Charging Batteries from Generators

Carbon Monoxide Poisoning

Seemingly growing is a problem of battery chargers and, more recently, inverter/chargers, which will not run satisfactorily from ac motor generators. In some instances the chargers produce only a fraction of their expected output. In some instances they produce no output at all.

Despite this problem existing virtually since basic battery chargers and ac generators were invented, and unrelated to quality or price, the vendors of each product are more likely than not to claim (a) they have never encountered the problem before; and or (b) a fault with the other’s product is causing the problem.

But the problem does exist and there are several possible causes. In earlier and still often encountered instances, the generator is simply too small for the charger. This occurs because it is not remotely obvious that a charger putting out 500 watts needs close to a 1000-watt generator.

Conventional Battery Chargers

Typically hardware/autopart store units (although truly up-market conventional chargers, such as the Woods range exist), such chargers are reliable but extremely inefficient (70%-75% is typical). The inefficiency alone necessitates a generator about 30% larger than otherwise required. A further factor, explained below, requires the generator to be a further 30% larger again.

The alternating current (ac) produced by mains supplies and ac motor generators will run things like water heaters, electric oven and toasters, incandescent globes, soldering irons, etc with no problems at all. A generator designed to produce a constant (say) 3000 watts will drive any of the above as long as the maximum load does not exceed 3000 watts. All the above loads are purely ‘resistive’. That is, that they partially oppose the flow of current and generate heat when doing so.

Conventional battery chargers however consist mostly of coils of wire wound around an iron or similar core. These loads are called ‘inductive’. Inductive loads behave oddly with alternating current in that the current flowing through the load gets out of step with the voltage pushing it. The effect is called ‘Power Factor’ and is usually expressed as a number between 0 and 1.0. A power factor of less than 1.0 has much the same effect as pushing a kid’s swing before or after the optimum time – in that the same amount of push has less effect on the desired result.

Conventional battery chargers have a power factor between 0.65 and 0.7. Driving them requires a generator that makes extra current available. This extra current is not actually ‘used’ but it must be available. It is sort of ‘shunted to and fro’. Electricity suppliers hate this effect because whilst they have to make the extra current available it does not register on their meters because it is ‘utilised ’ but not consumed.

As noted previously, conventional battery chargers requires (a) generators to be 30% larger to cope with their inefficiency, and (b) a further 30% to cope with power factor. The big battery chargers used in the larger caravans and motorhomes put out (say) 30 amps at 12 volts (360 watts). This needs, as absolute minimum, 650-watt generators to drive them. But as most ‘650-watt’ generators can only work that hard for a minute or two, a 1000-watt unit is far from overkill. Most conventional battery chargers are smaller than this. They are typically 8-10 amps, hence almost any generator is able to drive them – albeit inefficiently.

Generator Big Enough – but the charger still does not perform.

With some generators, adverse power factor can prevent the generator starting up or developing full power into a battery charger. A quick and dirty fix (but one that almost always works) is to connect a 100-watt incandescent globe across the generator. It’s not light produced that does the job, but simply that the globe, being purely resistive, tricks the generator into working properly. A better, but costly fix, is to add power factor correcting capacitors to the input of the battery charger (a licensed electrician will know what this means and probably why you want them). This also overcomes the need for the 30% larger generator needed to overcome the adverse power factor.

Be aware also that many ‘hardware-store special chargers’ are hugely sensitive to incoming voltage. They may well produce their claimed output when driven from the mains, but less than half that if driven at 220-volt from a generator in need of attention (or just a lousy generator!).

Inverter/chargers (and others)

In more recent times, apparently similar problems have arisen with the increasingly popular inverter chargers and associated chargers using switch-mode technology. But here, whilst the symptoms tend to be similar, the cause is totally different.

Switch-mode devices are reasonably efficient (85-90%) and have a power factor close to 1.0. Extra generator sizing is not required. But what is required with these devices is clean electricity. They want 50 Hz (cycles/second) not 50 Hz with an overlay at 100 Hz and another at 150 Hz etc……………and that’s what they are likely to get from petrol-driven generators.

On each power stroke of the engine, a generator speeds up. The increase in speed is very slight, but acceleration (the rate of that increase) is very high indeed. This rapid and ongoing speeding up and slowing down generates a harmonic series of multiples of 50Hz – right up to 5000 Hz or more. Being fed with dirty ac tosses the electronic circuitry of many switch mode devices. This can and does cause protection circuits to cut off that supply. This is a problem that I feel needs addressing by generator makers (Honda’s inverter generators do not suffer from it, nor do Mastervolt’s or Fischer Panda’s), as an ever-increasing proportion of equipment uses switch-mode power supply technology.

One (crude but simple) solution is a heavy flywheel, the inertia of which mechanically dampens the acceleration and deceleration that is the cause of the problem. Diesel engines are so prone to this problem that their makers have no choice but to use a heavy flywheel. Another fix is a flexible rubber coupling, between the engine and the electrical generating bits, that absorbs the changes in speed. Yet another is a ‘harmonic damper’ on the crankshaft. This is like a rubber coupling combined with a very small flywheel. It was invented by Dr Lanchester around 1905 and used in some cars ever since.

In Australia, Power Protection Systems (suppliers of Mastervolt etc) has designed a simple electrical modification (to the Dakar inverter charger) that fixes the problem. It was designed specifically with Onan’s 3600 petrol generators in mind, but Power Protection Solutions’ Bob Wisniewski says it enables the Dakar unit to work with the many other generators that exhibit similar problems.

Their fix partially cleans up the dirty ac and partially tricks the inverter charger into accepting any ‘noise’ that remains.

At present, Power Protection Solutions is incorporating this $75 modification only into its Dakar 12/1500-65 inverter charger. This makes sense as the company supplies this unit to Australia’s largest selling motorhome builder. You can contact Power Protection Systems on 07 3283 7800. They are friendly and know a lot about stuff like this. (Curiously, the 100-watt globe trick often works with this problem too. But not as reliably. And certainly not as elegantly.)

A heap more information on all such electrical and other problems are published in Collyn’s books. These are available direct from the CMCA Head Office (and also at CMCA Rallies).