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Old 19-04-2006, 16:45   #1
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New Batteries Needed

While I am not a rookie with regard to most sailing issues, I'm afraid the discussions on this forum regarding electrical matters are embarassingly far over my head. I would appreciate some advice for one who is electronically challenged.

My boat is a 27' sloop with a house battery and a starter battery. Other than starting, my electral uses are various lights (usual running, steaming, anchor, below deck lights), two 12v fans, stereo, vhf, autopilot, depth, speed, log, bilge pump, pressure water, water heater, mascerator (which is not used) and that is about it. Both batteries are old, and quite dead.

I spend virtually every weekend on the boat between now and late November and about half of the time spend the weekend on the hook, as well as a few longer cruises of about a week in duration. I don't tend to spend more than one night in any one anchorage so my batteries get charged every day as I use the engine either to leave an anchorage or to approach one, if not both, even if I might otherwise do it under sail, just so I can charge them. I am pretty careful about not using anything electrical at anchor other than the anchor light, the fans when necessary, and short periods of reading light, vhf for weather, and stereo use. When in the marina I do have an AC battery charger.

I have not bought new marine batteries and would like to ask a few questions:

1. What is the difference, in terms of pros and cons, between AGM and Gell house batteries?

2. For both the starter and the house battery, would anyone be kind enough to recommend a specific brand and, if possible, a specific battery.

I am willing to spend what it takes to get something that will give me the best overall batteries but my ignorance leaves me a bit confused with the choices out there.

Thank you for any assistance.
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Old 19-04-2006, 18:54   #2
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You say your batteries are old. If you got good service from them and they worked to your satisfaction, you might be advised to just replace what you have. That way they will fit without problems.

Otherwise you want to make sure that your house battery is a deep cycle battery. Trojan make good batteries. Many batteries marked "Marine" are not deep cycle so be sure that's what you get. The starter battery can be a good automotive type battery.

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Old 20-04-2006, 01:51   #3
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For a "starter" your boat size is going to dictate what you put into your boat to some extent. However, you work out how much you are going to draw in x period of time. Work out how much current each item draws. It should be marked on the device or in it's manual etc. Then muliply each device by the period of time it maybe on for over that toal of X period. The answer is the total amps you take from the battery system. You want a battery size that can supply that amount, yet still have at very minimum, 50% of it's charge left at the end of that x time.
Gel or AGM batteries have one distincted advantage. They are sealed. You don't have to do anything with them and they can be underwater if something like that should ahppen. This maybe a benifit to you in a small boat. You don't have to worry about heel, and you can place them down in the bilge for stability, without having to worry about them being flooded with seawater. They also recharge just slightly faster, which could also be helpful.
Flooded batteries, meaning a battery with caps and you top them up with water, have that has a negative. You can't ever have them get wet with seawater. They are possibly slightly larger in physical size compared to the AGM etc. But if well looked after, they last a little longer than AGM etc.
The last thing is the "well looked after" issue. You need to get a good battery charger. A three step charger is what you will need. A good charger will keep batteries close to their full performance a lot lot longer than a trickle charger. A trickle charger is next to useless at maintaining a battery to correct charge and the battery will fail prematurely if left on one of these long term.

For God so loved the world..........He didn't send a committee.
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Old 21-04-2006, 04:11   #4
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Thank you much gents. I try to research things in advance but my work schedule, the distance I have to travel to work on my boat and the miserable weather lately has me rushing to get her ready for launching this year. Soon, with some luck, she will be back in the water where she belongs. Fair winds guys.
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Old 21-04-2006, 08:51   #5
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Deep Cycle, Gel and AGM

There are three popular battery types used in boating applications these days: Wet Cell, Gel and AGM.

Wet Cell
Wet cell batteries can be broken down into Automotive, Deep Cycle and Maintenance-Free categories. All lead acid batteries shed plate material with each discharge cycle so thicker plates last longer. Thinner plates may provide more instantaneous current though.

