Words and Images by: COLLYN RIVERS N8054

Battery charging via a petrol generator’s 12 volt output can be slow and shallow or, often, barely works at all. This article explains why, and how to achieve it successfully.

A common experience while camping is seeing and hearing a frustrated RV owner attempting – often all day – to charge their auxiliary battery from the 12 volt dc output of a petrol generator.

This seems reasonable to do so, as that outlet may be labelled ‘Battery Charger’. In reality that output is typically 13.6 volt dc and intended for running 12 volt dc appliances with or without a 12 volt battery. That voltage is unregulated. It falls more or less proportionally as loads are applied.

And there is a second limitation. Most generator’s 12 volt outlets are limited to 8 amps. Even if the voltage was adequate, 8 amps is just not sufficient for charging RV batteries.

A battery is charged by applying across it a voltage that is higher than it has at the time. The higher the voltage difference, the quicker the battery charges. For a typical  lead acid battery the maximum safe= charging voltage (that ensures a safe difference) is about 14.4-14.7 volts. For most AGM batteries it is 14.2- 14.4 volts.

If attempting to directly charge a close-to-fl at 100 amp hour lead acid battery, the charging load will reduce that unregulated voltage. As a result that battery is likely to take five to six hours to reach 50 per cent charge (12.2-12.3 volts). So charging progressively slows. It is unlikely to exceed 60 per cent charge in a full day charging operation such as this.

As many rigs have a battery bank of 250 amp hours or more, it is impossible to charge successfully this way.

THE GENERATOR CHARGING SOLUTION

The solution can be simple, but surprisingly few owners think of it. It is to use a high quality multi-stage 230 volt ac battery charger driven from the generator’s 230 volt ac output. Given an adequately sized generator, that charger can be rated at whatever the batteries will accept.

A typical lead acid deep-cycle battery can be charged at 10-15 per  cent of its amp hour capacity, an AGM at 20-25 per cent. LiFePO4s can be charged at massively high rates but the specialised battery chargers that can do so are too costly for RV use. Most owners compromise (but may use large amounts of solar). The LiFePO4 batteries will charge from 13.6 volt, but a mere 8 amps slows charging.

THE GENERATOR REQUIRED

Most portable generators produce their advertised rated output for only a few minutes. They are limited to 80 per cent of that output for continual use. For most practical purposes a 1000 watt generator is really an 800 watt generator.

Do not attempt any form of battery charging via a $99 chain store generator. They produce voltage spikes that can and do damage switch-mode battery chargers (or may not run them at all). They are also extremely noisy and polluting.

Generators tend to be most fuel efficient around 70 per cent load (ie., 700 watts for a nominally 1000 watt unit). Allowing for losses and power factor (see below) this is sufficient  to power a high quality multi-stage 30-35 amp charger.

A charger of the correct size should be capable of fully charging a 12 volt 100 amp hour battery from 20 per cent to about 90 per cent charge within 3-4 hours. It can take the battery to 100 per cent but that takes a probable two hours longer. It is rarely worth attempting.

If you are planning to run air-conditioning all night, you will need a 2000 or 3000 watt generator and 50-70 amp plus charger, along with a 700-1000 amp hour battery capacity. This is only practical in coach conversions, etc. It is best done via an Onan or similar generator as the smaller Honda/ Yamaha types are not intended for such long-term use.

THE BATTERY CHARGER

High quality chargers are costly but it’s essential to have one. Do not skimp on this! Cheap battery chargers are inefficient, so will cost you far more long term in fuel. They may also damage your batteries. 

High quality chargers work quickly, deeply, safely and reliably. The top brands are typically 90 per cent efficient.

ISSUES THAT MAY ARISE

A curiosity of some ac appliances including battery chargers is that they may require a generator of 30- 40 per cent more output than the load that they drive. They do not actually consume that extra output – but it needs to be available (it is sort of constantly ‘borrowed and returned’).

This effect (called power factor) can also 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 power a 100 watt or so soldering iron by the generator. It’s not the heat that does the job – the soldering iron (being purely resistive) tricks the generator into  working properly! This used to be done using 100 watt incandescent globes, but they have long since been withdrawn from sale.

A permanent (but more costly) fix is to have an electrician fit powerfactor- correcting capacitors to the battery charger. The electrician will know why you want to do it.

This reduces the need for the larger generator otherwise needed. It is only worthwhile with large battery chargers (of say 70 amps and over). Power factor (often shown as PF or pf) is usually shown in charger specifications. It will be a number (typically from 0.7 to 1.0.) The closer it is to 1.0 the better.

Collyn writes and publishes books on all aspects of caravan and motor home usage, and also solar for both RV and home/property solar systems. Most are stocked by and available directly post-free from CMCA NHQ. They can also be obtained directly from the publishercaravanandmotorhomebooks.com  (postage is applicable).

Category: Technology
Written: Wed 01 Feb 2017
Printed: February, 2017
Published By: