I don't know what you have for a generator or a battery charger. I do know that many portable generators do an awful job of powering many conventional battery chargers. Sometimes a charger will barely run at 1/4 it's rated output when hooked to a portable generator. Yeah - nothing to do with solar but I figured I'd mention it anyway. This problem took me by surprise when I first hooked to solar and tried to use a generator to charge my batteries when it had been cloudy for a week. Here's what I regard as a nicely written article from Australia that's low tech and easy reading.
CHARGING BATTERIES FROM GENERATORS
There is a known problem of battery chargers and, more recently, inverter/chargers, that will not run satisfactorily from motor generators. In some instances the chargers produce far less than their expected output or even none at all. This issue also affects running AC induction motors from generators (and for much the same reasons).
This problem has existed almost since basic battery chargers and AC generators were invented. It is not necessarily related to quality or price. Despite this, the respective vendors tend to claim (a) they have never encountered the problem before; or (B), and sometimes true, that the fault lies with the other’s product. This problem exists and the respective industries are well aware that it exists. If it is denied, the person claiming this is either new to the job, or is denying something known to be true.
There are several possible causes. The most common is that the generator is simply too small for the charger (or that the charger is too big for the generator. This occurs because it is not remotely obvious, to those who do not understand electrics (nor always to those who do), that a 400 watt charger may require a 1000-watt generator. It may well operate with less, but not well.
Conventional Battery Chargers
Typically hardware/autopart store chargers are extremely inefficient (70% is typical). This alone necessitates a generator 30% larger than otherwise required for that alone. But a further issue necessitates the generator to be larger by an extra 30% again.
The alternating current (AC) from the electricity grid, and AC generators, will run things like water heaters, toasters and soldering irons, etc with no problems. A (say) 3000 watt generator will thus drive any such load up to 3000 watts. This is because all such loads are purely ‘resistive’ (for this purpose it is not necessary to know what 'resistive' strictly means).
Conventional battery chargers consist mostly of coils of wire wound around an iron or similar core. These loads are called ‘inductive’ - and behave oddly with alternating current in that the current flowing through them 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.
This power too must be available from the generator - so yet another 30% (minimum) must be added to its required size. On top of all this, most low-priced generators can produce their advertised output for more than a few minutes - most are limited to 80% for continual use.
In practice, to drive any conventional charger (i.e. those using big and very heavy transformers) needs a generator of twice the rated capacity of that charger.
Many ‘hardware-store special chargers’ are hugely sensitive to incoming voltage. They may well produce their claimed output when driven from a grid supply, but less than half if driven from a generator in need of attention (or just a lousy generator!).
Generator Big Enough – but the charger still does not perform.
With some generators, adverse power factor can prevent an otherwise adequate-sized generator starting up at all, or developing full power into a battery charger.
A quick and dirty fix (but one that almost always works) used to be 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. Grid voltage incandescent globes are no longer sold in many countries - but a small soldering iron will do instead.
A better, but costly fix, is to have power factor correcting capacitors added to the input of the battery charger (a licensed electrician will know what this means and probably also why you want to do it). This also overcomes the need for the 30% larger generator needed to overcome the adverse power factor but may cost as much as buying the switch mode type of charger described below.
Switch-mode chargers
Since 2000 or so, apparently similar problems have arisen with some chargers, and also inverter-chargers, that use switch-mode technology (these are much smaller and weigh a fraction of that of conventional transformer-based chargers).
Here, the symptoms tend to be similar, but the cause is usually different.
Switch-mode devices are reasonably efficient (plus 90% is common) and have a more favourable power factor. A 1000 watt generator should be able to run a 650 watt switch-mode charger. But far from all will because switch-mode devices demand 'clean' electricity. And that from cheap generators, and even at least one costly brand, can be very 'dirty' indeed.
What happens technically is this: on each power stroke, a generator speeds up. The speed only varies slightly, but its acceleration (the rate of change of that increase and decrease) is high. On the following compression stroke, it slows again. This rapid and ongoing speeding up and slowing down generates a harmonic series of multiples of 50 Hz – up to 5000 Hz or more. The resultant 'dirty' AC may cause charger protection circuits to cut off the supply.
The cause of the problem is that the flywheel, the inertia of which is intended to mechanically dampen the acceleration and deceleration, is simply too small.
Diesel engines are so prone to this, 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. But none of the Inverter generators, such as those made by Honda, Yamaha, Robin etc, do not suffer from this, nor do Mastervolt’s or Fischer Panda’s. Most generator vendors will however almost always deny responsibility - often and correctly claiming correctly that their products will drive most electrical loads without problems.
This situation has to change because an ever-increasing proportion of equipment uses switch-mode power supply technology.
In Australia, Power Protection Systems (suppliers of Mastervolt etc) has designed a simple electrical modification (to the Dakar inverter charger. It partially cleans up the dirty AC, and partially tricks the inverter charger into accepting any ‘noise’ that remains. It was designed specifically with Onan’s 3600 petrol generators in mind, but Power Protection Solutions’ Bob Wisniewski says it enables the Dakar charger to work with other generators that exhibit similar problems.
They are a friendly mob and know a lot about stuff like this. The 100-watt globe trick sometimes works with this problem too, but not as reliably.
