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Old 10-26-2019, 02:41 PM   #1
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Mercury 6.2 L alternator and Firefly batteries

I am just going to do a copy and paste from a conversation I had elsewhere. I will comment after the copy below:

"Grant October 12, 2019 at 11:45 am
Rick – you might want to reconsider holding off on that alternator upgrade – your Mercruiser 70A unit is internally regulated at a fixed 14.4V – and your $2,000 worth of new Fireflys are not going to like that long-term. They have specific float voltage requirements of 13.4-13.5V, and other criteria – see manual. They’re a great battery, but I wouldn’t make the investment until I had all my charging sources sorted out to deliver the appropriate charge profile they recommend…

So I did go into the Merc manual. My first mistake is the alternator is 72 amps hot and 65 amps cold. But so far I have found nothing about voltage being fixed at 14.4V.

I assume the above gentleman has done his homework and what he says is true. Is there anything that can be done in the above scenario so as not to cook the batteries. By the way, I am not considering an alternator upgrade currently as it will affect my Merc warranty and confirmed in a phone call to them in Wisconsin.
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Old 10-26-2019, 03:18 PM   #2
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Measure the actual voltage. What is it?

If it is too high, put a diode isolator in, it will drop volts by about 0.7-0.8.
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Old 10-26-2019, 03:34 PM   #3
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Ski is right above, but... you will get very slow charging if you install a diode. And it has to be a heavy duty one! At least 75 amps.

I can’t imagine a simple alternator upgrade to a high output, externally regulated alt such as Balmar would violate the warranty. I would just do it and if something happened to the engine, just swap the old alternator back before I told Merc.

You are spending a lot on very nice batteries. Upgrade your alternator so you can use them right.

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Old 10-30-2019, 11:27 AM   #4
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Don't know the laws in BC, in the US modifications to the alternator would not affect the warrantee for the rest of the engine by long standing federal law.

The internal regulators on car alternators (which is what you have there) do not have such a well designed setpoint. Usually they start hi, around 14.4 then drop as they heat up. This is intentional, since a constant 14.4V on a flooded battery in a car will dry it quickly. If is it well regulated at 14.4V, and you use the boat normally, it might be OK for the Fireflys. These have a bulk/absorb voltage of 14.4, and if you are running only 1 - 6 hours at a time, that is a good charge. If you were running 24 hours day after day, that is float - and should be set lower.
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Old 10-30-2019, 02:38 PM   #5
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I did call Merc in Wisconsin and confirmed the voltage output, so the 14.4 v is accurate. The Mercury guy said Mercury hasn't tested their motors with Fireflies so he said he couldn't comment. In actual fact, he had never heard of Fireflies.

I then went to the Firefly manual, and here is what I found:

"Operation & Charging

 The Oasis battery can be operated in a partial state of charge for long periods of time without sustaining any permanent damage.

 The Oasis may emit gas during the first 10-20 charge cycles. This is normal.

 The maximum recommended discharge current is 0.7V for extended periods of time to ensure the longevity of the battery. The FF battery discharge faster than that for short periods.

 If you have hardware that requires a Peukert constant to be entered, use 1.07.

 For a complete charge cycle, charge the Oasis to 14.4V with temperature compensation (bulk phase) and continue charging until the charging current drops to 1.5A(absorption phase, time will vary). You DO NOT need to fully charge the Oasis each cycle in order to maintain the capacity and only need to perform a complete charge cycle when you want maximize the capacity for the following discharge cycle.

 For charging sources that may be charging the battery for an extended period of time (solar, or an alternator if motoring for a while); set the float voltage to 13.2V or less. The Oasis does not require a float charge. But, if float charging, due to the Oasis’s longer projected lifespan, it is important to keep the float voltage at or below 13.2V to ensure the battery lasts for as many cycles as possible. Reset to bulk phase: for programmable charging sources, adjust the “reset to bulk phase” to occur if the battery voltage drops below 12.0V for >1 minute.

 The optimum operating temperature for a lead-acid battery is 25°C (77°F). As a rule of thumb, every 8-10°C (14-18°F) rise in temperature will cut the battery life in half."
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Old 10-30-2019, 02:58 PM   #6
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I'd want to test what your alternators do in practice. Knowing Mercruiser, I'd expect those alternators are some kind of Motorola or Mando unit. My older 55A Mandos on my Mercruiser 454s are internally regulated, but output varies a bit with engine room temperature. Usually they're around 14.3 - 14.4 volts at cold start up to as high as 14.9 volts in cold weather from what I've seen. By the time the engines are good and warmed up, voltage settles down to between 13.9 and 14 volts and stays there.

I figure 14 volts isn't exactly optimal for the engine start batteries that get warm in the engine room (they'd probably be happier at lower voltage after a couple hours of running), but my house batteries are outside the engine room and stay cooler. 14 volts is above their recommended float voltage (13.8 volts for my AGMs), but below absorption voltage. So I figure it's not enough to hurt anything, even after a few hours of running.

Of course, 14 volts is also lower than ideal for topping the batteries off, but even at 14 volts these batteries accept a decent bit of current, so they'll get to full (or at least very close) eventually. Of course, I don't really expect 100% charge from engine runtime anyway, that'll normally happen during a generator run or on shore power.
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Old 10-30-2019, 06:05 PM   #7
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Quote:
Originally Posted by rsn48 View Post
 For charging sources that may be charging the battery for an extended period of time (solar, or an alternator if motoring for a while); set the float voltage to 13.2V or less. The Oasis does not require a float charge. But, if float charging, due to the Oasis’s longer projected lifespan, it is important to keep the float voltage at or below 13.2V to ensure the battery lasts for as many cycles as possible. Reset to bulk phase: for programmable charging sources, adjust the “reset to bulk phase” to occur if the battery voltage drops below 12.0V for >1 minute.

There's a current thread on cruisersforum.com where Bruce Schwab/Ocean Planet (the US Firefly distributor) says the recommened float voltage has changed recently. 13.4/13.5V, something like that, but you can check that thread out to be sure.

Would have thought alternator charging while underway, while at the same time running other DC loads, wouldn't often be the same as a long-term float using a charger and shorepower or generator. Just a thought...

-Chris
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Old 10-30-2019, 06:42 PM   #8
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In any internally regulated alternator, the set point is at very best approximate. I be astonished if that alternator maintained 14.4V over load and temperature. It would e one of a kind. But again, float is for long term, meaning days at a time. Unless you run your motor for days at a time, you are just giving the batteries a good bulk or absorb charge which is usually set to something like 2 to 6 hours. It isn't going to be as good to them as a properly regulated charge system, but won't do them damage immediately either. Lifetime will be shortened by some amount.
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