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Old 03-20-2008, 09:14 PM   #1
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Need help sizing batteries and charger

I've spent a bit of time reading pages of threads and haven't come across exactly the help I've needed. If somebody would help me with the math and forumulas to correctly size and choose batteries for my 1960 Safari, I'd appreciate it. I'm purchasing 12V batteries (I've read the 6V arguments and have decided to stick with one or two 12v's). I'm running a furnace, a few light bulbs and maybe a coffee maker in the morning. I figure the furnace takes 3.5 amps when running. The light bulbs are 15W x 12V which is about 1.2A (if several of them are on...maybe 3.6A but only for part of the day). THe furnace would only run about 50% of the time too, so I'm guessing that I plan for a 5A draw that seems like it would be pretty conservative.

I'd also like to run for about 48 hours before needing a recharge.

5A for 48 hours = 240 Amp hours (Is that right?)
Is Amp hours how they rate batteries?
What else will I need to know before I head to the store to buy them?


Thanks for the help.

Doug
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Old 03-20-2008, 09:15 PM   #2
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Doug, where are you locating the batteries? If inside the coach, there will be a need to vent them.
Ideally, two AGM group 27 size batteries would work for you, but the 1960 has a factory size of a single 2EE battery.
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Old 03-20-2008, 11:02 PM   #3
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240 AH at 12v is 2880 watt hours. You will need about one pound of battery for every 10 watt hours. That means 288# of battery, which is about 4+ group 27's. Your 48 hours is close enough to the rule of thumb that capacity should serve your needs for a typical weekend off grid - 3 days + 2 nights with maybe a bit of furnace in the morning to take off the chill.

For charging, plan on 20 amps max current for each 100 AH battery (typical for a group 27 size).

Realistically speaking, a trailer can only handle about half that many batteries but a good converter at 80 amps is no problem. That means you will either need to adjust the lifestyle a bit or charge the batteries more often.

A bit extra capacity in the charger/converter usually isn't a problem as they don't push the voltage in a way that would fry the batteries with excess charging current.

For longest life, you want each battery discharge cycle to be at least 10% but no more than 50% (down to the 12.4v to 12.0v range measured after they have been resting a half hour). You want to charge them promptly and fully with a good 3 stage charger. A complete charge can take more than 8 hours so most camping recharging is only to get to the 90% or so charge level.

Keep in mind that everything batteries is +/- 20% as cycle to cycle charge, temperature, current use profiles, age, and other factors can all make this sort of difference. Don't get bogged down in needless precision. Use your experience as a guide and ultimate test of what works for you.

As Terry notes, code specifies a vented battery compartment - even for AGM's.

Also plan on a good storage manager for your batteries. The PD+Chargewizard or WMCO converters do this as does a BatteryMinder+.
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Old 03-21-2008, 09:57 PM   #4
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I think my estimate was a little high on the demand I would need. I think I'll start with two 12V batteries. I'm heading to the store tomorrow to shop for them. Will the batteries be marked with amp/hour ratings or watt/hour or is there some other rating I am going to need to be aware of?

Ok...that brings me to a charger. Most of the year I'll be on shore power or the trailer will be in storage. I'm looking to do this as economically as possible. Should I consider a solar panel? Should I get a converter that provides 12V when on shore power as well as a charger? Can somebody recommend an affordable one? What are the things I need to consider when purchasing a coverter/charger (sulfation, 3 phase, etc.)

Thanks a ton for the help. I've got 10 days before we leave for spring break!

Doug
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Old 03-22-2008, 04:29 AM   #5
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Doug,

Good luck with the battery search. I know that the batteries at Sam's Club have amp-hour ratings. The Cabela's private label (imported by Universal Battery) have no rating. I don't understand how you can offer a "deep cycle" battery, and not put a rating on it. Be careful at Sears Automotive, some of the sales clerks don't know the difference between the CCA and amp-hours.

Do as much research as you can beforehand. Sales clerks are great at telling you what they don't know.

For a converter/charger, most people use either a Inteli-power made by Progressive Dynamics, or a WFCO. Either should cost around $150-$180.
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Old 03-22-2008, 08:14 AM   #6
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Quote:
Originally Posted by crdouger
...
I'd also like to run for about 48 hours before needing a recharge.

