DUAL-BATTERY DIAGNOSTICS – Why your aux battery is NOT charging?


My dual-battery isn’t charging. I had a solenoid system installed in my 4×4 just over a year ago. The setup seemed to work fine the first time, but now there’s very little power in my auxiliary battery when I get to camp. On my most recent trip, I spent anywhere between 3 to 4-hours driving between camps, but by the following morning the battery was flat and my fridge wasn’t running.

I’ve heard reports that a solenoid-based dual-battery system doesn’t work with some vehicles, and that I should switch to a DC-DC system instead. But before I fork out thousands of rands on a second system, can you offer some advice?


We hear these complaints on a daily basis, and the answer is surprisingly simple: You need a 220V maintenance charger.

Sometime back, a leading safari rental company ran into a similar problem with their fleet of vehicles. Each vehicle was fitted with a National Luna Intelligent Solenoid dual-battery system, along with a wet-cell 105 Amp deep cycle battery. Initially, this setup worked well, but as the months went by, the auxiliary batteries weren’t providing enough power – even after a long drive. But the problem had nothing to do with the vehicle’s alternator or solenoid dual-battery system, and everything to do with the fact that the batteries weren’t lasting.


  • Deep cycle (wet cell) batteries are NOT necessarily the best choice, and
  • All batteries need maintenance!


The 105A deep-cycle (wet cell) battery is arguably the biggest selling unit in the dual-battery market, but if you look at the graph attached, you’ll see that in many cases this battery is not an ideal choice.

The graph shows typical recharge rates for a 105A deep cycle battery at various alternator outputs. There are two vitally important lessons to be learnt here…

  • It takes more than just a few hours to restore full power to a deeply discharged deep-cycle battery.
  • Because a 20 amp DC-DC charger limits the current output to 20 amps, in many cases (if your alternator’s output is 13.7 V or more) it slows down the recharge rate, and becomes a limitation, and not an advantage.

So how does all this relate to battery maintenance?

Well, most holiday travellers don’t drive for more than a few hours a day, which means most deeply discharged (deep-cycle) batteries are in a permanent discharged state. Or to put that another way: Their auxiliary battery is sustaining long-term damage and losing its ability to hold a charge.

This is why your dual-battery system worked the first time, and perhaps even the second, but if you weren’t maintaining the charge between trips, your battery’s ability to hold a charge would’ve diminished with every passing month. Contrary to popular belief, it takes a surprisingly short period of time for a battery to sustain permanent performance damage if left in a discharged state.

Likewise, in the case of the safari rental company: after spending considerable money on fitting DC-DC systems, the problem got worse (due to the limited 20A output), and was eventually solved by going back to the National Luna solenoid system and simply upgrading the batteries from wet-cell units, to dry-cell (AGM) batteries. The rental company also invested in a series of 220V maintenance chargers, and since then, their dual-battery problems have been solved.


So to summarise: If your dual-battery system is giving you troubles, there’s a very good chance your battery is to blame – even if it’s less than a year old. If you suspect your battery is in a partially discharged state, you need to invest in a 220V maintenance charger before the battery suffers permanent damage, along with a subsequent loss in performance.

This information was confirmed in 2016 when National Luna conducted dozens of laboratory tests on a variety of battery types and split-charging setups. The results of those tests led to the recent launch of a brand new National Luna Maintenance Charger, built specifically for overland travellers.

The National Luna Maintenance Charger is designed for motorcycle and 4×4 applications; the charger features an 8-step recharge process that includes an analysis of your battery’s charge state, along with the option to select a boost mode in cold-climate conditions.

More importantly, the charger is programmed so that it can be left (connected to your battery) for an indefinite period, and you never have to worry about the battery heating up, or “over” charging.

Like this article?

Share on Facebook
Share on Twitter
Share on Linkdin
Share on Pinterest

You might also like