Automotive and Marine Starter batteries have thin plates and are designed to put out high current for a very short period (95% of full charge) and then receive immediate recharge. Starter batteries take a discharge down to 50% on the chin and may never recover fully.

As Wheels mentioned, Deep cycle house batteries are designed to be drained down to 50% of capacity and typically serve low current needs (lights, nav equipment, etc). You can discharge a good Deep Cycle down to 50% more than 1,000 times. Deep cycle batteries, as long as they have the current output to do the job, don’t suffer from high current use (i.e. starting the diesel).

As Phil says, many marine batteries are not heavy enough to last very long in house applications. As far as I know there is no minimum requirement for labeling a battery “Deep Cycle”. Look for a good 100 Ah deep cycle battery to weigh 75 – 85 pounds, or buy the heaviest battery you can find with the same rating if you’re looking for long life.

One concern with wet cell batteries is that they loose about 1 percent (more as the weather heats up) of their charge a day. When you charge a wet cell battery it will release oxygen and hydrogen. Oxygen good, hydrogen bad (highly explosive). Need to store these guys somewhere with proper ventilation.

Wet cells need to be topped up from time to time with distilled water. Don’t buy maintenance free wet cells. Boat chargers over charge them causing them to vent (they have a safety venting valve) which greatly reduces their usable life.

Gel cells use a semi-solid electrolyte gel. They are constructed with lead internally and don’t require the structural antimony present in wet cells, allowing a greatly reduced self discharge (less than 1/10 of a percent per day). A wet cell may be good for about a month without charging before permanent damage is done, gels can sit much longer. Gels have good current output and can work as starter batteries or house batteries. Gels charge faster and although a properly charged deep cycle could last longer than a Gel, given typical charging regimes on boats, Gels tend to last longer because they charge faster. Gels need no maintenance and as Wheels mentioned they can survive submersion. Over charge them though and they’re toast.

These are like gels but they have a glass mat between the plates flooded with electrolyte (Absorbed Glass Mat). They are like gels in almost every way except that they have an improved output but are more susceptible to over charging (never put more than 14 volts on one of these).

I would recommend you buy two of the same type battery (both Wet, or both Gel/AGM). Gels charge with a different pattern than deep cycle (wet) batteries and they have a different discharge profile. Combining both batteries in parallel for charging or starting (when the starter bat is not cutting it) can shorten the life of one or both batteries if they are dissimilar. Here are two possible setups:

The Bargain Setup (Deep Cycle)
Try an Automotive Starter Battery and Golf Cart House Battery, both are mass produced with high quality low price units available. Make sure that you get a good quality Automotive or Marine starter battery; a boat will shake a cheap battery’s internal lead structure apart. Also don’t discharge a starter battery more than 95% if you want it to have a long life. Golf cart batteries take untold months of abuse on the links and tend to provide great service as house batteries at a value price.

The Pricey No Maintenance Setup
Two AGMs. No maintenance, no battery dissimilarities (interchangeable and combinable), great for house and good for starting but not cheap.

Human history becomes more and more a race between education and catastrophe. -- HG Wells
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Old 22-04-2006, 06:05   #6
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Thank you Randy, that was very helpful.
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Old 26-04-2006, 16:53   #7

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Simply put:
Gel: Obsolete technology. Pass.

AGM: Sealed, can't spill acid, can't eat your clothes. Also can't add water if you've boiled some electrolyte out. Expensive, typically 30-40% higher than wet acid. Can also take a charge at 25% of their capacity ("C") rate, instead of the 20% of wet cells, so you can recharge them 25% faster--assuming your alternator/regulator are sized for this.

Wet cells: Did I mention spilled acid?<G> But still the most bang for the buck. Some care needs to be given to tie-downs and spills and hydrogen gassing during charging. And, wet cells will take permanent damage after 30 days without a charge--so you must maintain them over the winter. In contrast, an AGM cell can be left for six months with no care, and take NO damage. So, if you plan to leave the batteries aboard over the winter, with no charging, go with AGM for that reason alone. But the price difference can make wet cells the right choice for a lot of folks.