5A for 48 hours = 240 Amp hours (Is that right?)
Is Amp hours how they rate batteries?
...
I don't think that's right. The math is right, but the concept is a bit off. I like to calculate based on the per centage of a day that you use appliances and see how many days the battery will last.

If you've got three 1.2 amp bulbs that you'll run maybe 10 hours a day max, that's 36 amp hours per day (that's high on the hours, but I think low on the bulbs--some fixtures have 6 bulbs each). IF you run the furnace 50% of the time, that's 12 hours times 5 amps, or 60 amp hours per day (you can see why members are bailing out of the standard furnace and going to catalytic--no fan). I don't think you'll run the furnace this much--add some quilts! Anyway, this puts you right at 100 amp hours per day.

Typical marine batteries, designed for deep discharge, are 105 or 115 amp hours. You shouldn't routinely discharge more than about 80% on these (LEIPPER is more conservative than I am--more on this below). If you get the 115 amp hour battery, that gives you 92 amp hours available, per battery. If you tone down the furnace a little, you can go one day per battery at your anticipated rate of consumption.

My practical experience with an Overlander (and Caravel to some extent) is that if you don't run a furnace, you can go about 3 days per battery, in the winter (long dark). You really can't carry more than two batteries, even in a big Airstream. If you limit yourself to 50% discharge, you're doing it to extend battery life, but you'd be carring a lot of weight around and essentially not using it. Over the years I've converted up from one battery to two. But I boondock a lot and I've found that you can never quite anticipate your electrical needs. Afte quite a few trips, I'm now firmly in the camp that one battery and a generator is a much better combination. Plus, if you're moving locations a lot (every day or every other day), the tow vehicle charge line can keep the trailer battery charged.

Regarding depth of discharge--automotive batteries do not like slow, continuous discharge and will not last. It's probably true, however, that this style battery will last longer if discharged only down to 50%. Marine batteries, on the other hand, are designed for just the kind of use you'd put it to in an Airstream. Plus, they can withstand deeper discharge levels. I think that you can be too careful with batteries--modify your whole life schedule to keep them watered, charged properly, etc. And after all that care, if you screw up just a couple of times and completely discarge them (and you will), all that special care and feeding will be for naught. I'd rather use them and then replace in 2-3 years when I've mistreated them enough.

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Old 03-24-2008, 07:59 AM   #7
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Very helpful

Thanks guys. I feel like I am on feet. I've got two 125ah batteries installed underneath my bed where the old hot water heater used to be and hopefully in a day or two I'll have them boxed in and vented outside. I need a charger/converter and I think I'm ready to hit the roads. Thanks for all the help.

Doug
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Old 03-24-2008, 09:50 AM   #8
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Quote:
Will the batteries be marked with amp/hour ratings or watt/hour or is there some other rating I am going to need to be aware of?
In most cases you will see Amp Hours at a current rate that drains the batteries in 20 hours. That's about a 5 amp drain for a 100 AH battery.

You may also see Reserve Minutes and this is the number of minutes you can pull 25 amps from the battery. Multiply by 25 and divide by 60 to convert this to amp hours at a 25 amp rate.

These two will differ as a change in current draw from 5 amps to 25 amps will make about a 20% or more difference in available capacity.

The suggestion to use watt hours for your 'engineering' is because that is a proper energy measure and eliminates errors that may arise if you have a bank made from differing battery voltages. AH ratings always just assume battery voltage and that can lead to errors if you aren't careful.

But don't worry about these specs too much. Just get biggest and heaviest that will fit and are designed for RV service and will serve your needs.

Note that the AH capacity very much depends upon current (look up Peukert) and also on how that current is drawn, As Zep notes, constant steady draws tend to make more of an impact that bursty draws. That is a reason a DVD movie in the evening can have a big impact on battery capacity!

BatteryUniversity.com cited studies that indicated longest batter life at the 50% discharge point each cycle. The key is not a rigid adherence to such findings but rather the knowing that how you use and maintain the battery will influence its age. That is a way you can understand some of the anecdotes about battery life. It also provides information you can use to make changes if you are not happy with the battery life you get.

