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DRAFT / EXPERIMENTAL / OPINION
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:
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.
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)
This option is requires no HVD, but has two possible drawbacks:
Similar to the diode isolator setup above, but with MOSFETS instead of diodes. This means no4) voltage drop.
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.
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.