Installing a proper marine regulator and matching the capacity of the alternator can do more to extend your charging capacity and battery life than you might think. That also means running the regulator's "sense" wire to the battery itself rather than tying to to the alternator output, as some cheap installations with automotive type alternators do.
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Old 26-04-2006, 18:59   #8
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How have you determined A. That gel-cell batteries are obselete? They are quite good, given a good brand. B. That wet batteries give the best "bang for the buck" without defining what that exactly means and backing it up with some facts?
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Old 26-04-2006, 21:02   #9

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The primary advantage of gel cells was that:
1-There was no liquid electrolyte
2-They would not suffer from self-discharge like wet cells did

But the drawbacks were that the "gelling" agent changed the chemistry, requiring different charging voltages & schemes. And, it reduced the overall capacity. With your experience with Link I expect you're well aware of that. Folks who used gel cells without making accomodations could hurt them.

Then AGM cells were produced, with all the advantages of gell cells BUT with improved capacity (because there is no gel agent "diluting" the electrolyte) and charging/voltage requirements that match conventional systems and wet cells. Bottom line? Anything a gel cell could/can do, an AGM cell can do better. With less special care, and more capacity per cubic inch. There is no longer *any* advantage to the older gelled electrolyte technology. That makes it obsolete. It may still be "good", but it is OBSOLETE, as in, there is a better product to replace it.

Even the makers of traditional "gel cell" powered devices, like emergency lighting and alarm systems and UPS systems, have largely switched to AGM technology even though their new cells still look like the old gray/black "gel" cells are many users are unaware of the difference. AGM technology simply beats them in every way.

B. Bang for the buck:
Wet cells are still much cheaper than AGM cells. Typically 2/3 of the price, sometimes only 60% of the price. And for a given cubic foot, they typically have 10% more capacity, although that will vary with the case design and other factors. They are also WAY more popular, so they are stocked and sold in larger numbers, and far more companies make them. All of that means again that the wet cells are sold WAY cheaper in terms of amp-hours-per-dollar or per cubic foot.
The only drawbacks to wet cells, are that they can and will spill acid. They cannot accept a charge at the same rate as AGM cells, 20%C compared to 25%C, and by the way gell cells have an even slower charge acceptance rate, another point against them. And, they self-discharge and without equilization that gets even worse. AGM technology prevents the sulphating that builds up "sludge" in the bottom of wet cells and helps stabilize the plates, physically. So with a wet cell, you need to beware of the acid, beware of the hydrogen, and make sure you don't let them sulphate--they can't sit 30 days without a permanent loss of capacity.
But, in routine use and with routine care, the wet cells will give you 30% more amp-hours per dollar. Literally, more bang for the buck.

You can verify all the numbers independently by checking with the various battery manufacturers. Pricing from the stores and the dealers. WHile you're doing that, note that the most common AGM battery name is the "Optima", which is not simply an AGM but a spiral-wound AGM. Flat plate AGM batteries are available for 10-20% less than the Optima line but much harder to find. Aside from that, Optima can be treated as any other AGM--except, because it is a "circle" in a "square", it loses another 5-10% of capacity simply form not filling the available space with battery cell.

As I mentioned before, sealed cells of any kind are also a trade-off. If you have an alternator accident and overcharge a wet cell--no problem, you can add water. If you overcharge an AGM, it permanently loses capacity because it is sealed. So wet cells are more resistant to charging abuse or misuse, as well. That doesn't necessarily make them better, but is one of the points to consider. Wet cells also typically have longer guarantees, in both deep cycle and SLI batteries.
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Old 27-04-2006, 06:12   #10
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For what its worth, the sales people in the three chandleries I spoke to knew not a fraction of what you folks have posted. I ended up buying two dual purpose AGMs even though it meant going from the 1000 amps of my old starter battery to 660 amps with the new. I am scheduled for launching today or tomorrow so I should find out this weekend how well it works out.

One additonal question: with dual purpose batteries, should I continue the practice of using a dedicated starter and a designated house battery or should I alternate the two?
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Old 27-04-2006, 10:23   #11

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Part of the "newer" concept of using separate starting and deep cycle batteries is to use a smaller starting ("SLI") battery to save money and weight, and reserve it for starting alone, along with using a single large deep cycle battery so it that goes flat you can still start from the reserved lighter cheaper starting battery. So, if you already have twins you've lost part of that advantage--having a smaller cheaper dedicated SLI battery.
You do have "matched" twins in the sense that batteries from the same production run usually are most similar in performance and they can be used as a pair with the least waste. I don't think I've ever heard a stunning technical reason to decide how you want to use them in that situation.<G>
You could alternate, i.e. #1 on odd days #2 on even. Or simply use both for starting and running all the time, but alternate which one you "save" for starting when the engine is off. Anything that balances your usage pattern should work out about the same in the long run.
The big unmentioned factor is how do you charge them? How is your charging system set up? If it treats "the batteries" as one bank, one sensor, all the time, then you will do better by running down one battery (or both) and then recharging the same, i.e. don't discharge #1 but then charge BOTH, since you'll be overcharging the one that hadn't been discharged.
I hope that reads the way I think it does, holler if it doesn't seem clear.<G>
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Old 27-04-2006, 11:48   #12
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The idea with a dedicated starting battery is it really is easy to drain down your battery and you just want to be sure your starting battery will - start the engine. A true staring battery will deliver more cold cranking amps that a deep cycle battery will and you don't want to deep cycle a starting battery in any case. Also note you don't ever want to discharge even a deep cycle battery more than 50% as it shortend the lifespan far quicker if you do. They don't last forever.

I connect both of mine to the alternator and place an isolator between them so they both charge but using the house battery won't discharge the starting battery too. I have a combiner switch betwen them in case I need the house bank to start the engine should the starting battery fail.

I would not rotate them.
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Old 27-04-2006, 15:03   #13
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Sjs and Hellosailor, separating fact from misunderstanding

The concept of alternating two battery banks is one to gain an economic optimization which historically has worked in the alternative energy applications and not in marine applications. The differentiating caveat is that in order to gain the economic optimization when switching to the other full battery is that the discharged one must immediately and completely be recharged before left to stand. Most vessels do not have sufficient space or can handle the weight due to necessarily having to completely double the requisite capacity for this type of operation.

Performance tests have shown, and continue to show, that one cannot generalize an overall superiority amongst AGM, gel-cell, or flooded-cell lead-acid battery types. It comed down to specific brands and models. Because they all follow lead-acid electrochemistry the proper charge voltage is directly related to temperature, specific gravity of the electrolyte, and charge acceptance (a multiplicity of factors, again not to be generalized with validity). Therefore, it is widely misunderstood that AGM and gel and "wet" batteries should be charged with different voltages. Now it IS posssible to make (and they ARE made) flooded-cell lead-acid batteries having the SAME specific gravity electrolyte as AGM and gel-cell lead-acid batteries they will then resond properly to the same charge regimen as far as voltage applied versus time all the way to float.

Your assertion, "Anything a gel cell could/can do, an AGM cell can do better. With less special care, and more capacity per cubic inch" is false. There are several brands of AGM batteries which do not perform as well as good brands of gel-cell batteries OR as well as good brands of flooded-cell batteries.

For years I have had the benefit of using battery testing equipment having the ability of testing for Amp-hour capacity as well as true energy capacity, i.e. Amp-hour-Volts and the opportunity to test many battery types to verify the veracity of the battery marketing department claims ( such claims often are not the claims of the electrochemists which designed the batteries working for the same company....not unusual for other types of products, in general).

