AGM vs Sealed Lead Acid Battery

[goboatingnow]

AGMs suffer from misleading advertising

Firstly charge efficiency , AGMs ranges from 85-95% just like the variety of Wet LAs.

Secondly theres no evidence that there is an improvement in absorption phase efficiency over wet LAs

Charge rates are not "20 times". typically in fact 2 times ( C20/10 for wet LA T105s and C20/5 for AGMs, source Trojan)

Self discharge is only of interest to a small number of boaters, few leave their batteries disconnected for long periods. Most are on shore power chargers or solar.

AGMs will most definitely gas if overcharged, the rate of the gas combining is finite and can easily be exceeded, especially in absorption phase. This results in permanent loss of capacity and its not capable of being determined by the user. Its one of the most common causes of premature failure.

AGMS need careful charging, especially approaching C20/5 rates. Good temperature compensation is a requirement. Modern regulator alternators often now are set at 14.2-14.4 volts and without compensation, overcharge AGMs. Modern alternators often have in built temperature compensation, but actually in a boat this can cause incorrect charging voltages as the batteries unlike a car are usually not in the same enclosure.

AGMs hate trickle charging

AGMs are very sensitive to excessive absorption phase overcharging and incorrect phase duration. They require good quality smart chargers with temp compensation. Many boats dont have this and most dont have it for their alternators.

AGMs were developed for the military aviation market, they are very good at handling vibration , case leakage (bullets etc), inverted positions etc. All features that have little applicability.

Wet LAs give off approx 0.25 cubic feet of hydrogen per 100Ah, typically this is generated during the absorption phase, A phase that many people never actually achieve. Hydrogen being very light, rapidly leaves the battery space. There is no specific venting requirements in general, other then not making the battery area airtight. Batteries are regularly under bunks in boats etc.

Wet LAs properly tied down will not spill, even inverted spillage is slow, and anyway then its the least of your worries.

Wet LAs have the advantage of being cheap and cheerful. They can be resuscitated after abusive treatment, their internal status can be monitored and excessive gassing can be compensated for. The remain the best compromise between cost and function for marine leisure use.

DAve

[deckofficer]

Originally Posted by Vasco

How are you going to charge the batteries?

While on the hook, 2KW of solar panels, while sailing, props pitched in astern and using regen on the two controllers.

[deckofficer]

DAve,

I wouldn't charge a flooded any faster the 0.1C. So if you have a 500 a/hr bank that works out to 50 amps from the charger or alternator. On that scooter, the bank was a little 48 volt at 16 a/hr and I used a 48 volt golf cart charger to bring to full charge from depletion in about 25 minutes. Can't do that with a flooded.

[Dulcesuenos]

Interesting thread. Our new cat has 3 Napa AGM's 90 a/hr i think, and one deep cycle marine starting for the motor. All 4 are fed via a charge controller from the 200 watt solar bank, and a Ampair wind gen. Also the Outboard has a 25 amp alt on it. The Solar alone charges the batteries very rapidly and goes into a mode where the gauge needle bounces back and forth as I think it is supposed to.
Going thru the paperwork it looks like the AGM's have been warranteed out every 12-14 months. I am not sure if they did this faking bad batteries, to keep them new or if they actually did fail. If they failed yearly I would like to find out why .What can I do to check them and see what their integrity is? There is a large charge controller on built in, and there is a battery charger that can be switched on while hooked to shore power. The batts are only 7 months old at this time.

[goboatingnow]

Originally Posted by deckofficer

DAve,

I wouldn't charge a flooded any faster the 0.1C. So if you have a 500 a/hr bank that works out to 50 amps from the charger or alternator. On that scooter, the bank was a little 48 volt at 16 a/hr and I used a 48 volt golf cart charger to bring to full charge from depletion in about 25 minutes. Can't do that with a flooded.

but you have no idea, if the battery is gassing and loosing capacity, also AGM charge rates for small capacities are better then for very large AH agms.

The other things you will not see is proper charge rates and charge efficiencies for AGMs for a lot of manufacturers as they will not be held to those specs, where such spec's are quoted they are often very conservative, ( like Trojan). AGMs tend to be marketed with loads of extravagant general claims, but the specs say otherwise or make no reference to it at all.

Some like Odessey, can be charged at 1 or even 1.2C, but the key is accurate shut-off. Odessey only reccomend this to bring the battery quickly to 80%. In practice this is very hard to do for big banks and few people can charge a 600Ah bank at 1C. ( also any faults at this type of rate and theres loads of big bangs).

I dont deny that AGMs have a set of particular advantages as, do GELs and even LiFepo4 ( which I love). But taken on a cost/feature basis open cell Wet LAs offer the best compromise for most marine users.

