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Old 11-26-2007, 09:55 AM   #1
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Cold weather battery bank maintenance

I have been boondocking out at the wildlife refuge for about a week or so now. Daytime temps in the 40's nighttime lower 30's (but down to 22 one night). My power usage is fairly minimal -call it one reading light for about 8 hrs a day and the stereo pretty much all the time. I have read that most people are getting by when boondocking by charging for 90 minutes or so daily, however, this isn't getting the bank (2 batteries) charged. Is this most likely a temperature issue or should I be looking elsewhere for the problem? If this is temperature driven issue, are there charts I can get to figure charge time from? Thanks!
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Old 11-26-2007, 10:36 AM   #2
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a google for "lead acid battery charging" got lots of resaults...
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Old 11-26-2007, 10:41 AM   #3
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Rodney,

At those temps, I'd be running the furnace right much. You didn't mention that as a power use - but if you are, that would likely explain where much of your battery useage goes to.

Pat
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Old 11-26-2007, 10:45 AM   #4
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Quote:
Originally Posted by pmclemore
Rodney,

At those temps, I'd be running the furnace right much. You didn't mention that as a power use - but if you are, that would likely explain where much of your battery useage goes to.

Pat
It would, but I'm not running the furnace. The only other hidden drain that I know of is the igniter on the refrigerator, but that should be minimal I would think.
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Old 11-26-2007, 12:02 PM   #5
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the tables are specific to EACH manufacturer of batteries...

there are LOTS of brands but only a few makers, although many brands do tweak specs to their liking.

lots of variables (plate thickness, mass, total internal/external resistance, blah blah) but regardless the tables are available for many online.

or you can go to battery college...

Welcome to Battery University

no specific tables in this link but hey it's college.

my understanding is that liquid lead acid deep cycles loose about 20-25% of their available capacity....

going from 75 degrees F to 32 degrees F

that's battery temp not air temp.

since the battery has a thermal mass it holds/looses heat at a different rate than air.

so short periods of warming daytime temps/cooling night temps have less effect on the core battery temp.

charging rates are also reduced at cooler temps.

one of the reasons for the development of 'temperature compensated' charging systems that are growing in popularity.

additionally the charge meters most of use aren't very accurate.

as a result 90 minutes on the genset might give the appearance of a 'fully' charged battery.

but that reading is usually surface charge only. and gradually drops as the plates absorb/release to a uniform state.

it is unlikely that even in the summer 90 minutes a day will fully juice typical lead acid batteries, unless the charging system has been optimized for this.

again depending on drain/usage a better (summer) approach would be to charge for 3-6 hours every 3rd/4th day.

in the winter this might translate to 3-6 hours every other day, if using the furnace.

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Old 11-26-2007, 05:01 PM   #6
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What are you charging your batteries with? You need a good 3 stage charger that is rated 20 amps for each 100 AH of battery (40 amp charger for typical Airstream TT). As 2air noted, plan on a 3-6 hour charge time but even that will only get you to 90% or so. A proper lead acid battery charge is a 12 hour or longer affair to get a complete charge done.

You need enough battery so you don't pull them below 50% (about 12v after resting with no load for a half hour). - i.e. if you can't go a couple of days the way you like to use energy without the batteries going below 12.0v, then you need to look at reducing consumption or adding batteries.

You'd be surprised how a stereo "pretty much all the time" can take out of your energy budget. It is the lower power continuous loads that surprise folks. The amp or two of all the detectors and alarms can add up, too, over a couple of days.

Solar can help extend time between charges but you should still apply a rigorous and full charge on occasion to equalize the batteries and keep the electrolyte stirred up.
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Old 11-27-2007, 09:07 AM   #7
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Quote:
Originally Posted by Leipper
What are you charging your batteries with?
I am using a three stage charger I installed this spring. My energy usage is not out of line to my summer expenditures which was on one battery vs the two I am using now. Last night I ran the generator for about 3 1/2 hours. At the end of the charging period the bank was in much better shape. I suspect that between the cold reducing the power in the batteries and lengthening the time required for charge, I had misguessamated the charge time required from what I had read others were doing. I plan on giving the bank another extended charge this evening and then reevaluate. Thanks for all the thoughts on this.
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Old 11-27-2007, 11:00 AM   #8
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Whatcha using for heat, R? If you have a cat heater, remind me which one, and how you like it.

Or did you just suck it up and put on another layer?

Pat
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Old 11-27-2007, 11:09 AM   #9
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Quote:
Originally Posted by pmclemore
Whatcha using for heat, R? If you have a cat heater, remind me which one, and how you like it.

Or did you just suck it up and put on another layer?

Pat
I am embarrassed to admit it, but I still do not have the furnace operational. What I am using is a propane space heater that is wall mounted (not catalitic) and thermostat controlled along with a little buddy heater. The little buddy by itself appears to be capable of keeping the inside at about 68 degrees or so when the outside temp is around 40. The wall mounted heater only kicks on when the inside temp goes below 60 or so (I don't have this dialed in yet). After the long weekend (4 days) I am comfortable with the safety issues related to venting and it is pretty comfortable although I will be buying some throw rugs for that pergo floor and some slippers.
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Old 11-27-2007, 12:11 PM   #10
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Quote:
I plan on giving the bank another extended charge this evening and then reevaluate.
good plan. batteries change as they age.

two batteries in parallel should give you about 15% more capacity than just double a single battery due to halving the current draw on each (re Peukert's law). On the other side of this is that battery capacilty from cycle to cycle can vary by this much. There isn't much precision when it comes to lead acid batteries as far as predicting capacity over time.

but - if your batteries are aged, like due to excessive discharging, sitting around without a full charge, not being desulfated, or just plain old, then you aren't going to get the life out them you expect. That is a key indicator for when to replace batteries: when they just don't provide the life they used to. It is also a good time to re-evaluate battery use and management practice to see if any of your habits contributed to early failure and if any changes in use or management could help extend battery life.

