Will My Outboard Overcharge My Battery ? Please Help !

[OrangeCrush]

I am trying to resolve a few questions I have before starting a pretty long cruise where I will need to be able to keep my batteries charged for lights, radar, etc.....

I have a 15hp 2002 Mercury 2-cycle outboard on my Pearson 26. When I got it, the charging wires did not produce any current. After replacing the rectifier, the wires produced current ranging from 2-3 volts at idle to 17 or 18 volts (maybe higher) when revving up. Based on some limited research, it seems like this is normal for (what I believe is) an unregulated alternator system. It is my impression that in this arrangement, the battery acts as a regulator and keeps the voltage down, at least until it is fully charged.

So I have a few questions about this.

1) Since my engine is small (low amps?), how likely am I to overcharge my deep-cycle battery? I may be motoring for some extended periods and I will surely go some days without motoring at all. If I do overcharge, will I blow the rectifier again or are there some other consequence? I can deal with less than ideal battery life, but I wouldn't want to fry any electronics. But fuses would protect that, right?

2) Can I unplug the charging wires when the batteries are getting full to avoid overcharging? Should I stop the motor before unplugging or plugging in? Is there any way to know when voltage is getting too high without putting a voltmeter to the battery terminals?

3) Is there some in-line regulator I can install so that I don't have to think about it too much?

4) I have one large new deep cycle battery and one older smaller one that I have been using as a backup. So far I have been using them separately, switching back and forth and charging them up alternately with a plug-in smart charger. Would it make sense to wire the batteries together and treat them as one unit?

While sailing, I was running on the nearly discharged backup battery the other day, and I started to lose some of the more demanding electronics to low voltage. When I started up the motor and throttled up, the electronics came back to life. After running for 15 minutes and cutting the motor, it was another 15 minutes or so before they started dropping off again. This seemed to me like a good sign, that the motor was adding a charge to the battery at about twice the rate of consumption (for what I had running at the time)... is there any problem with this logic (besides being highly oversimplified)?

Thanks a lot in advance, and please forgive my general ignorance of 12v DC electronics!

[skipmac]

You didn't say how large your batteries are. If large like 200 amp hour then you would have to run the outboard nonstop for a week to overcharge them.

[OrangeCrush]

Yeah, I don't remember exactly but I believe the larger of the two is 160 or maybe 200 amp hours. But what if I go out with them fully charged from having been plugged in? Will the outboard start overcharging them right away?

How did you come up with an idea of how quickly they would charge? Obviously I will need to hook up to shore power once in a while since I don't expect to be motoring constantly. But I want the outboard to help offset the power usage as much as possible without hurting anything.

Is there a danger to plugging and unplugging or should I just leave them plugged in all the time, so whenever I am motoring I add something?

[skipmac]

If you start out with the batteries fully charged and motor for many hours you might have a problem. I am guessing that you are correct in assuming your outboard only puts out a few amps, maybe 5-6 tops so it would take many hours to over charge a battery.

As the battery is charged the voltage of the battery increases. Charging a battery takes place when the charging source (alternator, battery charger, whatever) voltage is higher than the battery voltage. The more charged the battery becomes, the higher the battery voltage goes so the difference in battery voltage and the charger voltage gets smaller and the charging goes slower. So even if your outboard will put out 5 amps it probably won't do that into a battery that is charged up.

I would get a digital volt meter and check the voltage at the battery when the outboard is revved up and making 16-18V. That is high enough to damage a battery but I am guessing when the battery is hooked up to the motor it will not put out at 18V.

[OrangeCrush]

Thanks, skipmac. That sounds pretty logical. I will check it with a voltmeter.

So, I'm guessing no harm to the motor (rectifier, stator, etc.) from disconnecting the charging wire? Any danger to electronics if I were to accidentally let it overcharge?

And I take it hooking the batteries up parallel is not advisable since one battery is newer and bigger than the other. Is that correct?

