SJ23 Tech Tip E04, (Updated 2018-03-14) Bob Schimmel


Pulse Charge a Flooded Battery - Desulphate the plates & measure state of charge.
- Installation, Charger & Battery State, Manufacturers.

I learned about battery sulfation (liquid filled) during the mid 1990s while trying to determine why our telephone equipment operated for such a short time from their "perfectly good" standby batteries.  Telephone battery plants are typically sized to operate the equipment for 24 hours, by which time commercial power is usually restored.  In the ensuing investigation I discovered the reason for the failure; that many technicians are ignorant of battery maintenance and the majority of them are downright scared of the acid, fearing an explosion if they equalize charging the battery plant or perform a load test on them.  The sailing population is likely no different.  Yet regular maintenance like monthly equalize charge, monitor the charge current, voltage, electrolyte levels and keeping the terminals greased and cases clean, are simple and effective things to do.  However, even with all this maintenance, certain batteries still didn't perform to specification and I discarded them as manufacturing duds.  Little did I know!

That's when I learned that sulfation is a natural by-product of liquid filled battery operation and sooner or later it will kill a battery, especially for a stored battery and to a lesser degree for an infrequently used battery.  It occurs normally on battery plates during the charge/discharge cycles.  Some lead sulphate is not dissolved back into the electrolyte during the charge cycle.  This is especially true if a battery is not charged to its rated voltage or is left discharged for a long time.  Over time sulphates build up to cover much of the plates until the  battery efficiency is reduced to the point where it is assumed to be dead.  This process is the main reason why over 80% of batteries "fail".  In this state the average life of a liquid filled battery, depending on usage, is 48 months.  Some last only six months.  Apparently only 30% of these batteries actually last to 48 months.  Suffice it to say that removing the sulphate deposits revived many telephone batteries.  So without delving into the depths of battery chemistry or electronics, what follows is a general information about plate sulphation and how to rid a battery of it.

Contrary to common belief, if a boat generator or alternator runs all the time it doesn't mean the battery is maintained in good condition.  It is possible to undercharge a battery if the engine is constantly run at a slow RPM so the alternator cannot charge at the optimum rate.  Idling for long periods with a standard alternator, especially if it is combined with a high electrical load, is a killer.  The result is a battery that is not fully charged so it will sulphate, ensuring a quick death.

The most effective solution is to pulse charge (high voltage and current) a battery using a "smart" battery charger.  There are web links to 2 manufacturers at the end of this Tech Tip.  TOP


The preferred method to connect a pulse charger or a solar charge controller to a battery is to take input power from a solar panel and then discharge the high voltage pulses into the battery, both charging the battery and desulphating the plates.  It is very efficient because when the sun goes down, the energy is retained in the battery.  The output of the solar charge controller can be left connected to the battery continuously.  Think of it as a technique for automatically charging a battery when the sun shines. 

Don't confuse a solar charge controller with a "smart charger."  A Smart Charger draws input power from 110VAC with a 12VDC output which is perfect to maintain a boat battery in winter storage or a vehicle battery.

The PWM solar charge controller must be installed electrically close to the battery to minimize power loss for the high voltage charge pulses.  Connect the controller 12V output leads directly across the battery terminals using the shortest wires possible.  Connect the solar panel output leads directly to the controller input leads.  Connect the outboard generator, wind generator and DC distribution panel leads directly to the system buss bars or battery, observing the correct polarity. 
If you hear electrical "noise" or a steady motor boat sound on the VHF radio, AM/FM radio or media receiver then the PWM pulses are leaking into the boat electrical system.  To block the "noise" traveling to the DC panel or VHF radio, install ferrite beads on the + DC cables feeding the (radio, distribution panel, outboard generator) as electrically close to the battery as possible.  (Tech Tip E02).  The wire must go through the ferrite bead and is more effective if you loop it through a second or third turn.  If a specific spot along the cable attenuates the RF signal more than another, tape it in this place.  You will have to experiment with this.  You can get a ready source of ferrite beads from a local electronics recycler.  Look for a box of computer power cables with 1" long "bump" (ferrite bead) near one connector.  Cut off the cable at both ends of the bump, remove the heat shrink, and drill out the old cable.  Now slide the ferrite bead over your power cable.  While a single bead should attenuate all the "noise," sometimes several are required. 

For absolute control you should install a switch for each power input line (solar panel, engine generator, wind generator, etc.) to isolate any one of them in the event of a trouble.  Install and label the switches on the power control panel.  All too often you see only the load switches labelled and very seldom are power input switches wired or labelled.  To minimize confusion out on the water, the power input switches should be labelled or shaped different than the load switches. 

