backchannel: DC-DC charging myths

Outbackjoe’s info is excellent.  Here I will address some of his points about what he calls DC-DC charging myths.

 The most commonly reported myth is that your alternator cannot fully charge a battery.

It can, given enough voltage and enough time, as on a powered cruiser (boat), that spends many hours a day running the engine.   To some degree, time can be substituted for voltage (ie absorption takes longer at lower voltages).

Having said that, Continue reading “backchannel: DC-DC charging myths”

backchannel: solar is a luxury

I generally don’t look at the username or other details of a poster’s presence when replying;  I stick to the “content of their [text] character[s]”, to paraphrase MLKjr.  This can cause me to waste time casting pearls before known swine.

So I didn’t realize that the post was by a daydreaming pothead who frequently bombs half-baked ideas at us in /r/vandwellers.  This particular one didn’t seem as moronic as usual (only confused and overconfident) so I engaged.  It had the title:  “How often do you drive your van? I think solar is a luxury. I’m thinking driving to work and to buy food ect is going to be plenty of time for the alternator to recharge 4 deep cycle batteries. Instead of having solar you could recharge with a system your van already has.

So here’s what I wrote before I realized s/he probably wouldn’t even remember writing it later:

Solar charging and alternator charging are different beasts that work better together than either do separately.

Solar isn’t mandatory if you can plug in a smart converter at work a couple times a week to get the bank fully charged.

People who want to charge from alternator only would be wise to look at lithium or carbon-foam chemistries which don’t care about partial stage of charge (the scenario you presented). Lead batteries die early from partial charging.







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: Lifeline batts and alternator charging

I follow the on Meryl and Me Hit the Road blog.  I read an article there today and thought some of the tech details would be worth talking about.  The article discusses the charging system[s] for the RoadTrek’s new house battery bank.

the isolator

[the battery seperator] is a “smart” switch – when the engine is running and the RV batteries need to be charged it connects the engine battery to the RV batteries and charges them through the van’s alternator. It also will charge the engine battery when the RV is plugged into shore power

This sounds like a dual voltage sensing relay (VSR).  Such a relay “senses” when either side has power to share and then connects the starter and house batteries.   The VSR is powered by the starter battery and is always “on” watching the voltage on both sides.

[A normal isolator (constant-duty solenoid) on the other hand gets power from the ignition wire so it is only “on” when the key is turned on. ]

Let’s unpack the quoted paragraph a bit; there’s a lot going on. Continue reading “backchannel: Lifeline batts and alternator charging”

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.