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hlkaulfman 12-27-2007 09:56 AM

Nissan alternator problem
I need the help of someone that understands batteries and DC electrical systems.

Here's the deal.

I have a 6hp Nissan long shank outboard in the motor well of my CD25. I had an alternator (read coil with rectifier) installed. When I measure the output of the motor in neutral I get something in the vicinity of 14.4 volts at something less than full throttle. When I put the motor in gear the output falls to about 10 voltsat full throttle. After extensive discussions of this situation with Nissan tech support, I am told that everything is working properly and that at 10 volts the battery is being charged.

Will a battery charge from a source from which the voltage is lower than the current charge in the battery (i.e. will 10 volts charge a battery that is registering 12.4 volts)? It was always my understanding that it wouldn't but, I am not sufficiently certain to argue the point with a professional technician.

I am also told that, due to the size of the boat, the motor is unable to achieve the rpms for which the motor is rated. They suggest switching from an 8 pitch prop to a 6. The idea is that this will let the motor operate at higher rpms and the alternator to put out more power.

The alternator is rated at 6 amps. I realize that this is not much but, on a typical day of sailing I am not using much power on a C25. I typically motor 2-3 hours in a day of sailing. Shouldn't that put at least 12 amp hours into the battery while a handheld gps, listening to a VHF, a depth gauge, and tiller pilot would not draw near that much?

I hope there is someone out there that understands all of this stuff and can explain it until a novice can understand. I definitely need ?DC Systems for Dummies?. I'll certainly appreciate any help anyone can give. I'm "bumfuzzled".

If you have any thoughts, let me know.

junkmeister 12-27-2007 10:22 AM

10 volts will never charge a 12 volt battery. You need in excess of battery voltage to charge. At 14.4 you will be charging OK. How much does the RPM drop when you are reading 10 volts. 10 volts is way below battery voltage which would indicate to me a large draw on the battery. There is a piece of the puzzle missing here. Also I'm not familiar with your engine. Larry.

k1vsk 12-27-2007 10:27 AM

The charging voltage must be greater than the battery voltage to "push" amps into the battery. A typical 12 V battery therefore requires more than 12V to charge it. If the 6 amp charger were operating at 14V, it would take 10 hours to recharge a 120 amp/hr rated battery from 50% charge to a full charge. If you don't fully recharge a battery often, it will quickly kill the battery.

HoffaLives 12-27-2007 03:31 PM

i agree that if your rpms aren't up your output will fall. the load you are drawing exceeds the power production of the charging circuit so the voltage drops. remember that the rated output of your charger is a theoretical maximum, and as engine rpms decrease, output falls off precipitously.
you will need to check the sum of the current draws of all of your electronics, but i really doubt that that alternator will be able to maintain a charge on a deep cycle battery. you will need to connect to a decent shore-based charger when you get to port.
note that the current demands of a partially-charged battery alone will easily overload your very meagre charging system.

Northbeach 12-28-2007 01:24 AM

hlkaulfman Lee

It sounds like some “tech” is talking out of both sides of their mouth to you. Everything is not working properly; a 10 volt DC output off an alternator will not replenish a 12 volt battery. Others have pointed this out. Your intuition was correct. Your charging source (alternator) must have a higher voltage output than the battery it is trying to charge up.
Secondly if “everything is working right” then why are you being told by this “tech” to drop down to a smaller prop to increase the revolutions in an attempt to boost voltage of the alternator’s output if the alternator output is fine in the first place?
When the Nissan outboard is in neutral you are reading 14.4 volts. When you are in gear, and under power, is your engine’s revolutions faster than they were when the motor was in neutral? If so your alternator output should be at least 14.4 volts. The alternator voltage output should remain stable within a fine range. It should not drop down. As the outboard slows down and eventually stops during the shutdown process the alternator output will fall off –that’s normal. But the alternator’s voltage output should remain stable throughout the normal operating range of your engine: idle, reverse, forward and full throttle.
To figure your electrical load you need to calculate your total amperage draw. Your VHF, depth gauge instrument and tiller pilot should have a wattage rating stamped on them or on listed in the paperwork. Divide the wattage rating of each electrical consumer by system voltage (your case a 12 volt system). This will yield your hourly amperage draw for each electrical consumer; then simply add them all up to come up with your total electrical demand in amps.
For example if your tiller pilot draws 40 watts of power then 40 (watts) divided by 12 (volts) yields 3.3 amps an hour. In that case your tiller would draw 3.3 amps of power each hour at 12 volts. If you had a fully charged 100 amp hour battery (meaning it will deliver 100 amps for one hour or one amp for 100 hours) that battery would last approximately 30 hours and 18 minutes at that draw down rate (100 divided by 3.3 answer is in time).
You may want to calculate your total electrical demands and compare it to the output capability of your alternator to see if you have any capacity left. I suspect a 6 amp alternator is very small, perhaps too small to run your electrical system and charge your battery at the same time. You may end up investing in a battery charger and the necessary power cords to hook up to marina power to keep your battery charged up.



