Alternator sizing

Maine Sail · Jan 26, 2009

Scott B said:
I am considering going to 4 golf car batteries to replace the 2 group 27's I now have. Last year we smoked the batteries on a few occasions ( dead flat) and had to us the engine start battery to get us going again. That was only after being out for a couple of days with running the engine for a couple of hours.

Am I right that with the 4 golf cart batteries ( 800 - 880 amphours) at 25% I'll need a 200 amp alt. as a minimum ???

We mostly cruise and anchor out on weekends, extended 3-4 day weekends and then get out for 1 1-week stint. We are hooked up to shore power during the working week with an on-board charger.

Scott,
4 GC batts are going to be in 420-450 amp range not 800. A 90-100 amp alt should be plenty. I have owned two boats with four GC's house banks and 90-100 amp alts with no issues..

Anchor Down · Jan 26, 2009

Parallel Lines (of thought)

Um, Scott, that's 800+ amp-hours @ 6-volts, right? When you connect two 6-volt, 120 amp-hour batteries in parallel, you double the voltage, so will have the same 120 amp-hours @ 12 volts.

My math has those 4 golf cart batteries yielding 400+ amp-hours @ 12v.
If that's right, then a 100-amp alt. seems right.

Am I missing something?

Maine Sail · Jan 26, 2009

Scott B said:
A 6V GC battery has Ah of 208 - 220. When you connect them for 12v do you only still get the 208 - 220 Ah ? so with 4 gc's 2 x 208 - 220.

Interstate Batteries

Each set of two 6v combined for 12V will produce the 6V face value AH rating of one 6v. In a four batt bank you'll have a series wired 6V+6V = 12V/220ah wired in parallel to a series wired 6V+6V = 12V/220ah. Two pairs of 6V batts are wired in series for 12v then the two sets of 12V GC's are wired in parallel to make one big bank of roughly 440ah.

sailaway78 · Jan 26, 2009

Alternator

I feed four golf cart batteries, 450ah, with a 70 amp alternator. But I also run 3 80 watt solar panels. You can go a little bit bigger, but unless you go to a high tech regulator, you won't gain very much. Also, a 100 amp alternator is about the upper limit with a single belt system, although others will likely disagtee.

Don S/V ILLusion · Jan 26, 2009

Scott
Answering your question directly can be misleading as alternator rating is not indicative of charge rate. For example, some companies specify a charge rate at one rpm while others use a different standard making comparisons misleading. Regardless of the validity of their rating, the actual charge capacity is always a function of the regulator.

I recommend to people that they always consider it as a system comprised of batteries, charger source capacity (alternator or shore power charger) and regulator.

The conventional wisdom is 20% of the battery capacity be used as a reference for charging capacity - in your case with approx 400 amp cap., an 80 amp rated alternator but make sure whatever regulator you use (internal or ext) be similarly rated. Anything more is excessive and potentially damaging to the batteries..

Maine Sail · Jan 26, 2009

Don S/V ILLusion said:
Scott
The conventional wisdom is 20% of the battery capacity be used as a reference for charging capacity - in your case with approx 400 amp cap., an 80 amp rated alternator but make sure whatever regulator you use (internal or ext) be similarly rated. Anything more is excessive and potentially damaging to the batteries..

Balmar recommends the alt be sized to 25-40% of total battery capacity. On a 3/8" belt you can generally go to about 80 amps, though I have pushed it to 90 with no real problems. With single 1/2" belts you can go to about 110 amps +/-.

How would anything more than 20% of the banks capacity be potentially damaging to the batteries?

Benny · Jan 26, 2009

You would not want to drive a 200 amp alternator with a small diesel. The charging time is dictated by the charge regulator and the adsorption ability of the batteries. Look for somthing in the 80 amp range for a high output alternator.

Don S/V ILLusion · Jan 26, 2009

Maine Sail said:
How would anything more than 20% of the banks capacity be potentially damaging to the batteries?

The most common problem is overheating and cooking batteries at too high a charge rate. This is why many charging monitors have temperature monitors to ensure the charge cuts back when the batteries overheat.

Another potential problem occurs when bulk charging occurs at too high a rate (but does not result in overheating) such that the batteries evolve sufficient electrolyte too quickly such that constant addition of water is required - as many boaters may not check electrolyte level frequently enough, drying can occur resulting in exposing the top of the plates..

People often make the mistake of installing a charging system too large for the bank and don't understand that more isn't necessarily better which is why I suggest it be thought of as a system rather than individual components.

Ross · Jan 26, 2009

I think that a battery monitor should also be on your shopping list, at the very least a wired in volt meter.

