18650 pack charging problem - help please

[Momato]

Hi All

I have built a 4s2p 14.8V li-ion pack with 8 2600 mAh Tenergy cells. I have used this protection module. I have now balanced the pack with my Hyperion balancer, but when I try to charge at 5C (for a 5000 mAh pack), the charger (Hyperion EOS) stops charging immediately (as it normally does when cells are fully charged). However, the cells are only at 3.8V. I can only charge if I go down to 1C (i.e. for a 1000 mAh pack). Also, if I try to discharge, I can only discharge at 0.1A - not higher. Why does this happen? Is it the PCB that shuts down for some reason? The PCB was fine, when I used only 4 cells (I just upgraded to double capacity). Batteries are new.

Inputs are greatly appreciated.

Best regards, Morten

 

[rantanplan]

I don´t have experience with that specific PCB, but it shouldn´t make a difference for the protection PCB if the capacity has doubled. It has worked fine and within its specs so far, so the reason has to lie somewhere in the "upgrading process". My soldering skills are limited, so if that was my pack , I would check each connection, maybe some soldering went bad ...

Have you checked each cell before building the pack? Maybe you should check cell voltages in the pack during charge/discharge ... if they they are all look fine and identical: Next one with ideas please

 

[Momato]

hmm, will resolder. But I cannot figure out if a bad connection could give this error? Maybe some increased resistance due to a bad solder could make the charger think some limit has been reached? That could make some sense I guess.

I hate resoldering.... Specially when everything is taped up :sick2:

Anyone else has ideas?

 

[mdocod]

You aren't soldering on those cells directly are you?

Are the cells you are using already protected cells, or are they "bare" cells?

Soldering directly to these cells could have caused an internal short, and the charger is seeing the low resistance when cranked up and terminating the charge for safety reasons. Or if you are using protected cells, you may have damaged a PCB in the soldering process, which could cause bizarre behavior.

Either way, if you are using a soldering iron on lithium cobalt cells, you are probably causing damage to the cells.

 

[Momato]

Nope, these are unprotected cells with solder tabs. I measured the resistance from each balance harness wire to each battery and the + side was up to about 250 kOhm. I isolated it to the PCB, and one of the IC's looks semi-cooked. Dunno if I can get another easily or just get a new PCB (expensive shipping from US on a 10$ PCB :mecry

 

[Wattnot]

You're trying to charge at 5C? You must mean 5 amps, I hope. Even so that's still WAY too much. The guideline for lithium is a 1C charge rate. Some of the newer chargers and lithium POLYMER batteries can take a higher rate. Lithium ION's, what you're using, have a higher internal resistance. The charger will respect that and back down.

Nothing is wrong here. You have a fantastic charger (I'm assuming your Hyperion is your charger too) and I'm sure it's working fine. When I use my Triton and try to charge even unprotected 18650's at 1 amp (well below 1C) the charger never gets above 600ma the entire time.

 

[Momato]

Sorry, forget about my "C" rating. Whenever I tried to charge/discharge with the correct current it stopped - only worked on the lowest setting (takes forever). The PCB in the pack (one of the FET's) is visibly toasted so the protection turns the charging off when there is more than about 0.1A current running.

The charger works fine with my other (same design) pack. A new PCB is ordered from battery-junction

 

[mdocod]

They should be called weld tabs, not solder tabs, they are made of metal and will conduct heat very quickly, I'd still be very leery of soldering a li-ion pack. Even with tabs.

Nope, these are unprotected cells with solder tabs. I measured the resistance from each balance harness wire to each battery and the + side was up to about 250 kOhm. I isolated it to the PCB, and one of the IC's looks semi-cooked. Dunno if I can get another easily or just get a new PCB (expensive shipping from US on a 10$ PCB :mecry