[Ex-Calif]

Originally Posted by OrangeCrush

1) Since my engine is small (low amps?), how likely am I to overcharge my deep-cycle battery? I may be motoring for some extended periods and I will surely go some days without motoring at all. If I do overcharge, will I blow the rectifier again or are there some other consequence? I can deal with less than ideal battery life, but I wouldn't want to fry any electronics. But fuses would protect that, right?

Your charging system should not overcharge your batteries, fry your electronics or blow any rectifiers.

I would do this simple test -
- Put a voltmeter across the positive and negative terminals of your battery with the engine off - a charged system should be about 12.5 volts or better.
- Start the motor and rev it up and see how high the voltage goes. You will now know approximately how much voltage the charging system will put on the system with the battery loaded - 14.5+ is a good target here.
- Turn on all your "normal" load and check the voltage again - this will show you what the fully loaded system will have in excess voltage for charging.

If you have a low amp charge system it is very easy to turn on enough electronics to have no residual voltage to charge the battery - i.e. you gotta shed load to charge the battery.

This will reassure you as to what is going on. Remember - the charging voltage has to be higher than the system voltage in order to achieve a charge.

Originally Posted by OrangeCrush

2) Can I unplug the charging wires when the batteries are getting full to avoid overcharging? Should I stop the motor before unplugging or plugging in? Is there any way to know when voltage is getting too high without putting a voltmeter to the battery terminals?

I wouldn't want to be cruising and have the hassle of unplugging and plugging batteries all the time.

You have a simple system and I would keep it that way. We have inboard power but I wanted to have a real cheap way of monitoring battery power. I bought something like the attached image and it sits in plain view at all times. It monitors voltage.

- With the engine running it should be about 14.X volts. Ours is actually 3 LEDs only - butif I bought one today I'd go with lights and digital readout like this one. I noticed one day the green light (>12.7V) did not come on. This immediately alerted me to a charging problem that I fixed.
- With the engine off it monitors battery voltage. Normally on a weekend sail it will go from green to amber in 2 days with about 210ah on board. In your case it could alert you to when you need to run the engine and fill up the battery.

Depending on your consumption I think you will have to run the engine daily at least.

Originally Posted by OrangeCrush

3) Is there some in-line regulator I can install so that I don't have to think about it too much?

I don't think this is necessary. I also think you should try and locate a manual for your engine/charging system so you can troubleshoot it and have a better idea how it should work.

Originally Posted by OrangeCrush

4) I have one large new deep cycle battery and one older smaller one that I have been using as a backup. So far I have been using them separately, switching back and forth and charging them up alternately with a plug-in smart charger. Would it make sense to wire the batteries together and treat them as one unit?

I assume your outboard is electric start?

If so I would further invest in a 3-position battery switch. 1-2-All. Use the smaller battery for starting and the larger one for house. With engine running switch in both position. Sailing you are on house. When the voltage drops, switch to start - start the engine - then go to both to charge everything up.

If you want to keep it simple you could go with a simple on-off battery switch and wore the batteries in parallel.

The added benefit of the switch is to isolate all the electronics and from the battery at one common location. This is also a safety item. If something shorts, you want a chance at shutting off the battery supply quickly.

Originally Posted by OrangeCrush

While sailing, I was running on the nearly discharged backup battery the other day, and I started to lose some of the more demanding electronics to low voltage. When I started up the motor and throttled up, the electronics came back to life. After running for 15 minutes and cutting the motor, it was another 15 minutes or so before they started dropping off again. This seemed to me like a good sign, that the motor was adding a charge to the battery at about twice the rate of consumption (for what I had running at the time)... is there any problem with this logic (besides being highly oversimplified)?

It's oversimplified for sure. You don't want to deplete the batteries to the point that electronics start dropping off line. Bad for the electronics and the batteries.

The voltage checks I mentioned will let you know at what voltage your system goes critical - probably <12v.

[OrangeCrush]

Wow, Ex-Calif, thanks for the info. You're extremely helpful as usual.

