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 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 D250 provides staged (boosted) charging and the Smartpass provides Big Current during Bulk charging.¹⁾ 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 setup but these main features might be replicated by

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.²⁾
2. Secondary - stop the isolator from passing voltages >Vfloat from alternator to house bank during Vfloat. This should be a minimal problem with lead batteries³⁾ 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 teh Ctek combo addresses this issue.

Another challenge confronting both Ctek and DIY is the isolator's lack of control over current. In reality the combo might make 20A+120A (max theoretical) or 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 cost 1/2 to 1/3rd as much as the official combo to meet basic functionality.

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)

• Renogy 20A DC-DC, $104. Cole-Hersee continuous duty solenoid, $55. High voltage disconnect, $13.

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.

• Renogy 20A DC-DC, $104. Cole-Hersee 90A diode isolator, $69.

Similar to the diode isolator setup above, but with MOSFETS instead of diodes. This means no⁴⁾ voltage drop.

• Renogy 20A DC-DC, $104. Victron ArgoFET 200A isolator (100A for each “leg”, so both starter and house batt get ≤100A). $120.

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.

• Renogy 20A DC-DC, $104. EpEver 20A 100v MPPT, $80. Cole-Hersee continuous duty solenoid, $55. High voltage disconnect, $13.
• Renogy 20A DC-DC, $104. HQST 20A PWM, $19. Cole-Hersee continuous duty solenoid, $55. High voltage disconnect, $13.

• Renogy 20A DC-DC, $104. EpEver 20A 100v MPPT, $80. Cole-Hersee 90A diode isolator, $69.
• Renogy 20A DC-DC, $104. HQST 20A PWM, $19. Cole-Hersee 90A diode isolator, $69.

• Renogy 20A DC-DC, $104. EpEver 20A 100v MPPT, $80. Victron ArgoFET 200A isolator (100A for each “leg”, so both starter and house batt get ≤100A). $120.

• Renogy 20A DC-DC, $104. HQST 20A PWM, $19. Victron ArgoFET 200A isolator (100A for each “leg”, so both starter and house batt get ≤100A). $120.

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.