Updated April 2017
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 causes.
Charging via generator 12 volt output
In many a campground, generators plug away most of a day, vainly attempting to charge an RV battery. You can do it successfully if you know how and why.
Most small generators have a 12 volt dc output socket possibly marked ‘Battery Charging’. This output is really intended to run small 12 volt dc devices without a battery. Their output on light loads is about 13.6 volts. That voltage is not regulated. It can and does drop to 12.6 volts or less. Its usual maximum is only 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 nominally 12 volt output is not nearly enough for battery charging.
Successful battery charging via a 230 volt generator is feasible. It need a multi-stage 230 volt ac battery charger run from that 230 volt output.
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 rated output for only minutes. Their limit is usually 80% for continual use. In reality a 1000 watt generator is an 800 watt generator. They are most fuel efficient around 70% load. That is 700 watts for a 1000 watt unit. This is sufficient to power a quality 30-40 amp charger. Chargers this size can fully charge a flat 12 volt 100 amp hour battery within 3-4 hours. Adequate for typical small RV back-up.
To run air-conditioning etc you need a 2000 or 3000 watt generator and 50 amp + charger. These are best run from a Onan or similar generator. Consider also Dometic’s TEC 29 petrol generator. It generates a continuous 2600 watts. It is quiet and has low emissions.
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. They are hideously noisy and polluting. Worse – their electrical output is very ‘dirty’. They produce voltage spikes that can and do damage so-called switch-mode chargers (or may not even run them at all).
The late 2015 released Dometic TEC 29 petrol generator produces up to a constant 2.6 kW. It is quiet and has low emissions. Pic: Dometic.
High quality chargers are not cheap. Expect to pay $350 upwards. Do not skimp on this. Any savings on a battery charger will be wiped out by inefficiency – more fuel used in running the generator longer. High quality chargers work quickly, deeply and reliably. Some top brands are better than 90% efficient.
High quality Xantrex charger is not cheap, but charges fast and safely
Generator apparently big enough – but the charger does not perform
This arises with battery charging via a generator by adverse power factor. This prevent a generator’s full power being exploited by the charger. The generator may also be hard or impossible to start unless the charger is initially off.
A quick and dirty fix is to plug a 100 watt or so soldering iron across the generator. It’s not the heat that does the job! The iron (being purely resistive) tricks the generator into working properly.
A better, but costly fix, is to have correcting capacitors fitted to the battery charger. 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
Some problems arise with switch-mode technology chargers and inverter-chargers. These are smaller and far lighter than transformer-based chargers. Some work properly from the grid 230 volt supply but produce low output, or even none, from certain generators.
Switch-mode chargers are reasonably efficient (plus 90% is common). A 1000 watt generator should be able to run a 35 amp such charger. But not all will. Switch-mode devices demand ‘clean’ electricity. And that from some generators is too ‘dirty.’
The cause is that generator speed varies slightly but constantly, typically 50 times a second. This causes its output to be also multiples of 50 cycles a second. That ‘dirty’ ac can cause switch mode charger protection circuits to cut off the supply. In some cases the generator wrecks the charger.
Fixing problems with switch-mode battery charging via generator
The problem’s cause is that the generator’s flywheel is too small. Diesel engines are so prone to this they must have a heavy flywheel.
Generator vendors usually deny responsibility. Their products will drive most electrical loads without problems. Likewise will switch mode battery charger makers.
This situation is now mostly historical. Most generators used by RV owners are the quiet inverter type units. These, 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 an electrical fix (for Dakar inverter chargers). It partially cleans up the dirty ac. This partially tricks the charger into accepting ‘noise’ that remains. It’s designed for specific generators, but the company says it enables Dakar chargers to work with other generators with similar problems.
Another fix is a flexible rubber coupling between engine and electrical generating bits. This absorbs the speed changes.
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