DAve

[hellosailor]

Bob, you're a brave soul and I hope you'll come back to keep us posted on how that works out, year by year. We don't hear much about all-electric sailboats with prop charging.

[deckofficer]

Rather recently a all electric boat (no sails at all) just crossed the Atlantic on solar, averaging 5.25 knots 24/7. As I see it, easier to do with sails, plus I always wanted an all electric galley. No propane, diesel, kerosene, or gas, as the dingy will also be electric. My solar will be around 2kw, the solar on the above mentioned boat if memory serves was 20kw. My 96 volt battery bank will consist of (48) 2 volt, 700 a/hr AGMs for 67 kw/hr of storage.

[Maine Sail]

Originally Posted by deckofficer

DAve,

I wouldn't charge a flooded any faster the 0.1C. So if you have a 500 a/hr bank that works out to 50 amps from the charger or alternator. On that scooter, the bank was a little 48 volt at 16 a/hr and I used a 48 volt golf cart charger to bring to full charge from depletion in about 25 minutes. Can't do that with a flooded.

Many boaters are recharging wets at 0.20 - 0.25C or what ever they will accept, usually about 0.25C. I have lots of customers who's wets, mostly 6V, are going on 8+ years and a couple with over 11 years. The two with banks at 11 years are charging to max acceptance in bulk but also have solar which often get the banks through absorption while on the hook.

You would probably get even longer life if charged at 0.1C but at what a bank of 6V wets cost, just installed a 450 Ah bank last week and the batts cost $320.00, who cares if you get a few more weeks out of them at 0.1C when $320.00 is lasting quite a long time at max acceptance.

[Maine Sail]

Originally Posted by Dulcesuenos

Going thru the paperwork it looks like the AGM's have been warranteed out every 12-14 months. I am not sure if they did this faking bad batteries, to keep them new or if they actually did fail.

Unless that NAPA did not have a quality battery tester, like a Midtronics, Argus etc., then faking bad batteries has become very difficult.

Even Wal*Mart stores each have a $1700.00 Midtronics tester so you can't fake it. Those $1700.00 analyzers probably save Wal*Mart millions in bogus battery returns.

These testers measure internal resistance and even if you drain them dead before arriving at the store the meter can still tell the approx condition.

Even for AGM's every 12-14 months is very short life.

[twistedtree]

Originally Posted by deckofficer

While on the hook, 2KW of solar panels, while sailing, props pitched in astern and using regen on the two controllers.

So your 2KW of solar will charge at about 20A, right? That's about C/35 which is a very low charge rate. With 50% depleted batteries you would need about 20hrs of full sun charge. At a typical yield of 5 full sub hrs per day, that's 4 days to recharge, and that's without running your electric stove, hot water heater, or any other appliances.

How much charge current do you expect to get from prop re-gen while sailing, and at what loss of speed? I'd guess to get the same 2kw you would lose 2-3 kts?

Does this power system actually exist, or is it being planned? I'd be real interested to understand all the engineering behind it.

[goboatingnow]

I'd like to understand it as well. 67Kwh is in reality about a third of that, to have any sort of lifetime out of those very expensive cells. in practice youll have about 20Kwh ( not Kw/h) capacity.

Also why use AGMs, you cant charge at high rates, self discharge isnt an issue, whats the advantage. good quality 2v traction cells would be way way better and would withstand 50% discharges without effecting life time too much.

What are the electric motor power requirements. this boat must need 8-16 kw of motors at least, so in practice you have about 2-5 hours of engine use and a 4 day recharge cycle ( excluding regen). Sounds like a big big genny is needed

You can see this is why all "stored energy" electric drive systems fail. The fail is that the Wh/Kg of batteries is terrible compared to diesel. Diesel is simply a wonder of calorific storage. No matter what you do , one cant get around the terrible inefficiencies of current battery storage.

Of course once we improve those inefficiencies, we then come to the next physical problem, energy generation. Again PV is so poor compared to Diesel, that we need another revolution to find a comparable energy delivery system.

Transatlantic21 the solar boat, has 2x 5Kw panels, taking up 65M2 !, and 1040 Ah at 48 volts, split into two banks one in each hull.I'd like to see how they did it.

In practice unless your PV array can run the system, and generate excess ( the batteries are really irelevant)then the system will constantly be in deficit, the bigger the system the bigger the deficit and the only way to restore that deficit is engines.

I wont be holding my breath

[deckofficer]

Close, charging a 96 volt bank will be at 117 volts, so 17 amps into a 700 a/hr bank is a 0.02C charge rate, but that isn't important. The solar will produce on an average sunny day around 15 Kwh, my power usage without AC will be 12~15 Kwh per day. The motor controllers I plan on using are laptop programmable for the amount of regen, so with just a 0~5K pot, I'll be able to fine tune charge rate vs drag and speed loss. At 6 knots, each motor can produce close to 2kw, for a total of 4kw under sail. If in the Sea of Cortez during the hot season, then I would have an additional 9Kwh per day for AC, which would have me taking a day sail about once a week to stay comfy with enough electrons for AC.

