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Old 05-28-2015, 10:29 AM   #1
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Sizing battery based on usage

I have been reading a ton on the internet trying to figure out what I need to put my electrical system in place but I have run in to a few issues that I could use specific advice on.

My usage: I will be installing a furnace this week and it says it uses 4.6 amps 55 watts. Other than that we are only charging our cell phones. The interior lights (3 leds) are still tied to the starting batteries but i will eventually swap them over. Fridge is propane only.

I would not be surprised if we add things in the future, but I am trying to do this on the cheap so I am sizing the system for this limited usage for the time being.

Background info: I am in SW Colorado so lots of sun in the day. I do not have and do not want a generator. I do not have a converter, but I do have an 800w inverter jic.

My plan: Add a battery or batteries (size to be determined) and attach one 100 Watt Monocrystalline Solar Panel with a 30 amp PWM Charge Controller.

If I add panels in the future I would switch to a mppt controller, but again, I am trying to keep costs down for now so I can get it done and then take the bus out and enjoy it this year.

My questions:
1)If I only have one 100w panel, with the PWM controller be ok or will I loose a ton of efficiency?

2)How many AH do I need to make sure the furnace will function properly. I have read that they only discharge at a certain rate determined by the total AH of that battery. Because I need ~5 amps out (c/ would I need a minimum of 40 AH to have the proper output to run the furnace?

In a similar vain, if I want to run the furnace steady for 10 hours (worst case scenario camping with a 4 year old in the winter), that would use ~50 AH so the absolute minimum battery size would be 100AH, but I do not believe I would be able to charge that in the winter off a 100w panel so what size would you think would suffice to get use through 3 days of camping in the mountains (ski trip)?

THANK YOU for any help at all.
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Old 05-28-2015, 11:51 AM   #2
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You might look at PDBreske's recent thread on lithium batteries -- lead is recommended not to be discharged more than 50%, but you can run a lithium battery much lower. But it looks like up-front cost weighs heavily in your choice, so probably lead will be the way to go for now.

I don't think the season (temperature) matters for solar panel output. Well, it does a little bit, as panel efficiency goes down as the panel heats up. But mostly if you have clear sky, good sun, and seasonal adjustments to the panel angle, I'd expect the panel to perform equally well in the winter as in the summer.

If you could get the full 100W out of the panel through a 100% efficient charge controller at 13.8 volts, it's about 7 amps. If it would do that for 5 hours a day, you get about 35 AH out of it. So no, that won't keep up with the 50 AH daily furnace load. Also, running the furnace just 10 hours on a cold day and night might be pretty optimistic unless you've done a lot of insulating and air sealing. In addition to that, it might be good to add maybe 40% on the solar capacity beyond your load demand to cover inefficiency in the charge controller and battery, plus some more as insurance against cloudy days. It all depends on how big a problem it'll be if solar charging can't keep up with demand. If the consequences of a dead battery are minor then it can be designed with thin margins.

I think that a battery with enough capacity to run the furnace for hours like you desire will have way more maximum discharge rate allowance than needed, so you probably don't need to even worry about that. Runtime will drive the battery size more than maximum discharge rate will.
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Old 05-28-2015, 07:57 PM   #3
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AC to DC

when you say the furnace uses 4.6 amps, is that 4.6 amps AC or Dc,
I assume it is AC, which means you must multiply by 10 to get the DC amps needed,
which means it uses about 45 amps DC,
and since most batteries cannot be discharged at more than 10% of their max capacity you will end up needing a battery bank of 500ah or more to run it for a few hours per day,

If you tried to pull 45 amps from say a 200ah bank the voltage would drip to below 11v quickly and most 12v devices will shut off if the input voltage is below 11v.
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Old 05-28-2015, 08:49 PM   #4
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Sorry,
Atwood MODEL #8520-IV
BTU Input 16,000
Duct Static Pressure .20˝ W.C.*
12 Volt Amperage (AMPS) 4.6
Watts 55
Power Supply (VOLT DC) 12
Recommended Return Air 80 in2
MINIMUM RETURN AIR 65 in2

My guess was and still is 4.6 amps dc, but please correct me if I am wrong.
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Old 05-29-2015, 10:18 AM   #5
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Quote:
Originally Posted by 05Willys View Post
My plan: Add a battery or batteries (size to be determined) and attach one 100 Watt Monocrystalline Solar Panel with a 30 amp PWM Charge Controller.

If I add panels in the future I would switch to a mppt controller, but again, I am trying to keep costs down for now so I can get it done and then take the bus out and enjoy it this year.
A 100w panel at ~12vdc is only going to put out ~7-8 amps max (as family wagon already mentioned).. You could go a couple ways here. Personally, if I knew I was going to increase the solar array some day I would bight the bullet and buy an appropriately sized MPPT charge controller. You're just adding more cost by starting with a PWM, then replacing it later.

If you're dead-set on getting a PWM for now, you can go much smaller. Get a 10 amp model. You can get them for around $20 on eBay and Amazon.

The other option: no charge controller. Just connect the solar panel directly to the batteries when you need to charge them (assuming it's a 12v panel. You're planning on using PWM technology, so it must be). You'll be putting 8 amps max into the batteries at peak daylight. That's not a lot. If you're pulling from it daily you'll be ok. Don't leave it connected all the time, though. I used this technique with the deep-cycle battery in my boat. I'd connect the panel directly to the battery with alligator clips during peak hours, let it charge for a couple hours, then put the panel away when I was done.

All that being said, a single 100w panel isn't going to do you a whole lot. It sounds like you'll be needing a 300Ah flooded lead acid (FLA) battery bank for 3 days of furnace use.. If so a 100w panel will work as a trickle charger, not much more. You could probably skip the panel cost altogether and charge with a 110vac battery charger before you go. Otherwise, I'd recommend getting a larger panel, or more panels.

