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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.

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 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. Still, being able to bring a battery from say 11.5V back up to 13V would be useful.

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?

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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. (In contrast, PV/wind/offgrid installations are usually made from single 3.2V cells with external BMS)

Charging 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. Well, some equalizing current will flow, but it is absolutely not a practical approach.

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.

What can go wrong, then?

  1. Alternator being slightly off-spec (like, 14.8 or 15.2 volt). Lead-acids tolerate this to some extent. Some alternators adjust intentionally their voltage that high in cold weather or immediately after starting the engine. This is good for lead-acid batteries, but not so good for LiFePO4. Avoid aftermarket alternators unless you know exactly how these behave. Be aware that most of their owners (offroad/RV/marine enthusiasts) don't know either.

  2. Jumper cables are not to be used unattended. They are related to a significant percent of car fires even when used for their intended purpose. When used for a prolonged period (like 15 minutes or more) they can overheat, melt, lose grip, etc... you get the hint. Use adequate connections, rated for the power of the alternator.

  3. The alternator maximal (self-limiting) current may be higher than what your battery is happy with. Most modern passenger cars are equipped with 100-180 ampere alternators. At idle, they may produce as low as 1/3 of their rated power, making your calculations about the charge transferred less accurate.

  4. 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. Connecting the negative to the chassis instead of the negative battery terminal may or may not help.


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.

Other household electronics that use similar voltages (computer PSUs, etc...) are even less practical.

A 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).

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If you just hook the LiFePo up to a car battery, you'll get a lot of smoke from both the cables and the batteries involved, and it can also cause the LiFePo to ignite or explode.

The problem is that there's nothing to limit the current flowing into the LiFePo battery. A car battery can easily provide thousands of amperes of short-circuit current, and if you connect a LiFePo across it, you have created just such a short-circuit. Assuming that your LiFePo battery is fully discharged, which means it has a cell voltage of 2 Volts, the total voltage of the battery will be at 8 Volts (4 cells in series). The car battery, on the other hand, might have 13 Volts when fully charged. That's a difference of 5 Volts, which will cause a massive current to flow that's only limited by the circuit's total resistance (which is mostly in the wires connecting the batteries). If we now assume that you've connected the batteries using 1 meter of 10AWG wire, this will give us about 3.3 Milliohms of total resistance. 5 Volts across 3.3 Milliohms results in a massive 1500 Ampere current flowing into the LiFePo battery, for a total power dissipation of 7500 Watts (1500 Ampere at 5 Volts cable drop). In practice, it'll be slightly lower due to the batteries' internal resistance, but it won't change the fact that things will blow up almost instantly. It also won't be any better if the LiFePo battery isn't fully discharged, it'll still dump thousands of Watts into the cables and burn them (and the batteries too).

Even if you managed to limit the current, you still need a balancer to properly charge a LiFePo battery, otherwise individual cells might get over-charged and damaged.

TL;DR: Don't do this, the batteries and cables will blow up. Always use a proper charger.

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  • \$\begingroup\$ You know this answer is OK. But the OP specifically said "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." So I don't think the OP was proposing to connect a fully charged lead acid battery directly to a depleted LiFePO4 battery. It sounded more like the OP wanted to connect the batteries in parallel to run the car engine and charge the LiFePO4 via the alternator occasionally or maybe in an emergency. \$\endgroup\$
    – user57037
    May 28, 2021 at 17:58
  • \$\begingroup\$ @mkeith Even though LiFePO4 aren't as dangerous as Li-Poly/Ion, that doesn't change the fact that we're dealing with fractions of a milliohm of internal impedance here. This means that no matter how small the voltage difference between the batteries is, there will be immense heating - if the batteries don't catch on fire, the cables will. \$\endgroup\$ May 30, 2021 at 13:29
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    \$\begingroup\$ @JonathanS. if the voltages are close there would not be any big drama. In order for massive heating to occur you need high currents for a long time. But as soon as the batteries are connected together, the voltages will move toward each other. But mainly, I challenge what Mast said, that LiFePO4 are "bloody dangerous." Dangerous compared to what? \$\endgroup\$
    – user57037
    May 30, 2021 at 17:04
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Every time someone asks about this sort of thing, the overwhelming response is "only do it the proper way or your batteries will blow up and your house will burn down." You seem to understand that and specifically want to know what will happen or what your options are. In my opinion, that is a perfectly valid question.

I work with Lithium batteries a lot and also have read a lot of the manufacturer's literature. I am not a safety expert on the topic but I am not coming out of left field, either. LiFePO4 batteries are thermally much safer than Li-Ion/Li-polymer. But they still require proper charging for best battery life and for safety.