Bill Roosa · Jan 27, 2009

Belt sizing, storage vs production and other stuff

Hi Scott B
GC batteries work great, you will be very happy with them provided you can keep them charged.
You really can't design an electrical system by picking stuff that feels good however. The production-storage-use must be balanced

If they are standard Trojan batteries you will get 410 AH which will provide 205 AH MAX if you want to have them last more than a season or two. The more deeply you discharge a battery the fewer number of charge-discharge cycles you can get out of it before it dies. The number take an alarming turn around 40% discharged so I would not use 50% as a design value. As a "weekend cruiser" you should be able to get the batteries completely charged during the week and then draw them down on the cruise. Long time cruisers are not so lucky and most will cycle between 60-90% charged. This is yet another reason to tie all (less start) your batteries together into one big house bank. If you already have a start battery then you certainly don't what to have 2 house banks. One big bank will be discharged half what two smaller banks will.

How much motoring (hours) do you do on an ideal (sail out and back assumed) weekend?

That is one of the things that is going to drive your alternator sizing. You want to be able to charge the batteries to 90ish% with normal engine use if you can. this simplifies the whole charging regime and you get your motor working doing something besides supplying 5 HP to the alternator.
Charging to 90% may or may not be possible because of the loads you put on the boat while sailing or on the hook. I have a spreadsheet that takes the math out of it that I find very handy for just such purposes.
Once you have the loads down you may find that you, for instance, can run the microwave for dinner while motoring up the the final anchorage vs running the microwave after you anchor and save a lot of battery charging.
The order of things does matter in small systems where storage is the primary source of power. The spreadsheet allows you to play with when you charge also.
All this, and the battery acceptance of charge..... stuff that has already been talked about in the thread, has to factor in to get a happy boat.

Then you can go an pick components that make sense for YOU. If you do a lot of motoring then you can reduce the size of your alternator and battery bank. If you have lots of fe-mal-ees with 1000 watt hair power tools then you are going to have to have a more robust system no matter how you motor. The possibilities go on and on and only you will be able to figure out the answer.

So, figure out the weekend electrical load and charge plan then determine the size battery bank that will support that without breaking the batteries or the budget.

email me for the spreadsheet. It is free and comes with instructions.

william-roosa@us.army.mil

Maine Sail · Jan 27, 2009

Don S/V ILLusion said:
The most common problem is overheating and cooking batteries at too high a charge rate. This is why many charging monitors have temperature monitors to ensure the charge cuts back when the batteries overheat.

This is why one should install the optional temperature probes when installing a high output alternator. Too high a charge rate in terms of voltage is user error if the set up of the regulator was not done properly. A properly installed regulator with temp compensation on a HO alt should not charge at a voltage that is too high unless it is broken.

Don S/V ILLusion said:
Another potential problem occurs when bulk charging occurs at too high a rate (but does not result in overheating) such that the batteries evolve sufficient electrolyte too quickly such that constant addition of water is required - as many boaters may not check electrolyte level frequently enough, drying can occur resulting in exposing the top of the plates..

This is one reason I do not like external regulators that try to "force" amperage into a battery at a higher rate than the batts naturally want to accept. Even though the output voltage is fine, 14.2- 14.4, the regulator is trying to force more amps into the batteries than they normally would want to accept. If you add a HO system you should check electrolyte levels on a regular basis. The regulator/alt can only output higher than the batts accept for a short period of time then it will back off based on temperature. If you don't add temp compensation it won't do this and you can over drive your batteries and the alt regulator won't really know though the alternator will get hot..

Don S/V ILLusion said:
People often make the mistake of installing a charging system too large for the bank and don't understand that more isn't necessarily better which is why I suggest it be thought of as a system rather than individual components.

I still ask my original question based on your original statement of 20% capacity. Why is anything over 20% of capacity going to kill your batts if properly installed? In a properly designed system you can comfortably go to 40% of capacity and not cause issues. I know of many, many boats running Balmar 100's on banks of 200-240 amp hours and they still get plenty of life out of the battery bank and this is up to 50% of capacity..

Based on Scott's system he is plenty safe using an alternator between 80 & 100 amps. The GC2 batts he mentioned are 440 amp hours for four. An 80 amp alt is only 18% of that bank and a 100 amp is only 22% so there is no way he is over sized nor could be using a one belt system on his boat so it is a moot issue anyway. He would need a 175 amp alt to get to 40% of capacity which is on the high end of acceptable size.

