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Originally Posted by TMfor4ofus
... should I disconnect the batteries when sitting for even a couple weeks?
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Quote:
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Originally Posted by Bill
... it is often suggested that if you put the trailer in storage for more than a few days, you should unplug shore power, disconnect the battery from the entire trailer, and just let the battery sit there.
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Quote:
Originally Posted by BrucePerens
I am dubious that this is a good strategy, because the battery will self-discharge, and when it gets low enough, that will further damage the battery.
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Bruce -
Interesting thought. In my experience, self-discharge is vastly over-rated as a cause of problems. Why? Self-discharge is a very slow process. Online sources such as batteryuniversity.com and wikipedia agree that the self-discharge rate for lead-acid batteries is around 5% per month - a little more when it is hot, a little less when it is cold. When you multiply that out in a geometric progression, it shows that
even after 6 months of storage, a disconnected battery still has around 70% charge. This is well within the rule of thumb, often-repeated on this forum, that a battery should not be discharged below 50% capacity.
My personal experience bears out this conclusion. For the last 16 years, I have owned two cars which are each used for only half the year. Each of these cars is left unattended, with the battery disconnected, for 6 months of each year - no other measures are taken. One is in a very hot place (Phoenix area summer) and one in a very cold place (Maine winter). Every year, when I return to the unattended car, I simply re-connect the battery and turn the key, and the engine starts immediately. Even after 6 months of self-discharge, it has never failed.
By contrast, far more damage is done by leaving a battery connected to the phantom loads in the trailer for a week. In that time, the phantom loads will completely discharge the Group 24 or Group 27 battery used in most TMs. Sadly, I have experienced this result as well - leaving a battery connected to the TM's phantom loads will indeed harm the battery quite quickly.
So I still think that for mid-term storage (less than a couple months), it is sufficient to simply disconnect the battery. In that time frame, the degree of self-discharge is very small, and the risk of overcharge from an over-zealous converter/charger is eliminated. I also fully support the use of a battery maintainer for long-term storage. I use a Battery Tender, and as I have written, it seems to work well.
TECHNICAL GUDGE FOLLOWS:
If anyone other than Bruce and myself cares to delve into it, here is a good technical discussion of self-discharge.
http://mathscinotes.com/2012/04/batt...ischarge-math/
This discussion includes a family of self-discharge graphs that confirm my thoughts on self-discharge. One of the stated conclusions is that "A lead-acid battery can be stored for about a year at room temperature before it needs a recharge."
Bill