Battery safety

Discussion in 'Electrical Systems' started by duluthboats, Jul 31, 2002.

  1. duluthboats
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    duluthboats Senior Dreamer

  2. Jeff
    Joined: Jun 2001
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    Jeff Moderator

    Yikes!

    Not directly related, and I probably shouldn’t write this as it exposes my own lack of knowledge, but I bought a pair of marine gel cell “sealed maintenance free” batteries when they came out ~6 years ago and were supposed to be the latest greatest thing. When I first got them, the sides were a tiny bit swelled out, which was somewhat concerning but I bought them in a store and they wer all like that so I figured it was by design. Well, over the summer of use they got a little worse. Then a few months after I put them on a shelf to store them for the winter, I noticed that they had oozed near the terminals so badly that one of the terminals actually corroded right off. Someday I need to educate myself more about batteries as the finer points escape me (still not sure what went so wrong with those.)
     
  3. Polarity
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    Polarity Senior Member

    Jeff

    Get yourself a copy of Nigel Calders book "The boat owners mechanical and electrical manual". Amongst a million other things it has more than you ever wanted to know about batteries!

    Paul
     
  4. Willallison
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    Willallison Senior Member

  5. Portager
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    Portager Senior Member

    Boom Box Analysis

    I reviewed the account of the "boom box" incident and my guess is that the "regulated charger" probably does not have temperature compensation or if it does the temperature sensor wasn't attached to the battery. In high temperatures (and the battery charger will raise the temperature of the battery) a charger without temperature compensation will over-charge the battery and electrolyze the water. This produces hydrogen and oxygen gas inside the battery which flows out the vent holes. As this process continues the gasses will purge the air (nitrogen and oxygen) from the interior of the battery creating an ideal stoichiometric mixture ratio of hydrogen and oxygen. When enough of the water is consumed to uncover the plates in one cell then arching will occur and ignite the gas. Since the velocity of the flame front of hydrogen/oxygen is greater than the speed of sound the gas/air surrounding the combustion products can not get out of the way as fast as combustion products are produced. The resultant increased local pressure causes an increase in the burn rate, which causes increased pressure, which ... until all combustible products are consumed. Combustion that increases at an exponential rate is the definition of explosion.

    This illustrates the danger of vented lead acid batteries when combined with charging systems that are not temperature compensated. I couldn't design a better time bomb if I tried! IMHO if flooded lead acid batteries were developed today, they would be deemed too unsafe for general use and would be banned. This may sound harsh, but think about it! You have an open tub of acid that the Department of Transportation (DOT) will not even allow to be transported on highways with the acid in the cells and we mount them in boats! Then people buy the cheapest charger they can find (which is only suitable for use over a limited temperature range, nobody reads the safety warnings), which when used in hot weather turned these devices into a time bomb and the only thing that prevents them from going off is diligent preventative maintenance (i.e. adding water). Slight pause while I climb off my soap box.

    The link that Will provided is informative, but keep in mind that Victron Energy is not a totally unbiased reference. They are a distributor of Carbon batteries and WhisperGen Stirling engine generator/heaters. There information is accurate as far as it goes but their comparisons are a little misleading in that they compare their products to products that make their products look good.

    I think http://www.vonwentzel.net/Battery/ is a better unbiased resource, although he could be accused of going a little overboard. Don't know anybody else like that. Nope!:D

    As an engineer for a defense contractor, part of my job is to select the batteries that we use in the weapon systems that we develop and install in our customers boats. We take safety in our systems very seriously. When we use lead acid batteries we only use AGM batteries and we always use a positive ventilation system which makes a hydrogen fire or explosion impossible (and I never use that word). One thing I really like about AGM batteries is they are truly maintenance free and with a temperature compensated smart charger they are very tolerant of neglect and abuse.

    If you select batteries purely based on purchase price you will buy golf cart batteries at Sam's Club and I'll wish you luck. By the way, I'll be move my boat a few slips away. (I have finally learned not to argue with the, "Don't confuse me with facts, my mind is already made up" crowd.)

    The slightly more analytical consumer will look at the manufactures cycle life versus depth of discharge and decide that if you can believe the manufacturers claims then life cycle cost is a push and then buy what ever they prefer.

    If you very analytical you will understand that the cost of producing the power that goes into you batteries, especially the cost of power produced on board exceeds the cost of the batteries by several times. Therefore the higher charging efficiency (power out/power in) and higher charge acceptance (maximum charging rate) make AGM batteries significantly more economical than flooded or Gel cells. I got so tired of arguing battery economy on the "Trawler List" that I proved that it was cheaper to use Lifeline AGM batteries than to use free golf cart batteries unless you pay a flat rate for electrical power regardless of consumption (i.e. the all you can eat energy bar! We don't have them in California).

    If you are concerned about safety, then you will want AGM batteries and get a smart, temperature compensated battery charger.

    If anyone in interested in a long diatribe explaining how I arrive at the above conclusion, then just ask. :D I could also share the battery box venting technique that we have developed and use on all our systems.

    Cheers;
    Mike Schooley
     
  6. Willallison
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    Willallison Senior Member

    Victron certainly couldn't be described as unbiased - as you suggest - but it is interesting to note that the AGM battery that they rate above all others is one that they don't sell themselves - the Optima range
     
  7. Portager
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    Portager Senior Member

    Yes, it is interesting. Like I said they choose to make their products look better. Since they don't carry AGM batteries they choose ones that made Carbon batteries look better. The products page says, "For reasons beyond our control, we had to discontinue our range of batterys." Maybe at the next rewrite they will be less bias?

