Other household electronics that use similar voltages (computer PSUs, etc.) are even less practical.Ī good lead-acid 2- or 3-stage AC charger may or may not do the proper thing, but even if it does, for a depleted 100Ah battery, you will likely wait a day or two (most of these chargers are 3-7 ampere). In short, the jump cables approach is possible in emergency, with a good pair of cables and if you exactly know what you are doing, but is a good recipe for problems as well. Connecting the negative to the chassis instead of the negative battery terminal may or may not help. Some modern cars are quite unhappy about connecting something to the battery, because they use some internal accounting about the state of charge of the battery. At idle, they may produce as low as 1/3 of their rated power, making your calculations about the charge transferred less accurate. Most modern passenger cars are equipped with 100-180 ampere alternators. The alternator maximal (self-limiting) current may be higher than what your battery is happy with. Use adequate connections, rated for the power of the alternator. When used for a prolonged period (like 15 minutes or more) they can overheat, melt, lose grip, etc. They are related to a significant percent of car fires even when used for their intended purpose. Jumper cables are not to be used unattended. Be aware that most of their owners (offroad/RV/marine enthusiasts) don't know either. Avoid aftermarket alternators unless you know exactly how these behave. This is good for lead-acid batteries, but not so good for LiFePO4. Some alternators adjust intentionally their voltage that high in cold weather or immediately after starting the engine. It is possible and practical to use dedicated dc/dc charging devices like those made by Revolectrix or iSDT - with or without alternator running, as long as you don't deplete the car battery to the point where you can't start the (last) car.Īlternator being slightly off-spec (like, 14.8 or 15.2 volt). Well, some equalizing current will flow, but it is absolutely not a practical approach. (In contrast, PV/wind/offgrid installations are usually made from single 3.2V cells with external BMS)Ĭharging it from the car battery (without the alternator running) is not possible directly because of insufficient voltage of the lead-acid battery in any state of charge. Most 12V LiFePO4 batteries are designed as a replacement for a car/truck/boat/RV battery and in particular, EXACTLY to be charged by an ordinary car alternator that is limited at 14.2-14.6 volt. How can a 12V LiFePO4 100Ah battery get a charge from common automotive or household electronics? For example, can it be hooked up to a car battery with jumper cables, then the car occasionally started and left to run for a bit to keep the car battery charged, using a multimeter to check charger and charging batteries' voltage? If not, for the sake of learning, could someone explain why this is not safe or doable? Still, being able to bring a battery from say 11.5V back up to 13V would be useful. I also realize charging will not be very efficient or cannot 'top off' the battery without a dedicated charger, because of how a dedicated charger varies voltage to charge at highest possible amperage first and then holds voltage constant and reduces amperage to top the battery off. I understand that to charge this battery, the power source for charging must be near the correct voltage and within a safe range of amperage. I plan to charge a 12v LiFePO4 battery using a charger specifically designed for that purpose, but I would like to know alternative methods of charging this battery without a dedicated charger.
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