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Old 02-04-2012, 08:04 AM   #1546
HillbillyNinja
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Quote:
Originally Posted by geode View Post
What is your point?

Bumping to 24VDC means he can use a smaller wire, but Ohm's Law is Ohm's Law ... a refer that uses 50 watts of power per day uses 50 watts of power per day on either 12VDC or 24VDC ... bumping to 24VDC doesn't mean he needs less ah per day to run his refer.

Plus, it would be the only item on the whole rig using 24VDC. That means he'd need a combiner to generate 24VDC from his batts, and that just means he needs another chunk of $$$ to spend on buying and installing that.
Ah, but see using ohms law, 50W at 24V is 2A draw, meaning he can run twice as long as a 50W at 12V, which would draw ~4A. I work with batteries for a living and can tell you that a light 2A draw on a good set of deep cycles can run a long time to 50% SOC.

You wouldn't need a combiner to get 24VDC out of two batteries, you'd just need to wire them in series. You would have to change the charging system though, and depending on if Geek went solar for the bulk of his charging duties and turbine(wind while cruising down the road) he would need nothing more than a cheap charge controller. Of course, I'd add an inverter, because 24VDC is also better for running AC through an inverter to power consumer electronic gadgets.

I would set up all my house electronics on 24VDC. It's lower current draw has many benefits. Of course, this isn't my rig, and my opinion on the matter is skewed because the batteries I work on last 20 years and cost half a million plus. (NiH2).

JMHO. YMMV
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Old 02-04-2012, 09:19 AM   #1547
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Originally Posted by geode View Post
??

as an electrician I have to say I find your math confusing.

If the unit draws 50 watts, then 12V * 4a = 48, as does 24V * 2a.

Two Group 31 batts = 220 AH @12VDC or = 110 AH @ 24VDC. Since you can only draw down to 50% battery capacity that leaves you with 110AH @ 12VDC / 55AH @ 24VDC. 110/4=27, 55/2=27.

either way it's the same thing, he gets 27 hours of run time no matter which voltage he uses.

So where does he get any benefit.
You are making the mistake of halving the capacity when combining two batteries in series. So it should be 24vdc/220ah of which 110 ah are available for disharge cycles. At approximately 2A, that's a 55 hour run time over the same system at twelve volts.

Now, from personal experience, deep cycles can run much longer than thief rating the lower your current draw. Most manufacturers will have specs and ratings for lower rate discharge capacity. Nameplate is typically a c/2 rate discharge.

(yes I'm ignoring Perkuets Formula for the moment)


If the only item in the van using DC is the fridge, then yes, he doesn't need a combiner, but if his Espar, lights, inverter, and other DC items are all wired as 12VDC, then having a low draw device such as the fridge as the lone 24VDC item seems a bit odd.[/QUOTE]
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Old 02-04-2012, 10:36 AM   #1548
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I don't know squat about RV power systems, but I did play an EE while at school...

Let's see if I'm following what you both are saying with this example:
  • 2 identical batteries. Each 12V, deep cycle, rated at 100Ah.
  • Wire them in parallel, you get the equivalent of a 12V, 200Ah battery. Everybody agrees.
  • Wire them in series, HBninja thinks it is equivalent of a 24V, 200Ah battery, and Geode thinks it is equivalent of a 24V, 100Ah battery.

Pretty sure Geode is correct, and is supported by the well known, time-tested engineering theorem, TANSTAAFL, There Ain't No Such Thing As A Free Lunch, (also known as the law of conservation of energy)

100Ah at 24v is twice the power of 100Ah at 12V, with twice the batteries, as it should be.

But 200Ah at 24V is four times the power at 2x the number of batteries. If that where true, you could just keep going forever...
  • 10 batteries in series would be 120V at 1,000Ah (100x power for 10x cost)
  • 100 batteries in series would be 1,200V at 10,000Ah (10,000x power for 100x cost)
  • 1,000 batteries in series would be 12,000v at 100,000Ah, (1,000,000x power for 1,000x cost)
Pretty sure it doesn't work that way... But if it does, OPEC is in real trouble!

HBNinja is right on his other point, supported by Perkuets equation... Deep cycle batteries do better (more total usable power) with lower current draws than higher current draws, so there is some advantage in serial wiring vs. parallel wiring.But the difference is nowhere near as profound as above.
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pfb screwed with this post 02-04-2012 at 10:49 AM
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Old 02-04-2012, 11:21 AM   #1549
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Originally Posted by pfb View Post
I don't know squat about RV power systems, but I did play an EE while at school...

Let's see if I'm following what you both are saying with this example:
  • 2 identical batteries. Each 12V, deep cycle, rated at 100Ah.
  • Wire them in parallel, you get the equivalent of a 12V, 200Ah battery. Everybody agrees.
  • Wire them in series, HBninja thinks it is equivalent of a 24V, 200Ah battery, and Geode thinks it is equivalent of a 24V, 100Ah battery.

Pretty sure Geode is correct, and is supported by the well known, time-tested engineering theorem, TANSTAAFL, There Ain't No Such Thing As A Free Lunch, (also known as the law of conservation of energy)

100Ah at 24v is twice the power of 100Ah at 12V, with twice the batteries, as it should be.

But 200Ah at 24V is four times the power at 2x the number of batteries. If that where true, you could just keep going forever...
  • 10 batteries in series would be 120V at 1,000Ah (100x power for 10x cost)
  • 100 batteries in series would be 1,200V at 10,000Ah (10,000x power for 100x cost)
  • 1,000 batteries in series would be 12,000v at 100,000Ah, (1,000,000x power for 1,000x cost)
Pretty sure it doesn't work that way... But if it does, OPEC is in real trouble!

