1
\$\begingroup\$

For some background, most if not all modern charging solutions for lithium ion batteries use a CCCV model for charging:

CCCV Charging

As the diagram explains, we charge at a constant current, and then "suddenly" switch into a constant voltage mode. This "sudden" transition seems unnatural, and feels like it's more likely to be related to simpler electrical design for chargers rather than "better" charging.

So my question is, are there other charging models? For example:

  • Could the charger stay at CC for longer and go above the target voltage, and jump into a CV mode at the target voltage, leading to a quicker charge?
  • I've heard it leads to a lower battery life if batteries are fast charged near 100% of their capacity, could the CC mode start ramping down earlier as the battery gets full?

EDIT: The existing answers have prompted me to do some more research and I've found:

  • My two suggestions for improvements were indeed "silly", but I had found other alternatives, such as CP-CV charging (constant power, constant voltage).
  • Temperature is a big killer - there are charging techniques that vary the current during the CC phase to keep within temperature bounds. This allows them to use a higher base current.
  • This is a fantastic article that also details the chemistry: https://www.sciencedirect.com/science/article/pii/S2590116819300116
\$\endgroup\$
  • 5
    \$\begingroup\$ Your second paragraph seems wrong : CCCV is constant CURRENT first. Can we do "better"? First, define better. What do you want to improve? \$\endgroup\$ – Brian Drummond Jul 9 at 11:02
  • \$\begingroup\$ Haven’t we had this exact question up before, say 6 months ago? \$\endgroup\$ – winny Jul 9 at 12:47
  • \$\begingroup\$ @winny can you link it? I did a search and couldn't find anything \$\endgroup\$ – Hannesh Jul 9 at 23:58
  • \$\begingroup\$ @BrianDrummond you're right, my bad. I've switched it around. Better is easy to define here, either faster charging while keeping cycle life constant, or a higher cycle life at the same charging rate. \$\endgroup\$ – Hannesh Jul 10 at 0:01
0
\$\begingroup\$

The modes you are describing are little bit opposite.

we charge at a constant voltage, and then "suddenly" switch into a constant current mode.

  • First Pre-charging happens(Constant voltage)
  • Majority of power is fed through constant Current
  • When battery voltage reaches specified voltage level, it shifts to Constant voltage charging method(Tickle-charge).

First Pre-charging happens(Constant voltage), then majority of power is fed through constant Current and then when battery voltage reaches specified voltage level, it shifts to Constant voltage charging method.

So answering your questions:

  • All this process is related chemical composition which after researches came up with this kind of charging for the maximum life and utilization of the battery. You can do as you said, but it may lead to lower life of battery.
  • CC at near 100% capacity: What happens is that at near 70-80%, internal resistance of the battery comes into the picture, since at high CC mode would mean drop in the resistance to be same, but the internal voltage of the battery is increasing causing the terminal voltage to increase and we don't want that to happen beyond certain limit. So your charger would automatically shift from Constant current to constant voltage mode.
| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Good answer! If we knew the internal resistance, does that mean we could go above 4.2V? As the internal voltage would still be below 4.2V as you mention in your comment. \$\endgroup\$ – Hannesh Jul 10 at 0:07
  • 1
    \$\begingroup\$ That actually does not remain constant. It depends on various factors. As battery ages, it might change. Many research have been done to identify the internal resistance so as to improve the profile. But i have not seen them being used until some years back. Example: Electric vehicles have been using Li-ion batteries as power source and they need a very good profile for their batteries to increase lifetime and fast charging(many reasearch have been done in that area). I think you can find something there. \$\endgroup\$ – DivB Jul 10 at 8:25
  • \$\begingroup\$ I have heard of constant temperature-constant voltage charging too. Charging batteries faster without damaging them. \$\endgroup\$ – DivB Jul 11 at 10:13
3
\$\begingroup\$

You should not exceed the maximum charge current of the cell, or the maximum output current of your charger, so you need a CC stage if you want the fastest charge. You can of course charge more slowly.

