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I have a 12 volt power supply unit with 15 percent up or down voltage selector so I can achieve little more than 14 volts current draw is 30 amps maximum. It has over volt safety thermal safety and short circuit protection. Powered by 120v ac. The question is if I added a fuse 30amp and connected it to a car battery matching the voltage and then steady turn it up to 13.8 to 14 do you think I could charge and maintain the 12v battery? It wouldn't be a smart charger so @ 30amps im looking around 2.5 hours to reach capacity then shut it off? A.c to D.c 12v charge Screenshot_20220309-124050_Amazon Shoppi - EletriciansForums.net
 
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Lucien Nunes

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A few problems.

14V is not enough to fully charge a car battery that is in cyclic use (alternately charge and discharge). It is just enough to maintain the level of charge on permanent float i.e. when the battery is connected to the charger 24/7. The normal float voltage is 13.8 or so depending on temperature. At 14V the battery will charge slowly and will need 24-48 hours or more to reach full charge. It will never experience cell equalisation voltage (typically 14.5V) so with cyclic use some capacity might be permanently lost.

Even if the PSU went up to 14.5V, you still cannot charge at 30A all the way to full charge. This is where lead-acid batteries differ from e.g. lithium ion, which can be fast-charged to full. Lead-acid takes time to absorb the charge and abusive fast charging will overheat the battery, cause it to vent and lose electrolyte, and even buckle the plates causing a short-circuit. Charging a battery of say 80Ah at over 20A requires temperature feedback to the charger.

This is why there are smart chargers; most are 3-step. The first (bulk) step works at constant current, delivering the maximum that the charger is capable of giving (or the maximum the battery can accept). When a certain charge level is reached, the charger switches to constant (equalise) voltage and lets the battery approach very close to (bot not beyond) the fully charged point. It then monitors the current and when this falls to a certain level or a certain maximum time is reached, the voltage drops back to float voltage to prevent damage.

Your unit probably does not support constant-current regulation; it may go into foldback limiting or hiccup mode if the current gets too high. So you will have to see it simply as a float charger not really ideal for cyclic use.
 

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