[johnmathewson]

Has anybody had any problems charging Kokam 7.4v, 2100mah, 20c lipos in series (4s1p) using a Kokam lipo-502 charger? I've had 4 of these batteries fail as a result of overcharging where the casing became slightly inflated and spongy.

[JimMoss]

Not familiar with the charger ..... however as I understand it any and all lipo's will be damaged if you over charge ......

[4*40]

As long as you don't charge them over 2amps they shouldn't over charge and balloon.You probally have a bad charger.

[Wayne MIller]

Hi johnmathewson,

LiPo batteries can puff up if they are charged incorrectly, or discharged at a too high rate, or discharged too much.

I'm not sure about your specific charger, but if the batteries have been discharged too much and are "out of balance" (that is, one cell has a different voltage than another in the pack), damage may occur the next time the pack is charged. Note: With most LiPo chargers, they will abort the charge when you have a damaged cell.

If the charger tries to charge a pack with a damaged (or low) cell (very unlikely), the over charging can occur on the "good" cells. This is due to the fact that most chargers are designed to continue charging, until it "sees" a fully charged voltage for the complete pack. If one cell is damaged, the charger will never see the fully charged voltage. Thus, the "good" cells will be over charged and damage will occur to them.

As a note of caution, over charging any batteries can be very dangerous, they can explode or cause fire.

Hope the above is not too confusing.

Fly4Fun,

Wayne Miller

[johnmathewson]

Both the charger and the lipos are relatively new. The batteries have never been put on load so they haven't been over-discharged. There was no sign of bloating when the batteries were charged individually at 2 amps and 7.4v. The problem occurred when I tried to charge two of the batteries in series at 2 amps and 14.8v. According to the idiot lights on the charger, these batteries charged properly. The bloating occurred after the charging and was observed the following morning.

[Wayne MIller]

Hi johnmathewson,

The instructions I got with my charger stated not to charge LiPo's over 1 amp, but that may be a limitation for my particular charger.

Perhaps, someone else would know for sure.

Wayne

[Guest]

Originally Posted by johnmathewson

Both the charger and the lipos are relatively new. The batteries have never been put on load so they haven't been over-discharged. There was no sign of bloating when the batteries were charged individually at 2 amps and 7.4v. The problem occurred when I tried to charge two of the batteries in series at 2 amps and 14.8v. According to the idiot lights on the charger, these batteries charged properly. The bloating occurred after the charging and was observed the following morning.

Are you certain that the packs were of identical voltage when you started the charge? If not, their toast. You cannot series charge packs with a voltage difference.

When series charging 2 of my 5s packs (18.5V nominal) both packs must be within 0.01V-0.02V or I wont charge them in series. Since they are connected in the model in series (10s) and discharged in series they typically are close enough to series charge, but occasionally they are not.

If your packs were discharged seperately you are asking for trouble series charging without equalizing voltage first.

Your lucky all they did was puff, by your description I take it you were not monitoring the charge? tsk tsk tsk

[MRK]

From FMA's LiPo Handbook-Section 3, Page 9 (downloadable from https://www.fmadirect.com/site/fma.htm?body=rc_appNotes) it states:

"Use caution with chargers that automatically determine cell count. Keep a close eye on charging until absolutely certain that the charge has selected the proper cell count. Li Po cells are nominally 3.7V under load. Cell voltage at full charge is 4.235V. Two fully charged cells, for example, output 8.4V. Three partially discharged cells may be 2.8V/cell x 3 = 8.4V. If two nearly charged cells are put on charge to “top them off”, some auto-counting charger may incorrectly sense them as three discharged cells and set the charge voltage for three cells. The two cells will receive too much voltage and will definitely be damaged. It is the user’s responsibility to assure a charger’s voltage is properly set."

If you were using the "Auto" setting on your charger instead of selecting the exact number of series cells and the cells voltages were already at or near their fully charged levels then the charger didn't correctly determine the number of cells with the result being they were overcharge and ruined.

Mike

[johnmathewson]

I find it difficult to understand the reason why it is so critical to have pack voltages to within 0.01v prior to charging them in series. One would have to have an expensive voltmeter to read to that degree of accuracy. Perhaps the differential should have been 0.1v? The same rationale could be applied to individual cells in pre-assembled packs? How would one measure cell voltages in these packs considering they are sealed in shrinkwrap? I can understand the problem if the packs are connected in parallel where different pack voltages would generate loop currents. If the interconnection of lipos is so critical one wonders if their use in model aircraft is worth the risk and expense.

[Guest]

Originally Posted by johnmathewson

I find it difficult to understand the reason why it is so critical to have pack voltages to within 0.01v prior to charging them in series. One would have to have an expensive voltmeter to read to that degree of accuracy. Perhaps the differential should have been 0.1v? The same rationale could be applied to individual cells in pre-assembled packs? How would one measure cell voltages in these packs considering they are sealed in shrinkwrap? I can understand the problem if the packs are connected in parallel where different pack voltages would generate loop currents. If the interconnection of lipos is so critical one wonders if their use in model aircraft is worth the risk and expense.

Because the charger only sees the overall voltage, and could care less of individual cells. The further apart the pack voltages when series charging the more likely you are to overcharge one of the cells in the packs. You may get away with it at 0.1V but again that is simply pack voltage, you dont know what the cells are doing...and in my experience that variance will increase during a charge.

For instance, say pack 1 is 7.4V and pack 2 is 7.7V or 15.1V in series....charge the packs in series (4s, so you are going for 16.8V full), pack 1 is now at 8.25 and pack 2 is at 8.55 and is overcharged (4.275V/cell) and you have caused damage to the pack. When discharging separately its very easy to have a 0.3V difference in the packs, and without checking it could be anything.

Add that to the fact that some of the cells in the pack may be imbalanced and the possibility to overcharge a cell in a pack is further increased.

You can purchase a voltmeter at Can. Tire for about $20 that will read to X.XXV on a discharged 5s battery. I went and purchased a Fluke voltmeter for about $100 on Ebay, model 81438 is the best bang for the buck but only available at Sears in the US or on Ebay

Now when playing with multi-parallel packs (my packs are 5s4p 5.3Ah batteries) you gotta run balancers. Thunder Power makes the best one, but if you dont run TP batteries you will need to make a connectors to hook to whatever you are using. I would not buy a large pack without balancing connectors to check the series banks of cells for balance. TP is also shortly coming out with a new balancer that can handle x2 5s packs (or any combination less than that) and will automatically equalize the packs and balance them for safe series charging...right now you gotta do it all manually.

Series charging is something not too many people do as its riskier than parallel charging, but more chargers can deal with large packs in series than in parallel. I would avoid series charging until you are comfortable in you understanding of the technology.

Its too bad about your packs, there is a lot of info out there about charging and what to do and what not to do. Lipos make a great power source when used properly, and can last a long time....but make one mistake and they are usually toast