>>If you discharge your battery it directly reflects how long it will >>last. The shallower the discharge the longer the battery will last. >>Its actually best if you never approach 50 percent > > If Edd's figures are correct: > a GEL will give you 27% more power if carefully managed, (10% discharge) > and a AGM more than double. > a GEL battery will deliver three times as much current over its life time > as an AGM at 100% discharge and 46% more at 10% discharge. > Seems to me that the cost /amp of a AGM is VERY high compared to a GEL > > Am I missing something here? or the figures amiss? > Philip
REPLY Yes you are missing the particulars of the charging regime. Also the rate of discharge. GEL batteries are great for moderate discharge rates. They are also good for long shelf life and low self discharge. BUT GEL were originally developed for quite apecicif applications not for driving inverters nor for being recharged with a genset by a fuel consumption conscious boat owner. For boat applications it has ben said that you are not getting good efficicency once the charging goes into absorption mode. This is quite true. If you were to try and drive a charger right throuhg absorption and float mode th egenset would run prettty near round the clock. This is totally illogical if th eidea is to reduce genset run time. Secondly Gel batteries will not absorb maximum charge rate when only discharged lightly meanin gless than 50% DoD. What happens is th einternal resistance is high enough so as to limit the acceptance rate to half as much as compared to if the batttery was taken to 80% DoD or better 100% DoD. A third issue is rate of discharge. GEL electrolyte is formed by adding a gellin gagent to the liquid electrolyte. If you read th efine print in an engineering spec yo uwil ldiscover the gelling age4nt is silica - also called sand (finely ground) This gelling agent is non conductive. The result being a reduction of free electrons in the electrolyte. In othe rwords you cannot extract as high a discharge from a GEL cel las you can with a wet cell or even an AGM battery. Inverter use pretty much dictates there will be high surge rates from time to time. IF the GEL bank is large enough you may not notice much change unless you repeatedly demand high surges from the inverter. But ig you have a substantial fridge or freezer load you may find the inverter will stal lout sooner than a comparable installation using Good deep cycle flooded wet cells. As regards to charging if you live on the hook for long periods of time and mostly charge with a genset then you are likely to cut short the charge cycle when you have replendished the battery by 80%. This will result in sulfatte build up and a curtailment of the battery capacity. Furthermoer if you only do shallow discharge the acceptance rate will be such that you end up running your genset for longer period each charge session. If you stay mainly in marinas and plug into shore power most nights, it isn't as much of a problem since th eshoewr power runs constantly and you geneally charge to completion of the float cycle. So it really depends on how you use the boat. Someone by name Von Wenzel did a cost analysis of AGM versus flooded cells. Google the name and you will likely find his website and the spread sheet. You will be able to run your own cost analysis online by plugging in th eright numbers. Bottom line is the initial cost is only part of th total cost over the service life of the batteries. To find the best and most cost effective solution you must factor in all the variables including generator run time, genset oil and filter changes and total fuel burned. Not just the inital outlay for batteries. Realistic use of a battery bank may dictate a smaller bank and 80% DoD for 1500 cycles. A much large bank with much less depth of discharge may look good with 8 years or 3ven 10 yesrs theoreticla use nb ut chances are youy will never realize more than 5 years and sometimes much less if the charging regime is wrong. Your argument is predicated on "more power if carefully managed" but your comments and suppositions do not address the issues of how to achieve a closely matched charging system nor on what exactly constitute careful management. Different systems will have different system design. best regards Arild _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
