Battery charging via a Generator – how to speed it up


In many a campground, generators plug away all day as owners vainly try to charge their RV battery from the 12 volt socket of their 230 volt generator. Even if (misleadingly) marked ‘battery charger’, that output is only suitable for and intended to run your small 12 volt volt lights and appliances (directly) without a battery.

Technically, its output on light loads is about 13.6 volts, but that drops to a level far too low for charging an RV battery – particularly AGM or LiFePO4 batteries as they can and do cause that voltage to drop rapidly. Because of this, a generator can charge a flat (say) 100 amp hour battery to about half charge within 5 or 6 hours. But that’s about all. Thereafter charging virtually ceases.

Successful battery charging via a 230 volt generator is nevertheless feasible. What is needed is a high quality 230 volt battery charger run from the generator’s three-pin plug 230 volt output.

A good inverter generator charges batteries quickly and safely via a high quality charger from its 230 volt output Pic: Honda

A good inverter generator charges batteries quickly and safely via a high quality charger from its 230 volt output Pic: Honda

What type charger for battery charging via generator?

Most portable generators can maintain their rated output for only a few minutes. Their limit is usually 80% for continual use. In reality, your 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 will fully charge a flat 12 volt 100 amp hour AGM battery within 3-4 hours. Conventional lead acid batteries will take longer as they are unable to speedily absorb the charge.

Onan 2800 watt generator uses about 1.4 litres/hour Pic: Onan

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 can and sometimes do damage so-called switch-mode battery chargers (or may not even run them at all).

Dometic’s TEC 29 petrol generator produces up to a constant 2.6 kW. It is quiet and has low emissions Pic: Dometic

Dometic’s 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 cheap battery charger will be wiped out by its inevitable inefficiency as more fuel is burned in running the generator for 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

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

Generator big enough – but charger does not perform

This can happen with battery charging by using a generator that has so-called adverse power factor. It is an effect like two rowers in a boat pulling out of synchronisation. It precludes the available power being fully used. The generator may also be hard or impossible to start unless the charger is turned 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. There are other (costly) ways of fixing this, but its only worthwhile 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

Issues may arise with so-called switch-mode technology chargers and switch-mode inverter-chargers (these are smaller and lighter than transformer-based chargers). Switch-mode chargers are reasonably efficient (plus 90% is common). Some work just fine from grid power, but may produce little, or even none, from certain generators. Switch-mode devices are fine – but need ‘clean’ electricity. And that from most cheap generators is ‘dirty.’ The cause is that generator speed varies slightly but constantly, 50 times a second. This causes it to overlay multiples of the 50 cycles a second output. This particularly happens if it runs out of fuel and splutters to a halt. This usually cause the switch mode charger’s protection circuits to cut off the supply, but may even wreck the charger.

Fixing this problem may vary from difficult to impossible. Generator vendors usually deny responsibility because their products drive most electrical loads without problems. The battery vendor too may denies responsibility as their products work just fine on clean electricity. Each thus blames the other. Fortunately, this situation is mostly past – as many RV owners buy the quiet inverter type units. These, made by Dometic, Honda, Yamaha, Robin etc. do not suffer from this.
A DIY fix that often works is to include a flexible rubber coupling between the engine and electrical generating bits. This absorbs the speed changes.

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