Updated December 2016

Battery charging via generator is often slow and shallow or barely work at all. This article shows just why, and how to find and fix the usual causes.

Charging via generator 12 volt output

A common experience in many a campground is hearing a generator plugging away most of a day in vain attempts to charge an RV battery. You can do it successfully once you know how and why.

Most small generators have a 12 volt dc output socket that may even be marked ‘Battery Charging’. With most generators however that output is intended to run small 12 volt dc devices of up to about 80-90 watts directly – (i.e. without a battery). Their output on light loads is about 13.6 volts. It is not regulated so it can and does drop to 12.6 volts or less at its usual maximum of 8 amps.

Such output may charge a flat 100 amp hour battery to 40% or so within 6-8 hours – but that’s about it. From thereon charging virtually ceases. That generator’s nominally 12 volt output is fundamentally unsuitable for battery charging.

Successful battery charging via generator however is readily possible. All that is required is a high quality multi-stage 230 volt ac battery charger driven from the generator’s 230 volt output.

Honda Jeneratör

A good inverter generator charges batteries quickly and safely via a good quality charger from its 230 volt ac output. Pic: Honda.

 What type charger for battery charging via generator ?

Most portable generators produce their advertised rated output for only few minutes. They are limited to 80% for continual use. In reality a 1000 watt generator is, for most purposes, an 800 watt generator. They tend to be most fuel efficient at about 70% load (i.e. 700 watts for a 1000 watt unit). Allowing for losses this is sufficient to power a high quality 30-40 amp charger – capable of fully charging a flat 12 volt 100 amp hour battery within 3-4 hours – adequate for typical smallish RV back-up.

If you are planning to run air-conditioning etc you will need a 2000 or 3000 watt generator and 50 amp plus charger. These are best run from a Onan or similar generator. The smaller Honda/Yamaha type are not designed for long-term use. Also consider Dometic’s 2015 released TEC 29 petrol driven generator. It can generate a continuous 2600 watts. It is very quiet and has low emissions. 

Onan gen

Onan 2800 watt generator uses about 1.4 litres/hour. Pic Onan. 


Avoid cheap generators and chargers

Do not even think about doing this with a $99 chain store generator. Apart from being hideously noisy and polluting their electrical output too is very ‘dirty’. The produce voltage spikes that can can and do damage so-called switch-mode chargers (or may not even run them at all).


dometic TEC 29

The late 2015 released Dometic TEC 29 petrol generator produces up to a constant 2.6 kW. It is ultra-quiet and has very low emissions. Pic: Dometic.


High quality chargers too are not cheap. Expect to pay $350 upwards. Do not skimp on this. Any savings on a battery charger will be wiped out by their inefficiency. This results in more fuel used in running the generator for longer. High quality chargers work quickly, deeply and reliably. The top brands are typically better than 90% efficient.


Battery charger Xantrex 201 small

High quality Xantrex charger is not cheap, but charges fast and safely 

Generator apparently big enough – but the charger does not perform

This can arise with battery charging via a generator by so-called adverse power factor. It can prevent a seemingly adequate generator’s full power to be exploited by the charger. In some instances the generator may be hard or impossible to start unless the charger is initially turned off.

A quick and dirty fix (but one that almost always works) is to also plug a 100 watt or so soldering iron across the generator. It’s not the heat produced that does the job! The iron (being purely resistive) tricks the generator into working properly.

A better, but costly fix, is for you to have an electrician fit power factor correcting capacitors to the input of the battery charger. This reduces the need for the much larger generator otherwise needed. It is only worthwhile however with battery chargers of 50 amps or more. It is otherwise cheaper to buy a high quality charger.


Problems with switch-mode battery charging  via generator

Problems arise if you attempt battery charging via generator with some chargers and inverter-chargers that use switch-mode technology. These are much smaller and weigh a fraction of that of conventional transformer-based chargers. Some will work perfectly from the grid 230 volt supply but only produce low output, or even none at all, from certain generators.

Switch-mode chargers are reasonably efficient (plus 90% is common). A nominally 1000 watt generator should be able to run a 35 amp such charger. But not all can because switch-mode devices demand ‘clean’ electricity. And that from cheap generators, and even one costly brand, can be too ‘dirty.’ 

What happens technically is that a generator’s speed constantly varies slightly at a typical 50 times a second. This causes its output to be not just 50 cycles per second, but also at many multiples of that. The resultant ‘dirty’ ac may cause switch mode charger protection circuits to cut off the supply. In some cases the generator can even wreck the charger.


Fixing problems with switch-mode battery charging  via generator

The cause of the problem is that the generator’s flywheel, intended to mechanically dampen this effect, is too small. Diesel engines are so prone to this that their makers have to use a heavy flywheel. Another fix is a flexible rubber coupling, between the engine and the electrical generating bits.The flexible coupling absorbs the changes in speed.

Generator vendors almost always deny responsibility. They will claim their products will drive most electrical loads without problems. Likewise will switch mode battery charger makers.

This situation is now mostly historical as an ever-increasing proportion of generators used by RV owners are the quiet inverter type units, such as those made by Dometic,  Honda, Yamaha, Robin etc, do not suffer from this. Nor do Mastervolt’s or Fischer Panda’s.

In Australia, Power Protection Systems (suppliers of Mastervolt etc) has designed a simple electrical modification (to the popular Dakar inverter charger). It partially cleans up the dirty ac. This partially tricks the inverter charger into accepting any ‘noise’ that remains. It was designed with a specific petrol generators in mind, but the company says it enables the Dakar charger to work with other generators that exhibit similar problems.

Buying Books

If you found this article of value you will find Collyn Rivers’ books even more so. They cover topic like this in depth (and in plain English). These include the best selling Caravan & Motorhome Electrics, The Camper Trailer Book, Solar That Really Works (for RVs), Solar Success (for homes and property systems) and the all new Caravan & Motorhome Book.

Do please also add a Link from this article to any related forum query. This assists to advise others.