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Fast battery charging from generators – cheap, effective and relatively simply

by Collyn Rivers

Fast Battery Charging from Generators

Fast battery charging from generators is cheap, effective and relatively simple. Few people, however, know how to do it. In many a campground, generators plug away for hours on end in vain attempts to fully charge their batteries. This article by RV Books’ Collyn Rivers explains how to get it right first and every time.

A generator can provide fast battery charging

Most 120 and 230-volt generators also have a DC 12 volt and (typically) 8 amp output. Pic: Honda.

Almost all portable generators have a nominal 12-volt output. This outlet is primarily intended for powering 12-volt appliances directly (i.e. with no need for a battery). It typically supplies up to 8.0 amps. The ’12-volt’ output is typically 13.65 volts on light loads. Such output will partially charge a deeply discharged battery at 5-8 amps. Once the battery is 40%-50% charged, the current drops to an amp or so. Even if that outlet is marked ‘battery charger’ it may take a day or more to reach 60%-70%. It should eventually reach full charge, but may take a week or more to do so.

The simple solution

A cheap way of achieving fast battery charging from generators is via a basic 20 to 30 amp (230 volts) chain-store battery charger. You connect this 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 may reach 100% in even less time. Do not attempt to charge lithium batteries via a cheap charger. While normally rugged, (as with AGM batteries) they can be damaged or wrecked by over-voltage charging.

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

Switch-mode technology issues

A known problem with fast battery charging from generators relates to some early so-called ‘switch-mode’ chargers and inverter/chargers. Some work well from grid power but not well (or even not all) from most 230-volt generators. The problem is not necessarily related to either unit’s quality or price: it is that a switch-mode power supply converts power by using electronic switching devices that rapidly turn on and off. Much like an engine’s flywheel, storage components (such as inductors or capacitors) supply power when the switching device is in non-generating brief states.

Switch-mode power supplies are highly efficient. They are widely used in computers and other sensitive equipment requiring a stable and efficient power supply. Switch-mode devices, however, require ‘clean’ electricity. That from basic petrol generators (the ultra-cheap eBay specials), however, is ‘dirty’.

Such generators speed up during each power stroke. They slow again on each compression stroke. The generator flywheel’s resistance to changing only partially dampens such rapid changes (they lack the size and mass to do so sufficiently). The consequent rapid speed changes cause voltage spikes on the electric output. Sensing ‘dirty’ electricity may cause a switch-mode charger’s protective circuits to limit output, or even switch itself off.

An owner faced with this (not uncommon) issue has a further problem. While the cause is the generator, each vendor may claim their product works fine (albeit not together). Each vendor tends to blame the other’s product. To avoid this, always buy both from the same vendor insisting they must work with each other.

This issue primarily affects low-priced petrol generators. It does not happen with inverter-generators such as the Honda/Yamaha type products.

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.

Fast battery charging from generators – the power factor

A further issue called ‘power factor’ causes alternating-current (i.e. grid power) chargers and inverter chargers to charge at lower rates than had been expected. Power factor causes the alternating current to peak at a different time than the voltage. It’s rather like rowers sculling out of synchronisation. The same action and energy input has less effect. Power factor is expressed as between 0 and 1.0. The higher the better.

Fast battery charging from generators - cheap, effective and relatively simply - rv books, Collyn Rivers, rv books for beginners, solar books, best rv books, rv repair books, solar success, caravan and motorhome electrics, solar that really works, The caravan and motorhome book, why caravans roll over, how to choose and buy an rv

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

Most battery chargers have a power factor between 0.65 and 0.7. This necessitates using a generator that is correspondingly larger. Worse, most such chargers are only 70% efficient. A 12 volt, 30 amp (360 watts) such charger may thus need a 1000 watt generator to run it.

Switch mode chargers originally had poor power factor, but many now are 0.85-0.9 (i.e. 85%-90% efficient). Providing you avoid the cheap ‘specials’ this is largely a historical problem.

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. It is not efficient but that resistive load tricks the generator into working as intended. An electrician can install power factor correcting capacitors to fix the problem, but the cost of doing so may be similar to buying a higher quality (that will power factor correction already built-in).

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, travel trailers, campervans and motorhomes.

That needed to fix them (and particularly avoiding problems in the first place) is in Caravan & Motorhome Electrics. It is also in 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 now in eBook and paperback versions. Many auto-electricians use them as working guides. The Caravan & Motorhome Book covers every aspect of RV usage.

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Electrical (general)
Batteries & Battery Charging
Trailer/Tow Vehicle Behaviour & Suspension
General
Travel
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Between 1959 and 1961 Collyn Rivers and Antony Fleming drove a Bedford QLR off-road truck twice the length and breadth of Africa studying road surface conditions.

A 43 minute film about their adventure, produced and narrated by Antony Fleming, has been viewed on YouTube more than 1.6 million  times!

Picture from Africa journey. Caravan and Motorhome Electrics by Collyn Rivers

To view the film, and also read Collyn’s telling of the tale, just click the button below.

Africa 

Cover of How to Choose and Buy an RV by Collyn RiversWhether you’re buying your first RV or updating to a bigger, more comfortable rig, there are a great many things you need to know and consider.  This book breaks things down so that you can decide what you want and whether or not what you’ve found is right for you. It’s technically sound and comprehensive so that you can proceed with complete confidence.  How to Choose and Buy an RV is the ultimate guide for those times when you want to, well, choose and buy an RV.

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Cover of Solar Success by Collyn RiversSolar Success explains how to design and build home and property systems that ensures affordable solar success first and every time.

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Home Page - rv books, Collyn Rivers, rv books for beginners, solar books, best rv books, rv repair books, solar success, caravan and motorhome electrics, solar that really works, The caravan and motorhome book, why caravans roll over, how to choose and buy an rvThe Caravan & Motorhome Book covers every conceivable aspect of camper trailer, travel trailer and fifth wheel travel trailer, campervan and motorhome usage. Like all of Collyn Rivers’ books it is technically sound yet written in plain English.

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This is Collyn Rivers’ newest book.  Building on his years of experience and extensive knowledge of engineering and all things travel trailer and RV, this is the book you need with you when shopping for an RV.

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Collyn Rivers has done it again! – if you need to know anything about motorhome electrics, then this book is for you… Collyn must be, without doubt, the foremost authority in Australia on this subject.

The Wanderer (Campervan & Motorhome Club of Australia Ltd)

Why Caravans Roll Over….and How to Prevent It​

Should be compulsory reading for everyone involved with Travel trailers: Design / Engineering, Marketing, Journalism (print & electronic), Regulations, Inspections, Driver-Training, Travel Trailer Clubs, Towing and Modifications.

Colin Young

Founder and CEO, Caravan Council of Australia

Solar that Really Works!

Collyn is an accomplished engineer who knows how to design and build successful systems. It is refreshing to see practical advice written in plain language when there is so little solar information around. . . this is an essential text . . . you won’t find a better guide.

Alan T. Gray

Earth Garden Magazine

The Caravan & Motorhome Book

Without doubt, this is the best book we have ever read . . . it will be treasured, and read on a regular basis.

The Wanderer (Campervan & Motorhome Club of Australia Ltd)