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”

datapoint: excess power

My rig’s 12v house power system is literally divided into two sections:

  1. essential loads – battery charging, ventilation (fans), core lighting, refrigeration, networking
  2. nonessential (opportunity) loads – everything else:  laptop charging, spot lighting, lithium battery charging, NiMH charging,

The opportunity loads run on a dedicated circuit that automagically shuts on/off with the availability of excess power.  Note:  in practice this almost always means excess solar power Continue reading “datapoint: excess power”

Lithium vs. flooded banks for beginners

Note:  for comparison purposes there’s a link in the text below to Trojan batteries on Amazon.  Buy them locally to get the best price.  There’s no shipping and you’ll be able to trade in the old cores.

From a thread on /r/vanlife:

=======

Thank you for the thoughtful response.

I don’t disagree with your technical analysis. I am pushing back on ideas like lead is “cheapo”, that Li is “doing it right”, and that large power storage capacity is necessary for “actual living”.

I think it’s worth talking about in public a bit more to help Continue reading “Lithium vs. flooded banks for beginners”

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!

another newbie perfect storm

  • AGM batteries?  check
  • massive inverter?  check
  • wants to run A/C off grid? check
  • wants to charge batteries from idling vehicle?  check

I’m going to get some of the Renogy AGM deep cycle batteries. Yes, I know wet cells will give me more bang for the buck, but my situation does not allow for them. At all.

I’ve already got a 2000/4000w inverter.

The inverter will power a CPAP, charge a power wheelchair, and even run the AC so the lines need to be pretty robust. I was thinking I might skip the breaker box and run a direct fused line from the batteries to the inverter instead of going through the breaker box.

I also need to leave whatever opening I need to add solar at a later date, though I have little idea of what making accommodations future solar looks like.

Before you ask, I know that the AC will eat the battery power pretty quickly. My current plan is to have an auto-starter on the van that will track the battery charge and, when the charge hits 50%, will turn on the van and start charging the batteries and running things off the alternator.

 

Let’s take these in order.

AGM batteries cost about 2x as much, generally have less capacity and lower voltages during Absorption.  I would like to strangle the first youtuber that started telling people to buy AGM for vans and campers.

Inverters are problematic for both philosophical and practical reasons.  They encourage people to think that inverters pull power from thin air, forgetting that it comes from the battery bank (at ~10x the amperage).  “All I need is a giant inverter and I can run everything I want!”  No, you probably can’t.   And inverters waste some power as heat;  most rules of thumb mention 10% losses at their full efficiency, and much more when underloaded.

Off-grid A/C is possible but not practical. Most people don’t have the cash, skills, or stubbornness it takes to get it working.

Charging from an idling alternator is bad idea because:

  • it’s hard on the alternator (no air movement to cool it).   Doubly so with AGM since charging current can be higher than with FLA
  • it’s hard on the engine, particularly direct injection engines
  • it’s unlikely to get lead-acid batteries fully charged, because Absorption takes a long time at voltages most stock alternators will not produce.

 

The good news is many folks chimed in to try to save the poster from himself.  One problem with the greater awareness of vandwelling is there are more clueless folks in the mix now.

 

 

 

 

perfect storm of bad assumptions

It is easy to get burned out answering the same questions over and over.  It’s one of the reasons I started work on the RVwiki in general and the Gentle Introduction to Solar article in particular;  collect the info and point folks there.  With any luck they’ll read it before making uninformed and $$$ decisions.

Let’s consider what has gone wrong in one situation.  I want to shed light on Continue reading “perfect storm of bad assumptions”

GC2 update

I mentioned earlier that I’d picked up some abused Duracell GC2 off craigslist for testing and for use as cores if it came to that.

Since then they have been on my DIY converter setup and are acting fairly normal.  They come up to Vabs and bubble as one would expect.  They hold Vfloat at C/200 and don’t plummet immediately when charging is removed.  They handle light loads without exploding.

Still, I assume capacity is seriously compromised.   If I had an inverter I’d hammer it a bit and see how bad they crater but for now I will upgrade the Duracells from completely hosed to mostly hosed.

[youtube https://www.youtube.com/watch?v=_LlQSMu5Mso&w=560&h=315]

LiFePO4

I am currently working through the monster Ca&S LFP battery thread; after a couple of weeks I am well over 1000 posts in.  That’s maybe a third of the total.

So that no one else has to summarize the info I am making a wiki page on it.  Bit of a mess right now, but here’s the takeaway for boondockers so far:  If we

  1. start with balanced cells; and
  2. charge to 13.8v; and
  3. have sub-C (“fractional C”) charge/discharge rates

then we don’t need a balancing BMS.

Regular charge controllers with configurable setpoints can work fine,  Vabs would be 13.8v with a short a period as possible and “float” would be set to something lower than that (13.2 – 13.4v).  LFP batteries don’t need or use a real float so the lower voltage basically stops charging.

In a typical day the bank would charge to 100% SoC when the sun comes up, stop charging, drop under normal house loads to ~80% SoC  and hold there as long as the charger can support it.