Battery charging via generator 12-volt output – updated May 2019

In many a campground, generators plug away all day, vainly attempting to charge an RV battery. You can do it successfully if you know how and why. The article refers to 230 volts ac but where relevant (as in the USA), 110 volts ac equally applies.

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

Most small generators have a 12 volt dc output socket possibly even marked ‘Battery Charging’. This output, however, is really intended to run small 12 volt dc devices without a battery. Its 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. The usual maximum current is only 8 amps.

Such low voltage may charge a flat 100 amp hour battery to 40% or so within 6-8 hours. But that’s about all. From thereon charging virtually ceases. That voltage and current output are not high enough for battery charging fully or speedily.

Successful battery charging via a generator is nevertheless feasible. You do it by using a high-quality multi-stage 230 volt ac battery charger run from the generator’s 230 volt output.

What size generator?

Portable generators produce their rated output for only a few minutes. Their true output is usually 80% (of that) for continual use. In reality, a 1000 watt generator is thus an 800-watt generator. They are typically most fuel efficient around 70% load. That is 700 watts for a nominally 1000 watt unit. This is sufficient to power a quality 30-40 amp charger. Chargers this size will 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 an Onan or similar generator.

Battery Charging via Generator, Battery Charging via Generator
Onan 2800 watt generator uses about 1.4 litres/hour. Pic Onan.

Avoid cheap generators

Do not even think about battery charging via 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 (see below) or may not even run them at all.

Battery Charging via Generator, Battery Charging via Generator
Dometic TEC 29 petrol generator produces up to a constant 2.6 kW. It is quiet and has low emissions. Pic: Dometic.

What type/size charger

High-quality chargers are not cheap. Expect to pay $250 upwards. Do not skimp on this. Any savings on a battery charger will be wiped out by inefficiency – more fuel is used in running the generator longer. High-quality chargers work quickly, deeply and reliably. Some top brands are better than 90% efficient.

Battery Charging via Generator, Battery Charging via Generator

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

Problems with switch-mode battery charging via generator

Some problems arise with switch-mode technology chargers and switch-mode inverter-chargers. These are smaller and far lighter than transformer-based chargers. Switch-mode chargers are reasonably efficient (plus 90% is common). Some work properly from the grid 230 volt supply but produce low output, or even none, from certain generators. This is because all switch-mode devices demand ‘clean’ electricity. And that from some generators is ‘dirty.’

The cause is that generator speed varies slightly but constantly, typically 50 times a second. This causes it to overlay multiples of 50 cycles a second. That resultant ‘dirty’ ac may 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 varying generator speed problem is usually that the generator’s flywheel is too small. Generator vendors usually deny responsibility. They claim their products will drive most electrical loads without problems. Switch mode battery charger vendors too may deny responsibility as they are just fine running on clean electricity.

Fortunately, this situation is 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 or Fischer Panda.

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 the engine and electrical generating bits. This absorbs the speed changes.

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