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Most decent battery management systems out there employ balancing, overdischarge protection, overcharge protection, etc.

Almost universally they state that the charging voltage is just 4.2*N_cells. This is fine, but it is clearly a lower limit voltage for charging to happen. How high can I provide?

Do I need to build a buck converter to provide exactly that voltage, or can I just rectify 220V mains and push that in, and the BMS will limit current accordingly?

How about the case of regenerative braking in EV systems? If I rectify the output from motors, that voltage will surely go over the 4.2*N_cells figure. Is that ok?

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    \$\begingroup\$ You basically want to use your BMS as a battery charger. Don't. Regenerative EV systems don't just blindly rectify and apply the regen voltage to the motor. \$\endgroup\$
    – DKNguyen
    Oct 19, 2021 at 12:27
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    \$\begingroup\$ Relying on the BMS for this is like relying on the airbags when you park your car. \$\endgroup\$ Oct 19, 2021 at 22:28

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No.

4.2V for a lithium cell is not a lower limit, it is the upper limit. For increased cell life, a lot of products will only charge to 4-4.1V and not discharge fully either.

A battery charger does not just apply voltage to a Lithium battery. There has to be some form of current limiting implemented. Different batteries will have a maximum charge rate listed, as well as a pulse charge rate. This is usually around 1-2C depending on the chemistry. If you use a proper CC-CV charger, the voltage will be determined by the output current, the pack voltage, and the internal resistances of the pack.

If you rectify 220V and put that into a battery, the battery will happily accept it, until of course your breaker blows or the 100s of amps the battery would accept bursts it into sweet Lithium smoke.

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  • \$\begingroup\$ You misunderstood my question completely. It is plainly obvious to me that a lithium battery cannot be charged above 4.2V. I assume everyone knows this. What I mean is that if I apply (for example) 3.8*N volts to the BMS, it will not charge them to 4.2V per cell. If I apply the specified 4.2*N volts exactly, it will. Question is: As power sources giving some specific voltage number like 4.2*10 volts are rare, how much above this figure can I give to the BMS, and trust that it will limit the charging to 4.2V per cell and limit the voltage accordingly? \$\endgroup\$
    – Elmore
    Oct 19, 2021 at 12:24
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    \$\begingroup\$ BMS =/= Charger. OV protections in a BMS will usually disconnect the cells from the input, not change the voltage for you. There are plenty of DC-DC and AC-DC converters available for 4.2*N volts (I make them lol). There are also dozens of ICs that make it simple (relatively) to build one yourself if COTS products don't suit you. You'd be much better off just charging the cells to 3.8V a piece. You lose less than 10% capacity. \$\endgroup\$
    – Stiddily
    Oct 19, 2021 at 12:39
  • \$\begingroup\$ Answer to question is : None. Sources supplying the correct voltage are readily available, labelled : Li-Ion Battery chargers. \$\endgroup\$ Oct 19, 2021 at 13:28
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No. A BMS is not a charger. If the pack has 2.7V per cell and you connect 4.2V per cell to BMS, only the series resistance of batteries and BMS are limiting the current and most likely current exceeds safe battery charging current and hopefully the BMS overcurrent protection shuts down the charging.

And it is not the job of BMS to stop the charging at 4.2V, it is the job of charger. The BMS is there as a safety mechanism if charger breaks or pack is connected to something else which is not a charger.

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No it's never OK charge lithium batteries over 4.2 volts as it has already been mentioned. At first glance it looks like you will need to do the following:

  1. Read more about the BMS you want to use for your application to work out how and what it can actually do.
  2. Work out how you want to step-down to a more manageable DC voltage from 220V.
  3. Use a DC/DC regulated, dedicated lithium battery charger IC that will provide the correct constant current (CC) and constant voltage (CV) charging curve suitable for lithum battery technology. This will include safety measures to ensure 4.2V charging voltage is not exceeded, and will detect the end-of-charge current to cut-out the charging process etc.
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  • \$\begingroup\$ You too misunderstood my question completely. It is plainly obvious to me that a lithium battery cannot be charged above 4.2V. I assume everyone knows this. What I mean is that if I apply (for example) 3.8*N volts to the BMS, it will not charge them to 4.2V per cell. If I apply the specified 4.2*N volts exactly, it will. Question is: As power sources giving some specific voltage number like 4.2*10 volts are rare, how much above this figure can I give to the BMS, and trust that it will limit the charging to 4.2V per cell and limit the voltage accordingly? Most BMS do not mention this \$\endgroup\$
    – Elmore
    Oct 19, 2021 at 12:25
  • \$\begingroup\$ I think your definition of a "BMS" is a bit too broad and general to make sense with regards to your specific question about charge voltage. It does depend on the scale of the energy storage, the watage etc. but for small scale 1 - 10 cell based battery packs, the protection, charge and management will be done by different "sub-circuits". The PCM (Protection Circuit Module) for instance will only handle over-charge, over-discharge, over-current etc protection, and not do anything about correct charging of Lithium. \$\endgroup\$
    – citizen
    Oct 21, 2021 at 9:27
  • \$\begingroup\$ The dedicated charging circuit (linear or switched) will handle mostly the charging method, like Li-pol, Ni-cad etc and associated timings, threshold detection etc. \$\endgroup\$
    – citizen
    Oct 21, 2021 at 9:28
  • \$\begingroup\$ Then there could be more sophisticated battery management (BMS) circuitry included (completely separate, depending on the requirement and scale of energy storage etc) that will monitor via fuel gauges and/or thermistors, the actual state of charge/discharge of the battery technology so that a user can know at all times the state of charge of the battery storage system. \$\endgroup\$
    – citizen
    Oct 21, 2021 at 9:31
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    \$\begingroup\$ So you can see, even though a "BMS" may monitor battery voltage, battery temperature, current flow in/out of the battery and other "stuff" amongst a few parameters, it will mostly have nothing to do with ensuring the correct charging method is used for the specific battery technology (li-pol, Ni-cad, Nimh, Pb etc). Which is why the question about charging 4.2N or 3.8N cannot be solely taken in the context of a "black box" BMS that does everything already, and all you have to do is add a rectifier and/or buck and the BMS will just limit and to "out of the box" everything to ensure safety etc. \$\endgroup\$
    – citizen
    Oct 21, 2021 at 9:39
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Oh man, the answers here are painful. This might be a good answer for you.

For example, quick charging pumps 18v into a li-ion battery to boost the wattage input but keep the amperage at max 1-2A. THe higher the amps the more it will heat up and it will cause thermal runway. Yes you can pump 220v into a battery, it will only take as much as it can, but it will explode and have thermal runway very shortly after you connect it, because outlets have the potential to deliver 2000W, however, they can handle up to 30w if done properly. Voltage is only taken in, as much as the battery needs, so if you charge a battery with 30v and 0.1 amp, youll be supplying it with 3W and you must limit the voltage to stop delivering at 4.2V to not overcharge the battery. This is already solved by modern chargers and BMS. Buck convertor is a must for 220v, most efficient if you limit it to 4.15, so you dont have to worry disconnecting it at any point.

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    \$\begingroup\$ The answers are painful (or rather, realistic) because the question is painful as well. While you could increase the charging voltage (especially at higher SoC's) to lower charging time and keep it safe with elaborate charger designs, it seems that the OP is not referring to something like this. Instead, the OP seems to want to apply an obscenely high voltage to the pack terminals, without regulation or limiting, with only the BMS to save him from fire. At those voltages fets might just die when they open, leaving him with nothing. Let's prevent anyone from ever using batteries this way. \$\endgroup\$
    – Thijs
    Feb 21 at 14:32
  • \$\begingroup\$ I distictly do not want to do that. Most SE users lack any reading comprehension it seems. My question is solely about the design of the BMS system. I know perfectly well how a li-ion cell works. BMS system might let all supplied voltage in. It might also introduce a buck converter that adjusts the voltage for you. \$\endgroup\$
    – Elmore
    Feb 22 at 20:45

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