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Battery charging with a generator?

Unquestionably other than using solar,
when your away from mains power, the next best and quietest way of charging a battery or battery bank is to use a 240v generator, and I'm talking about one of the better quality quieter camping ones, not the noisy industrial building site type ones.

Now you may be thinking that the best way to charge your batteries is via the 12V battery charging outlet that most generators have built in...........

However trying to charge a battery using the built in 12 volt charger is almost a waste of time, most of these are what is called an emergency charger, and are limited to 8 amps maximum, 6A on some models, and this really is a waste of time for regular battery charging, at only 8 amps per hour going into the battery, its going to take many many hours to get the batteries up to anywhere near reasonably charged!

And it gets worse, these are what we call a linear charger, so as the voltage rises in the battery, the current backs off, so the battery may be only half charged and the generator might be only putting in 3 amps per hour, REDUCING continually!! As I said, a waste of time really.

A much better way is to connect a 240v Smart Multi Stage Battery Charger up to the generator and re-charge the battery at a much faster rate.....Like if it's a 25A charger, then that's what it puts into the battery, continuously, until the battery doesn't want/need to take any more......This is real battery charging.

I often tell people that when charging batteries with a generator and 240v battery charger it's a time and money thing, in that the more money you spend on a larger good quality battery charger, the less time you need to run the generator for, and that's always a good thing.

Now here is how it works...

First up charging our batteries up to about 85-90% is around the best we can expect to get if we want to run the genset for as short a time as possible, to get to the full 100% mark it would take a considerably longer time, and at this stage it is really just wasting fuel, and creating extra unnecessary wear and tear, and noise.
It really is best to leave the full charging cycle for when you have mains power, or maybe once a week via the genset you could run it longer if necessary.

All of the chargers I talk about below have indicator lights that show what stage in the charging process they are up to, so it's easy to know when you can shut down your genset, and we can do this as soon as the adsorption stage is reached, this comes immediately after the boost or bulk mode is finished.

Please note both the ePOWER chargers by Enerdrive, and the CTEK brand are very good and reliable chargers, though both have different case types, the CTEK units are built a bit more for tossing around in the dirt and don't mind a bit of sand and rain etc, while the ePOWER units are little more civilized and better suited for permanent mounting away from excessive dust and moisture.

ePOWER Chargers  CTEK 7000       

ePOWER Chargers, 20A, 40A, 60A                                         CTEK 10A

Now for the smart battery charger you need to decide on the size that will best suit your needs, below is a rough guide to size and performance.....

Charging times with different sized battery chargers.

So as an example lets say you have taken out 100Ah from a 200Ah battery and want to recharge it by running the genset and a 240v battery charger. And if your only taking 50Ah out of say a 120Ah battery, you could halve these times, or with a larger battery, maybe only run the generator once every 2 two days, there are many ways to adjust things to suit your particular needs, the main thing is to size things up properly and get it right the first time.

Using a CTEK 7A charger it will take about 15 hours running the genset to get the battery back up to about 90% charged.

Using a CTEK 10A charger it will take about 10 hours running the genset to get the battery back up to about 90% charged.

Using a CTEK 15A charger  it will take about 7 hours running the genset to get the battery back up to about 90% charged.

Using a 20A ePOWER charger it will take about 5 hours running the genset to get the battery back up to about 90% charged.

Using a CTEK 25A charger it will take about 4 hours running the genset to get the battery back up to about 90% charged.

Using a 40A ePOWER charger it will take about 2.5 hours running the genset to get the battery back up to about 90% charged.

So as you can see, the larger the amps out-put of the charger, the faster the batteries will be charged, and the shorter the generator run time will be.

This is assuming that you have batteries that can take the larger charging currents, like AGM's (Adsorbed Glass Matt), or a larger battery bank that is capable of taking the higher charging currents.


Just a word of warning regarding letting generators run out of fuel, it's best not to let the genset run out of fuel while the battery charger or any other electronic device is attached, have you ever heard a small petrol engine as it is running out of fuel, and how it revs up and slows down and then revs up again etc.
Can you imagine what is happening to the voltage and 50Hz while this is happening, this is not good for the constant 50Hz that is required for most electrical goods, well of course it's no longer 50Hz, it's 60 and then 40 and then 60 and then.......well I'm sure you get the drift, the newer inverter models don't suffer as badly, but it's still not good to be running them out of fuel with electrical appliances still connected.

For other RV related articles you may find interesting please check my Tech page

Copyright © Val Rigoli.      

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