Mailport May 1, 2000 Issue

Mailport 05/01/00

Battery Burnup
In your January 1, 2000 issue, Mr. Dale Botwin reported his experience with an inexpensive Group 24 starting battery getting very hot and emitting a strong rotten egg odor. Mr. Botwin went on to inquire how so much heat was generated.

This past spring I experienced a similar incident aboard my boat. After several days away from the dock, during which we lived off our battery bank, we returned to our slip late in the day and plugged into shorepower. We hit the sack early, and were awakened a few hours later by a nasty, pungent odor. After searching and sniffing for a while my nose lead me to the engine compartment, where my battery bank is located. Upon lifting the engine room access hatch there was no doubt about the source of the odor. The smell was overwhelming, and I could hear a battery boiling from 10 feet away. Being tired, and not thinking too clearly, I immediately turned off the battery charger (a Heart Freedom 10 inverter/charger). In a few minutes I could no longer hear the battery boiling. After airing out the boat we returned to bed.

My main battery bank consists of two group 27 Prevailer gel cells, one 4D Prevailer gel cell, and one 8D Prevailer. The two Group 27 batteries were about seven years old, and the other two batteries were about five years old. I am aware that this configuration is not ideal, but such is the nature of boats. The batteries were, however, connected properly, with all terminals being at most within 18 inches of each other, and the positive and negative connection at opposite ends.

When I investigated the situation in the morning I found one of the Group 27 batteries quite hot, and still boiling a little. I then realized that while disconnecting the charger had greatly reduced the boiling, the “good” big batteries continued to discharge through the “bad” battery. A quick voltage check of each battery told the story. All of the batteries but the hot one had voltage reading in the high 12V range; the hot one read between 9V and 10V.

What had happened was that one of the Group 27 batteries had lost a cell. When a battery looses a cell, it acts like a resistor in parallel with the other batteries—and in series with the battery charger. With the battery charger connected, most of the current from the battery charger is dissipated as heat through the “bad” battery. The circuit breaker does not trip because the battery charger is not putting out more than its rated current. The problem is that instead of the charger current being used to charge the batteries it is used to heat the “bad” battery.

When the battery charger is disconnected, the "good" batteries will see the "bad" battery as a load, and continue to discharge through that load—producing heat. The loss of water in the "bad" battery was most likely a result of the heat and boiling rather than the cause of the battery going "bad."

Jeff Huntington
Cruising on Sand Dollar
via e-mail


Dale Botwin's letter brings up the danger of connecting batteries in a parallel circuit without adequate precautions. Mr. Botwin speculates that his ferro-resonant charger supplied the current that overheated his Group 24 battery. This may not be the case.

Two non-identical batteries (one Group 24, and one size 4D) were connected in parallel—probably with big chunky wires and, of course, no in-line fuses between the batteries. If one of those two batteries drops in voltage, due to age and sulfation and all the other nasty electrochemical things that happen to lead-acid batteries, then the remaining battery will supply as much current as it can to try and bring the first battery up to the voltage of the second battery.

In a parallel circuit, both elements maintain the same voltage and they push current back and forth in order to maintain that voltage.

A lead-acid battery has very low internal resistance; that's why it's so good for delivering high current (discharge mode) to starter motors. But this also means that a huge amount of current can flow through a lead-acid battery in charging mode. Now, an AC-powered charger, like Mr. Martin's ferro-resonant unit, is designed so that it cannot deliver really large currents to connected battery. But a parallel-connected lead acid battery can deliver a huge current to its parallel partner.

Here's what I believe happened. The Group 24 battery suffered an internal failure. The reported open-circuit voltage of 9V-10V hints at a shorted cell. The parallel-connected 4D battery, being bigger and stronger, tried to “recharge” the Group 24 battery back up to 13.8V, delivering lots of current in the process. This, of course, heated up the Group 24 and the water solute in the electrolyte heated up to vaporization. I would expect to learn that the 4D battery was severely discharged in its attempt to recharge the Group 24.

