Join Date: Sep 2008
Location: Victoria B.C. Canada
Thanked 94 Times in 85 Posts
Rep Power: 8
While it is probably true that the batteries will last twice as long if depth of discharge is kept to 50% the actual AH from the deeper (80%) discharge will be greater. So while the batteries will survive less cycles each cycle will give you more AH between charges. It is commonly accepted that 80% SOC is the practical upper limit for alternator charging without excessive engine run time. If you take a 400 AH bank the available AH between 80% SOC and 50% SOC is 120 AH. If you draw the same bank down to 20% SOC usable AH becomes 240. Half the cycles but theoretically twice the AH usable for each cycle. So in the case of someone who cruises and charges with only the engine to 80% SOC the cost /AH would be the same each way. But when you do run the engine to charge there will be more engine run time to put the greater amount of AH into the bank. And if you have other charging methods available, shorepower charger when dockside and solar or wind when away the engine run time is less of a factor. And the person who only draws their bank down to 50% SOC has the occasional reserve of going lower when necessary while the person drawing their bank down to 20% SOC has no real reserve.
The second link you posted relates to RVs and their situation is different as they will be charging to 100% SOC more often as their engines are always charging when they are on the move. So they are able to use from 100% down to 50% where we who don't want to run our engines for many hours will only use from 80% down to 50% SOC.
I don't think there is anything new in what Casey wrote, just another way of looking at cost vs AH using the same battery charisterics we've known all along. The only difference in numbers I see is that Casey states that 90% is the practical upper limit for engine charging while most accept this to be 80%.
I will be interested in Trojan's answer to your email.
Living aboard in Victoria Harbour
Last edited by mitiempo; 05-09-2010 at 01:02 AM.