HBNinja is right on his other point, supported by Perkuets equation... Deep cycle batteries do better (more total usable power) with lower current draws than higher current draws, so there is some advantage in serial wiring vs. parallel wiring.But the difference is nowhere near as profound as above.
I thought we were wiring 220AH deep cycles in series? If using 110AH, you are correct. I might need to read up and make sure I'm not confused. If you wired ten in series, you'd have 120VDC(dangerous) at 220AH, rated at 50% Depth of Discharge cyles, netting ~100AH.

You're right though, there is no such thing as a free lunch.
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Old 02-04-2012, 11:35 AM   #1550
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Maybe I misread as well... I'll go back and take a look.

In either case, the total power available when wired either in series or parallel should be nearly the same, with a slight advantage to the series setup because of the lower current draw and how lead/acid batteries work. But not 27 hours vs. 55 hours. More like a 10% advantage, 27 vs 30.
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Old 02-04-2012, 01:52 PM   #1551
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Hey, Geek! Here's your rooftop snow removal method.

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Old 02-04-2012, 01:52 PM   #1552
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So how do you guys feel about wind generators







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Old 02-04-2012, 02:15 PM   #1553
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Any of you gents have suggestions when it comes to shopping for snow-chains?

I got the Sprinter stuck today for the first time

Luckily everyone from the 4 vehicles behind me got out and pushed (they had no choice because I was blocking the way ) so in 30 seconds I was going again.
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Old 02-04-2012, 02:50 PM   #1554
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Quote:
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So how do you guys feel about wind generators







Charge the batteries with the wind while you drive? Guaranteed you will get a moving violation ticket for breaking the laws of physics.

Good luck with the panels. If you go that route, I did a ton of research. You will want to get an MPPT controller. I went with the Morningstar SunSaver MPPT 15L. Good prices thru Ecodirect.com

Quote:
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Any of you gents have suggestions when it comes to shopping for snow-chains?

I got the Sprinter stuck today for the first time

Luckily everyone from the 4 vehicles behind me got out and pushed (they had no choice because I was blocking the way ) so in 30 seconds I was going again.
Full chains ride pretty harsh at any reasonable speed. You might want to look into cable chains.

Plan B: always carry a jug of kitty litter & a shovel. This will get you out of some icy spots.
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Old 02-04-2012, 03:10 PM   #1555
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Before we left this morning I loaded up both the snowboarding and the snowshoeing gear.



Good thing... when we got about 5 miles from the ski resort traffic came to a standstill (i.e. it was bumper-to-bumper traffic to the resort ). Turns out our local hill got 2 feet of snow while every other ski hill in Colorado only got a few inches.

We went snowshoeing instead.



Beautiful day for a drive







Time to learn about ways to get myself unstuck if I get it wrong again like I did today... luckily I got it stuck in Nederland and not in the middle of nowhere.
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Old 02-04-2012, 03:40 PM   #1556
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Quote:
Originally Posted by geode View Post
A) ??? ... a group 31 size deep cycle battery is 110ah capacity. I referenced a group 31 in my example. so it's either 220 ah @ 12VDC, or it's 110ah @ 24VDC.

B) Perkuets Formula accounts for the lower draw = more capacity. It is can actually be predicted if you really need to know ahead of time.
Yep, I misread earlier. Somehow I had 220AH capacity batteries in mind and couldn't figure out how you had gotten to 55AH from a 2S configuration. This is why I really should drink coffee on the weekends

If Geek just made the back of his van look like this...


He'd have no problems with his fridge.
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Old 02-04-2012, 03:47 PM   #1557
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Quote:
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Any of you gents have suggestions when it comes to shopping for snow-chains?

I got the Sprinter stuck today for the first time
Lots of emergency vehicles (Firetrucks, ambulances) use automatic chain systems. Mountain volunteer fire departments really love them. You wouldn't think they would work well, but actually are pretty awesome. I think ONSPOT even makes one specifically for the Sprinter chassis, so no fabrication work.



Not cheap, but a lot less expensive than making your sprinter a 4x4
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pfb screwed with this post 02-04-2012 at 03:54 PM
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Old 02-04-2012, 03:51 PM   #1558
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Lots of emergency vehicles (Firetrucks, ambulances) use automatic chain systems. You wouldn't think they would work well, but actually are pretty awesome. I think ONSPOT even makes one specifically for the Sprinter chassis, so no fabrication work.



Not cheap, but a lot less expensive than making your sprinter a 4x4
+1
All the school buses and ambulances here run them.
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Old 02-04-2012, 04:01 PM   #1559
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Hey, the instachain video even has a Sprinter in it!

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Old 02-04-2012, 04:14 PM   #1560
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Yep, I misread earlier. Somehow I had 220AH capacity batteries in mind and couldn't figure out how you had gotten to 55AH from a 2S configuration. This is why I really should drink coffee on the weekends

If Geek just made the back of his van look like this...


He'd have no problems with his fridge.
He also wouldn't notice the weight of the LLPro in his cabinets any more.

I had occasion to tour an MCI switching center once. Their battery room was huge and full of clear, open-topped 48V batteries with automatic electrolyte management systems. (Basically a float that turned on a stream of demineralized water to top off the battery.) They also had a 1.5MW Cat-powered diesel generator w/ a 15k gallon underground fuel tank to keep the batteries charged in the event of a major power failure.
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