How to speed up battery charging without violating its operating requirements

These days, it’s not just how long you can run on a standalone power source that matters, but also how quickly you can recharge it. People often complain in the comments that their batteries—lead-acid or lithium—charge slowly, but don’t understand how to speed things up using "legitimate methods." Today I’ll try to explain why this happens and what you can do about it.

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A few words by way of introduction.
I won’t tell you anything particularly new or unusual; many of you already know all this perfectly well. But experience has shown that quite a lot of people simply don’t pay attention to it—and they should.


Lately, we’ve been having well-known problems with the power supply. I don’t think I need to explain what they are or remind you who’s responsible, but the problem exists and we have to deal with it.
So, in an attempt to solve the problem, we go to a store or order a large battery and an inverter online, along with a charger, of course. We get everything, put it all together, and try to make it work. But power outages are only half the problem. Sometimes the intervals between them are quite short, and we need to recharge our battery during that time—only to find that the battery just won’t charge quickly.

Most often, people use chargers like the ones shown in the photo below. I won’t get into the details of their circuitry or other design features, but I do want to draw your attention to... their wires. On most of these chargers, the wires are not only thin but also long—and some users even extend them further.


Now I’ll explain what the mistake is and how to fix it, thereby speeding up battery charging. There will be graphs, but they’re needed to illustrate the charging principle and its characteristics.

Here, for example, is a graph of a lead-acid battery charging. We’re interested in the point where the red and blue curves intersect. The blue curve shows the voltage at the battery terminals, while the red one shows the charging current.
As you can see, the voltage rises as the battery charges, which makes sense, while the current is held constant, so the line at the top is straight. As soon as the curves intersect, the current starts to fall while the voltage remains unchanged.
The first phase is charging at a constant current, CC.
The second phase is charging at a constant voltage, CV.

You can also see that the current falls during the second phase, but time keeps ticking along at the same rate, so the charging rate clearly slows down.



This is because the battery has internal resistance. In reality, things are considerably more complicated, but let’s simplify as much as possible and call it internal resistance anyway.
In simplified terms, a battery can be represented as an ideal cell with a resistor (Rp) connected in parallel to account for self-discharge, and an internal series resistance (sorry for the tautology), Rs, connected in series.
The first one doesn’t concern us much right now, but the second is quite critical in this case.


What does the internal series resistance do? First, it causes losses. Second, the voltage drop across it makes the charger think it’s time to switch to the constant-voltage charging phase, even though the voltage across the cell itself hasn’t yet reached the charge termination voltage.

Here are two graphs as an example: the same battery and the same charger, but the charging currents differ severalfold.
The first graph shows that the second charging phase is very short, which is good: it means our battery is charging as efficiently as possible.


But then we increase the charging current. The battery should charge proportionally faster, right? Not so fast. As you can see, the second charging phase has become much longer.
This is due not only to the chemical processes involved, but also to the effect of internal resistance.


You’ll probably ask what the wires have to do with this. The wires are connected in series with the battery, so they increase its internal resistance. In the diagram on the right, they’ve been added to our battery. And the longer the wires, the greater their effect on the second charging phase.


In other words, instead of charging the battery, we end up just heating the wires. This applies mostly to lead-acid and “regular” lithium-ion batteries, since their charging curves are similar.
The situation is somewhat better with lithium iron phosphate batteries: their curve is very steep at the end of charging (and discharging), so by the time charging finishes, the current has already dropped quickly to a low level, and wire resistance has little effect.



Now let’s look at our charger and battery “in cross-section.”
The current-setting circuit stabilizes the charging current and handles the first charging phase.
The voltage-setting circuit limits the output voltage and handles the second charging phase.

For example, say the wires from the charger to the battery are 1 meter long, have a cross-section of 0.75 mm², and carry a charging current of 8 A. The voltage drop across the wires (assuming they really are 0.75 mm² copper) will be about 0.36 V, which is quite a lot in my opinion. And what if the wires aren’t copper, or are longer than one meter?


If you’ve connected powerful inverters, you’ve probably noticed that their wires often get hot during operation. The solution is simple: use thicker wires and make them as short as possible. It’s exactly the same here: toss out the Chinese wires made from a mixture of foil and cardboard, and fit proper wires with a decent cross-section, shortening them as much as possible while you’re at it.


The previous solution is for those who “don’t know their way around” electronics; this one is for the more advanced.

If you separate the current-setting circuit from the voltage-measurement circuit, you can eliminate the effect of the wires altogether. This is called a “four-wire connection.”
There are some complications, though. For example, separating the positive sense wire from the power wire is usually no trouble, but the negative side can be trickier. Even so, separating just one wire—that is, “virtually” turning a two-wire setup into a three-wire one—will halve the effective wire length to the battery (without any compensation tricks).


Unfortunately, both three-wire and four-wire connection schemes have one significant drawback: if a sense wire breaks, your charger will keep charging the battery until all its electrolyte boils away or the battery turns red-hot.
To prevent this, connect the power and sense wires through a resistor whose resistance is much higher than that of the wires, but still much lower than the resistance of the sense input, so it doesn’t introduce a measurement error. I think a value around 100–300 Ω is sufficient for most applications.


In this case, if a wire breaks, the sense input will still remain connected to the power circuit. The voltage may rise, but only slightly, and not enough to harm the battery.


Of course, you won’t speed up the charging process dramatically, but if your lead-acid battery takes around 5–7 hours to charge, saving half an hour is definitely worthwhile.

By the way, when testing various batteries, I use a four-wire connection, and I can say that charging really is noticeably faster this way than with a regular charger.

This article draws on material from these reviews:
YR1030, milliohm meter for measuring low resistance and battery internal resistance
Universal battery holder with four-wire connection

That’s all. I hope this article helps you get through the challenges. As always, I’ll be glad to answer your questions. I may be slow to reply, but I do try to answer everyone.