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Author Topic:   Charging VRLA-AGM Batteries with Older Outboard Motor
Brys13s posted 04-02-2007 07:00 PM ET (US)   Profile for Brys13s   Send Email to Brys13s  
Hello again. Since my boat, motor, and service manual are not close by at the moment, I was wondering if somebody could help me. It's time for a new battery and I was thinking about switching from a wet cell to an valve regulated lead acid aborbed glass mat battery (VRLA-AGM). From what I've read so far, VRLA-AGM batteries are somewhat sensitive about [charging voltage]. My outboard is a 1987 Mercury 35-HP two-cycle. Does this engine even have a voltage regulator? And if it does, what is the output supposed to be? Thanks,Bryan.
jimh posted 04-03-2007 09:30 AM ET (US)     Profile for jimh  Send Email to jimh     
VRLA-AGM batteries are sensitive to the charging voltage. Here is an excerpt from the technical data about VRLA-AGM batteries from a manufacturer's data sheet:

How do VRLA batteries recharge?
Are there any special precautions?

While our VRLA batteries accept a charge extremely well due to their
low internal resistance, any battery will be damaged by continual
under- or overcharging. Capacity is reduced and life is shortened.
Overcharging is especially harmful to any VRLA battery because of
the sealed design. Overcharging dries out the electrolyte by driving
the oxygen and hydrogen out of the battery through the pressure
relief valves. Performance and life are reduced.

If a battery is continually undercharged, a power-robbing layer of
sulfate will build up on the positive plate, which acts as a barrier
to recharging. Premature plate shedding can also occur.
Performance is reduced and life is shortened.

Therefore, it is critical that a charger be used that limits voltage.
The charger must be temperature-compensated to prevent underor
overcharging due to ambient temperature changes. (See Charging
Voltage vs. Ambient Temperature chart on page 11.)

Important Charging Instructions

The warranty is void if improperly charged. Use a good constant
potential, temperature-compensated, voltage-regulated charger.
Constant current chargers should never be used on VRLA batteries.

The chart mentioned shows that the charging voltage in a 70- to 80-degree ambient temperature range must be limited to 14.60-volts, and that the optimum charging voltage is 14.30-volts. Your outboard charging system needs to be able control the voltage output in this narrow range in order to properly charge a VRLA-AGM battery.

I don't have any data about the voltage regulation of you 1987 Mercury motor, but my impression of older outboard motors in general is that their battery charging output is typically not extraordinarily well regulated. They were designed in an era when most boats used an ordinary flooded cell lead acid battery. In a flooded cell lead acid battery if overcharging occurs the electrolyte which is boiled away can be replaced. In a sealed battery overcharging is more of a problem.


Brys13s posted 04-03-2007 08:04 PM ET (US)     Profile for Brys13s  Send Email to Brys13s     
Thanks Jim.Next time I'm at the boat,I'll check the voltage with a meter at the battery at different rpm's.If this engine will run without the battery connected[I think it will]then I will also check the voltage output at the terminals with the battery disconnected.The reason I am thinking of switching to a VRLA-AGM is they are better suited to the sometimes harsh ride of the 13 sport than a lead/acid wet cell.
jimh posted 04-05-2007 01:18 AM ET (US)     Profile for jimh  Send Email to jimh     
The best way to determine the charging voltage is to let the engine charge the battery for a while until the battery terminal voltage rises and the charging current begins to taper off to a low value. How low? I'd say probably less than 1/10-th of the charger's output current maximum rating. So if you have a 10-Ampere charging current rating, the current should be down to around 1-amp. Measure the battery terminal voltage under those conditions. It should be a good indication of the charger voltage output.

The window between the "optimum" charging voltage and the maximum is small. On the other hand, you can charge the battery at a voltage below optimum. You just won't get the best results. Charging with a voltage that is consistently too low can cause problems, too.

Charging a battery is a process of reversing a chemical reaction, and, to get the chemistry just right, the voltage has to be controlled somewhat precisely.

jimh posted 04-18-2007 09:38 AM ET (US)     Profile for jimh  Send Email to jimh     
Inside of a valve-regulated lead-acid (VRLA) battery you have the same chemistry as in a flooded cell lead-acid battery. If the voltage in a cell exceeds about 2.39 volts, the chemical reaction drives hydrogen and oxygen out of solution and into a gas. This is what occurs when the cell is "over-charged" and is called out gassing. This produces a loss of electrolyte in the battery

In a flooded cell battery which is vented to the atmosphere, the hydrogen and oxygen gases can vent from the battery. In a VRLA battery they are retained in the battery, where a catalytic converter tries to change them back into water. As long as there is not too much gas produced, the battery remains sealed and no electrolyte is lost. The VRLA battery cannot be completely sealed or the pressure could build up to a dangerous level, so a valve regulates the pressure and allows out-gassing if the pressure is too high. When out gassing occurs there is a loss of electrolyte in the VRLA battery. A flooded cell battery can be refilled with water and recover from loss of electrolyte due to over charging. A VRLA cannot.

The exact voltage when gassing will occur depends on temperature. For a 12-volt battery it is nominally

2.39 X 12 = 14.34 Volts

The charging system should be designed to not exceed this voltage.

Unfortunately, most older outboard motors have a permanent magnet alternator, and the voltage output of the alternator will increase with engine speed. Unless there is a voltage regulator circuit, the charging voltage will rise and often will exceed 14.34 volts. Therefore, if you use a VRLA battery with an older outboard motor that has an unregulated or poorly regulated battery charging circuit, there is a good chance you will subject a VRLA battery to chronic over charging. Depending on the design of the VRLA battery and its regulating valve, this could result in loss of electrolyte. Loss of electrolyte, of course, reduces the capacity of the VRLA battery and eventually results in failure.

Because they are openly vented, a flooded cell battery is subject to some loss of electrolyte, and thus it require occasional refilling with distilled water and recharging afterwards. A flooded cell battery may also produce some gas vapors which are corrosive, and consideration should be given to the effect this may have on its surroundings. For this reason, most flooded cell batteries are mounted in battery boxes which can contain any corrosive vapor or spilled electrolyte.

Mister S posted 04-19-2007 12:23 PM ET (US)     Profile for Mister S    
Jim, Thanks for an understandable explanation of this. It appears that you've done your homework.

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