Outboard motor battery charger control

rporter · October 2018

The motor is a 6hp Tohatsu with what they refer to as an 6amp 'alternator' but in reality it's a magneto coil and rectifier. The battery is a 12v approx. 75ah deep cycle battery. I've tested the output and the rectifier correctly limits the voltage output to around 14v as the engine speed increases.

This is on a sailboat. The problem is if I motor for several hours or all day, there is nothing limiting the amps being pumped into the battery. It's all about a year old and I'm afraid I have already started to damage the battery.

The boat also has a 100w solar panel that is connected to a Morningstar SunSaver controller. Typically not connected when the motor is on. That works fine at controlling the charge from the panel. I contacted Morningstar to see if the outboard charger could be connected to the SunSaver to control the charging and was told definitely not. The two systems can be connected in parallel after the charger but not before.

I have looked quite a bit and have not found any small device that will act as a controller to cut the charging from the outboard when the battery is fully charged.

Any help or suggestions will be appreciated.

mike95490 · October 2018

6 amps ought to be safe for any flooded 12v 75ah battery, regulated to 14V. That's about what is seen in a car on a long trip.
I don't see a viable way to be damaging the battery from even 6 hours daily running.

rporter · October 2018

A car of course has a regulator that stops charging the battery when it's full.

Let's say it's putting out 4 amps for 6 hours, that is 24ah. During the day while motoring the only load is a speed/depth instrument that draws 0.1a. Seems that it could be easily overcharged.

After a two week trip the electrolyte level was very low (I didn't think to check it during the trip) prompting me to think it had been over charged while I motored all day for two or three days and the fluid had boiled off. The battery doesn't charge up to the same level as previous. It used to be 12.85 when fully charged. Now it's 12.62.

Estragon · October 2018

When the battery is full, it should take very little current at 14v, maybe an amp or two, but not six.

How warm/hot was the battery? Was it full when you measured the 14v limit?

rporter · October 2018

What would reduce the amperage going into the battery? The charger keeps pumping out 4 to 6 amps. It doesn't know anything about the state of the battery. It only limits it's output to 14v.

I think the battery was full. I measured it prior to going out one day before I installed a permanent voltmeter so typically the battery is fully charged. I didn't check the temperature of the battery. I can read the voltage to two decimals so I can see what the voltage is while the engine is running and at the speed that I usually motor it's around 13.5v.

Estragon · October 2018

The battery itself should limit current when full, or nearly so. The charger doesn't push out amperage, it pushes voltage. The "flow" of amps depends on loads, like opening a tap. At 14v (assuming that is actually limited), a properly functioning 12v nominal lead battery near fully charged is like an almost closed tap. At a given water pressure (voltage), there's a limit to flow (amps).

rporter · October 2018

If that is so how are batteries overcharged?

mcgivor · October 2018

Batteries are overcharged by applying a voltage higher than what's recommended, this will induce current to flow into the battery, being that it can no longer store any more energy, assuming its fully charged, the energy generates heat which consumes the electrolyte and damages the plate material. So if the maximum voltage is regulated to a value less than or equal to the battery’s required voltage, it cannot theoretically overcharge if in good condition.

jonr · October 2018

> I've tested the output and the rectifier correctly limits the voltage output

Rectifiers don't limit voltage and I suspect that this is all this engine has (the limited capacity of the coil and the battery itself provides some regulation). If so, then yes, there would be some advantage to adding a voltage regulator (a shunt regulator is common).

Also note that a typical charging voltage (say 14.4V) will eventually overcharge a battery. This is why a float voltage (say 13.3V) is used in more sophisticated charging systems.

https://www.tohatsuoutboardparts.com/Electrical-Kits.html

Outboard motor battery charger control

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