Properly designed battery chargers include isolation diodes in their output circuits so that multiple batteries can be serviced by the same charger, without this dangerous condition.

Batteries should not be left permanently connected in parallel. A temporary connection, as a boost to engine starting (the "1 and 2" position on the main cutoff switch) is fine. Fusing the parallel connection would be counter-productive, since during engine start-up you want a high current flow.

Cedric Walker, PE
New Orleans, Louisiana


Dale Botwin's battery problem is actually not that unusual. I have made repairs on several vessels over the years with similar problems due to the combination of vastly dissimilar batteries wired in parallel. Mr. Botwin's supposition that he had a shorted cell is borne out by his float voltage reading of 9V to 10V. He is missing 2 volts, the equivalent of one cell.

As to his other problem of the overheating of the battery, I regard that as a somewhat self-inflicted wound. First of all, it's not a good idea to combine batteries of vastly different ampacities in parallel. The 4D will have a capacity of more than twice the Group 24. If the smaller battery fails due to a shorted cell that battery receives not only the amperage from the charger but also the very considerable amperage from the larger battery. With a 4D battery or greater that amperage is more than enough to really begin to heat things up. This can be a real recipe for disaster.

I firmly believe in the use of battery isolators for multiple batteries and fusible links in the hot lead between paralleled batteries. If a 50- to 100-amp fusible link had been in the circuit between the plus terminals that paralleled both batteries there would have been no problem. Unfortunately, less than 5% of the vessels I see everyday have that protection.

Bob Hill
Bob Hill Marine
Miami, Florida


I also had a battery undergo "thermal run away" while leaving my boat unattended with the charger left on. My charger was an expensive West Marine 40 amp charger (high frequency charger, build by Statpower) that advertised the ability to be left on at all times, keeping a maintenance charge on the battery. This became my standard practice while leaving the boat. The primary advantage of leaving the charger on was that the batteries were maintained at full charge and thus did not experience any self discharge and subsequent sulfate formation.

While I was concerned about leaving the charger on, it had numerous overload protection devices so I felt relatively comfortable. One day, while inspecting my batteries, I noted that one battery was very hot to the touch and very low on water. I filled the battery with distilled water and left the boat for a few hours to do some errands. When I returned my charger was off and the suspect battery was still hot to the touch. I could not get the charger to come back on. After taking it to a reputable electronics repair shop they advised my that my charger had been "fried" by the short circuited battery. Despite the fuses that were present to protect against this, they did not prevent the problem. Despite claims of battery charger manufacturers I have since NOT left my main charger on while being away from the boat for extended periods of time.

Both Dale and I wondered what had caused this problem. I did some research and found out that one of the ways in which batteries fail is that the active material on the battery plates breaks loose from the lead grid of the plates and falls to the bottom of the battery case. Once enough of this material has fallen to the bottom of the case it builds up and "shorts out" across the positive and negative plates. When this short occurs, the amount of current that the battery can absorb increases dramatically and high levels of current start to flow through the battery. These high levels of current generate heat in the battery due to the resistance of the sulfated material in the bottom of the battery. If amperage levels are not limited, excessive levels of heat can occur.

This heat also has the side effect of increasing the "boiling off" or gassing of the electrolyte in the battery. A small amount of gassing during the charging process is normal. However high levels of heat, as a result of shorting out the plates, increases this gassing to such levels that the electrolyte is boiled off. During this process hydrogen and oxygen gas is given off. It is the hydrogen that smells like rotten eggs.

The probable root cause of the problem was the shedding of material to the bottom of the battery case. Shedding can become a problem if a battery is left in an undercharged condition for long periods of time. Plus a battery will self discharge itself at a relatively high rate. Tests on my own lead acid batteries indicate they can self discharge from a full charge to about a 50% charge in about a month. Therefore maintaining a battery continually at full charge minimizes the shedding and maintains full battery capacity.

