Updated 31 May 2017

Speeding battery charging from generators is cheap, effective and relatively simple. This article by Caravan and Motorhome Books’ Collyn Rivers explains how to do it.

Many 230 volt portable generators have a nominal 12 volt output. This outlet is intended for powering 12 volt appliances directly. It typically supplies up to 8.0 amps (with or without a battery). The 12 volt output is typically 13.65 volts. This will partially charge a deeply discharged battery at 5-8 amps, but then drops back to a mere 1.0 amp once the battery is 40% or so charged. Even if that outlet is marked ‘battery charger’ it may take day to reach even 50%-60% charge. Despite this, owners may have generators plugging away for hours in vain attempts to fully charge.

The cheapest solution for speeding battery charging from generators is to use a basic 20 to 30 amp (230 volt) charger connected to the generator’s 230 volts ac outlet. This will recharge a 100 amp hour battery from deeply discharged to about 80% within about six hours. A more costly multi-stage (dc-dc) 20 amp charger will achieve close to 100% in even less about the same time.

For a fuller explanation see dc-dc charging on this website.

Speeding battery charging from generators – switch-mode technology

Another known problem with speeding battery charging from generators relates to some early switch-mode chargers and inverter/chargers. Some work from mains power, but produce little or no output from 230 volt generators. The problem is not necessarily related to either unit’s quality or price. One otherwise excellent generator was prone to this problem but for all, the cause is/was as below.

Switch-mode devices require ‘clean’ electricity. That from many generators is ‘dirty’. All generator motors accelerate on each power stroke, and slow on their compression stroke. The generator flywheel’s inertia (resistance to change) is intended to dampen such rapid changes, but lack flywheel size and mass to do so sufficiently. The rapid speed changes overlay voltage spikes on the output. This so-called ‘dirty’ electricity causes the charger’s protective circuits to limit output or switch itself off. 

Whilst the cause is the generator, each vendor invariably claims their product works fine (but not together). Each then blames the other’s product. Always buy both from the same vendor. Insist they must work with each other. 

In Australia, Power Protection Systems (supplier of Mastervolt etc) has a simple modification. It cleans up dirty input and tricks inverter-chargers into accepting any remaining ‘noise.’ It was designed specifically for Onan’s 3600 petrol generators and Dakar chargers, but is claimed to work with other generators.

(Honda/Yamaha type inverter-generators are free of this problem).

Speeding battery charging from generators – power factor

Most chargers and inverter chargers (switch-mode or otherwise) have a poor so-called power factor that necessitates a more powerful generator than usually believed.

Alternating current runs things that produce heat (water heaters, toasters, soldering irons) with no problem. Most 2500 watt generators provide that output for a short time, but may be limited to 80% (2000 watts) for continuous use.

Conventional (transformer) battery chargers, induction motors etc however have wire coiled around iron cores. This causes the incoming current to get out of step with the voltage. It’s like two rowers sculling out of synchronisation. The same action and energy input has less effect. This is called ‘Power Factor’ and expressed as between 0 and 1.0. The higher the better. (In technical terms, the voltage and current peaks are ‘out of phase’.

Rowers eps

Adverse power factor is like two scullers (Ampy and Volty) rowing out of syncronisation. They do the same work as if they were together, but the boat will not go as fast. Pic: ‘Concentration’ – copyrightdreamstime.com

Such battery chargers have a power factor between 0.65 and 0.7. This adverse power factor necessitates using a generator that is correspondingly larger. Worse, most such chargers are only 70% efficient. A 12 volt, 30 amp (360 watt) such charger may thus need a 1500 watt generator to run it.

Switch mode chargers originally had poor power factor, but many now are 0.85- 0.9 (85%-90% efficient): it is advisable to check. If they are, that 12 volt, 30 amp charger should run comfortably and continuously from a 1000 watt generator.

Power factor correction

A charger that has really poor power factor can prevent an otherwise adequate- sized generator developing full power. A quick and dirty fix, that often works, is to run a 100-watt incandescent globe or soldering iron at the same time. That purely resistive load tricks the generator into working better.

A permanent fix is to have power factor correcting capacitors added to the input of the charger. Whilst that works, the cost of doing so may be much as buying a charger that has power factor correction already built in.

Speeding battery charging from generators – further information

This article will hopefully help speeding battery charging from generators and other battery-charging problems. Many similar issues bedevil electrical, battery and solar systems in camper trailers, caravans, campervans and motor homes. That needed to fix them (and particularly avoiding problems in the first place) is in my book Caravan & Motorhome Electrics. Consider also the associated (now 4th edition) Solar That Really Works (for cabins and RVs) and/or Solar Success (for home and property systems). These books are bought globally. Auto electricians use them as working guides. The Caravan & Motorhome Book covers every aspect of RV usage.

It really is worth buying one or more of my books as there is a limit on how much can be explained in article form. Their cost will be repaid multiple times by getting the system right first time. They are unusual in that I have both an engineering and a writing/publishing background of over 50 years. This enables me to combine technical accuracy with writing in plain English. See Bio.