0
\$\begingroup\$

I need some help in clarifying my understanding on charging Lithium Polymer batteries.

Context:

I am trying to build DIY vacuum cleaner bot. For that I need a LiPo batteries which bot could charge itself automatically by locating the charging station.

My Understanding about charging Lithium Cells:

  1. BMS can protect the battery cells and balance them.
  2. BMS has charger circuitry to charge the cells as well.
  3. If I apply the DC power input to BMS input it can distribute among cells and charge them appropriately
  4. I only need some DC supply with BMS to charge batteries.

Are these true?

When I searched on the web I have seen other chargers which charges battery in CCCV mode, they are lot complex and bulky.

If I apply input to BMS using DC DC buck/boost converter then can it charge the batteries using BMS ?

I am beginner in battery charging/discharging subject please help me with some pointers in correct direction.

Thanks in advance.

Edit: Add Part Numbers

  1. BMS : LiFePO4 Battery Management Systems (4S 10A)

2.(DC-DC-Converter)[https://robu.in/product/10a-dc-dc-step-down-adjustable-constant-voltage-module] : It says can be used for battery charge.

  1. High-power LED driver.

  2. Lithium battery(or lead accumulator) charge.

  3. Vehicle-mounted power supply.

  4. Low voltage system power supply.

  5. 6V, 12V, 14V, 24V battery charge.

  6. On-board laptop power supply.

  7. regulated power supply. 8.Low voltage power supply system

  8. Battery: 3.2V 32650 / 32700 6000mAh LiFePO4 battery Nominal Voltage: 3.2V Norminal Capacity(0.2C): 6000mAh Charging Cut-off voltage: 3.7V Discharging Cut-off voltage: 2.5V Max.Charging Current(Continue):1C Max.Discharging Current(Instantaneous):3C Cell's Diameter: 32mm Cell's Height: 70mm Cell's Weight: 147g Charging Methods Standard: 0.2C*6.5Hrs Operating Temp Charge: 0ºC~45ºC Discharge: 0ºC~45ºC Storage: -20ºC~60ºC

Effectively my question is: If charger does the CC-CV steps to charge battery with BMS then can I use the DC-DC Converter (CC-CV) to charge battery pack with BMS? I am going to make 4S2P (2 cell connected in parallel, each such cells are connected in series 4 times)

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Can you give part number of BMS you're considering? As I know most BMS can't do CCCV, they only monitor and protect. Also please share the battery configuration in terms of how many in parallel and series. \$\endgroup\$
    – EarthLord
    Commented Sep 26, 2021 at 5:44
  • \$\begingroup\$ Added the part numbers. Question is can DC-DC converter act as charger with protection from BMS ? \$\endgroup\$
    – zero
    Commented Sep 26, 2021 at 10:44

3 Answers 3

0
\$\begingroup\$

Between the answers and comments, you should have everything you need but let me try and be more direct on the issue with what you are proposing. And I understand why you are pursuing this, simply by reading the high level verbal specifications, it seems as if you can hookup a random DC supply, a BMS and a battery and magically all will work. But it is more nuanced than that.

So your assumptions are:

  1. BMS can protect the battery cells and balance them.

Yes, BUT most BMSs protect the cells at a fixed voltage, which may be too high. As far as balancing, most BMSs balance at very low current, unlike a dedicated balance charger that actively balances the cells during charging.

  1. BMS has charger circuitry to charge the cells as well.

There is nothing "magic" about charging a cell, just apply a higher voltage than the cell to "push" current into it. BUT, depending on the supply, the current may be higher than the cell can take, and then you will run into serious trouble. The BMS has nothing special in it to control current, only cut it if excessive.

  1. If I apply the DC power input to BMS input it can distribute among cells and charge them appropriately

Not quite - the BMS simply lets the current through as long as individual cell voltages are under the max limit and charge current is under the limit. BUT, the limit control is not very tight; a BMS with a 20A charge limit may cut at 35A or higher, it does not have the fine control necessary. Also, it does not distribute the current other than the balancing, which is very weak to be effective during charging.

  1. I only need some DC supply with BMS to charge batteries.

It could work but not reliably and as you know (or should know), lack of reliability when charging LiPo batteries is rather dangerous.

In summary, you are taking a bunch of components and depend on their "emergency" features for normal operation. For example:

  • You depend on the power supply capacity to limit current when charging, but unless the supply was meant to be run at maximum, this is dangerous. If the supply can provide more current than the battery can take, this is dangerous. If the supply burns (they do) and it shorts, the battery will now be shorted, this is very dangerous. You can see where this is going.
  • You depend on the BMS to stop charging the batteries. But the BMS will stop charging if the total battery voltage is over a limit OR if any of the cells goes over the limit. Also a proper charger will drop the current as you get closer to the full voltage, the BMS does not do that. If the BMS stops charging because one cell is full and the rest aren't, your battery is not charged and the BMS can only slowly balance the cells.

So you can see that even though in very narrow cases, you could make this work, there are just too many things that can go wrong with your approach.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for the answer. I understand it now. \$\endgroup\$
    – zero
    Commented Sep 26, 2021 at 12:54
2
\$\begingroup\$

No, a BMS is not a charger.

A BMS will not implement charging.

Connecting a battery with or without a BMS directly to a DC voltage is wrong.

Lithium cells need to be safely charged and that happens with a charger that implements CV-CC charging and will stop charging when batteries are determined to be fully charged.

\$\endgroup\$
3
  • \$\begingroup\$ If DC-DC converter has CC-CV mode then can it charge battery? Like any power supply with CC-CV mode can it charge the battery? BMS will take care of cutoff once battery is charged. I edited question with parts details. \$\endgroup\$
    – zero
    Commented Sep 26, 2021 at 10:46
  • \$\begingroup\$ Main purpose of BMS is to provide protection from error situations, such as overcharge and overdischarge protection, overcurrent protection, and also manage charge balancing. The job of charger is to charge a battery safely and terminate charge when done, not the job of a BMS which will do a cut-off to prevent overcharge. A proper charger has end of charge detection, a CC-CV power supply does not. Please note that incorrectly charging lithium batteries is not safe, as they can explode or start a fire, if you use wrong charger for your batteries. Charging lithium batteries is complex. \$\endgroup\$
    – Justme
    Commented Sep 26, 2021 at 11:28
  • \$\begingroup\$ End of the charge can be detected by BMS and then cut off the input supply. Does this work? I am confused only because I have seen youtube videos with DC-DC converter and BMS to charge battery. They claim BMS takes care of balancing and cutoff, DC-DC converter just provides supply with CC and CV. One more question is it possible to charge whole pack at once or it should be each cell charging using proper charger which connects to each cell parallel to apply charge \$\endgroup\$
    – zero
    Commented Sep 26, 2021 at 11:47
0
\$\begingroup\$

There are many different BMS that all have different sets of features. Some BMS only provide information on state of charge. Other BMS might also do passive or active charge balancing of cells connected in series and also monitor state of health and temperature.

The charging process is not necessarily the main task of the BMS. Batteries could be charged with CCCV in conjunction with a BMS that takes care of battery health, balancing and so on. But depending on the requirements there might be a single IC solution for charging and battery monitoring. So think about your specific requirements and take a closer look at some BMS datasheets to find a possible solution.

\$\endgroup\$
1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.