I now have an relatively inexpensive maintenance charger that I leave connected to the batteries (Battery Pal by Guest, $54) while I am away from the boat. This is not a typical automotive "trickle charger" that feeds a constant low amperage current to the batteries. Rather it varies the amperage from 0 to 3 amps so as to maintain a constant battery voltage of 13.5 volts (for lead acid wet cells). This prevents the formation of sulfates on the plates due to self discharge of the batteries and is just below the "gassing voltage" so that the electrolyte is not boiled off. Several other companies sell marine battery maintainers that are powered by AC or solar panels (Batt Cat, ICP and Pulse Tech)

This "maintenance charger" is limited to a 3-amp output and has a fuse on the DC side of the circuit. Even if the safety protections in this were to fail, it would not supply enough current to gas the batteries and would not result in the loss of an expensive piece of equipment. It was somewhat consoling to hear that I was not the only one who has had problems with a runaway battery.

Dave Demski
Cypress, Texas


Inexpensive Belt Tensioner
Re: the discussion of belt tensioners in the January 15, 2000 issue, a V-belt tensioning method that I have used for lo these 55 years of boat ownership and cruising comes from an old Farmers' Almanac article. Using ones thumb and index finger, grasp a V-belt midway between sheeves. If the belt is properly tensioned one should be able to twist it 90°. Any more, the belt is too loose; any less, the belt is too tight. Spacing or number of sheeves on a single belt doesn't seem to matter. With multiple sheaves, make the "check" between the pair of sheeves most distant from each other. It works with all sizes of V-belts, in the dark, in cramped spaces, and is simple!

Dick Sargent
via e-mail


Home-made Alternator
Last spring I did some work on my uncle’s 28' Hunter. He wanted to live aboard and all manner of electrically thirsty devices were added. I determined that the low output alternator that came with the Yanmar would have to be run ad nauseam to keep up with the demand of the over all system(s).

A replacement was needed and it fell on me to determine which one to install. When I priced marine alternators I thought the supplier was having me on. In addition, they required, according to him, an external “smart” regulator.

“I don’t think so,” I mumbled to myself.

So, I went up the street to our local auto supply house and spoke to Old Bert who I knew well and who has been in the auto-electric business since just before Herr Diesel was born.

“Yep,” he said. “Know what you’re after. Off a school bus, she is.”

We searched through the books and decided that a Delco-Remy part #A1358 would do the job.

“And the regulator?” I asked.

“Don’t need one,” he replied. “She’s built right in. Just attach a heavy wire from this here terminal to the positive side of the battery.”

It took a few minutes, perhaps an hour, to get the new alternator installed. I had to run a drill through the mounting boss on the engine, about a 1/2 mm larger, and do some filing in the adjusting bar. It took some time to run the wire (I wanted to leave the original wiring in place in case we had to put the original alternator back on) and the rest of the afternoon to get the pulleys sized the way I wanted them and lined up perfectly. I decided to use a ratio of 1:1.9 or so. 1:2 is not a good idea because of potential wear patterns. A new belt was needed, but that was right off the shelf.

Here are the results. The output of the new alternator at 1000 rpm engine speed, is just over 100 amps. At 2500 rpm the alternator is cranking out 135 amps. At full song, which doesn’t bother the alternator at all, we see just under 150 amps. You can imagine how that cuts down on the running time of the engine.

It ran that way all last summer around the Montreal area (sailing more than motoring) and then it took my uncle and me up the St. Lawrence river, across Lake Ontario, through the Erie Canal system and the Hudson river (motoring 10 hours a day) and as far as Atlantic City where I got off the boat to come home and he continued on.

It ticked merrily along all winter and made the return trip to Montreal. When I asked Uncle Donald about the alternator he seemed surprised I would ask. “Why, perfect,” he replied.

I guess the thing still thinks it’s on a school bus, happily motoring through the rural roads of Canada. Only in the summer time.

Barry Webb
via e-mail

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