AGM for weekenders

Off-grid charging AGM is challenging and expensive;  doing it as a weekender can be easier and less expensive since we can ward off battery-murdering sulfation with vigorous and timely charging just before/after the weekend outing.


  1. make it through the weekend outings with power for light-to-medium loads

  2. without murdering AGM batteries due to undercharging, by using a minimum of C/5 (40A) charging.

Scenario:  camper has 200A of AGM and 200w of panels.  The panels are not intended to fully charge the batteries, only to support the batteries over the weekend and get them back to at least 12.7v during the day.  Vfloat would be nice.   Campground is an hour away from the house.   Batteries are fully charged at the house just before leaving and immediately upon return.  When not camping (ie, not cycling), 200w of panel will keep the house batteries maintained.


  • there is a huge cost difference between fully charging lead batteries to manufacturers specs in the field and keeping them alive i_n the field and fully charging at home._   We aim to exploit that difference.

  • Configurations with B2B (aka DC-DC chargers) can be fed at home with 12v power supplies, since they will step up voltage.    Configs with isolators will require a smart converter.

From most to least expensive, not including battery costs:

for reference:  full time off-grid ($830 - $2400)

So you want to do it the Hard Way.  :-)

  • 6+ 100w panels, $600+

  • charge controller[s], $160+

  • isolator ($70)

The lower price is for optimal insolation in places like Phoenix.  Salt Lake City would be approx 2x as much, and Seattle 3x as much.

CTEK Smartpass-based b2b option ($795)

The SmartPass provides real current while driving, and also gives us a way to pass current to the isolator/b2b system.  No RV-style converter needed.

  • 2 @ 100w panels, $200

  • D250SA, $250  (text references to the S model replaced by SA, per the correct  advice of StolidSentinel).

  • SmartPass 120S, $250

  • 75A 12v power supply to feed SmartPass/D250S when at home ($75)

  • shore power port $20


CTEK-based b2b top ($650)

In this model we don’t have a way to pass lots of current to the battery while driving, so we use a big converter at home.

CTEK DIY-“Smartpass”-based b2b option ($615)

Here we use an isolator and HVD to emulate smartpass functionality

  • 2 @ 100w panels, $200

  • D250SA, $250

  • isolator ($70) and high voltage disconnect ($10)

  • 75A 12v power supply to feed SmartPass/D250S when at home ($75)

  • shore power port $20


Renogy-based 50A b2b option ($595)

This Renogy has MPPT, and we can feed it at home with a power supply.  The higher output meets AGM minimums without needing an isolator/smartpass.

As a bonus, one could increase panels up to about 600w on this unit (12v panels in parallel).

Renogy-based b2b option ($595)

This Renogy has no built-in MPPT so we are buying our own.   At this price we might be better off with the 50A setup above, unless we want to run a single 24v 200w panel or something.

Renogy-based DIY-“smartpass” b2b option ($570)

This is a CTEK Smartpass-style solution for $225 less than CTEK.

DIY isolator option ($550)


Bonus:  some numbers for charging flooded or lithium batts.   These numbers are already lower than the above because flooded require half as much current. Of particular note:  the 20A limit b2b chargers meet flooded charging minimums and therefore do not require additional isolators/smartpass.  B2b also boost voltage so you can use a 12v power source on them at home instead of needing a pricier converter/charger.

In addition, it would save about $200 on the battery purchase over AGM.  Overall, a flooded battery + charging setup costs about half what an AGM charging setup costs.

CTEK-based b2b for flooded ($520)

Remember that 20A meets flooded requirements;  no converter needed, just feed the b2b.

  • 2 @ 100w panels, $200

  • D250SA, $250

  • 40A 12v power supply to feed D250S at home, $50

  • shore power port $20

DIY isolator option for flooded ($500)

The hardcore among you might feed the MPPT with a 12v power supply at home using an A/B switch, saving about $70.

Renogy-based b2b option for flooded ($465)

This Renogy has no built-in MPPT so we are buying our own.