camaraderie 12-28-2007 01:52 AM

Northbeach:"If you had a fully charged 50 amp hour battery (meaning it will deliver 50 amps for one hour or one amp for 50 hours)"
That is not precisely correct. A 50 amp hour deep cycle battery is rated at the amp hours it can deliver over a 20 hour even discharge cycle to 10.5 volts. In this case it can deliver 2.5amps continuously for 20 hours before dropping to 10.5V (fully discharged).
It will NOT deliver 50 amps for one hour. It will deliver one amp for more than 50 hours.

Since the quoted 3.3amp TILLER Pilot draw is HIGHER than the rated 2.5 amp 20 hour will NOT last even the 15 hours you quote. This is often an overlooked area when considering sizing battery or alternator capacity needs.
The rule is: If your hourly amp draw exceeds the battery capacity divided by wll get less than your rated amp hours from your battery and should size up accordingly or plan on recharging more frequently.

That said...I totally agree with the rest of your post.

Northbeach 12-28-2007 02:22 AM

Thank you for the correction. This information and your "rule" is this a marine application or does it carry over to all batteries auto-aviation? Does it vary depending on battery type lead acid/nicad-or whatever eles is available. Electricity interests me although I am not an electrician.

HoffaLives 12-28-2007 03:20 AM

"When the Nissan outboard is in neutral you are reading 14.4 volts. When you are in gear, and under power, is your engine’s revolutions faster than they were when the motor was in neutral? If so your alternator output should be at least 14.4 volts."

he stated that the output of 14.4 volts was at near full throttle in neutral, and if his engine is bogging in gear that can account for decreased output. i understand that these small outboard chargers are really quite marginal.

TohatsuGuru 12-28-2007 08:21 AM

The charging system produces unregulated voltage and the engine only produces it's 5 amp maximum output at 5000 - 6000 RPM. The problem is an application error from using too high of a pitch propeller for the load. To achieve the best possible performance from that engine you will need to drop down in pitch. There are two different 6 pitch props available for that model engine. I would suggest going to the following one as it sounds like you are far below the max RPM.

camaraderie 12-28-2007 10:30 AM

Northbeach...there are a variety of specs available to use for any battery and mfrs. tend to emphasize THE particular spec that is most important for a particular application. For batteries must provide starting power so the 20 hour discharge rate is not very important though it could be figured out. Instead the mfrs. focus on CCA's or cold cranking amps. The construction of an auto battery is designed to maximize the short term amps avsailable. Such batteries are great for cars but in a boat will only last about 1/10th of the time as a deep cycle battery.

In a boat with deep cycle batteries...the longer term potential of the battery is more important so the mfrs. focus on the 20 hour discharge AMP hours and have defacto agreed on this as a means of comparing different batteries. Another important spec is the "# of cycles" which tends to be related to how much lead is in the battery. (Note...When comparing cycles between mfrs. make sure the depth of discharge is the same in counting a cycle.)
You can make a cheap battery with the SAME amp hour rating that will only last 1/2 as long as another with better construction. So...those are the 2 specs most commonly quoted on deep cycle batteries for marine AND other deep cycle use (golf cart batteries, RV house battries, fork lifts, trolling motors etc.) My "rule" applies to all deep cycle batteries of flooded, AGM or Gel "lead/acid" type...but you may find that such batteries in use for other industries use a different defacto standard to compare...commonly used are the 8 hour, 20 hour and 100 hour amp hour ratings. You may also see a RESERVE CAPACITY rating in minutes/hours which is a measurement of how long a battery will last under a 25 amp load before hitting 10.5 volts.
In theory, nicad, L-ions and other types specs should reflect the same type of specs but I cannot speak to that as my experience is primarily with boat systems and these type batteries are not presently in use in boats.

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