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wwilson 02-10-2001 06:13 AM

House battery capacity
I''ve seen factors of 3 to 4 X used for the calculation of house battery bank capacity to daily electrical load. Ignoring, for the moment, other complicating factors such as on-board generating devices, # sustained days at load, etc., is anyone fimiliar with a rule of thumb for a first order estimate of AHr storage capacity to daily use?

walt123 02-10-2001 06:28 AM

House battery capacity
total amps used x hours x % natural battery [includes system leakage] loss per day and figure in a temperature factor and type of batter used, wet or gel

dazeoff 02-11-2001 12:51 PM

House battery capacity
Have a look at Nigel Calders book on boat electrics. I have 2 x 200A/hr gels, and a seperate starting battery. Whatever you do it never seems to be enough, NOW I have a wind gen. and solar and still use the engine 3/4 of an hour each day. The fridge is the killer - but I have to keep the beer cool.

wwilson 02-13-2001 12:53 PM

House battery capacity
thanks dazeoff-

I found the source of the recommended batery sizing factor. West Marine Advisor recommends a storage capacity of 3 to 4 times the average daily use in AH. I''ll get Calder''s book and see what he has to say as well before spending any money on batteries!


munchimike 02-15-2001 10:38 PM

House battery capacity
You gentlemen are being a little too cerebral about this; and eventually the factor that none of you have mentioned will catch up with you......attrition. The life of your batteries will decrease at an ever increasing rate, the charge capability will diminish, and who knows how much resistance increases in all of your wiring as the copper takes a load for a few years?? Don''t do it, don''t even try; there will never be a perfect formula. Get 6-volt batteries and up your amp hour capability. Get a bunchofem. Mark each one for startup date. Measure the charge every year,....replace as necessary. Get more than you''ll need, keep experimenting,..stop calculating.

peerlessg 02-18-2001 11:38 AM

House battery capacity
Nigel Calder estimates that, including what he calls fudge factors, you need battery capacity equivalent to 4 times your daily usage, i.e., if you need 100 amps per day then according to Nigel you need 400 amp hours. He gives a detailed rationale for all of this. Notwithstanding his considerable expertise and experience, I think he is being quite conservative in his assessment and I personally be comfortable with 3 times daily usage requirements.

kimberlite 02-18-2001 04:28 PM

House battery capacity
since your batteries are dead at about 50%
3 times your daily use only gives you a day and a half before dead batts.

Constantin 03-07-2001 08:06 PM

House battery capacity
There are a number of factors that influence the size of the battery bank. The usual consensus is a battery capacity around 3-4x of daily needs (although Surrette has a guide based on boat length). Calder advocates 4x daily need, or 4x max temporary load, whichever is greater. AGMs or Gel-cells may allow higher temp loads, so the maximum temporary load factor can be reduced to 3x with them.

A battery is considered "dead" when it only retains 80% of its rated charge. Thus, unless you use cheap Trojans or other bargain batteries, it''ll be a while before you wear down the battery to this point. To go to one extreme: the Surrette/Rolls CS series can sustain 3,200 cycles at 50% Depth of Discharge before it''s "dead". That''s 9+ years of daily use, assuming one discharge to 50% per day. The AGM batteries I use are rated for 1,000 such cycles...

Ideally you won''t discharge the house bank deeper than 50% on a daily basis, cycling it between 80% and 55% (if you used the 4x rule) since this allows bulk charging (much higher replenishment rates than at higher states of charge) which minimizes engine runtime, fuel consumption, and wear. A 4x battery bank also gives you three days of charge if the generator/alternator fails.

Have one starter bank and one house bank. Keep the two circuits completely separate except during charging. The two banks should have similar battery types in them (Gel, AGM, Flooded) although using an isolator/eliminator by Ample Power gets around this.

Have a look at my battery page for more info:

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