Another note: if you get an MPPT charge controller, purchase solar panels that are meant for grid-tied installations instead of the crappy 12v ones. They're typically ~36v and cheaper per watt. The higher voltage means less line loss. MPPT can manage the high voltage. PWM charge controllers cannot since they need the panel voltage to be closely matched to the charge rate of the batteries.

Quote:
Originally Posted by family wagon View Post
You might look at PDBreske's recent thread on lithium batteries -- lead is recommended not to be discharged more than 50%, but you can run a lithium battery much lower. But it looks like up-front cost weighs heavily in your choice, so probably lead will be the way to go for now.
Yep. LiFePO4 is superior, but lead acid can be found local and cheap.

Quote:
Originally Posted by family wagon View Post
I don't think the season (temperature) matters for solar panel output. Well, it does a little bit, as panel efficiency goes down as the panel heats up. But mostly if you have clear sky, good sun, and seasonal adjustments to the panel angle, I'd expect the panel to perform equally well in the winter as in the summer.
I'll emphasize this a bit more: Since you plan on using this system largely in the winter, make sure your panels can be tilted towards the sun to maximize their output.


Quote:
Originally Posted by 05Willys View Post
Sorry,
Atwood MODEL #8520-IV
BTU Input 16,000
Duct Static Pressure .20˝ W.C.*
12 Volt Amperage (AMPS) 4.6
Watts 55
Power Supply (VOLT DC) 12
Recommended Return Air 80 in2
MINIMUM RETURN AIR 65 in2

My guess was and still is 4.6 amps dc, but please correct me if I am wrong.
Yep, 4.6 amps. The furnace uses a very typical 12vdc motor. All the blower motors in my bus use the same amount. Chances are that the one in your furnace is interchangeable with your blower motors as well. You could probably replace the single speed motor with a 2-speed motor. On low it would only draw ~2-3 amps. It'll pull less heat from the furnace's heat exchange, though.

Do you have room for a wood stove? They require 0 amps.
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Old 05-29-2015, 10:56 AM   #6
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I have 2 spaces that I am looking at for a wood stove like the Dickinson 00-NEWSF Solid Fuel Newport, but I would like the propane furnace as a backup and the primary for the spring and fall when a wood stove is usually overkill.

I have a shorty so I don't think it will take much to keep it warm. A small 500w electric heater did fine at 32* on our first trip out, but we had power then and I do not plan on that again.
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Old 05-29-2015, 11:29 AM   #7
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my bad, as I looked it up the furnace it is 4.6amps at 12v,
the pwm is not bad, the mppt is more efficient at lower sun angles and lower temperature, most get 5% to 20% more power with an mppt but with a single panel that you can turn and tilt several times per day you would probably get only 5% more power from an mppt,
To know how long your battery will last depends upon how many minutes per hour on average the furnace is pulling power, it will last a lot longer if it runs 10min per hour than 30min per hour,
be sure the battery you get is a true deep cycle battery usually this means it does not even mention cca on the battery,
you never want to pull more than about 10% of the AH capacity, this means on a 100ah battery a 10amp load is max, but 5% or less is even better,
just guessing here but assume it runs 30min per hour, so for 10hrs,
5amps x 30min/hr x 10hrs, + 50ah used,
assume at least a 10% inefficiency in the system means it uses ~55amps,
you would want imho at least a 120ah battery,
100 watt panel puts out about 7amps, so at 5hrs per day of sun and you are putting 35AH back into the battery, assuming another 10% inefficiency and you are down to a 30ah charge,
you will probably want 200 watts of solar,
On the cheap means you would want to live in a colder bus with a thicker sleeping bag,
most devices I have seen overstate the amps needed so it probably only pulls 3.5 to 4.0 amps but until it is used that is a guess.
the calculations above will allow you to play around with different numbers of panels and batteries to see what will work for you.
some will use a solar panel hooked up directly to a battery but you must be careful, the battery will need a max of 14.8v to fill the battery without causing damage, most panels will put out 16v to 18v,
the cable run from the panels to the charge controller should be 10ft or less, controller to battery 3ft or less, battery to furnace probably 3ft or less, be sure to look at a wire size chart,
I assume you know where you will be parking the bus since tall trees and hills will decrease your daily available sun
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Old 05-29-2015, 11:36 AM   #8
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Quote:
Originally Posted by bubb, the real one View Post
just guessing here but assume it runs 30min per hour, so for 10hrs,
5amps x 30min/hr x 10hrs, + 50ah used,
assume at least a 10% inefficiency in the system means it uses ~55amps,
Wouldn't it be more like 5amps X (30min/hr) .5hr X10hrs = 25ah used?

Thanks for the info, I agree with a lot of your points.
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Old 05-29-2015, 04:00 PM   #9
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Originally Posted by 05Willys View Post
Wouldn't it be more like 5amps X (30min/hr) .5hr X10hrs = 25ah used?

Thanks for the info, I agree with a lot of your points.
Ah! Good catch. That sounds more accurate.

So you might be able to get away with a 150-200Ah battery bank for a 3-day heating session. The solar panel will likely still not keep the batteries fully topped up, but it'll help to keep things running for longer.
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Old 06-13-2015, 09:51 AM   #10
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Turns out this question is purely academic and a bit of a waste of time because I bought 2 6v GC2s at sams club for $170 total and I now have 208 AH to work with.

I also have 280 watts @24v (should have ~20a peak) going on the roof in the form of two unisolar panels, so they should be pretty well topped off unless it is raining on our whole trip.

Now I need to figure out what to do with the batteries.
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