Is a 40 amp DC-DC charger better than a 25 amp unit?

Not necessarily. National Luna explains why…


I’ve got my heart set on a DC-DC setup, purely because I like the ease at which solar power can be added to the system. My question is: What size DC-DC charger do I need? Should I be considering a 40 amp unit instead of the typical 20- to 25 amp charger? I know there’s a price difference, but is bigger better?


The limitation of any 20A or 25A DC-DC system is the fact that the battery’s recharge rate is restricted to just 25 amps or less. This is particularly important during the ‘bulk stage’ of the recharge process.

On that note, the following graph depicts the recharge curves of a 25A versus a 40A DC-DC charger. This test was conducted using a 140Ah AGM battery that was discharged by 110Ah (78%).

Recharge rates of a 25A and 40A DC to DC system. Note the dotted lines at the 2-hour mark and how the benefits of the 40A charger start to tamper off at this point.

You’ll see from the graph above that the NLDC-40 restores up to 30Ah more than the NLDC-25 after just 2-hours of driving. Following that, the NLDC-40’s charging curve starts to taper off.

That said, although the 40A DC-DC system appears to be the “silver bullet” of vehicle-based, auxiliary-battery power, the benefit of using a 40A charger is only noticeable during the ‘bulk’ stage of recharging. Once you include the ‘absorption’ stage in the overall process, the total recharge time is similar for both products.

This is because the total charge time is determined by the battery’s technology, and not the charger itself. Simply put: The battery is the limiting factor.

“Total charge time is determined by the battery’s technology, and not the charger itself. Simply put: The battery is the limiting factor”

Unfortunately, there’s no way around this, as most lead-acid batteries require considerable time to reach full charge. In contrast, a lithium battery will accept far more energy during its ‘bulk charge stage’ than a lead-acid battery, and ultimately, reach full-charge quicker.


However, the economics of using a lithium battery (within a vehicle-based application) should be strongly considered. Thankfully, the NLDC-40 and NLDC-25 both support Lithium-ion and LiFeP04 batteries, as well as regular Flooded, Gel, AGM and Calcium batteries.

So, from that perspective, using a 40A charger is noticeably beneficial if your driving habits are typically around the 3-hour mark or less. Beyond that, the two products start to offer similar results. However, there are other considerations, too…

Because lead-acid batteries require time to recharge, no matter what dual-battery system you install, it’s advisable that an Intelligent Maintenance Charger be used to service the auxiliary battery and restore it to full capacity after each trip.


As mentioned before, solar power plays a big role in the decision to install a DC-DC system, where most top-tier products include a built-in MPPT solar regulator.

Having a solar panel connected to your auxiliary battery negates the need to drive your vehicle on a daily basis. In many cases, solar power will be responsible for keeping the battery full, and may even be enough to supply your total power needs.

This feature is particularly important for motorhomes, caravans and off-road campers, where you’re more likely to camp for a prolonged period, rather than drive your vehicle everyday.

A typical installation of a DC-DC charger.

With this in mind, the NLDC-40 features a 600W MPPT solar regulator, as opposed to the NLDC-25’s 375W MPPT. Meaning, your panel array can be much larger when using the NLDC-40; and, because its solar input is rated to 42V, the unit supports newer, more efficient solar panels that generally produce higher voltages.

What’s more, the NLDC-40 boasts a second DC input that can be connected to an external power supply. This is common in mobile homes where Mains Power (220V AC) is converted into DC current, and then subsequently connected to the NLDC-40’s separate auxiliary DC port. This utilizes the NLDC-40’s powerful 5-stage smart-charge algorithm that effectively creates a 220V Maintenance Charger.


Although a 40A DC-DC charger will reduce the bulk-charging time, the real benefits are realized in applications with larger battery banks or installations with large solar arrays.

On that note, here’s a short list of when a 40A DC-DC system should be the preferred choice, and, when it shouldn’t…

A larger 40A DC-DC charger is better when:

  • You need the extra charge quickly during the ‘bulk stage’ of the charge, despite the battery not reaching full capacity.
  • You have a large battery bank where the ‘absorption stage’ is comparatively short to the ‘bulk stage’ of the recharge process.
  • You are powering a load (a fridge or another appliance) at the same time as charging the battery.
  • You want the extra solar capabilities

A larger 40A DC-DC charger is NOT better when:

  • The battery is not capable of receiving high currents, which is often the case with smaller GEL and Lithium batteries.
  • Your application is mostly stationery, where you rely predominantly on AC power.
  • Your solar array is relatively small (150W to 350W).

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