The first project is to set up the dingy for electric propulsion. For this I will use the same type of PM pancake brushless motors as for the big cat. These motors are rated at 12kw cont., and 30 kw for one minute. The dingy will be the minimum wetted surface Dux Hammerhead that due to the tunnel hulled, cat design are able to run at 60 mph with just 50 hp, very efficient hull. Now the math on the dingy is (2) of these motors mounted on top of lower drive from a 115 hp outboard, can produced for 1 minute 80 hp and 32 hp cont. This should make for one very sporty RIB.

[goboatingnow]

Originally Posted by deckofficer

Close, charging a 96 volt bank will be at 117 volts, so 17 amps into a 700 a/hr bank is a 0.02C charge rate, but that isn't important. The solar will produce on an average sunny day around 15 Kwh, my power usage without AC will be 12~15 Kwh per day. The motor controllers I plan on using are laptop programmable for the amount of regen, so with just a 0~5K pot, I'll be able to fine tune charge rate vs drag and speed loss. At 6 knots, each motor can produce close to 2kw, for a total of 4kw under sail. If in the Sea of Cortez during the hot season, then I would have an additional 9Kwh per day for AC, which would have me taking a day sail about once a week to stay comfy with enough electrons for AC.

Some numbers here don't make sense. what is the expected consumption of the motors under normal motoring conditions, if you 24 hour consumption is similar or above you daily production, you will run a deficit. Given the low recharge rate, it means you will have no motoring ability for 3/4 days. I cant see this system functioning without generator support.

Also can you really regen 2Kw per shaft. I know the motor may be theoretically capable of it, but will the prop actually propel it. I suspect the motor will act as a shaft brake. The effect on sailing speed will also be very noticeable. Our simple towed genny, causes a 0.5 knot reduction.

The other thing is, asuming the regen generates as specified, where are you going to store it. in practice you have about 20-25Kwh of capacity, yet the last 20% will be at a much lower acceptance. Most of the regen power will be lost.

typical charge efficiencies will be probably around 85%, but in absorption phase, also more then 50% is lost

Sounds interesting, like to see the math though. it sounds like you've just used max theoretical figures rather then real world numbers.

Dave

[twistedtree]

At a C/50 charge rate (0.02C), I don't think you will ever be able to get the batteries up to their bulk voltage. My house battery bank (house as is terra-firma house) charges at C/22 and the battery manufacturer says I'm at the lower limit of charge rate. At C/50 you will barely be overcoming the self-discharge rate, and is just slightly more than what's required to maintain float voltage.

To get 15kwh/day out of your panels you are planning on 7.5 full sun hours per day, and 100% efficiency. I see you are in So Cal, but I think you might still be overly optimistic on both assumptions. Also, keep in mind that during absorption, the batteries in most systems can't accept the full output potential of the solar array, further reducing the portion of your ideal 15kwh that can really be captured. However C/50 is typically the terminal charge current for absorption, so I think in your system the batteries will always be able to accept whatever your panels can put out.

On the regen charge assumptions, I could see how you could get 2kw from each motor/gen if you were being towed at 6 kts. But going from free-wheeling at 6kts to generating 4kw at 6kts, the tow vessel would have to put out an additional 4kw of energy to maintain speed. It's physics, and there's no way around it that anyone has found so far. Under sail, your speed will drop significantly, and power generated will also drop until the amount of lost speed equals the power being generated. In practice I think you will find your boat slows to 3kts or so and power generated is 1-2kw total.

There's another thread where a guy repowered with a battery electric propulsion system and is finding actual performance to be so far off from the theoretical performance we was sold, that the system is nearly unusable. I just worry that you will sink a ton of money into a system only to find it's actual performance is quite different than expected.

[Captain Bill]

At the Miami boat show in 2005 there was a hybrid cat. It had an electric motor in one hull, batteries in the other and a big genset on the bridge deck. It could put the motor in regen mode and he had about 1000W of solar if memory serves me. I forget the brand, but I think it was a South African cat. In any case, it was delivered to Florida on its own hull. It crossed the atlantic with a sister ship that was Diesel/Sail. The captain claimed that the two boats never got out of site of each other and most of the trip was sailing and on occasion motor sailing. If I remember the numbers he said that the hybrid completed the trip on 85 gallons of Diesel and the other boat used about 350. He said that at 8 knots in the trades he lost about .5 knots in regen mode slowing to about 7.5. While certainly an improvement in fuel consumption it was not capable of going all electric.