  • HIGH CHARGE CURRENT INSTALLATION - For a high charge current or quick battery charge recovery it is OK to connect the outputs of multiple Morning Star controllers in parallel.  The outputs of solar panel(s), wind generator, alternator or other mix, must be electrically isolated from each other by being wired to their respective controller.  However, the negative leads must be connected to each other so all charge voltages have a common reference of 0 volts.  It prevents stray current flowing between them.  This high charge capacity may not normally be required on a pocket cruiser, but sometimes you have a desperate situation that requires you to charge your batteries very quickly and it is good to know that your system can be wired for this.  One time I extended Panache's solar panel output to a buddy's boat to charge his near dead battery.  It took only an hour to raise the battery above 12V, after which he started his diesel and we fixed his charging problem back at the sail club.  Its really no different than boosting another car.  Few boats are equipped to do this though.

  • WILL ANOTHER CHARGING SOURCE BACK FEED INTO MY CONTROLLER? - Another charging source (outboard generator) can be connected in parallel with a Morningstar charge controller.  The controller does not need to be electrically isolated from the battery IE: using a diode.  For best performance when using other charging sources, ensure the charging source and Morningstar controller are set to charge at the same (or close) voltage.  If the supplemental charging source is set to a higher charging voltage than the Morningstar controller, the controller may temporarily go into a fault state when the battery voltage rises higher than its set point.  The controller can automatically recover from this state when the battery voltage drops to a lower level.

I've heard of a totally dead battery being revived simply by connecting a pulse charger across it.  When the sun light is weak the charger will simply take longer.  This is similar to an electronic camera flash with weak batteries, the recycle time to the next flash becomes longer as the battery voltage decays.  A pulse charger has similar circuitry.  If the solar panel output is too low then the pulse charger cannot charge.  (Eventually that stupid rabbit will die too!).  TOP


The most popular battery on an SJ23 is likely a liquid filled deep cycle 12V battery.  If you want this battery to perform for many years it is advisable to keep it charged all the time.  If you are going to buy a 115VAC battery charger for home use, regardless of output amperage, consider one with an automatic regulator or what is commonly referred to today as a "smart charger".  Keep your smart charger connected to the deep cycle battery during the winter and your battery will be ready for the summer.  If the charger is fully automatic there is no need to check it because the charger will switch to maintenance mode when the battery is fully charged.  This will greatly improve the condition and life span of a deep cycle battery.  With regard to battery maintenance, there is little difference between a smart charger and a dumb charger that is operated manually.  Either can maintain a battery charge but most of us have better things to do than monitor a battery being charged!

Smart chargers are available at many automotive stores or battery retailers.  A word of caution on leaving a trickle or similar manual charger connected to a liquid filled battery for a long time; it will eventually overcharge the battery, "boiling" it dry in the process.  It is better to use a charger equipped with an automatic set back of the charge voltage.  Slow charging over a long time is better than to ram a charge into it as fast as possible.  A high charge current over extended time will also heat a battery which may warp the plates.  The following at-rest voltages apply to most batteries types.  Expect the voltage readings to be .2 to 1 volts higher on a gel type battery.


Level of Charge

At Rest Voltage

Fully charged. 12.6 V or more
75 to 100%. 12.4 - 12.6 V
50 to 75%. 12.2 - 12.4 V
25 to 50%. 12.0 - 12.2 V
0 to 25%. 11.7 - 12.0 V
0% or dead. 11.7 V or less


Level of Charge At Rest Voltage
100% 13.00 V
90% 12.75 V
80% 12.50 V
70% 12.30 V
60% 12.15 V
50% 12.05 V
40% 11.95 V
30% 11.81 V
20% 11.66 V
10% 11.51 V
0% 10.50 V

NOTE - Whether you use a sophisticated battery monitor or a hydrometer to check the state of your liquid filled battery, when to check the battery voltage is important.  Battery voltage is probably the closest equivalent of a gauge to measure the charge level.  The at-rest voltage is the most accurate reading to take.  This can be accomplished by taking the reading in the morning before the sun illuminates the solar panel or by removing the charge current.  In general the battery voltage must be measured after the charge voltage has been removed for an hour or two and with no load current.  (Don't want to get up that early?  Cover the solar panel with a towel).  The voltages shown above are open circuit readings and correlate to the battery' state of charge.  Measure with a digital voltmeter as the scale on most analogue meters cannot accurately show 1/10 V.  Print this chart and stick it close to your boat battery as you will never remember these voltages at the time of a trouble.

Here are some guidelines to charge a dead battery.  A dead 700 cold cranking amps (CCA) battery requires at least 8-10 hours of charging time using a 5 A charger.  Most deep cycle batteries are rated around 500 CCA so a 5 amp charger will take approximately 7-9 hours to charge the battery.  TOP

Links to Solar Charge Controller Manufacturers

If you would like more information on pulse charging, go to one of the following web sites where you can also order a controller.
  • Morning Star Corp. - The SunGuard 4 Solar Controller is rated at 4.5 amps input from the solar panel and is likely the optimal unit for the small electrical system of a San Juan 23, far outperforming my linear shunt regulator.  It worked great on Panache till I replaced it with an MPPT charge controller and two 35W panels in 2020.  See Tech Tip E01.
  • PulseTech's. - These pulse chargers may include a high efficient solar panel so may be a good way to go if you don't have one.


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