Skip to content

Lithium Dual Battery Charging With CTEK D250SE & Smartpass 120S


CTEK, who makes high-quality chargers and auxiliary battery chargers (like the D250SA and Smartpass 120 that I just installed) is going to be releasing some new stuff that will handle the “new” battery types such as LiFeP04, Li-Fe, Li-iron, and LFP. This is going to be HUGE for the overland crowd or serious camp guys who want the latest and greatest on their rigs. Of course, lithium ability isn’t just about having the latest tech, it’s about saving weight. The current lithium battery offerings exceed 50% weight savings vs standard batteries or AGM, with increased usable capacity because of their ability to discharge deeper without damage, and recharge much faster.


If you’re unfamiliar with dual battery setups, you’ll need to read my post about what it is and why you’d want it, but for now, let’s take a look at some of the specs on the new CTEK units.


Starting off with the D250SE, you gain the ability to easily add another battery to your rig and know that your starter battery is never in jeopardy of running out of power because of your fridge or electronics. The D250SE provides up to 20A of battery charging power to your second/auxiliary battery while the engine is running. To help keep things topped off while you’re parked, the D250SE also includes a built-in MPPT charger right into the unit.

To “tell” the charger what type of battery you have so it charges properly, there are certain wires on the unit that you leave disconnected, attach to positive (+), or ground (-).


If 20A charging isn’t enough, you can also add on the CTEK Smartpass 120S, for up to 120A (140A charge rate is achieved when the D250SE and Smartpass 120S are connected together). Luckily, you can add the Smartpass 120S later if you want, so you can start with 20A and upgrade down the road as needed.

My favorite part about the Smartpass 120S is what it offers while driving; you can attach consumers (fridge etc) to the consumer output (not the auxiliary battery output, but the connection indicated with a “bulb” on the 120S) to power them from the alternator while driving. This allows the auxiliary battery to charge faster.


For example, if you have been sitting all night drawing power from your auxiliary battery. When you start your vehicle, the CTEK D250SE and Smartpass 120S will begin charging your battery up to 140 amps. If you have a fridge and other electronics hooked up and drawing 40 amps, your battery is only receiving a 100 amp charge. However, with the Smartpass 120S, you can power the electronics straight from the alternator so your auxiliary battery receives a full 140 amp charge, getting you back to 100% faster. Keep in mind that this is a very simplified breakdown of battery charging, but the result while utilizing the Smartpass 120S output is faster auxiliary battery charging.


This is a pretty sweet upgrade to their onboard dual battery management system; and if you’re looking for an all-in-one system that just plain works, the CTEK 250SE and Smartpass 120S is the way to go. Check out the full specs and release date (250SE is scheduled for the end of September 2019, 120S for October 2019) here.


CTEK Smartpass 120S

From CTEK on the D250SE

The D250SE is a fully automatic, 5 step charger that supplies up to 20A of power to any 12V lead acid or lithium* service battery from 40–300Ah. It has selectable charge algorithms for AGM and lithium* batteries and can use power through its dual input from alternator, solar panel and wind power. When the service battery is fully charged, the D250SE will automatically redirect maintenance charge power to the starter battery. The D250SE can maintain a stable output up to 20A to vehicles fitted with smart ECU controlled alternators and also has a temperature sensor for optimized charging, regardless of weather conditions.

*) 12V lithium batteries (LiFePO4, Li-Fe, Li-iron, LFP

With award winning Swedish technology, the dual input D250SE allows you to enjoy your vehicle more with maximized battery capacity as the ideal charger for vehicles without access to a mains power supply.

CTEK info on the Smartpass 120S

SMARTPASS 120S charges and supplies the service battery and equipment with 120A, separates critical equipment from non-critical equipment ensuring that radio, emergency lights and navigation systems always have power.


SMARTPASS 120S is a 120A Power Management Solution which distributes, controls and maximizes the available energy from your alternator to power service batteries and consumers. SMARTPASS 120S handles all 12V batteries between 28–800Ah. The SMARTPASS 120S’s Battery Guard function ensures that critical equipment such as radio, emergency lights and n avigation systems always have power and take priority when the voltage on the service battery is low disconnecting non-critical consumers. The unit’s Battery Guard will protect your service battery from total discharge and built in over temperature protection reduces charge current before the battery temperature gets too high. SMARTPASS 120S is compatible with the D250SA and D250SE 20A on board chargers.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: