I happen to own a power drill/driver that runs on a Li-Ion battery and ships with a charger that charges it fully in 35 minutes and claims to charge it to 70% in 15 minutes.
According to answers to this question the highest charging current for Li-Ion batteries is about 1C which with losses taken into account means that charging time should be at least more that one hour. This is consistent with my experience of using other devices like cell phones - they take about 1.5 hour to fully charge.
How is charging a Li-Ion battery in about 35 minutes possible then?
I wrote the long answer to the prior question.
Your drill battery and charger quite possibly combine several of the aspects that I described there that could enable fast or apparently fast charge.
Firstly I said:
- LiIon batteries can be safely (enough) charged at the rate advised by their manufacturers. Faster may be possible and may be "safe" but all guarantees are off and shorter life or instantaneously very short life are definite options.
- Standard spec is 1C max charge.
That is, the industry practice is to charge at 1C max BUT individual manufacturers are free to push the limits. Issues are thermal, mechanical and chemical (at least). As I said - lower battery life may result.
I also said
- There are new lithium based chemistries and new mechanical arrangements which allow lithium based cells to be charged at faster rates. If the manufacturer says it is so it indeed may be. I've seen apparently standard LiIon cells with 2C charge ratings but the norm is 1C max. (see above)
Which is just what you are reporting - its entirely consistent with the prior answer - just not industry standard and it suggests that you may get short cycle life or lower than expected capacity.
A major reason may be that the manufacturer is in fact extending cell life by rating the cell at a lower capacity than standard and not charging it fully. If they rate it at about 60% of actual then:
Say full capacity is 1 Ah to simplify calculations. Any capacity produces the same results.
60% capacity = 0.6 Ah.
Charge at constant 1C = 1A.
Time to reach 0.6C at 1C rate = 0.6 hours = 40 minutes (Claimed 35)
Time to reach 70% = 0.7 x 0.6 x 60 minutes = 25 minutes (claimed 15)
So lets get daring and charge at 1.6C for 1st 15 minutes when capacity is low. At this level the delta voltage between Vin and Vcell is smaller and heat losses are lower. If we manage 70% capacity in 15 minutes we need to add 30% in (35-20) = 15 minutes. 15m is 15/35 = 43% of the total 35 minutes charge time but we need to add only 30% of the charge so a rate below 1C is acceptable for his last part.
In practice some mix of the above is probably used.Say
Derate battery to say 75% to 80% of full possible capacity.
Charge at > 1C for first70% of charge- tapering current under charger and not battery control so it drops to under 1C at say 70% of battery capacity. Battery is thus charged hard when at low capacity and at a decreasing rate with charge level and is never filled. End result may well be an extended cycle life.
Or they may do something quite different :-).