3.24.2014 – Monday
There has been little doubt that these past few months have been Winter. Little chance of mistaking it for any other season. Records are adding up and it’s reasonable to say that it has been consistently cold. The silver lining has been the lake ice of Lake Superior. More ice means the liquid water is locked away and we have more sun and fewer lake-effect snow showers.
2014 data from NWS Marquette:
- February averaged 5.6°F
- 78 consecutive days were below freezing
- 5 days this winter the high was below zero
- 20 days in February had a low below 0°F
- -28°F was recorded on 2/28/2014
The off-grid tech we have in the un-heated garage faired quite well this winter. As a result, I think we can endure future winters without much worry. Despite the cold, the lowest recorded temperature at the battery terminal (where the sensor is bolted on) was still in the twenties. The garage is fairly well insulated and the generator exhaust vent gets closed off when we depart – sealing up the garage quite well. An interesting observation the Ol’man and I noted was that the exhaust fans for the generator (two 100 cubic-feet per minute 110V muffin fans) were not working as efficiently as hoped. The solution was to crack the service door just a bit to let fresh air in to displace the air vented by the fans. The garage is very tight with doors shut and windows latched.
Before departing we like to make sure the batteries have a charge somewhere between 85-100% in the winter months. After some data mining I assembled the table and drew up the graph in this document: Freezing Point Depression. Below is the graph.
The document was my effort to determine what it would take to damage our battery bank. The short answer is that it is almost impossible to freeze-damage our batteries given the lowest observed temperature in the garage.
We charge up to 85% or more before departure because of the lake-effect skies. It may take 10 days for our panels to collect 100 Ah in winter. The 12V LED lighting requires about 30 Ah (at 12V DC) to operate each day. In 10 days without sun (not an uncommon event) we’d find our battery bank down 300 Ah! The battery bank is rated at 1540Ah at 80°F. Ever wonder why batteries get ratings at specific temperatures? I recorded some data from our battery bank (via amp-meter and specific gravity) and found that at 30°F the batteries are down to roughly 50% of the 80°F rated capacity! The reaction required to transform chemical energy to electrical energy gets inefficient as the temperature drops. In deep winter, 300 Ah becomes 40% or more of the battery bank capacity.
My data set only has three samples and it would be nice to get another point below freezing. However, given how well this graph has aligned with casual observation, I’m not too enthusiastic about drawing up below-freezing electrolyte in a glass bulb for an additional dot on a graph.
While cold batteries may last longer, warm ones sure work better. If you happen to be in the process of deciding where to place batteries in an off-grid system, hopefully I’ve given you some useful information. To finish up, here are a few parting shots of winter. Despite the cold I enjoyed winter this year and the lake ice made for some fun family outings. None-the-less, I’m happy to move on to maple tapping.