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DIY Ctek D250 + Smartpass setup

This article is about the Ctek combo and possible approaches to duplicating the major functionality for less money and/or better specs.

This article assumes you want and the bank will accept >20A of alternator charging, and that your alternator can handle it. If you only have a 80Ah AGM then the combo setup is massive overkill. The bare D250 would handle that with ease.

the official combo: ~$654

The Ctek D250-series DC-DC charger charger and Smartpass MOSFET isolator combine to solve an expensive problem: how to get meaningful current and staged charging at the same time. Meaningful current with the Ctek combo means “up to 100A (or 140A) during Bulk.”

the problem the Ctek solution addresses

Isolators are great at providing Big Current during Bulk stage. But since isolators pass alternator voltage through they cannot charge the house battery to voltages higher than the alternator can put out. Because of voltage sag1) the charging voltage at the house bank will likely be somewhat lower.

DC-DC chargers can boost alternator voltage to the required Absorption voltage but doing so with Big Current can be impossible or at least very expensive. So DC-DC chargers are usually relatively small.

Digression: this means that for short drives the isolator will likely return more Ah to the battery ban k than DC-DC. On longer drives the DC-DC will both return more current and charge at a more appropriate voltage for the battery chemistry.

What if we paralleled an isolator with a small DC-DC to get the both of best worlds? That is what the Ctek combo does.

how it works

The D250 provides staged/boosted charging and the Smartpass provides more during Bulk charging.2) Combined this means you get increased current during Bulk without needing a giant DC-DC charger.

The setup is pricey; as of Aug 2022 the D250 is $328 and the Smartpass 120 is $326. Total $654. There are other features in this setup3) but these main features might be replicated by the approaches described below.

Although people tend to fixate on the MOAR AMPS!!!! aspect, there is a more subtle benefit to a small DC-DC charger + isolator combo: charging current will be very high only at first when the alternator temperature is lowest and most able to handle it.4)


The two devices are paralleled. One boosts voltage for proper charging and the other simply passes alternator voltage. The chief challenges are:

  1. Primary - stop higher charging voltage from leaking back to the starter battery, creating an endless loop. Ctek handles this inside the Smartpass by using MOSFETS.5)
  2. stop the isolator from passing voltages >Vfloat from alternator to house bank during Float stage. This should be a minimal problem with lead batteries6) and with driving times limited to a few hours. It could be a problem with Li batteries and many hours of driving. Note: it is not clear that the Ctek combo addresses this issue. The only approaches known to prevent excessive voltage during float are the combo DC-DC and the solenoid + HVD in conservative mode (see below).
  3. isolators have no control over current. In reality the combo might make 20A+120A (dead house bank, big alternator, cool temps, Bulk stage) or it might make 20A+1A (when battery bank is approaching alternator voltage). There is no guarantee that the DC-DC will be making 20A, either. If your bank is only accepting 15A then having 140A theoretical on tap will not matter.


The approaches below meet the Ctek combo's basic charging functionality and cost 1/2 to 1/3rd as much. Read details on starter battery maintenance and self-jumpstarting features if available. Also be aware of limitations and trade-offs:

There are no solutions. There are only trade-offs. ― Thomas Sowell

20A combo DC-DC + isolator: $172

This is the simplest and cheapest option, but requires ingenuity to keep boosted voltage from leaking back to the chassis. In this case the isolator (solenoid) is turned off by the HVD at Vfloat (conservative) or just below alternator voltage (power-maximizing).

The HVD mentioned here is actually LVD/HVD. You can use it to make the isolator a VSR, turning on at 13.2v (engine running) and turning off at just under alternator voltage, if Valt > Vfloat.7) Alternatively, the HVD might turn off >Vfloat.

If you wanted to self-jumpstart set the connect voltage much lower so the two are connected anytime the key is one. Or wire a separate momentary ON to energize the isolator.

20A combo DC-DC + diode isolator: $173

This option is requires no HVD, but has two possible drawbacks:

  1. often requires moving alternator-to-battery wire
  2. drop of ~0.5v. Perhaps not a big issue since the DC-DC is boosting.

20A combo DC-DC + MOSFET isolator: $224

Same technology as the Smartpass.

Similar to the diode isolator setup above, but with MOSFETS instead of diodes. This means no8) voltage drop.

20A DC-DC + 20A solar controller + isolator: $191-$252

The main benefit of a separate MPPT is we get to normal PV input limits, 100v+. We also may have more control. One can also drastically reduce cost by using PWM with nominal 12v panels, or using MPPT with cheap used higher-voltage panels.

20A DC-DC + 20A DC-DC + 20A solar controller + diode isolator: $192-$253

20A DC-DC 20A DC-DC + 20A solar controller + MOSFET isolator: $243-$304

50A DC-DC combo chargers: $265-$328

50A is a big step down from 100-140A, but it is not clear how long the official combo would sustain >50A in anyhow. The 20A DC-DC will be consistent but the isolator's current will drop ~linearly as bank voltage rises to meet alternator voltage.

  • Kisae DMT1250, $328. The major advantage over the Renogy below are 50v PV input instead of 23v, and a preference for alternator charging at 50A.
  • Renogy DCC50, $265. Probably the most common DC-DC charger in 2022.

The Renogy does maintain the starter battery, so self-jumpstarting should not be necessary. Kisae documentation does not describe this feature.

Note that a constant 50A for hours may be harder on the alternator than a brief 100A falling to 20A when the bank reaches Valt.

lower voltage due to resistance in wiring and connections
until battery voltage == alternator voltage
jumpstarting, starter battery maintenance
just started the engine
one-way like diodes, but with less voltage drop
which can usually tolerate high floats
and it probably is
or minimal
electrical/12v/diysmartpass.txt · Last modified: 2022/09/10 13:26 by frater_secessus