If you exceed the maximum voltage of 4.2 V on the cell, at best you dramatically shorten the battery life, at worst the battery catches fire. If you stop charging when you reach 4.2 V per cell, you haven't fully charged the battery by some margin, so you need a CV stage.

So if you want something less than a fast charge, to something less than full charge, you don't need to do CC followed by CV. If you want a fast charge to a full charge, without hammering your battery life, then CC followed by CV is the way to do it.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ When you say "should not exceed maximum charge current of the cell" and "should not exceed maximum voltage" - you're implying that these specifications of a battery, graphed in 2 dimensions, would form a perfect square, bounded by max voltage and max current. But I don't believe that batteries are this perfect. Perhaps a charger could charge at a higher current while at 3.7V, and reduce the current at 4.1V, which might lead to a longer life? \$\endgroup\$ – Hannesh Jul 10 at 0:05
  • \$\begingroup\$ @Hannesh No, you caught me on a bad day. I've thought carefully about what I think are your misconceptions, and will update my answer, watch this space. No, better still, I'm going to add a completely new answer. \$\endgroup\$ – Neil_UK Jul 10 at 7:43
3
\$\begingroup\$

We must understand that the context of the question is a commercially successful energy storage system., and 'commercial' is perhaps the most important word here.

The manufacturer has to anticipate the users' needs, to product a product that is usable, cost effective etc etc. Different users have different needs.

Let's take a very extreme user, NASA. It's not practical to replace batteries in service, and satellites are required to have decade lifetimes. This means lifetime is more important than anything else. Satellite LiPos are therefore charged to around 3.92 V, and as a result, exhibit decade lifetimes, even under many charge/discharge cycles per day.

Charging to only 3.92 V gives very low energy density. If a battery had a multi-decade lifetime, it would not be suitable for hand-held tools, phones, laptops where the product lifetime is sub-decade, and there is a premium placed on run time, and purchase price.

Consumer batteries therefore are rated to have a capacity as high as possible, while lasting for a reasonable time. Any student of specifications will recognise that neither 'as high as possible' nor 'reasonable' are specifications. However, locating a 'knee' in a cost/benefit graph, and operating just on the right side of it generally gives a reasonable tradeoff.

In the case of LiPos, there is a fairly hard lifetime/capacity knee at around 4.2 V. Chemists understand the degradation mechanisms of the cell system, and it is voltage sensitive. Any time spent above 4.2 V is very detrimental to the lifetime of the cell. Personally, I lie to my charger that I have LiLo batteries, so it only charges to 4.1 V. Tweaks to the chemistry means that some premium cells these days are rated to 4.3 V.

An important part of the 'commercial' aspect of the system is ease of use. '4.2 V constant voltage' is easy to communicate, to understand, and to implement. Give users too many choices and it will complicate the uptake of the product.

With current, it's more complicated. Again there will a graph of lifetime versus charging current for any particular cell construction, but the knee is not so sharp, and the rated charging current varies by more than an order of magnitude across various cell constructions. Temperature is also a confounding factor, one car manufacturer controls the temperature of their cells when fast charging, and the concensus seems to be that they get much better lifetime than most other car manufacturers who don't.

There is certainly a market where 'fast charge' can be sold at a premium. Whether they achieve this performance by using a more expensive cell construction, or a lower capacity cell, or a more limited cycle lifetime, is up to the manufacturer. However, I don't see any open discussion of the tradeoffs, at least in the commercial market, other than a fairly crude division into 'energy' cells and 'power' cells. I'm sure that sophisticated buyers, like militaries and space companies, would be able to have proper discussions with the manufacturers.

The choice of charging current limit is therefore hypothecated on lifetime. 'If you want a 'standard' lifetime, for this particular cell construction, then you charge at up to xC'.

It may be the case that tapering the charge current down as the voltage increases results in a faster charge while yielding the same cycle lifetime. I don't know if manufacturers have done that experiment. It's certainly open to do the investigation. However, the communication of any successful results would complicate the supply of chargers to the commercial market. It's far clearer to say 'maximum charge rate is 1C', or whatever it is for that particular cell construction, especially as the benefits would be likely to be a small fraction of the differences you get switching between 'energy' and 'power' cells.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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