time is charged in turn.
Combiners work well when two different charging sources alternately feed
into a single battery bank, or when one bank should be disconnected first to
prevent over charging.
Echo chargers work well as long as all connected batteries can tolerate the
duration and voltage levels experienced in a typical charging cycle and
assuming the battery technology and chemistry is the same in all cases.
Starting batteries do not normally require frequent recharging like a house
bank does. Use a small separate charger for start battery(ies).
Regards
Arild
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