calculating how many hours it SHOULD take for a battery to finish charging?

So I'm the proud new owner of two new MNKE Li-ion IMR 3700 mAh cells (26650) and I am charging them from at about 500mA on a USB charger and about 600 mA on another charger. I started charging them at about noon an it's about four hours later now and they're still charging.

I know nothing's wrong with the cells, but I'm really just curious as to how I can calculate how long a full charging should take. is it simply:

is that how you calculate that stuff? that's really all I'm asking, but I'm no Electrical Engineer so I wanted to ask haha.

thanks!

how are u charging with usb ?
u have shown theoratical values.. real values will differ significantly during the initial charge phase and the ending charge phase..

also if u calculate the time to be 7.4 hours give it some 20% more time as charger inefficency..
so 7.4+.2*7.4=X is the time it will take for full charge from full empty..

also have a good voltmeter at hand. without it u may not be able to ensure either saftey ...

buy a cheap dt830d multimeter and use it.. ur liion battery is charged when its terminal voltage under no load is 4.2 volts.. and this voltage is maintaned for almost half an hour..
see data sheets of various liion cell from sony.etc..

tobrien said:

Code:

3700 milliamp hours
------------------
500 mA

= 7.4 hours

Click to expand...

This estimate will always be to low, the actual charge time will be longer due to the CV phase, probably about 0.5 hour.
If the charger is less than perfect CC/CV voltage, the time will also be longer (Some chargers reduces the charge current, before the CV phase is reached).

USSR said:

how are u charging with usb ?
u have shown theoratical values.. real values will differ significantly during the initial charge phase and the ending charge phase..

also if u calculate the time to be 7.4 hours give it some 20% more time as charger inefficency..
so 7.4+.2*7.4=X is the time it will take for full charge from full empty..

also have a good voltmeter at hand. without it u may not be able to ensure either saftey ...

buy a cheap dt830d multimeter and use it.. ur liion battery is charged when its terminal voltage under no load is 4.2 volts.. and this voltage is maintaned for almost half an hour..
see data sheets of various liion cell from sony.etc..

Click to expand...

"Wow!" I'm glad I found this thread. With the purchase of three (3) torches, two (2) independent double-battery sets (18650s)... one of which included a charger and the harvesting of eleven (11) 18650s from laptop batteries, I went to work charging all the cells. Here's what I found...

• the first two (2) cells - that came with the charger - lit up the RED light on the charger and... within about 2.5 hours, the RED led changed to GREEN. Therefore, I pulled those two (2) cells and put them into the 'charged' box
• with each of the subsequent pairs of batteries I charged, the led started of RED but never changed to GREEN - even after 3 hours. This includes one set of NEW cells as well as the harvested cells

All batteries have now been charged - at least this is what I may have been fooled into thinking by a 3 hour charge. So, to test the usability of each battery, I cycled each one through a torch. I was impressed at how bright the resulting beams were - in my backyard, at night. But the question remained... "Did each of the 'other' cells require MORE THAN 3 hours as to why the RED led didn't change to GREEN on those cells?" I also thought that the charger 'broke' after charging the first two cells.

I ordered an Equus 3320 Auto-ranging DMM from Amazon which should be here in a couple days.

Thanks to the OP and USSR's response, I've now been pointed in the right direction and an answer to my question(s) above.


Take care

QuePsi83 said:

"Wow!" I'm glad I found this thread. With the purchase of three (3) torches, two (2) independent double-battery sets (18650s)... one of which included a charger and the harvesting of eleven (11) 18650s from laptop batteries, I went to work charging all the cells. Here's what I found...

• the first two (2) cells - that came with the charger - lit up the RED light on the charger and... within about 2.5 hours, the RED led changed to GREEN. Therefore, I pulled those two (2) cells and put them into the 'charged' box
• with each of the subsequent pairs of batteries I charged, the led started of RED but never changed to GREEN - even after 3 hours. This includes one set of NEW cells as well as the harvested cells

All batteries have now been charged - at least this is what I may have been fooled into thinking by a 3 hour charge. So, to test the usability of each battery, I cycled each one through a torch. I was impressed at how bright the resulting beams were - in my backyard, at night. But the question remained... "Did each of the 'other' cells require MORE THAN 3 hours as to why the RED led didn't change to GREEN on those cells?" I also thought that the charger 'broke' after charging the first two cells.

I ordered an Equus 3320 Auto-ranging DMM from Amazon which should be here in a couple days.

Thanks to the OP and USSR's response, I've now been pointed in the right direction and an answer to my question(s) above.


Take care

Click to expand...

You really need to check the voltage of each Li-Ion cell ... Fully charged is about 4.20 volts ...3.6 volts is empty , 3.7V is about 20% full , 3.85V is about 50% full , 4.00V is about 80% full ... A table showing the voltages is somewhere on CPF.

When I say "Full" , we don't know how many ampere-hours capacity it is unless we do a discharge test ... Good makes of battery will be pretty close to their advertised capacity though.

So , it is essential to have an accurate Voltmeter when assessing the state of charge of Li-Ion cells.

The life-expectancy of a Li-Ion cell is not only dependant on how many charge/discharge cycles it has seen , but it is also age-related ... An old unused Li-Ion might very well be useless unless it has been stored at the correct state of charge at low temperature.

Good luck.
.

HKJ said:

This estimate will always be to low, the actual charge time will be longer due to the CV phase, probably about 0.5 hour.

Click to expand...

Just to add a bit to what HKJ said here. At higher initial charge rates (CC) the CV stage will be proportionately longer. At lower initial charge rates, the CV stage will be proportionately shorter. Due to this, charging at a higher initial rate will always be faster, but not as much as you would think.

The above only applies to chargers that actually use a CC/CV algorithm. Many/most "consumer type" chargers do not. Some are basically CC only, and some are simply CV. Both usually monitor voltage so as to roughly determine when the cell is charged. This is not necessary when a CC/CV algorithm is used.

tobrien's formula in the first post will work, for the most part, with CC only chargers. The CV only type take considerably longer, regardless of the initial charging rate, as the charging current begins to decline in value, as soon as charging commences.

Also, the CV stage is proportionately longer with LiCo cells. LiMn, and especially LiFe cells, have a proportionately shorter CV stage. I believe this is, in part, due to these chemistry cells having a lower IR.

Dave

45/70 said:

Just to add a bit to what HKJ said here. At higher initial charge rates (CC) the CV stage will be proportionately longer. At lower initial charge rates, the CV stage will be proportionately shorter. Due to this, charging at a higher initial rate will always be faster, but not as much as you would think.

Click to expand...

To see an example of this check my SP2 review, this charger can charge with 0.5A, 1A and 2A and I show curves for all 3 current where it is easy to see the different length of the CV stage.


Generally I have charge curves in both battery reviews and charger reviews. The battery reviews are done with a nearly ideal CC/CV voltage charger at 1A, making it possible to see the difference in CV time. In the battery reviews I uses a couple of different batteries, i.e. the time is not directly comparable.