My motor is pull-start, so that makes things a little simpler I guess. But as for estimating my actual consumption and production potential, I'm at a bit of a loss. Unfortunately I don't have a great amount of time before we leave to figure out my usage patterns, but at least it will be a while before I get to the more remote parts of my trip. [Leaving from NYC, the biggest section away from shore power is about 60 hours offshore up to eastern Maine, then 4 days at anchor (no usage, we'll be on land) then about two day trips before we get back to marina country.] I'm hoping to avoid making special trips motoring around with no electronics running just to charge the battery. My biggest fear is that we just won't be able to bring up the charge at all once it gets low and I'll be out there in the fog without instruments.

I will definitely set up the voltmeter you mentioned. Can I wire that up to one of my switches or does that need to go directly to the battery?

[OrangeCrush]

Oh I just noticed that the voltmeter shown plugs right into the DC socket.

[Ex-Calif]

Figuring out your consumption is a bit of a paper exercise best done in excel. the formula is pretty simple.

Amps drawn * hours per day used = amps/day

Then calculate half your available storage power - in my case 210 * .5 = 105

This will let you know roughly how much time you have between charges. I am gonna guess your 9.9 has a 6 amp charger. If it were to charge the whole 105 amps it would need to run 17 hours.

I don't know your engine but some have a 35 amp charger. That would obviously be great.

The good news is that you are probably only going to run an amp deficit of 20 amps (total guess) but with 6 amps available you still are looking at 3 hours a day of motoring. The big killer is going to be lights. Instruments are dead cheap in terms of current. Incandescent lights at night - reading, navigation, anchoring etc. consume a lot.

In terms of the battery monitor, I simply wired a lighter plug into my distribution panel. It has an in-line fuse to protect it. The lighter plug is also handy for running my ipod speakers.

[OrangeCrush]

Great info, thanks. I will do my best to figure out what is actually going on. I'll upgrade the smaller older battery if I think my storage capacity will be too low. If I do that, should I connect (and leave connected) the two batteries in parallel?

I already have the lighter socket so that shouldn't be a problem.

If I do start overcharging out of carelessness, what will happen? Will there be any signs besides the voltage meter? Is there a voltage limit (on the indicator) where you would unplug the motor charger, or should I simply turn on some more lights to draw some of the excess voltage? (Assuming I need to motor at that moment.) I still can't quite understand if it's safe to just unplug the charger from the battery. Seems like all that current with nowhere to go would cause something to blow.

I bought some 12v compact flourescent bulbs but they don't seem to work on my system. Have you had any luck with these?


Thanks a lot!

[Ex-Calif]

For simplicity in your system - two batteries of the same size can be wired in parallel. The concern with this is draining all your battery power and being unable to "electric start" your engine. You have a rip cord so no issue there.

I would not advise wiring two different capacity batteries together. I would be concerned, but somewhat less so of batteries of different ages also. I say this and purists will agree. However, I added a third battery to my system and after 3 years seems no ill effects - they are all the same 70 ah batteries.

I am going to go out on a limb here because I am not expert in unregulated alternators as exist on your engine. The limb is that it would make no sense to have to disconnect your batteries at any time. Our club has 8 rescue boats with electric start and outboard power. No one ever disconnect those batteries. The designer of the system must have known what he is doing.

The best answer is to read the manual. Without a manual I would run the engine until I get a nice big fat green light on my newly purchased $20 battery monitor and wait for the LED voltage readout to come down to 14.5 volts or thereabouts. If the voltage never came down to that level I would be concerned and learn more. With the engine running all day long at 14.5 volts you will never have a problem.

If the voltage settles in at your previously reported 17 volts you would have a serious issue with wet cell batteries (caps and battery fluid) as the high charge rate will boil off the water. On a closed cell you won't lose any chemical but the internal heat could build up and damage the battery internally.

