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Posted: Wed Jan 28, 2009 9:44pm

Post Subject: battery charging

I have a 240v engine driven generator - if I run the engine and charge the batteries with the 12v alternator, can I also use the 240v generator to put additional charge in using the mains generator at the same time or will this cause damage? It could be quite useful to get the batteries charged up on short runs.


Posted: Thu Jan 29, 2009 11:48am

Post Subject: battery charging

Dear Steve. There is no way a 240 volt generator can charge batteries. A 240 volt generator feeding a battery charger can or a 240 volt generator with a "12 volt" outlet might. I must assume that you propose to use the mains generator to feed a battery charger. First of all , without a lot of research and reference to the manufacturers of the equipment I can not give you a totally definitive answer but it is unlikely that running both at the same time will damage anything. If there is a candidate for damage it would be the battery charger, but I doubt it would be. You need to understand what is happening with your charging systems. Taking each individual system in turn and assuming discharged batteries. The alternator will start charging at its maximum output at a voltage around the 13.5 volts. This voltage is being controlled by moving magnetic fields so can not be influenced by the voltage regulator. As the battery stars to accept a charge the charging current drops and the voltage rises until the voltage reaches about 14 to 14.5 volts (dependant upon the alternator and its regulator). At this point the alternator's voltage regulator starts to turn the alternator on and off very, very fast so the output voltage averages 14.whatever volts. If by some magic means the battery voltage managed to rise above this figure the voltage regulator would just shut the alternator down. Now the charger. I assume you have a multi-stage marine charger. The first part of the charging will be similar to the above, but once the charging current starts to fall the charger will raise the charging voltage to maintain the charging current flow. It will do this for a period based on time or battery voltage, when it decides the battery is charged enough, and then it will probably drop the voltage to about 13.8 volts. At random times many chargers will decide to raise the voltage to 15 volts plus and go through an equalisation cycle. What is likely to happen is that both the alternator and charger will both be delivering charge whilst the voltage is below 14.something. Once the voltage regulation starts in the alternator I suspect the battery charger will start raising the voltage and so shut the alternator down and the warning lamp might glow. No damage should be done but exactly what happens and when will depend upon the individual settings in the alternator’s voltage regulator and battery charger. If by some chance both were set to “regulate” at the same voltage both would continue to charge. Tony brooks

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