From time to time I see folks complaining/questioning there battery resting voltage after charging there 18650 with a hobby charger .
A lot of people simply cant understand why there battery wont hold 4.2v after charging .
So I will attempt to enlighten :
Have you noticed voltage sag when turning on a device and monitoring voltage ? Lets talk AA for a moment , and say you have a 1.75v Lithium cell , in your flashlight @ 2A current draw that battery can sag , bellow 1.4v under enough load ..
Now the same goes for Li-ion under load , a good quality cell , @ lets call it 1A discharge , may sag to 4v or even lower , cheap nasty cells I have seen sag all the way down to 3.5v under 1A load .
The same thing happens when charging , but in reverse , rather than dropping voltage , it is increased under the load of having current pushed into it .
Remember , batteries are designed so that energy flows out , not in , so energy needs to be forced in , this creates a resistance load from the battery that needs to be overcome by the charger , usually in the form of increased voltage [ with me so far ? ]
Now a lot of folks using hobby chargers run higher charge currents than say the usual Chinese charger [ lets call it the much loved WF-139 ] .
So what folks need to understand is that it takes time for a battery to normalize after and during charging , and that Hobby chargers @ higher current dont give the battery the time or opportunity to normalize during the charge cycle .[ So many charge 0.8A to 1A , so lets call this the Charge rate for the Hobby Charger ]
So once completed , it may take an hour for the battery to normalize its resting voltage , which due to the Hobby charger behavior , may be quite a bit less than many expect .
What is the issue ? The hobby charger maintains its charge rate longer , there by charging faster , and also not allowing the battery to normalize during charging .
This is where the Cheap Chinese chargers excel :
All the chargers I have , tapper the charge current , so even if the charger were to start @ 1A charge rate , it begins to tapper the charge rate giving the battery time to normalize under charge , and its charging at a higher rate at a time when the battery is more accepting [ empty ] of higher charge rates ..
The Cheap charger may have tapered down to 300mA by the time you get to 4v on the battery voltage , 200ma by 4.1v and by the time you approach 4.2v it may be doing 60mA and have a terminating charge rate of 40mA [ most chargers are less than this - but Im going with highest one I have as an example = charge cycle current ]
Anyhow , what this means is , the Cheap charger [ Compared to hobby chargers ] does a really good job of charging the battery , and in the real world [ rather than some ones imagination ] , may actually stress the battery less , than the hobby charger .
And lets not forget , if you have good quality batteries , they wont mind being charged to 4.2v . Whether you charge to 4.15v or 4.2v , the better the battery , the less it will matter , as the batteries are under the greatest load / stress near completion .. And if you complete the charge gently [ Cheap Charger ] , then how much difference will 0.05v make ?
But with the Hobby charger , it will stress the battery more , how do we know , because it sags !
If you charged your 18650 to 4.2v with the hobby charger and it sags to 4.15v , then it had to be under enough load/stress to sag back to a unstressed state , and the Cheap charger terminates at 4.2v and the battery holds 4.2v , does that tell you that the battery was under far less stress ?
You dont need to be a rocket scientist , to understand that , these cheap chargers are maybe not so bad as some would have you believe ...
Now if you have a $200 Hobby charger that can be fully programmed for the charge rate , that's fine , in fact its fantastic , but when a $8 Chinese Charger from HK can do the same thing , and save you $192 , then I cant knock or put down the cheaper product that does the job .
Life is , to each there own , but the king is , knowledge ! [ I dont know everything , but hopefully I have shared some of the little I know ]
This is a very light weight explanation ..
And hopefully , explains that , Hobby Chargers are maybe not the best option for the 18650 , possibly if you insist on using one , you may reconsider the reasons for such , and maybe its time to lower the charge rate if you suffer from a lot of voltage sag on completion , and maybe you stressing the battery more than you thought [ lots of sag ]
The battery is under the greatest stress near or at completion , and this is where you need to be gentle
And empty battery is more accepting of higher charge rates , than one that is not empty
Hobby chargers are wonderful things , but like any tool , when misused or misunderstood
Are cheap chargers so bad [ if you have a good one ] as some would have you believe , or is it simply a - Mine cost more than yours thing ? [ so it must be better ]
Price makes right !