Imagine it this way - Your charger is like a hose filling up a sealed container from the top. Your current load - lights and all, is a spigot at the bottom draining it out. The amp rating is analagous to the size of the hose - i.e. how much water can I push through. The voltage is analagous to the pressure. The batteries contain 12.5 volts static, less when depleted. The hose has to be able to push at 14.5 volts or water (amps) don't flow into the container. If your charger can push 6 amps at 14.5 volts and you are drawing 2 amps with the current load then you are filling the container at 4 amps per hour. When the container gets full, the voltage will still be 14.5 with the engine running but no current will flow other than the 2 amps you are currently consuming.

So the key is to be looking at charge voltage. Look for 14.5v or thereabouts and life is good.

[ViribusUnitis]

Don't unhook an alternator from the battery while the engine is running. The alternator needs the battery to keep everything working properly. Without the battery, the alternator won't have a reference voltage, and can easily dammage the rectifier as the voltage spikes.

[topi]

Originally Posted by Ex-Calif

Your charging system should not overcharge your batteries, fry your electronics or blow any rectifiers.

I would do this simple test -
- Put a voltmeter across the positive and negative terminals of your battery with the engine off - a charged system should be about 12.5 volts or better.
- Start the motor and rev it up and see how high the voltage goes. You will now know approximately how much voltage the charging system will put on the system with the battery loaded - 14.5+ is a good target here.
- Turn on all your "normal" load and check the voltage again - this will show you what the fully loaded system will have in excess voltage for charging.

Hi, I guess the voltages depend on the type of battery used. I have normal Lead Acid batteries (the starting battery is a car battery and the main battery is a Lead acid battery for marine apps). I hear there are also Calcium type of batteries, which should be charged at different voltages. When my motor is running and there's no load on the batteries, I get 13.1V or slightly higher as the voltage, is this too low to charge the main battery? It might be a symptom that my alternator is not working fully. I also have a 36W solar panel set up that is directly connected to the main battery for charging. One day in bright sunshine I observed +15.2V on the main battery. Is there a risk of overcharging and boiling the battery at these voltage levels?

[Ex-Calif]

Above 12.5 you are charging the batteries. 13.1V no load indicates a fairly full charge state on the batteries and as far as the solar panel goes (36/15.2=2.2a) - 2 1/2 amps will not cook the battery.

Both the voltage and the amperage are important. In a mechanical sense imagine amps as the flow and the flow creates friction. Friction creates heat. The battery easily can handle 10amps continuous flow at anything from 12-15 volts. Starters and windlasses generally draw lots of current but only for short periods.

[OrangeCrush]

Thanks again for all the help. I got the voltmeter and tested the system like you said. It creeps up to about 14 or 15 volts while throttling up then levels off. Seems about right, I guess.

I decided to replace the old one battery with two new ones and connect them in parallel. It seems like for cost effectiveness (I am way over budget already) and because of the potential of accidentally overcharging that wet cells are the way to go.

I have been doing a bit of research and without going into too many of the details, I leaning towards buying a pair of Sears DieHard Group 29 Deep Cycle/RV batteries. http://www.sears.com/shc/s/p_10153_12605_02827582000P

Does this seem like a good choice? Obviously there are better batteries out there but most of them are way out of my price range. Also I care less about battery life and more about holding a charge for as long as possible between proper shore power recharges. They are rated at 115 amp hours each. A month and a half from now I won't be nearly as concerned with my batteries in the long or short term. I want to get through this trip successfully.

Remember, I don't need the battery for starting. I need it for house power, including gps and radar. I will probably want to have the gps on for a lot of the time. I am going to be 'off the grid' for at least 5 days at some points, but I am expecting a couple hours of motoring a day to make up for at least a good part of my usage.

Is there anything wrong with my assumptions? Is there a better battery out there in the price range? I could buy just one Platinum battery for the price of these two and get slightly fewer amp hours. For some reason hooking these two up in parallel seems like a better choice to me, but I am open to suggestions.

Opinions? Thanks again!