    Cheers;
    Mike Schooley
     
  8. lockhughes
    Joined: Jun 2002
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    lockhughes ElectricGuy

    Mike -

    batts storage.

    For my little ferryboat cattlemaran, I imagine the batts installed in a central nacel (sp?) "slung" fore-and-aft between the hulls, under the bridgedeck. Accessible from above through removeable deck plates, with the nacel itself just well ventilated at the bottom and sides, and having no other purpose...

    Designed so that any explosion would be directed sideways and down to open air and water, instead of up (passenger deck) or being in the hulls

    Sound OK?

    Lock
     
  9. Portager
    Joined: May 2002
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    Portager Senior Member

    Positive Battery Vent

    Is a cattlemaran where you use the carouses of stuffed bovine for the hulls? :D

    Why do you want to use a Catamaran? At displacement speed 80% to 90% of the drag of a well designed hull is due to skin friction. Therefore, to reduce your largest drag component you want to minimize wetted area. The minimum surface area solution is a monohull. Perhaps you have a preconceived solution in search of an application. I hope not because this is generally the recipe for disappointment!

    Some people believe that because multi-hull sailboats are much lighter than monohull sail boats that multihulls are the lighter solution for everything. The fact is when you remove the sail everything changes! You no longer have a large sail so you no longer need a huge ballast weight for stability. For example, rowing sculls are probably the most efficient craft I know of. They have very limited power and the hull forms has been developed over many many years. To the best of my knowledge, the fastest sculls are monohulls. Their are catamaran sculls also, but they are marketed as a more stable option (for paranoid non-swimmers), not for speed or efficiency.

    If you insist on a Catamaran, then you want the batteries as low as you can get them. Put half in the bilge of each keel.

    Hydrogen is lighter than air, so a passive battery compartment vent must go continuously up from the top of the battery box. It is important to avoid any traps that could accumulate hydrogen and very important to eliminate any spark sources in the vent path. Static electric discharges, lightning and people lighting or tossing cigarettes near the vent opening have been known to ignite the hydrogen.

    I do not like passive vents! They rely on the buoyancy of the hydrogen to vent the gas, but the buoyancy force is very weak until the concentration levels reach about 20%. Since mixture all ratios of hydrogen and oxygen between 5% and 95% are combustible you are assured that a passive vent will collect the hydrogen until it reaches a combustible concentration (this is not a good thing). In Safety 101 our first objective is to avoid combustible mixtures and we only resort to eliminating ignition sources as a last resort. The objective of a positive vent system is to provide a sufficient air flow to dilute the hydrogen gas and keep it below the combustion threshold. With AGM & Gel cell batteries, because the recombinant process if ~98% effective out gassing is very low so the positive vent flow rate is very low. We use a small AC blower (tried DC blowers but they have much shorter life span) to blow air into the battery box (if you configure it to suck air through the battery box, then you need an explosion proof blower). This blower is activated by a relay connected to the charging circuit so that whenever the battery is being charged the blower runs. A diode between the relay and the battery prevents battery voltage from activating the blower. This is a simple, nearly fool-proof system that eliminates the possibility of explosions by avoiding combustible mixture ratios. I say nearly fool-proof because if the blower fails and the user doesn't repaid it ...

    Safety 101: It is far better to eliminate the possibility of an explosions than to design around the results of an explosion. If you do design to direct the blast, will require a very massive blast shield.

    Cheers;
    Mike Schooley
     
  10. lockhughes
    Joined: Jun 2002
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    lockhughes ElectricGuy

    Re: Positive Battery Vent

    Hi Mike
    Hehe... you knew what I meant!... I won't use "cattlemaran" any more, but I am searching for a word other than "pontoon boat" or "catamaran"... it's a marketing thingee, ok? <wink>

    Mike,Mike,Mike... everythings a compromise in boat design, no?
    I'm as interested in creature comforts (flat,stable ride, and walk on access/egress, for example) as I am in pure efficiencies...
    I am also struck by the variety of eboat ferryboats out there.

    Many I have found are cats:

    http://photos.groups.yahoo.com/group/QCYCTender/lst

    Why as-low-as Mike? No such thing as low-enough? I'm primarily interested in 1) ease of access/replacement and 2) getting them out of the hulls...

    Gawd! LIGHTER than air - that's a forehead-slapper Mike. I've forgotten my High School physics/chemistry (though the incident with the mustard gas was pretty memorable <grin>)

    Hence my thought about *seriously* good flow-through ventilation - eg a batt trough open to the bow and stern and at the sides... Steady 10kts of air (less friction, I suppose) when underway. ... OK. Not when charging with shorepower... Hmmm!

    Lock
     

  11. Portager
    Joined: May 2002
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    Portager Senior Member

    I heard, "Don't confuse me with facts my mind is already made up.

    Have you seen this link? http://www.umwelteinsatz.ch/ibs/solship2.html Take a look at Sonnenschein. They appear to have actually used the Cat to reduce the wetted area. This design looks like it might be efficient. Note were the control station is. This would probably improve visibility. OTOH if their design is so great why did they go out of business?

    You can't be too rich or have your center of gravity too low. The lower CG will improve the ride in rough seas. Besides AGM batteries like the bilge. They can operate totally submerged and the live longer if you keep them cool. Isn't the bilge usually cool?

    You only need to vent the batteries when charging. Good airflow while underway will help keep them cooler and that is a good thing, but your not in the Sahara or Gobi. Since you only need positive ventilation while charging and you'll be charging mainly at the dock, why not use an AC blow and avoid the KABOOM situation? My motto is, "One explosion can ruin your hole day." :D

    Cheers;
    Mike Schooley
     
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