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Old 04-25-2018, 05:54 PM   #21
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12V vs 24V System

My plan calls for a 24V system. Why? Because it's more efficient for my purposes.

The only electrical components on my build are the lights, the water pump, and an air conditioner. I have forty 3.2V/30 amp hour LiFePo4 batteries, which, when wired up eight in parallel (24V and 30 amp hours) and then five of these cells in series (24V and 150 amp hours), will give me 3,600 watts.

Since I plan on running the air conditioner off these batteries sans generator, I need maximum efficiency everywhere. My plan also calls for three 300W/32.48V/9.24 amp solar panels, which means I want my battery bank as close to their voltage as possible to minimize energy loss.

So basically I have 30V solar panels charging a 24V battery bank, which is more efficient than charging a 12V battery bank.

My water pump is also 24V, and there are plenty of 24V LED light strips out there, so for my purposes, 24V is the most efficient since I want to go without a generator.
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Old 04-25-2018, 05:56 PM   #22
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I should also say that going 24V does have a disadvantage, the 24V inverters I want are more expensive than their 12V counter parts. But I can live with that.
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Old 04-26-2018, 07:39 AM   #23
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Not necessarily telling you to change anything, but for those seeking knowledge FFR.

Quote:
Originally Posted by TheArgobus View Post
I have forty 3.2V/30 amp hour LiFePo4 batteries, which, when wired up eight in parallel (24V and 30 amp hours) and then five of these cells in series (24V and 150 amp hours), will give me 3,600 watts.
Watts is an instantaneous measure of draw, not a capacity. Maybe you mean watt-hours, but no need to convert, best to just stick to AH.

Also a serial/parallel design should not have more than 3 series strings, or you will get balance problems.

What is acting as your BMS?


> Since I plan on running the air conditioner off these batteries sans generator, I need maximum efficiency everywhere.

You will find your bank is way too small, you will need a genny when your solar isn't pumping out 800-1200W.

IMO you should have gone with 180AH cells, minimum, even 540AH will run aircon for just a few hours before needing charging and genny running aircon - size your genny + charger to be able to do both at once.

> So basically I have 30V solar panels charging a 24V battery bank, which is more efficient than charging a 12V battery bank.

Not a significant factor IRL

Most people will want lots more DC devices, often 12V is all that's available, or the 24V versions are **lots** more expensive. DCDC down converters (buck) will be required, which add cost and reduce energy efficiency.

But the 24V issue is a minor one.

The big problem is thinking you can run aircon off batteries for any length of time.

I advise you to test everything before finalizing your installation.

And consider using lead batts, going to LFP only when you have more IRL experience.
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Old 04-26-2018, 08:33 AM   #24
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Kinda off topic but very interesting stuff (to me at least - hopefully to others).

All of this is specific to my setup. 24V lithium battery (28.4VDC is absorb voltage, 27.2VDC is float voltage), Sunpower E20 435watt panels (~72 volts), and Morningstar TS-MPPT-60.

I had to go do some research on MPPT efficiency. I like research so thanks!

In the case of the Morningstar TS-MPPT-45/60, there is some efficiency gained with a 24V battery. From the manufacturer documentation, the controller is about 97% efficient converting 66 volts from the array for a 24V battery and about 94.5% efficient with a 12V battery (and 98.5% efficient with a 48V battery).

I attempted to dream up the worst case and, I think (again, in my case), it is during the winter months when the array is struggling to produce 1000 watts. With a 24VDC battery bank, the controller would output (theoretically) 34 amps (@ 28.4V). If it were a 12VDC battery, it would output 66.5 amps (@ 14.2V - ignoring that the TS-MPPT-60 maxes out at 60 amps). The efficiency gain of the higher voltage battery bank is then about 1.5 amps (at 24VDC).

If all that is correct then I don't put this is in the category of significant. However; the small things do add up eventually. I probably waste this much energy just leaving a light on because I'm too lazy to get up and turn it off. Of course, that is luxury that having an excess of power provides. I wasn't nearly as wasteful in my first motorhome that had a small capacity electrical system.

The specs for my inverter list 85%-90% efficiency for both the 12V and 24V models. Those look like very round numbers to me.

Using 85% (maximum lithium discharge) of a 300 Ah battery (255Ah), assuming A/C drawing 1650 watts, and 90% efficient inverter (~1800 watts). My math shows that you will be able to run the A/C for about three hours when there is no solar input. Sound about right??

Obviously, there is all sorts of complexity with A/C duty cycle and such. Ignoring that, I think three hours is respectable. It doesn't help when it is 100 degrees and 100% humidity all night but that is really a problem that requires shore power.

The problem for some/many of us is that a single A/C simply isn't enough. Having to double (or triple) the A/C power consumption really makes the numbers difficult.
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Old 04-26-2018, 08:48 AM   #25
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The efficiency rating for the inverter on the box is best case scenario, won't get near that IRL.

And even with a good start capacitor, the inverter will need to be sized way higher than the continuous discharge power.

And it's pulling power even during cycle-down.

At high-amps discharge, the safe cutoff LVD setpoint will need to be higher than normal, aiming to cutoff at 10-15% will take a pretty intelligent BMS or you're really taking chances drawing too low.

Basically anything under 600AH is just time-shifting genny usage a little bit, plus all that energy needs to get put back into the bank.

And a nice quiet inverter genny is likely more quiet than the aircon anyway.

So why not save the thousands of extra dollars and just run the aircon straight off the genny in the first place?
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Old 04-26-2018, 09:10 AM   #26
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Quote:
Originally Posted by john61ct View Post
At high-amps discharge, the safe cutoff LVD setpoint will need to be higher than normal, aiming to cutoff at 10-15% will take a pretty intelligent BMS or you're really taking chances drawing too low.
Obviously, we need the person that brought up this subject to comment on what equipment is being used. I did a lot of guessing in my previous post.

