Questions on current settings

T

tankahn

Newly Enlightened
Joined
Oct 14, 2003
Messages
89
Hi, I am not able to find what looks like a simple question so I hope you can help if I am mistaken.

I read that its safe to charge li-ons at 0.7C. If I have a 18650 cell with 2000 mAh capacity, then I can charge at 1.4A. Am I right with this assumption? Also if it is an IMR battery can I charge it at 1C even?

I connected it to a hobby charger, put the settings at 1.4A but the charge would not go above 1A or sometimes lower. At this rate will it take a lot longer? I read from a battery tutorial site that charging at higher current even if safe does not mean a faster charge as topping charge will take longer.

The inability of the charger to charge at higher currents could be due to internal resistance of the battery. I could be charging a half filled battery with higher internal resistance than when it is empty. Is this the case here?

If so, would it be safe to charge at 1c with hobby chargers (I got both a Triton and a icharger) since it always adjust accordingly.

If I am using a improvised cradle and the contacts is oxidized. Will that create resistance and would affect charging time or terminate the charger prematurely? My icharger sometimes terminates prematurely at 3.9V termination voltage and I suspect it is due to oxidised contacts. Should I do a resistance test as when I fashion new contacts (eg with magnets).

Finally, a related question, the icharger is able to measure internal resistance of a cell. Is there a way to use this information to identify cells that is no longer viable. I have a 3 year old R123 cell that measure 4.12V after charging and does not fall a few days later (meaning its still good under conventional wisdom). But I know it is not since it would quit on high draw but it run fine with lower current Is there a range of resistance values that indicates a failing cell?

Many thanks for your replies.
James
 
P

pae77

Enlightened
Joined
Dec 18, 2005
Messages
494
Location
Honolulu, HI
It's possible that your hobby charger would not charge at the highest current you set because the voltage of the battery was already high enough that the charger went right into the CV phase of the charge cycle (skipping the CC phase). In the CV phase, the charger holds 4.20 volts and applies a gradually decreasing current until the current reaches 1/20th of the original current (or something along those lines). So if you start charging a battery that is only partially discharged, there is a good chance the hobby charger will go right into the CV part of the charge cycle and start charging at a current that is considerably lower than the current you set.

If you want to see the charger charge at the current you set (or something closer to it), try charging a cell that has been discharged to a fairly low voltage, like 3.6 or 3.7.
 

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