Shower thought: shallow cycling for car dwellers

I was thinking about solutions for car-dwelling folks with minimal resources:  not much room, starter battery only.

Seems to me quality of life for car dwellers would likely go up if they had an inexpensive/automatic/easy system for charging small devices through USB and running a fan or laptop off a ciggy lighter without hurting the starter battery (much).   No danger of running down the starter battery;  it’s actually likely that the starter battery would be healthier.

So consider the following:

big picture

10A system charges the starter battery gently and draws on it when fully charged. Bonus points for keeping starter battery topped up for reliable starting in the middle of nowhere.  This is basically an opportunity-only setup with no real meaningful cycling of the battery.

The** default approach** assumes starter battery is actually sealed (not just “sealed” or maintenance free).

  • Charge to 14.2v.  Goal here is to get voltage up high enough to harvest power without outgassing.

  • disconnect loads at 12.7v; battery stays fully charged

  • Reconnect at ???v (14.2?  14.0?  Something high enough to let the PWM controller get closer to Vmp and make the most power

An** more aggressive approach** to be tested on a separate system assumes ability and willingness to water batteries from time to time.

  • Charge to 14.4v for more power.  Some outgassing may result.

  •  disconnect at 12.5v - 12.6v (~85-95% SoC)


Whatever panel is handy and cheap.  Something like 75-100W would probably be the sweet spot.  Larger panels up to 130W could work but they are likely too large for easy car storage.  25W would put something into the battery.

  • used, hand-me-downs where possible

  • set out in the sun?  If set in the sun some kind of waterproof quick-connect to allow easy deployment.  SAE plugs aren’t loved much [on CRVL] but would be an easy and cheap solution.  I use them on my vehicles (bikes, scooters, truck) to quickly plug in AC chargers.

  • mounted to roof or trunk?



Configurable 10A PWM shunt controllers with USB and LOAD outputs are about $15 now on Amazon and eBay, ~$8 if one is willing to wait on the slow boat from China.  The ones I am talking about here are labeled CMTD-G but that info will of course change rapidly.  YT videos suggest they function as advertised.

MPPT is not warranted as the battery will never be cycled deeply;  that eliminates one of the MPPT’s key benefits.

About the controller:

  • configurable charging voltage.  Some appear to have a FLOAT stage and some do not.  I do not think it will matter since it will be shallow-cycled each day

  • configurable LVD/LVR

  • temp compensation 3mV/cell/℃.  At the controller rather than at the battery, but better than nothing

  • USB and LOAD turn off together, manually or by LVD


The 10A limit and small dimensions of passenger vehicles makes wiring easy and drama-free.  The controller will accept up to 8awg wire.

10AWG landscape wiring is cheap, flexible, UV resistant, and good for 15’ (30’ circuit length) between battery and controller with 3% loss, and up to 50’ (100’ circuit length) between panels and controller.

The latter extreme incurs up to 10% loss but I think it won’t make a practical difference. The PWM controller will already be running the panel at less than Vmp. The controller will see the lossy end and drive the panel higher until the controller sees the voltage it wants.   Losses could be minimized by having a long cable and a short cable made in advance;  use the shortest one that can see the sun.  Running a long cable to panels could be a real advantage if one wanted to park in shade but still harvest power.

the experiment

I’ve ordered a couple of the $15 controllers for testing.  I meant to get 2x 10A controllers but I accidentally ordered a 10A and 20A version of the same controller.  No biggie.

I already have a couple spare batteries (one starter, one marine) and a couple of small panels.  The marine battery has removable cell vents but the starter does not.  I ordered a set of caps and adapters ($15) to allow easy watering.

I’ve got spare 10awg landscaping wire.

I’ll set them up and run some loads on them to see how they do.  Phones and tablets can be charged off 2A, and most consumer grade laptops charge off 5A dc.