My B-Van batteries were only showing 12.06 volts when they should have been near full. Time to replace. I know they are at least 8 years old so there doesn't seem to be anything for me to look at as far as use there. The trailer battery is 3 years old and showed 12.6 after surface charged removed - that looks OK, too.
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Old 03-24-2008, 04:15 PM   #9
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Sizing the Converter/Charger

Quote:
Originally Posted by Leipper
For charging, plan on 20 amps max current for each 100 AH battery (typical for a group 27 size).
Hi Bryan,

From your statement, it seems a PD9245CV (45 Amp) Inteli-Power Converter/Charger with built-in Charge Wizard would be a good match for a pair of group 27 batteries. Would there be any advantage to spending a bit more for more amps? If so, what would that be?

Best Regards,
Walt
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Old 03-24-2008, 04:32 PM   #10
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Good question Walt. I'm also wondering how do I know that I'm using my batteries between 10% and 50%(or 80%) of their capacity. Is reading battery voltage a way to monitor the batteries?

Doug
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Old 03-24-2008, 05:11 PM   #11
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I get people angry by trying to answer questions like this - but what the hey ...

Quote:
Would there be any advantage to spending a bit more for more amps?
Extra capacity provides a means to power the fridge, alarms, and other things while still providing full battery charging.

Since decent RV converters don't run the voltage higher than safe for connected 12v appliances, their ability to provide too much current to the battery is somewhat limited. A deeply discharged battery will rather rapidly get its voltage and internal resistance to charging up and that keeps any excess current during that period from getting it too hot, which is the risk of too rapidly charging the battery.

Quote:
Is reading battery voltage a way to monitor the batteries?
There are still hydrometer diehards but proper use of a specific gravity measuring device not only requires many of the same preconditions as voltage measures but also runs the risk of contamination and the need to use appropriate hazardous materials precautions. There is little need anymore for single cell battery diagnostics and the modern DVM with a precision of .01v does a good job for measuring state of charge.

For modern RV batteries, I think voltage measures with appropriate interpretation serve most needs. As long as the battery has had no significant charging or discharging for at least a half hour, the voltage vs state of charge charts will tell you where the battery is with some consideration if temperature is very hot or cold. The key is to not run below 12.0v. 12.2v resting is considered the normal 50% point and 12.6v to 12.8v a full charge. I use 12.0v because of the half hour rest in a campout situation means the battery may not be completely at rest and it is a convenient guide.

Note that state of charge and battery condition are two separate things. I see a lot of confusion between these two. By watching how the voltage drops during your campout you will get a more realistic measure of the condition of the battery as well as how long before you need to get it charged. Those meters that compare energy in to energy out cannot tell you this. SmartGauge Electronics - battery metering is one that watches battery behavior to determine what you actually have left. Modern impedance testers (these are replacing load testing) will also provide both state of charge as well as condition measures.

See also the links at blog entries Batteries or Battery instrumentation or Hydrometers, ugh!
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Old 03-24-2008, 05:44 PM   #12
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How much battery capacity do I have left?

Hi Doug,

I'll start these sentences with "I" since they are my conclusions, observations and what I do vs. others.

I use the voltage to determine when it's time to re-charge. I'm not willing to pull & disconnect the batteries (60+ lbs. each) and then test for each cell the specific gravity with a hydrometer, run the temp conversions and average the results each time I might be getting low enough to re-charge. I do use a digital multimeter that will display 2 decimal places for the voltage (You won't really notice much of the draw down with less precision). I've given up on precision regarding battery charge state (Though my inclination is to go for that) for the ease and simplicity of measuring the voltage. I've searched other threads and other sources for anything cheaper, more precise and/or more practical & failed to find it.

********************************************
OOP's, I see Bryan has responded while I was writing this -- we seem consistent in our thinking on this subject.

Thanks Bryan, I'd forgotten about the other power consumption likely to be going on while charging.

Best Regards,
Walt
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Old 03-24-2008, 05:51 PM   #13
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Sweet

I've got a nice DMM that reads two decimal places AND I know how to use it. You guys are so helpful. I appreciate it a TON. I wish I could return the favor...and I guess my challenge is to find somebody else to help somehow. Now...I need to go our and purchase a converter/charger unless somebody has a used one sitting around they want to sell. I'm so close to the first trip with this trailer that I can taste it. My kids are excite too. Thanks for the help.

Doug
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Old 03-24-2008, 06:21 PM   #14
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Quote:
I've given up on precision regarding battery charge state
yeah, me too. The more I look the worse it gets. Everything seems able to change results by near 20% - temperature, discharge current rate, discharge continuity pattern, battery age, cycle to cycle variations, ... what else?
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