A car alternator can charge 4S LiFePO4 battery pack, but you need to monitor the current and voltage. If it is a small battery pack, the charge current could easily be too high for the battery. If it is a big battery, then it is very possible for the battery to overload the alternator, because the LiFePO4 batteries will accept very high currents without appreciable voltage rise. So there is danger that you will burn up your alternator.

Likewise, if the batteries are left on the alternator for a long time, they will get over-charged and lose capacity prematurely. Considering the expense of LiFePO4 batteries, this doesn't seem like a good idea other than some kind of emergency scenario. If possible, watch the voltage like a hawk and disconnect the LiFePO4 batteries when they get to the recommended maximum voltage.

Extreme caution is warranted if you are contemplating connecting a 4S LiFePO4 battery pack to a car battery. The voltages must be equal prior to making the connection to avoid excessive equalization currents. This is not a safe arrangement in the long term, and all previous comments about alternators apply if you run the engine.

One option for connecting different voltage batteries is to put a resistor between them to limit current. This is very inefficient, but may be workable in some cases. You have to do all the math to determine current and power dissipation in the resistor, etc. Most likely this would be done with a large power resistor. Once again, this would be something to do in an emergency only.

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    \$\begingroup\$ Look at this way even the cheep device manufacturers use proper chargers, they do this for a reason as it cost them more. If you do this name your favorite charity as beneficiary on your life insurance. \$\endgroup\$
    – Gil
    May 28, 2021 at 17:22
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    \$\begingroup\$ @Gil I re-wrote my answer. I don't think the OP or anyone else, really, will be particularly well served by your sarcastic comment about beneficiaries and life insurance. The fact is that as long as you do not exceed allowable charge current and voltage, it does not matter what source you use to charge a LiFePO4 battery. And I am not proposing that it be charged outside of the allowable range, and neither is the OP. This is an engineering site. People should be allowed to ask "what happens if I do this" even if "this" is not recommended. They want to know the consequences. Just tell them. \$\endgroup\$
    – user57037
    May 28, 2021 at 18:02
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    \$\begingroup\$ I think a resistor to limit charge with a low-voltage diode to bypass the resistor during discharge might "work" for some definition of work. The LiFePo battery would only charge very slowly and could never be fully charged and might suffer degraded life, but it would charge to a reasonable level and would discharge acceptably when needed. \$\endgroup\$ May 28, 2021 at 22:27
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    \$\begingroup\$ I appreciate recognizing this as a valid question. This answer is helpful but the other one, albeit having the bold 'will blow up' point, does help me understand the problems more in breaking down the engineering details of it (as in, why simply hooking up the car battery and LiFePO4 battery would result in high amps like a short circuit). I think your answer also gets at this point: regulating current is the main challenge and has the most severe effects. \$\endgroup\$
    – cr0
    May 29, 2021 at 1:29
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    \$\begingroup\$ I think blowing up either the lead acid or LiFePO4 battery is very unlikely. But if you connected a fully charged lead acid battery to a fully depleted LiFePO4 battery, I think there is a good chance for a bright blue spark and melted wire ends and small flying bits of molten metal shooting out in all directions. But in all likelihood, even though it would be stupid to do it, both batteries would probably survive OK (other than damage done to the terminals by massive sparking). The current would be large, but the voltage would equalize rapidly, too. The wire might get very hot and burn you. \$\endgroup\$
    – user57037
    May 29, 2021 at 23:45
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You can charge any (rechargeable) battery from any other battery or any other source provided that you can ensure that the charging current is within allowable limits for both batteries. There are a few ways to do this; the simplest is to use a resistor although a constant-current circuit would be better, and a switch-mode circuit better again since it would not require that the donor voltage be somewhat higher than the recipient.

Allowable limits includes not only current but minimum and maximum voltages too.

Other commentators have already indicated the hazards of going outside the operational limits.

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  • \$\begingroup\$ I have a LiFePO4 with a diva BMS that wants 14.6v to charge. I have a PV DC-DC converter that only outputs [email protected] without reprogramming it, gasp. The DC-DC converter is backfeed protected... But directly connecting an AC inverter to plug in a [email protected] charger would lead to some smoky voltage oscillation. So I guess I'll have to add a $20 relay to charge the battery when the PV has power... But that means when a 250A load spins up for a second, the power will cutoff. Maxwell to the rescue? \$\endgroup\$
    – Ray Foss
    Oct 26, 2022 at 2:52

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