I do have a BMS that handles the 85% discharge scenario. However; I'm not going to try running A/C from my battery. I am able to stay away from hot/humid places (for the most part), not everyone is - the whole bit about there not being a single right answer/solution.

Quote:
Originally Posted by john61ct View Post
Basically anything under 600AH is just time-shifting genny usage a little bit, plus all that energy needs to get put back into the bank.
600AH of what?? 12 Volts? He said he at 300Ah at 24V. That comes mighty close to 600Ah at 12V.

Quote:
Originally Posted by john61ct View Post
So why not save the thousands of extra dollars and just run the aircon straight off the genny in the first place?
The implication is that a generator doesn't cost anything. Obviously, there is a purchase cost but the operational costs can be fairly significant over a period of time. I did the numbers once to calculate the payoff time for the solar system on my last motorhome (please don't confuse this with the picture I painted above). Here:
Payback of a Solar Charging System - JdFinley.com

That post does not consider running the aircon (A/C) many hours a day (from generator power). I can only imagine the generator fuel costs of such a thing. Probably $600/month??? The potential for cost savings is a good reason to try and do it from solar - if in a situation where it is workable.
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Old 04-26-2018, 12:17 PM   #27
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Running off solar is one thing.

Running from batteries - for any extended period of time - is another.

I've seen plenty of successful examples of the former, but attempts at the latter only in S&B, never mobile, just not enough room on the roof.

And fuel costs do indeed add up, but just as with making aircon off DC **even possible at all**, the key is small BTU and only running for short periods.

Best of course is to just follow the 60's, head for higher altitudes, what's the point of living mobile if you don't move when the weather is uncomfortable?

Otherwise whether balancing an upfront investment of $6+K or a monthly fuel cost of a few hundred, it's still cheaper than paying for living on the grid.

Which is the only time aircon get cheap.
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Old 04-26-2018, 03:08 PM   #28
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I appreciate the replies guys, it helps iron out some deficiencies.

The mini split AC I have my eyes on draws 4 amps at 220 volts, and the inverter I want lists 87% efficiency but let’s just go with 80%. With a 150 amp hour bank at 24 volts, I have 3,600 watts—which, when inverted and loses 20%, gives me 2,880 watts. At 220v, that’s 13 amp hours, which essentially gives me 3 hours of full blown mini split a/c usage.

Or the way that I look at it, the AC requires 880 watts per hour to operate. With the 20% inverter loss, that’s really 1056 watts per hour, which is 44 amps at 24v. So about 3 Hours if I only have 150 amp hours.

I understand that the listed amperage is only for maximum blow, which mini splits do not run at most of the time, but for the sake of energy planning I’m going with maximum draw all the time.

Now that’s with no charging. Let’s say the three solar panels are operating at 70% efficiency, so instead of 900w, they’re at 630 watts, which, at 24 volts, is about 26 amps. That’s only half the draw of the air conditioner, so best case scenario I can get 6 hours of full blown ac in full sun with nothing else operating.

But I don’t need that. I’m not full timing, the bus, which is about 130 square feet inside, is only used at high elevations on the west coast, so the few times I’ll actually need the air conditioner I’ll be in full sun and it’ll really only be to take naps in during the afternoon. So for my purposes, it seems that running the AC off a 24 v bank works for me.

You brought up a good point about keeping the cells balanced. My plan is to use these little guys: http://www.electriccarpartscompany.c...ers-Equalizers

Now, some people (myself included) might be thinking golly gee, this is gonna be thousands of dollars just to go without a generator. And that’s fine, because my bus project is a long term project, and I don’t expect to even install the AC for a few years (thanks to the environment I’m in, it’s really not necessary). It’s more an experiment to see if I can make it as luxurious as possible, and fortunately the money and time isn’t that much of an issue for me.

Anyway, the point is, 24v makes more sense for me than 12v, given my austere electrical requirements.

Now, I have a thick skin, and I’m not an electrical engineer, so if any of this seems off to you and you’re pulling your hair out over how dumb I am, please say something! I won’t be offended, and welcome any critiques of how unrealistic or unreasonable any part of my plan is from you guys, given that most people on the forum are way smarter than I am about these topics.

EDIT: For instance, one deficient area is the charge controller. I have one of these (https://www.amazon.com/dp/B00FB3OPKM) but I don’t know how exactly the charge controller, and how many, will tie in to my system given the battery bank and solar panel size. The electrical system is pretty far down on my list of conversion priorities. The only reason I have the batteries right now is because of a soon-to-be-gone deal where I bought them for $20 a piece from a colleague.
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Old 04-26-2018, 03:45 PM   #29
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I love it TheArgobus!!! Go For It!! I'm in total agreement on the $$. I would much rather invest a goodly bit in a cool test. All the better if it uses renewable energy.

I don't know anything about mini-splits but what you said seems reasonable.

Remember that you can draw your lithium bank down to about 15%. Like lead-acid (50%), the entire capacity is not available for use.

Quote:
Originally Posted by TheArgobus View Post
With a 150 amp hour bank at 24 volts, I have 3,600 watts...
A tiny thing but, in the interest of accuracy, you then have 3,600 watt-hours.

Also, my apologies... I thought you said above that you had a 300Ah (@ 24V) capacity. Now I understand 150Ah (@ 24V).
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Old 04-26-2018, 03:48 PM   #30
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My advice on the charge controller is be sure it either comes with or you can set the proper configuration for lithium (different than lead-acid, agm, sla, etc.).
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