Generators - again

[PhilipH]

So this geezer tells me he has a 700 watt Honda generator that he plugs in to the 240v shore power socket and with his smart battery charger uses it to charge his batteries. Now I know that amps = watts/volts, but which voltage number do you use for the calculation? If it's 240 volts then it takes an hour to out in less than 3 amps, but if it's 12 volts then it's a very handy 50+ amps. Help - and why?

 

[Norman_E]

The 700 watt generator produces mains voltage at up to about 2.9 amps. The battery charger (which is in essence a transformer with electronically controlled output) runs on that voltage, transforming it to a little over 14 volts to charge the battery, and gradually dropping the output as the battery nears full charge, finally just providing a trickle charge. At 14 volts, 700 watts equates to 50 amps, but of course the charger is not 100% efficient. I would expect a 700 watt generator to be OK with a maximum 40 amp charger, and probably happier running a 30 amp charger, which would allow some safety margin.

 

[phuvf]

The figure of 700 watts is the maximum power the generator can supply.
This is usually supplied at 240v (so you can plug any domestic 240v equipment in). Your friend is plugging his whole boat into this generator. The maximum current the generator can supply at 240v will be around 3 Amps.
The battery charger on board will take 240V in and convert it to 12+v (usually around 14V) to charge the batteries. Battery chargers are usually rated by current output (for example, 40A) - this is the maximum current they can supply to recharge the batteries, at 12V - i.e. 12x40 = 480W.
If the charger was 100% efficient, the power in (at 240v) would equal the power out (at 12V) - hence 480W would be drawn from the generator - which equates to around 2 Amps at 240V. In practice, the current drawn from the generator would be higher due to heating of the battery charger, fans and lights etc. and the power factor of the load - what this means is that the charger may draw more like 3 amps when being used.

Hope this helps!

Nick Roberts

www.bayanblue.com

 

[William_H]

Some of the Honda and other generators provide a 12 volt output however this does not usually give enough voltage ie over 14 at full current to give any really good battery charging. ie anything like full power of 40 plus amps into the battery. The regulated output voltage is chosen so it will not boil batteries or damage gear running on 12 v o/p direct. This output voltage can only fall then with increase in current so reducing current into the batery.

By using the 240 volt output the specialised charger can deliver the required voltage ie over 14 volts at the max current into the batteries. You can use a stepped charger or you could use an old crude transformer rectifier type non regulated charger provided the output voltage and current capability are enough to get the required current into the batteries (hence max power out of the generators).
What is vital is an ampmeter to confirm you are getting a usefull current into the battery. Cos you don't want to run a gen with only a trickle going into the battery.

You need to be carefull with some of the cheap generators which do not provide a true sine wave and so can cause heating and losses or damage to some of the modern smart chargers.
The older Honda (and others) are fine in that they use a true alternator which must be run at 3000RPM to give 50 hertz AC. Just be carefull of modern Chinese gens which I think actually generate an AC (frequency wild) which is rectified to DC and then electronically switched much like an inverter to produce a square wave 50 hertz AC. good luck olewill

 

[cliff]

[ QUOTE ]
The older Honda (and others) are fine in that they use a true alternator which must be run at 3000RPM to give 50 hertz AC. Just be carefull of modern Chinese gens which I think actually generate an AC (frequency wild) which is rectified to DC and then electronically switched much like an inverter to produce a square wave 50 hertz AC. good luck olewill

[/ QUOTE ]The frequency of the "older Honda (and others)" is dependant on engine speed which fluctuates "(frequency wild)" on changing loads and can be a very rough output not only frequency wise but also voltage wise. The more modern honda EU10i and EU20i models utilise invertor technology and a lot of smoothing circuitry to give a clean stable AC output as do the Kipor Ti units (These are of chinese manufacture) The Kipor are so good Honda UK has obtained a court order against Wuxi Kipor Power Co. Ltd (aka Kipor) for using "Honda" state of the art technology (see here).

Seems to me there is no difference in the output between small Honda and Kipor generators. If there was I could not see Honda UK going to so much trouble to try to prevent the sale of Kipor in the U.K.
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[PhilipH]

So, Cliff for the sake of absolute clarity these small Hondas would be a useful source of battery charging, delivering 30-40 amps then? By the way this geezer hung the Honda from the end of the boom on an Atlantic crossing (sailing with twin headsails) when he needed to charge, so that it would not cut out if heeled and thinking it had run out of oil. Cool or what?

 

[William_H]

Thanks Cliff My point being that the straight alternator while frequency is not very stable (but they try to be with a governor) the output should be true sine wave so OK for the switch mode power supply of a modern battery charger. Some of "inverter " switchig type generators have an awefull wave form which can give trouble with electronics. The point I was trying to make is for purchasers to beware. ie if you can try before you buy. olewilll