One other issue you touched on that can't be over looked is the effect of temp on the alt. Most alts are rated cold which gives them better sales numbers. If a manufacturer offers hot ratings that is what will be the safe standard output. Even on a 100 amps alternator you will likely never see anywhere near 100 amps of output unless your bank was flat dead and then it would only put out close to 100 for a very, very short period. Often times your pulley configuration can also limit the output of the alt..

Here are the 20 hours specs for a few popular 6V batts including the Interstates you listed:

Interstate UAGC2N = 220ah X4 = 440
Sam's Club GC2 = 220ah X4 = 440
Trojan T105 = 225ah X4 = 450
Trojan T125 = 240ah X4 = 480
Sportsman GC2 (made by Superior) = 220ah X4 = 440 (Defender's brand)
Sea Volt GC2 (WM Brand) = 215ah X4 = 430

Ross · Jan 27, 2009

You won't harm the engine to run it a bit more and it may even be good for it Those short runs that you mention are not the best way to treat a diesel engine. Make believe that you have an auxillary generator on the boat and run the engine more.

Maine Sail · Jan 27, 2009

With a 3/8"

Scott B said:
It is only a 3/8" one-belt system or am I reading your response wrong. I'm sure it's an 80 amp alt.. I remember looking at it last summer when I tightened the belt, but one of those things that if you don't write it down gets misplaced in the memory bank. :cry: The Yanmar specs for the 3YM30 also list the 80 amp as an upgrade with a belt cover, which is also why I'm thinking it's 80.

Scott,

If it is 80 with a 3/8 belt you are essentially at your top end of what the belt is good for. I have used a 90 on a 3/8 and it worked well but then again I never discharged below 50% of capacity so it never actually had to work very hard.

I would upgrade the bank and add a battery monitor first and foremost. I think you'll find, after adding a battery monitor, that the 80 amp alt is sufficient but your motor run times are not. Short diesel motor runs are not really good for them. I am a firm believer in motor sailing for a while to let it fully come up to temp and burn off any internal condensation doing so would also allow more charging time. You can't put 220+ amp hours, or even close to it, back into a bank running the engine 45 minutes per day.. An 80 amp alt is MAX rated at 80 amps per hour but batteries don't work that way and will rarely if ever accept 80 amps continuously for more than a little while. You alt will also not put out 80 amps continuously even though that is what is stamped onto it. It's just like a stereo receiver that claims 200 watts won't play 200 watts your alt also won't put out 80 amps per hour. It would be nice but it is not the reality of battery behavior.

The output of the alternator diminishes as the batteries get full. This phenomenon is called acceptance. Batteries accept the most when they are totally discharged and the least when they are fully charged. As state of charge rises the allowable amp input diminishes. The last 20% of charge can take 10+ hours or more of run time no matter how big your alternator is.

When traveling on the hook it's a good idea to not count the 10-20% of capacity you can't effectively re-charge via the alt without ten+ hour motoring events. So if a 50% discharge from a 100% charge level on a 440ah bank is 220 amp hours. To get back to 90% state of charge would leave you with 396ah in the bank or 176 usable amp hours. Your real available & usable bank, when drawn to 50% capacity, is now only 176ah @ 90% state of re-charge. At an 80% re-charge you'll have a usable capacity of 132 amp hours before hitting the 50% threshold. You're probably a candidate for a solar panel.

Watch this video and pay attention to how quickly the alternator output/amperage backs off from about 30 amps of output once it senses the state of charge of the batteries to about 1.3 amps. In this scenario the batteries will not accept more than just one to two amps even thought the alternator initially tries to force it in at first. The batteries were at about 99-100% full when I shot this.

What Happens When I Start My Engine (LINK)

sailor5017 · Jan 27, 2009

Hi Scott:

I have been through this upgrade on several boats. As stated you really have to upgrade the alternator battery banks battery monitors etc to make a reasonably good electrical storage and charging system. It looks like your alternator is the stock one that comes with yanmar rngines It may be rated at 80 amps but my bet is you would be lucky to get 40 or 50 out of it in reality. you should consider upgrading to an externaly regulated 100 amp alternator( check out balmar stes) .Externaly regulating allows the alternator to be set up as a three stage charging unit. A good quallity 3/8 power belt will do fine and be able to handle the load
This will give you the ability to put a decent charge into your deep cycle batteries when thet are in the bulk charge mode, in a reasonable time.
also recomend some type of battery monitoring system, such as a link system of some sort, then you will be able to monitor the state of discharge or how many amps you are charging at.

Stu Jackson · Jan 27, 2009

Depends if you're gonna plug in or not, also

See reply
#20 here: http://c34.org/bbs/index.php/topic,4454.15.html

You may not neeed to increase your alternator size, even if you don't plug in ever!