on the CTEK D250S charger and smartpass

51nzw3voull._ac_ul320_The CTEK D250S-series charger is probably the most popular DC-DC (boosting) charger for vandwellers.  There are several reasons for this, I think:

  • it is a clean looking and simply labeled.

  • it really does do DC-DC charging from the alternator

  • it supports 12v nominal solar panels

In practice there are a few issues with this. 

  • The device has presets for “wet”/flooded (Vabs 14.4v), AGM (Vabs 14.7v), etc, but the setpoints are not configurable.

  • The built-in solar charge controller (claimed MPPT) is infamous for not working well.  It also has a low Vmax of 23v.  Some owners describe the controller as a bonus add-on that should not be counted on to perform.

  • The unit is pricey, usually north of $250.

  • DC-DC output is limited to 20A.  See below for smartpass.

  • normally people aren’t driving their vehicles long enough for them to get fully charged – it takes hours.   The last few hours require high-ish voltage but not a lot of current.  Note:  This is part of  why solar is so popularSolar + Alternator charging is even better.



The smartpass is an isolator intended for use with the D250S charger shown above.  It appears to have some additional functions beyond that of a normal solenoid:

  1. It does not backfeed boosted voltage to the chassis the way a simple isolator would

  2. it sends occasional, limited bursts of power back to the starter battery when the house side is being charged by solar or other means.

So the smartpass passes normal alternator voltage/current to the battery bank like a generic solenoid until current drops below 20A.  At that point control reverts to  the D250S which DC-DC boost charges to reach/hold Vabs (and eventually Vfloat if you drive enough).

[Updated to add: the paragraph above was inaccurate/misleading; no such logic is present.  As with any other isolator, the SmartPass will simply stop contributing when Vchassis == Vbatt because there is no excess voltage “pressure” to push its current.

We can glean a bit more information on how the ‘Pass functions from the manual (pdf):

end, update ]


DIY SmartPass


The Sterling Power BB1230-12 is a 30A DC-DC charger will full setpoint configurability. It runs about $275.

The 30A output might be sufficient charging, but if you wanted to DIY a smartpass you could also wire in a constant-duty solenoid with a high voltage disconnect like these on the D+/IGN wire.   You set the high voltage cutoff  near the starter battery’s Vfloat and the solenoid loses its control power if voltage goes higher.  So when voltage reaches your alt’s ability the DC-DC charger will be on its own.

Total cost $325 instead of $500+, and you get to pick your own setpoints.

Update Aug 2022

I finally figured out why the Smartpass acts this way and why it is $$$; per this document (pdf) it is a FET-based isolator.

See this draft article for a discussion of ways to reproduced Ctek functionality for less money.