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  #1  
Old 05-08-2010
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Casey Goes Rogue

This is page 272 of the second edition of "This Old Boat"


I've got to say I've been studying this for a few years now and his chapter on 12 volts is the best and most complete I've seen.

I hope he doesn't mind my posting a couple paragraphs that are a little off the beaten path as he covers:

Batteries, selection and sizing.
Amps, volts, ohm and meters
charging
solar
wind
110 volt

In short a very clear very complete and current section better than anything else I've seen.



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Old 05-08-2010
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This guy has different numbers?

Polk’s Top 7 Tips for Saving your RV Batteries
3. Reducing the battery’s depth of discharge will increase the life of the battery. A battery discharged to 50 percent every day (50 percent capacity remaining) will last twice as long as it would if it’s cycled to 80 percent (20 percent capacity remaining).
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Old 05-08-2010
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I just sent an email to Trojan and will post the answer.
6% is negligible 50% is a big deal.
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Old 05-09-2010
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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.
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Last edited by mitiempo; 05-09-2010 at 01:02 AM. Reason: add
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Old 05-09-2010
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The 90% upper limit is only realistic if you are only using the diesel to charge.
I sit at 100% pretty much every day thanks to a 80w solar panel, and I don't ever use the diesel just to charge, nor have I ever used shore power to charge.

If I was to go to 80% discharge I'd use the diesel to get me back to 85% or so, then shut it down and let the solar take the rest.
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Quote:
Originally Posted by mitiempo View Post
If you take a 400 AH bank the available AH between 80% SOC and 50% SOC is 120 AH. I
Are you sure about that number?

The 400 AH bank is probably 400 AH rated as discharged to 10.5 volts over 20 hours. Problem is the 10.5 volts is deader than the 11.8 volts we usually figure for a dead battery.
So:
Knock off 15% from the top because you are not going to get 20 hours but maybe 17 hours.
Knock off another 20 percent off the top as that is where the bulk accepance charge stops.
Knock off 50% from the bottom for reserver and you only have 15% left which is only 60 AH, pretty pitiful.

And of course this is assuming it is exactly 80 degrees out. The capacity goes down if the temperature is lower.
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David
I think you're wrong about that. For a 12 volt battery Trojan shows 100% SOC at 12.73 volts and 10% SOC as 11.51 volts. Suggest you get a good battery monitor like a Victron or Xantrex and with it set properly for a specific battery bank it is easy to compare bank voltage with SOC in % as shown by the meter. Many people are cruising with deep cycle banks and good battery meters and know where they stand in AH used/left as well as compared to bank voltage at any given time. True, batteries have to be left resting a while to make voltage all that meaningful, but if there was as little usable as you suggest I think we would have heard about it by now.
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Quote:
Originally Posted by mitiempo View Post
but if there was as little usable as you suggest I think we would have heard about it by now.
Of course you are right. Drop it another 50% for 32 degrees and the battery would supply nothing.

I think my numbers are based on the following math:
Aha! Jokes > Office Jokes > Want a day off work?

The number Casey and others I have seen is to size the house bank at 2.5 times the amp hour load expected between charges. So if your daily use is 100 amp hours and you want to run the engine once a day the house bank needs to be 250 amp hours. Double that to 500 amp hours if you want to draw the 100 amp hours for two days before charging.

Anyway that seems to be the rule of thumb.

Last edited by davidpm; 05-09-2010 at 09:00 PM.
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My batteries might get cool but not 32 degrees F. I live aboard and prefer it a bit warmer.
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