backchannel: solar calculators

from this old post I found while Googling a different topic:

I have about 364 watt [daily use] total. I don’t know how accurate that is becuase im guessing on hours used and wattage of leds but 364  is what i inputed.  Alt e gave me 300 watts panel, 310 ah battery, 25 amp charge controller. Renogy gave me 196 watt solar and 60 ah battery

Battery sizing

364w/day would be about 30Ah usable capacity needed, 60Ah usable.

Option 1:  Compact:  Trojan type 27 AGM, 89Ah.  My interpolation of published data suggests Continue reading “backchannel: solar calculators”

backchannel: this week in battery mistakes

not a deep cycle

> I put a single 1400Ah (700 useable) wet cell in my travel trailer at 170 pounds. No solar yet

Uh, no.  OP bought a marine battery with 1400 CCA.  Not a deep cycle, as s/he will find out within a year.


killing me softly

> 655ah of  batteries! … I got 960w of solar on the roof to charge them too!

  1. it’s a mismatched bank of one excellent AGM (Lifeline) and two other SLAs, presumably AGM.  Good luck with that, and my condolences to that nice Lifeline (excellent batteries, this example is ~$650)
  2. AGM requires C/5 charging current at a minimum.  Example:  lifeline says they require a minimum of .2C (C/5) charging or “the cycle life of the battery may be negatively affected”.  Ie, they will die early.
  3. The solar will provide  70A of charging on a good day (960w – at least 15% heat derating and DC-DC losses  = 816w,   816w / 12v = 68A.    .2C charging would be 131A for a 655Ah bank.   Kids in grade school can tell you that 70 is much less than 131.  About half.

backchannel: thoughts on gnomad home’s epic guide

FIrst off, much respect to them for tackling such an extensive article.  I don’t have the patience or talent to do it.

This post describes what we did with our own system based on our own research

A good start.  It does not claim to be gospel.

main components

The 400w Renogy kit is fine.  Could be sourced cheaper by the piece, but no biggie.

I like the 400w:155Ah ratio, especially since we are going to have an isolator installed.   The 155Ah AGM is ~$360.  For that money I would much rather have a set of Trojan T-105.    Longer life, lower cost.  If he really needs AGM I’d pony up another couple hundred for 6v AGM.  Still less expensive than the 12v by the lifetme kAh.  Continue reading “backchannel: thoughts on gnomad home’s epic guide”

backchannel: AGM @ 80% depth of discharge

I’ve seen similar claims before, but I haven’t done the homework until I was inspired by this one:

>they are design such that you can use more of it without damaging the battery’s lifespan. So comparatively, AGM compared to FLA batteries with the same rated capacity (the same amount of lead in them) will have more USABLE capacity.

I could think of no particular reason that vanilla AGM should enjoy more cycles at very deep depth of discharge (DoD) compared to flooded, excepting maybe the exotic carbon-foam variant.

So I started doing the homework.  It took less than a day because Continue reading “backchannel: AGM @ 80% depth of discharge”

backchannel: island of misfit [solar] toys

Our unfortunate fellow has already spent hundreds on some stuff that won’t play well together.  Each component is fine on its own, interestingly enough.

good news first

The setup (other than the oversized inverter) is sized rather well.  200w of panel, 20A of b2b charging, and 100Ah of battery.   Good.

the less good news

Renogy needs to be horsewhipped Continue reading “backchannel: island of misfit [solar] toys”

backchannel: new guy nails small solar setup

Most of my backchannel comments are lamentations about bad decisions and techncal ignorance.  But this newcomer did a great job designing a small system: and it’s worth talking about.

> Just need enough power to charge a phone and occasionally a laptop, power the roof vent fan, and a small rv fridge would also be nice.

It’s nice to see someone have a reasonable expectation of what can be powered from a small setup.  No electric cooking, no A/C, etc.   Carry on.

Here are the parts he picked out, and my comments on them: Continue reading “backchannel: new guy nails small solar setup”

backchannel: solar generators, charging AGM from alternator

OP is running a Yeti-style battery pack.

I charge a 20,000mAh external battery

Onlookers: 20,000mAh = 20Ah.

I’m out of the Van most of the day and charge that battery while I’m working

That is a perfect use case for such devices.  [not joking]

I do not yet own a refrigerator, but do plan to have a fridge and full house battery set up with AGM batteries within a few months hooked up with a solenoid to the alternator for charging. No solar panels for me as I’m stealth camping in New York City.

Something to consider when planning out the future setup: it will be tough to keep lead-acid batteries (including AGM) healthy with alternator charging alone. Possible workarounds:

  • charge the AGM periodically (2x/week or more if possible) from shore power at your work, using a converter. This will help limit damage from less-than-fully charging.
  • budget to replace lead-acid batteries more often.  Give real consideration to flooded batteries;  they are half the price and more resilient to abuse.
  • Normally I would encourage to a ‘dweller to grit their teeth and install a panel or two: the panel will provide high voltage, long duration charging to complement the high current, short duration charging from the alternator. BUT stealthing in NYC brings a special challenge — tall buildings make “urban canyons” that make direct sunlight a rarer occurrence than elsewhere.
  • Because of the urban canyon effect, I’d encourage you to think about switching to a battery chemistry that doesn’t care about state of charge. Both lithium and carbon-foam (a subtype of AGM) are much more tolerant about infrequent or incomplete charging.


backchannel: agm for vandwellers

Part of a response to a question I answered.  OP wanted advice on the short term only, and I’d overshared my thoughts on the long term.  I took it out and moved it here.

If you decide vanlife is for you, in the **long term** the biggest bang-for-buck in deep cycle batteries is 2x 6v golf cart flooded (wet cell) batteries in series, usually 200Ah+. They are able to take massive abuse and can be maintained (watered) when used hard. The purchase price is about half of what AGM cost, and they’ll likely last longer in this use case.

There are very few *actual* 12v deep cycle lead-acid batteries. The lead plates are so heavy they are not practical for humans to move. Hence splitting them up into two 6v batts. The 12v ones that do exist cluster on the high end: northstar, odyssey, rolls.

AGM weren’t developed for our uses; they were developed to for standby/backup applications where:

* normal liquid electrolyte would stratify when sitting still for long periods – this is not an issue for us because our batteries are agitated by vehicular movement.
* they could provide massive current suddenly when needed (ie, invert their power to AC to run cell towers when grid power goes out)
* would sit fully charged at float voltage (Vfloat) in perpetuity with limited damage

Our van battery banks are not still, do not stay fully charged, and generally speaking do not need the ability to dump massive current in a hurry.

exceptions to the rule

There really are situations where AGM is warranted but they are relatively rare in vandweller scenarios.

  • *  the ‘dweller installs the battery bank in a location where it is not reachable for normal maintenance
  • *  the ‘dweller needs to mount the battery on its side for some esoteric reason (or is 4×4 trekking so the RV itself can end up in weird angles)
  • *  the ‘dweller chooses to run heavier-than-normal loads like microwaves, electric coffeemakers, electric cooktops, etc. Note this means the battery can be drained faster (and bulk recharged faster), not that it provides more capacity.
  • *  the ‘dweller has has a chemical hypersensitivity.
  • *  the advanced ‘dweller is running a single battery for both house and starter systems (“shallow cycling”)  <– not for beginners!