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.