I have found, in general, that the claimed Amp-hour capacities are "overly optimistic" at the least with some delightful exceptions. What is most important, however, is that because, in general, AGM and gel-cell batteries have higher specific gravities than flooded-cell batteries (for consumer wet-cell batteries having higher specific gravity relates to more rapid internal degradation for the same temperatures over time than the other types yet they are made, therefore, for low temperature operation) they have higher operating voltage over their discharge curves and, therfore, have HIGHER Amp-hour-Volts capacity.

In general, flooded-cell batteries have LOWER Amp-hour-Volts capacity even though they CAN demonstrate higher Amp-hour (missing the Volts) capacity. Get it? They have LESS energy per pound capacity than either the AGM or gel-cell technologies.

If one ceases to look beyond the purchase price it would appear that, in general, wet batteries are cheaper than the others yet when alalyzed in terms of pennies spent per kilo-Watt-hours derived over lifetime that is not true either.

Because AGM and gel-cell technology provides more intimate contact between the active plate material and the charge carriers they exhibit a much lower internal resistance than flooded-cell batteries making a significant benefit in performance as well as increased cyclical efficiency. This, in addition to the higher terminal voltage under load pound-for-pound, or cubic foot, makes them superior starting batteries as well, therefore, spilling acid is definitely NOT the only disadvantage of flooded-cell constructed lead-acid batteries.

You have a misunderstanding of what contributes to "...sulphating that builds up "sludge" in the bottom of wet cells...." Because AGM batteries ARE lead-acid electrochemistry they still normally form sulphate during any discharge. Even flooded-cell batteries do not form such "sluge" when treated properly. I worked at a site which had huge expensive flooded-cell battery banks 25 years old. They had transparent plastic cases and you could SEE no such sludge. They were treated very well.

Wet cell technology can create batteries capable of standing well over 30 days without significant permanent loss of capacity. Here's the terminology: sulphation is a normal reversible electrochemical reaction. Suphpatation (note the spelling difference) is an irreversible product contributing to the permanent capacity loss of any lead-acid battery (all types).

Even comparing the best of the flooded-cell batteries against the worst of the AGM and gel batteries (well, almost the brand was really low) I have not measured a 30% advantage in Amp-hour capacity and absolutely not in Amp-hour-Volts capacity. Where did you get this? Do you believe everything that marketing people put on data sheets?

BTW: Optima batteries (which I do like) are not the most common AGM batteries. GNB batteries (one of the three major manufacturers of lead-acid batteries in this country for years), for example, have a huge customer base not known in the marine market.

A final comment regarding charge voltages: If you read my contributions: Understanding a model for battery charge acceptance on 13-01-05 and How "fast" you can reliably charge your battery. on 10-06-05 in this "zone" you might understand better some of the safe boundary limits for charging voltages. Of particular note is the concept of Amp-hour-law charging which dictates that there is no SINGLE correct charge voltage except for final float voltage. Keep in mind that all correct voltage points versus time are the SAME for the SAME specific gravity whether or not the battery type is AGM, gel-cell, or flooded-cell construction. This concept washes away the multitude differences advertised by the various marketing departments selling their batteries.

It is the creation of new classes of battery chargers which finally provides the voltage regulation and/or current regulation which allows the sealed-types of lead-acid batteries to achieve their designed long lives and superior cycling abilities.
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Old 27-04-2006, 15:07   #14
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O.K. I mispelled sulphatation (as opposed to sulphation, the normal formation during discharge).
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Old 28-04-2006, 15:44   #15
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I did not know, so had to find out how my charging system works. It turns out it charges both batteries at the same time, whether on engine from the alternator, or on shore power from the charger, regardless of what I do with the battery switch or charger. I must confess, I am still unsure what this will mean with my choice of AGM batteries but I have to live with my decision and figure out how to optimize it. Thank you much for the input.
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