Author Topic: Why do Lipos swell  (Read 2602 times)

Offline Ashley Hoyland

  • Global Moderator
  • FAI Class Poster
  • *****
  • Posts: 2,008
  • Flying Class: FAI Pilot (P)
  • Other Status: Contest Director
Why do Lipos swell
« on: 29, April 2013, 07:45:57 AM »
A question was asked on another board on this forum and I believe the reply is worth preserving, so I have copied it here:

Indeed how much should a LIPO puff before you take note?

LIPO's can puff for all sorts of reasons but generally there are two primary reasons, one it has some moisture in the chemistry or the pack has been stressed. The former usually means a slight softness nothing more and it generally resets fairly solid after flight and there are no issues noted during charging such as high IR, heating, loss of capacity, poor balancing or will not balance. If it just gets a bit soft but works fine it probably is fine. The only way to test is to discharge in a controlled fashion stressing it and watch the discharge curves, median voltages, ESR and capacity and temperature for this you need specialist equipment which OPTI does have and is always pleased to put packs through the tester if asked.

So your pack has puffed badly and stays that way, this is likely to be more serious and should be investigated. Again lots of reasons why this might happen but normally its highlighting a fault of some sort, perhaps self-inflicted perhaps not. Bad puffs usually mean stress and often are associated with heat in the pack (excessive heat). This can occur due to a capacity change in the chemistry for example one cell has degraded and this means the other cells are driving the under capacity cell (ohms law) and that cell gets hot. This could be due to vacuum seal failure, damaged chemistry, poor resistance (this is usually associated to a bad cell), poor tab condition (high resistance solder joint) etc.

Again very difficult to diagnose without the right kit but as a rule of thumb IR should be <10 on all cells ideally < 5mohms. Packs should always balance to +/-0.01v per cell and charge fully in around 60 to 80 minutes assuming not on a high power charge, ideally 1C max 2C in the field 3C absolute max.

What kills packs very effectively is over discharging and use in cold ambient. A pack at say 3 degrees C is going to stress like hell compared to a pack at 20 degrees C and in the winter time the incidence of returned packs is significantly higher purely due to people insisting on flying their packs really hard when they are stone cold WARM THEM UP!! Lithium does not like cold at zero degrees C its almost useless! Nitro at zero C on the other hand is working great.

Poor solder joints on the connections is another very usual way to stress a lithium pack, a high resistance means the energy is given up as heat and this in turn stress's the pack. Make sure you have a high wattage soldering iron.

Should you balance or not, I would say yes always balance charge as it will highlight any issues. It’s a good idea to have a charger that indicates the IR as this can show a failing cell with increased resistance.

Can I recover an over discharged cell? Yes you can but again you need the right equipment to do this safely. Some chargers have a recovery function and you need to use this feature to recover a cell properly, and if recovered it should be fine. We drive the cells to 2.7v on the discharger and they recover just fine.

We can offer a service to clients (any cell make/brand) to check cells out and produce a proper factual report on condition and their safety just mail me at sales@optipower.co.uk and we can respond accordingly.

If it looks like a rugby ball it’s likely it is no longer a lithium pack that you should use. If it’s too hot to touch there is something very wrong with it. If the pack has a high IR it is probably going faulty soon!

I hope this is of some limited help. :)