# How do DC/DC converters work as lithium battery chargers?

There's a lot of DIYs that utilize DC/DC converters to charge Lithium batteries. A quick Youtube search shows dozens of these DIYs. I was wondering how these home-made chargers work. Yes, DC/DC converters do provide constant voltage and constant current, but the mechanism of battery chargers isn't exactly the same? A typical charger would provide enough voltage and current to a battery to raise the voltage of the battery being charged and as the battery's voltage stabilizes, the charger would decrease the current.

For example:

Let's say we have a 10s 10 Ah Li-ion battery pack with a nominal voltage of 37 V and full charge voltage of 42 V. Now, charging this pack using DC/DC converter that could supply constant voltage of 42 V and let's assume we charge the battery at 0.2C which means 2 amps. Since the pack is receiving constant current of 2 amps, the pack's voltage will increase steadily until it reaches 42 V. The question here is that, will the DC converter output constant 2 amps even when the battery pack reaches its fully charged voltage of 42 V? If so, how these people are able to charge their batteries without running into big bangs or some kind of troubles? Normally a lithium battery charger starts with a constant current supply to the battery and then as the battery reaches its full charge voltage, the charger detects the battery's voltage and adjusts the current until the battery's voltage stabilizes.

Example of such DIYs:

How to Charge Lithium Batteries

The guy in the video starts with one of the many garbage counterfeit LM2596 modules, so I haven't watched further.

Lithium battery chargers usually work on the CC-CV principle, which means "Constant Current then Constant Voltage" (pic source).

In order to do that you need a power supply with two accurate feedback paths: one for voltage and one for current, both being able to limit the output.

It can be implemented in many ways, for example AC-DC switching converter, DC-DC converter, linear, etc. The important thing is that it has an I-V output characteristic like this black curve: constant current over the whole charging voltage range, and it keeps working at reduced current once the limit voltage is reached.

Another important characteristic for a charger is that it should accept being connected to a battery, which means not drawing reverse current, for example.

If you pick a random voltage output DC-DC converter, most likely it will have features not suitable for charging, for example:

• Foldback current limiting (red curve above) or other means of current limiting like shutting down or going into intermittent mode (hiccup), which means it can't act as a constant current source at all.

• Not designed to be connected to a battery. For example, if output voltage is higher than input voltage, a DC-DC buck converter will let current flow in the reverse direction through the top MOSFET body diode and send power to its input. Whether that's a problem depends on the circuit...

• It expects to startup into a capacitive load at 0V, usually with slow-start, so it will ramp up its output voltage. If the DC-DC is a synchronous buck, and the load is a battery, it may attempt to do stupid things like think output voltage is too high during startup and try to bring it down, which would be rather suicidal.

• If it's a voltage output DC-DC then the current limit is usually not accurate: as long as it protects the chip, it does the job, so the manufacturer won't spend extra production cost to make it accurate. Sometimes you have no way to set it, it's internal to the chip and set to the absolute maximum current the chip will tolerate. Using it as a constant current source means the chip will operate at its maximum power during most of the charge, without any derating, so it may overheat or run at lower efficiency.

The question here is that, will the DC converter output constant 2 amps even when the battery pack reaches its fully charged voltage of 42 V?

That depends which DC-DC converter you've got, so it's impossible to answer in a generic way.

If your DCDC was designed with the intent to be used as a battery charger, with accurate voltage and current feedback paths, it'll probably work as a charger, LED driver, and generic DC-DC converter. Otherwise, maybe, maybe not. It most likely won't be optimal.

Regarding DC-DC converter chips, if the current limit feature is intended to make a constant current source, it is usually clearly specified in the datasheet along with the expected accuracy and how to set it.

For constant current chips, usually sold as "LED drivers", it's the opposite: you get accurate current regulation, but inaccurate voltage. The output overvoltage protection's job is to protect the chip in case the load is disconnected: in this case there is no need for accuracy on the output voltage.

So why don't all DC-DC chips have a CC-CV mode? First, it costs extra. Then it also has disadvantages: for example if the load is not a battery but some electronics, constant current limiting is not desirable. If a chip dies and draws too much current, foldback or hiccup current limit protects the power supply while not feeding much power into output. Constant current on the other hand would keep feeding the maximum current into the shorted chip, with no limit to thermal dissipation, resulting in fire hazard or non-repairable damage to the board.

So DC-DC chips with a usable CC-CV mode are a minority.

Also if you have several cells in series, a proper charger would monitor the voltage on each, perhaps do the balancing itself or let the BMS do it.

• Thanks for the detailed answer. I found some DC/DC modules that output constant voltage and current. Both voltage and currents can be regulated. In fact, some of those DC/DC modules have inverse current protection and include instructions on how to charge batteries. But still this leaves me with this question, do these modules designs reduce the current after reaching the desired voltage of the battery or continue with whatever constant current is set?
– GIZ
Dec 25, 2023 at 14:52
• How would they do constant voltage if they did not reduce current once the target voltage is reached? Dec 25, 2023 at 14:54
• Not sure if they do CC. I presume they do. I think such modules provide fixed current and voltage rates according to what they're set to output by a user. But it seems they come with the mechanism necessary to charge batteries. Unless I get one and do my tests, I wouldn't be able to know.
– GIZ
Dec 25, 2023 at 16:02
• Maybe there's documentation? Dec 25, 2023 at 16:11
• Unfortunately no. I'm getting one soon and I'll run my tests on it. I could just assume now that the DIYs guys that use DC/DC converters to charge their batteries have the ones that are designed to charge Lithium batteries or they'll screw up their batteries otherwise.
– GIZ
Dec 25, 2023 at 16:42

How do DC/DC converters work as lithium battery chargers?

In short; they don't.

A longer answer is that they can be used within an overall design that is suitable for charging a lithium battery but, on their own they are generally unsuitable without additional components.

Yes, DC/DC converters do provide constant voltage and constant current

Unfortunately you cannot rely on that. Many DC-to-DC converters provide an unregulated output voltage and no current limiting whatsoever. A lot provide a regulated output voltage but, any current limiting they produce will not likely be accurate or steady enough for lithium battery charging.