Lithium Polymer Battery Packs

[audioguru]

The chargers for my electric RC airplane Li-Po batteries charge each cell to exactly 4.20V. The cells do not get warm when charging and a full charge takes 20 minutes so the average charging current is 3C. The batteries get warm when flying with wide-open-throttle with a discharge in 10 minutes (6C).

Some new Li-Po batteries are 6th-generation and are rated for a max charge of 12C (fully charged in 5 minutes) and a max discharge rate of 45C (discharged in 1.3 minutes).

 

[Sceadwian]

bountyhunter, I think you may have misread some numbers, this isn't a high power pack, high energy storage sure. But the calculated discharge rate is .3C and the suggested charge rate is 1C.... This is WELL within the capabilities of Lipo's without any heating or pack stress.
The hard part is balancing that many cells, and given they're both a series and parallel string you need extremely high quality cells, even the most well designed charge controller won't do jack if you don't have matched cells. Charging a pack of that size is actually pretty quick, but balancing needs to be done frequently.

 

[tom_pay]

I was going to make a temperature cut-off switch, in case of a fire when discharging the pack. Will a LiPo about to catch fire get hot? If so, how hot? And will cutting discharge current stop it from making a fire?

The hard part is balancing that many cells, and given they're both a series and parallel string you need extremely high quality cells, even the most well designed charge controller won't do jack if you don't have matched cells. Charging a pack of that size is actually pretty quick, but balancing needs to be done frequently.

You say I need matched cells, how do I match them? Is it just the same brand/type? I was going to balance the pack in two sections, a section of 4S3P and then another section of 5S3P with a proper, bought balance charger. This is because I cannot find a 9S charger, but 8S are readily available. Is this going to work/be safe?

Thanks

Tom

 

[audioguru]

I have used small Li-Po single and double battery cells for a few years. The manufactured chargers work perfectly and have never caused a fire.
Tutorials say that overvoltage charging, too low discharge voltage then attempted charging, over-current discharge or over-current charging might cause a fire.

I saw an RC model airplane have its brand new very expensive Li-po battery catch on fire in flight. The pilot landed it quickly, snatched out its burning battery and put it in a bucket of sand (water makes the fire burn hotter). It burned making a lot of smoke for about half an hour.

Lithium is almost the same as magnesium (burning flares and bright fireworks).

 

[bountyhunter]

I have used small Li-Po single and double battery cells for a few years. The manufactured chargers work perfectly and have never caused a fire.
Tutorials say that overvoltage charging, too low discharge voltage

The Li-Po cells from Valence Technology would grow internal shorts when they discharged too far. Unfortunately, they still contained PLENTY of energy and got extremely hot from the internal short and some burned the devices they were in. Overcharging/incorrect charging is not the only way to get them to go off, although it's probably the most common cause.

 

[tom_pay]

So even the expensive LiPos catch on fire.

I hadn't thought about extinguishing a LiPo fire, Lithium and water probably aren't the best of friends!!!

The manufactured chargers work perfectly and have never caused a fire.

I think I will forget about using a home made charger, buying a charger and using it twice sounds like a much better idea.

Other than, shorts, over charging/discharging and puncturing how else do they catch on fire?

Thanks

Tom

 

[Sceadwian]

audio-guru, what does that have to do with using a 9 series and 9+ parallel pack that the poster is talking about?

If I a Lipo fire really gets going the only way to put it out is via heavy C02 or chemical powder based fire extinguishers.

One other way top_pay is ambient temperature.. Lithium chemistry fire resulting thermal runaways are just that... thermal runaways. Ambient temperature and every ambient condition the pack will experience which includes waste heat from it's load must be taken into account.

To the best of my knowledge, the primary failure of all Lithium based batteries in laptops cell phones or any other device which has proven to show a fire based failure is OVER INSULATION.

 

[tom_pay]

Lithium chemistry fire resulting thermal runaways are just that... thermal runaways.

Ok, so a temperature sensor is a must. Is is possible to 'catch' the thermal runaway, preventing a fire, by disconnecting the load at a certain threshold temperature? If so what would it be? I was thinking 60C (140F).

To the best of my knowledge, the primary failure of all Lithium based batteries in laptops cell phones or any other device which has proven to show a fire based failure is OVER INSULATION.

This is kinda a bit of a bummer The packs will be used for underwater lighting, but the water should keep them cool and I don't have access to the shrink-wrap that goes on the outside of the battery pack. So it should be right?

Thanks so much for all of your help

Tom

 

[Sceadwian]

In your situations tom_pay I don't see this being a problem, you simply aren't using the pack in conditions that can encounter thermal run away. It's only a problem with packs that are discharged at multiple times their capacity rating where ohmic heating of the pack itself is high. You stated that you were only discharging at .3C the battery should never heat more than a fraction of a percent over ambient unless the battery is extremely well insulated.

 

[tom_pay]

That is a big relief...

I shall still put in a temperature sensor though. Just to be safe.

I guess it is now time to design some cut-off sensors and order some cells.

Are the cells hard to solder up?

Thanks so much

Tom

 

[Sceadwian]

They're very easy to solder if you get them with solder tabs. If you're doing your own tabs you better be real good wither soldering, or have a low power spot welder, over heating the anode/cathode can drastically effect the cell to the point where they won't ballance properly or are outright destroyed. Solder tabs directly from the factory are done right with spot welding equipment which causes no serious cathode/anode heating.