backchannel: AGM and “solar generator” misconceptions extravaganza

I gave up on the following thread in /r/vandwellers.  I share comments below in the hope it encourages others to dig deeper on their own.

one of you full time road warriors feels very strongly the other way and thinks the 80% is fine for AGM

It is fine for both AGM and flooded, assuming one accepts the trade-offs.   This has been my consistent position.   [See why I gave up?]

Discharging to 80% DoD is just another choice on the bank-size vs. duty cycle continuum.   This back-and-forth has not been about whether one can take AGM to 80% DoD (one surely can) but whether one can take AGM to 80% DoD but not do the same with Flooded.   They exhibit the same cycle reduction at deep DoD.  AGM does not possess special deep DoD properties.

Generally speaking:

  • 30% DoD = cheapest lifetime kW due to radically high number of cycles, but lots of “dead lead” (unused capacity and weight to lug around) and up-front expense

  • 50% DoD = a common compromise between cycles and bank size.

  • 80% DoD = most expensive kW due to drastically curtailed cycles, but uses the smallest practical bank and replaces it more often.

Pick your poison.

No matter the DoD one chooses, proper charging is critical, and even more so when subjecting the bank to deep DoD.   Moreso again for AGM.

> and he pointed me to battery university

B.U. articles vary widely in quality.  I wonder how many thousands of dollars in battery murder that article has caused vandwellers.

The sentence in question at B.U. is “AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life.”

But it doesn’t work that way.  No manufacturer publishes data claiming AGM has similar cycles at 80% DoD as the same line’s flooded has at 50%.   Don’t you think you would want to claim this for a product that cost 2x as much?

The DoD v. cycle chart for all lead-chemistry deep cycle batteries looks like this:

The numbers are different but the trend is consistent.  That above example is for the Trojan T-105 AGM, an apples to apples comparison with the T-105 flooded.   You will notice no miracles occurring at 80% DoD; AGM suffers just like flooded.

> I do know that the corporate guidance you mention is from folks that would obviously profit from folks buying more batteries than discharging deeper.

Discharging deeper will sell more batteries;  that is what drives the  $/kW trends above.

“One can live in the shadow of an idea without grasping it.” – Elizabeth Bowen

> Does not mean they are wrong

It’s published empirical data about DoD v. duty cycles.

> I was getting close to 5 grand for a 400 watt solar system to power a 6x12 box. It just started seeming absurd.

Yes, that is absurd.  It seems like a Brewster’s Millions scenario where  a person needs to intentionally waste as much money as possible.

> That's why I switched gears and took a fresh look at this years crop of solar gens, Camping world sells Natures gen to motor home folks

Solar gens are very expensive solutions, incorrectly named and often presented with very misleading specs bordering on fraud.

As it was memorably (if roughly) put in /r/vandwellers:  “These aren’t aimed at people that know what they are doing.”

Do they work to spec?  Sure, if one knows how to read and apply the spec.  Most people see what they want to see.

>so it was one of the ones I looked at and they have a new model coming out at the end of August with 3800 starting and 2800 running watts

That starting/running watts spec is completely unrelated to OP’s low-watt usage patterns (6 LED lights, on demand water pump for sink, 12v box fridge, and vent fan) but the marketing bait has worked.

The battery in that not-yet-existent product is a 12v 100Ah AGM.  If we pull 80% of the capacity of that battery we will get 20.5 minutes of runtime at that vaunted 2800w rate.

> that works on wind, solar and shore.

Any battery can be charged by wind, solar, and shore.   Wind power is famously ineffective for vandwellers;  here it’s “creeping featurism”.   Having a feature you don’t use is a needless increase in cost and complexity.

Even if you wanted to use wind, the manufacturer specifies

  • = 10mph sustained winds;  and

  • a tower 20-30’ above nearby obstacles _(ie, trees and van). _Tower not included.

The $500 turbine (with “our propriatary connection that only hooks up to our Nature’s Generator”) is also not included.   Their documention makes no mention of shunting in the devices or other overspeed control.  Hopefully they thought of that.

The charge controllers handle 300 watts

The $1000 (!) unit comes with 100w of panel and has PWM controller.

and they have a stand alone 12v power pod used to also expand the base system and each one adds 100ah battery, solar controller and another 12v output.

ie, proprietary walled garden hardware

**> For my 6x12 box** I really do think it's a fine.

No complaints from me.  I like to see people making their own decisions (good or bad) and spending their own money to make it happen.

> Its a lot less than I would have spent on the high-end plans

I’m not sure that’s a compelling argument.

> should handle my fridge, fan and lights. Bough one pod so can go to 600 watts which is prob all the roof space I have anyway.

We doubled down.  The “pod” (another 100Ah batt) is $500 and comes with no panels.  We are now at $1500 for a 100w:200Ah system.  Oof.

> Is it a mistake dunno, but will find out.

The more common scenario is OP experiences disappointment and disappears from /r/vandwellers.   I hope for a good outcome here.

> It is a gen as well

No, it’s not.  It does not generate power.

with a 30amp output on the back so if down the road I decide to replace it with a component system this could serve as a backup unit.

Ie, an DC outlet.

The lead batts also charge slower that AGM or lith so i
  1.  AGM are lead batteries

  2. AGM accepts more current in Bulk.  This does not appreciably shorten overall charging time since Absorption time is greatly extended at higher charging rates.  The difference between charging at the max and min current for lead batteries is about 12 minutes over many, many hours.

  3. AGM can’t charge faster in this context due to power input limitations (see below)

in essence I prob need a 600watt system to get the charging power of a 400 with the others.

So much confusion.

  • 400w of panel on flooded acts just like 400w on AGM or lithium.

  • 100Ah of flooded has a charging max of 20A;  below that AGM and flooded charge at the same rate.  20A is about 250w, and the unit’s max solar panel size is 300w;  on its PWM controller under perfect conditions that would be about 255w output.  The wall power charger is 90w.  100Ah of lithium, flooded, or AGM would charge at the same rate on this device because it won’t reach the max charge rate of any of them.

  • AGM requires greater current to charge than flooded, which this unit cannot provide.   The batteries are on a death watch from the very beginning.

  • when discharged to 80% DoD the 100w panel will be maxxing out at ~68W.  (11.66v x ~5.8A) <– have to guess at the Imp because (surprise!) the device manufacturer does not give the specs.   I will give the mfg credit for using poly panels here, which is a better fit than mono for PWM controllers.

> Hey anyway, just doing the best I can to make decisions that work for me.

I am 100% cool with that.  Not knowing is a-ok;   not knowing and spreading misinformation is not ok.