Quote:
I am using a three stage charger I installed this spring.
is this one rated for 40 amps (for 200 AH of battery)? Is the fridge on electric or propane when you are trying to charge the batteries (that can sap a lot of energy that would otherwise go into charging batteries)? Does the charger limit its bulk voltage so as to be safe for connected RV appliances (again, this can limit charge rates)?

Do take care with the heaters that don't vent combustion products like the little buddy. We loose a few folks every year who don't provide them with proper ventilation. (the last had only roof vents open in their RV)
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Old 11-27-2007, 12:55 PM   #11
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Bryan,

I am using the 4455TC ParaMode with TempAssure from best converter. I run the refer on LP when boondocking. The only difference in my energy use during charging is if I need something that runs on AC but that would cause the generator to put out more power to meet the demand.

I have been careful with the heaters since I have no desire to wake up dead. Having said that, I am starting to gain enough confidence based on usage to say that the ventilation procedure I am using is sufficient for the heaters in use. Of course, if I have to increase the burn rate(s) to stay warm, then its back to the drawing board. On the other hand, the 22 degree morning was one of the days of use so that is encouraging.
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Old 11-27-2007, 06:04 PM   #12
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Quote:
I am using the 4455TC ParaMode with TempAssure from best converter
What I have learned about that in a short search will probably get me into trouble. I can't find much technical data on the ParaMode but I do know Randy has a good rep at Best.

From what I can tell, the ParaMode is not really a battery charger. It will just up supply to as much as 14.4v for 4 hours and then drop back to float. It's claim to fame is a battery temperature sensor to help it get the float voltage right and avoid excess electrolyte loss during storage. It also does not appear to do any battery maintenance other than a proper float voltage.

Your experience with using it to bring up battery state of charge with the ParaMode does not support the idea of its being a good rapid battery charger, either.

It does look like a good converter and a significant upgrade from its predecessors. When I look at the hype for the ParaMode and compare to that for the charge wizard I note that the CW talks about charge level rather than time as a decision point for moving off the bulk phase. I also note the CW talks about a storage mode with sulfation prevention while the ParaMode comparison is a temperature compensated float only. (the CW periodic equalization is another means to compensate for temperature float voltage variation)

So, for 4 hour or less charging, the ParaMode is probably equivalent to the ChargeWizard. For longer than that, the CW could have an edge, especially since you can manually override its decision points.

Neither of these RV converters will bring a battery up as fast as a dedicated charger that uses higher than 14.4 volts for charging - but that higher voltage might be hazardous to connected equipment. I know several folks using a ~$50 Vector or equivalent smart auto charger to get around the converter limitations for short term genset battery charging.

I also tend to prefer the ChargeWizard approach to storage rather than just a float. I know some who use a Battery Minder PLus type device on their Airstreams when not in use for a couple of weeks and that appears to be another good solution. My experience with float based storage has not been good - either electrolyte loss or premature sulfation and its a very tricky balance. If your batteries seem to get old and stale too fast, it is probably storage maintenance as a primary culprit.
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Old 11-27-2007, 06:25 PM   #13
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Cold is the culprit. Batteries are rated at 80 degrees F. Their capacity drops off sharply at temps below 40. All the technical info above is correct, but the simplest solution is to keep the batteries warm. Years ago, I built a heated, vented battery box under the dinette seat of my '76 Caravanner that held two group 27 deep-cycle 12V batteries. It was heated by a 2" flexible duct that was connected to the duct heating the under-floor fresh water tank. The batteries remained at the same temp as the interior of the trailer. This set-up ran the trailer, including the 34K BTU furnace, for 20hours a day at temps that dropped as low as -18 degrees on one of our snowmobiling adventures. I would start the generator in late afternoon and run it for about 4 hours. I had a 3-stage home-made charger that could be turned down as the capacity of the batteries was reached. While the generator was running, I used a 1000W electric heater to reduce the load on the batteries. Now, with good gel-cell batteries, venting is no longer necessary so batteries can be located in any spare cabinet conveneint to the 12V connection in the trailer so keeping them warm is not such a hard thing to do. Darol
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Old 11-27-2007, 07:11 PM   #14
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Quote:
Cold is the culprit.
It is definitely a factor which is why some Airstreams have batteries inside. But there are several reasons why it isn't alone.

The capacity difference from room temperature to freezing is about 15% (see pacific power faq) and batteries are dense enough so that their temperature is more like an average over the day and not the night's coldest. So the difference most folks will see from temperature effects is likely to be quite a bit less, even with the batteries outside.

Other factors in the ten percent variance range include cycle to cycle battery capacity, short term (less than 6 hours) charge results, use patterns, and aging (sulfation). There may be others but these I know about.

One ten percent deficit isn't going to be very noticeable. When a couple of them add up together, it can be another matter unless you have a lot of reserve. I also note that colder temperatures tend to correlated with less daylight which also correlates with more lighting and other energy needs.

What I don't know is how a draw at low temperature influences remaining energy at higher temperatures. It may be that cold capacity is restored when warmed up but how much I don't know. (the contrast is the Peukert thing)

At least the cold weather is easier on your batteries as far as aging goes.
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