brief update :: musing on winter and batteries

12.22.2011 – Thursday

It seems that everything moves slower in the cold. It would appear that batteries are not exempt from this generalization. Recently I’ve observed that the battery capacity has diminished from the faceplate rating 1540Ah in summer to a meager 500Ah or so now that it is below freezing outside (the batteries are around 35-40°F). The reason for this drop in capacity has yet to be determined, but it would appear that the cold is responsible. I find it a bit surprising to see that much of a drop, but I suppose it is possible. It looks like I’ll have to wait until summer when temperatures get to around 70-80°F before I make progress on this topic. Once I get to the bottom of this I’ll write a post on it and cover the some of the more issues that I often get questioned on about the cabin. Until then, here is a link to a thread at NAWS  where I’ve been spending some time.

On the upside, I may have strengthened my argument for having an oversized battery bank relative to the size of the PV array since typical use is 150-250Ah a day. So much for a two day reserve however.

UPDATE (1/8/2012): I’m putting the Victron BMV 600s to good use this winter. By next winter I hope to know exactly how the batteries perform in the cold. Right now it appears that the batteries are indeed suffering a loss in capacity due to the cold, and are operating somewhere between 50-75% of the faceplate (80°F) Ah rating. I’m still collecting data in an effort to get to the bottom of this, but when it is around 32°F the batteries should still have 375-500Ah until 50% SOC (down from 770Ah at 80°F). For now I’m using Volts as a reference for when to recharge (the Victron measures to 0.01 V accuracy versus the 0.1V of the Xantrex control panel). Some time this summer I’ll share what I’ve been working on, including three really awesome battery capacity graphs and two charts (information sources will be included).


2 thoughts on “brief update :: musing on winter and batteries

  1. Bobby

    I’m following your articles, mate. Thanks for giving us an inside how solar systems performs in varions conditions.. I would like to know how your system performs on an extreme overcast day.

  2. offgridcabin Post author

    Oops. Sorry Bobby, I didn’t see your comment until now. It’s been a heck of a spring (I’d need another blog to elaborate any further).

    On an extreme overcast-thunderstorm day (requiring lights in the cabin to be on) there is zero output. That might not be too useful; here’s another comparisons: On a perfect day we’ll see 700-760W from a faceplate rating of 810W. On an overcast-is-it-going-to-rain-or-not day we’ll see about 70-90W pretty much all day long, and interestingly enough, power production will last up to an extra hour into the evening.

    So to sum up: about 90% rated efficiency is normal for clear sky, but only 10% for a typical overcast day, and nothing when the thunderheads roll in.


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