VAPEX Racing Ni-MH AA battery fail & meltdown - pictures - Page 1
Forum section: electronics / general

author: Fraser
author_rank: Super Contributor
timestamp: May 14, 2016, 06:31:42 pm
content: Those who have dismantled commercial battery packs will know that they usually contain a thermal fuse and sometimes an over current fuse. Are they important ? You bet they are ! Never rebuild a battery and leave out the original protection, or equivalent.To see what happens when a battery builder decides to not include any thermal fuse protection, take a look at the attached pictures.What happened ?1. VAPEX 12 AA cell battery pack rated at 2600mAh placed on a constant current power supply that has been set to 1.5A.2. Battery charges until it is at full charge3. Charging current continues to pass through the battery pack and is dissipated as heat from the cells4. The temperature of the cells rises rapidly5. In the absence of a thermal fuse, the cell temperature rises to a temperature so high that gas pressure inside the cell activates the safety vents to prevent explosive failure of seals or casing.6. The temperature rises so high that the heat shrink sleeves around the cells melts7. The melted heat shrink sleeves permit cells to short to each other8. Fully charged, overheating cells are shorted positive to negative terminal creating thermal runaway in the shorted cell.9. The combination of overcharge current and shorted cells causes the battery pack to effectively suffer catastrophic failure with much venting of gas and electrolyte.10. One very messy and extremely hot battery results. If the battery had been inside a piece of equipment, further electrolyte and heat related damage would likely have occurred.A thermal fuse would have prevented all this drama. Do not skimp on this essential protection in your battery packs. Constant current charge circuits and their related timers/ Delta V circuits can fail causing a severe overcharge condition.I hope this little demonstration is helpful to battery pack building newbies.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 14, 2016, 06:34:06 pm
content: More pictures

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2016, 06:50:19 pm
content: Whoah! Was that a 12s pack? And someone tried to fast charge it at 1.5 A? Are they crazy?

author: stmdude
author_rank: Frequent Contributor
timestamp: May 14, 2016, 06:53:05 pm
content: This is why proper chargers for RC gear have automatic shutdown, and in the slightly more expensive ones, thermometer.The reason why there's no fuses in an RC pack is because a fuse that wouldn't trip during normal use would be useless for the charging scenario.NiCd/NiMH routinely hit 50C+ during a race, and see peak currents of many hundreds of amps.Heck, if you push them, regular solder on 14AWG wire will _melt_ due to the internal resistance of the solder.. This is why packs are spot-welded..On a side-node, way back in the days, we charged our NiCd packs using resistance-wire, connected to a car battery. I.e, no shutoff or safety at all. You felt the battery with your hand, and stopped charging once the pack started heating up.Here's a pro-tip in this situation: Don't rest the battery on your arm _in bed_ whilst reading, and proceed to fall asleep. I have that scar to this day (some 25-30 years later).I have no idea how I survived my youth..

author: stmdude
author_rank: Frequent Contributor
timestamp: May 14, 2016, 06:54:52 pm
content: Quote from: IanB on May 14, 2016, 06:50:19 pmHuh? 2600mAh NiMH cells can be charged at 5+ amps without any ill effects. You just need to stop once the battery is full.. (Which didn't happen in this case)

author: Fraser
author_rank: Super Contributor
timestamp: May 14, 2016, 06:58:30 pm
content: It was a 12 cell Ni-MH pack tested to destruction deliberatelyit was supposed to be capable of a 2.3A charge for 50 minutes. This test was simulating the failure of the safety systems in a 1.5A fast charger in a piece of equipment. The present battery packs are all at the end of their lives and I was amazed to see no thermal fuses fitted. The new batteries will be fitted with current and thermal fuses.Fraser

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2016, 07:02:32 pm
content: Quote from: stmdude on May 14, 2016, 06:54:52 pmAssuming they are AA cells, that is not my experience. A 2600 mAh AA cell is optimized for high capacity, not high current. I've had a single AA cell vent when charged at 2 A before it even got warm. High power RC cells are usually 1800 mAh or less.If you then have a pack of 12 cells in series it is very unlikely they are all evenly balanced. One cell is going to get full first and then overheat while the others catch up.I know RC racers abuse packs, but the packs won't last very long if treated so badly.

author: stmdude
author_rank: Frequent Contributor
timestamp: May 14, 2016, 07:03:22 pm
content: Quote from: Fraser on May 14, 2016, 06:58:30 pmWhut? Where will you find fuses that doesn't trip when you _pull_ 100+ amps, but when you _push_ 2 amps ?

author: stmdude
author_rank: Frequent Contributor
timestamp: May 14, 2016, 07:08:48 pm
content: Quote from: IanB on May 14, 2016, 07:02:32 pmThe higher capacity ones have higher IR, hence, you can't pull as many amps from them, so they're not really suited for the more aggressive types of RC racing (won't handle the peaks).But, I haven't dabbled with AAs, mostly Sub-C for me. They might behave a bit different due to smaller thermal mass and higher IR.Quote from: IanB on May 14, 2016, 07:02:32 pmThat's only a problem with a fresh-from-the-factory pack (the good ones actually come balanced and matched from the factory, but lets disregard that). Once it's been through a few cycles, the cells start matching each other due to the memory effect. (Takes a little longer on NiMH than NiCd)Quote from: IanB on May 14, 2016, 07:02:32 pmThey typically last a season. ~200cycles I would say.They're not toasted by then, but they've lost a noticable amount of capacity (as in, won't last a full race)

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2016, 07:10:40 pm
content: Quote from: Fraser on May 14, 2016, 06:58:30 pmI don't know what "Mas. charge" means in that data sheet? The "fast charge" figure would be the normal design spec.But those specs are for a single cell. You cannot necessarily extrapolate from a single cell to a 12-series pack and expect the same protocols to apply.

author: IanB
author_rank: Super Contributor
timestamp: May 14, 2016, 07:12:06 pm
content: Quote from: stmdude on May 14, 2016, 07:08:48 pmI think Sub-C have different characteristics than AA. They are typically much more capable of high current, high power applications.

author: TheSteve
author_rank: Unknown
timestamp: May 14, 2016, 07:49:04 pm
content: What is this pack used for? With a name like "VAPEX Racing" it does make it sound RC related. As previously mentioned nearly every RC related battery pack made will not have a thermal fuse as it would never handle the current requirements.

author: Fraser
author_rank: Super Contributor
timestamp: May 14, 2016, 08:09:43 pm
content: The pack is powering nothing more exciting than a camera trap for wildlife video capture. Maximum draw on the pack is only 1A with illumination on.They are custom built for a broadcast company that I am assisting. Whoever built the units used the VAPEX racing packs presumably to facilitate faster charging ? He/she should have fitted a thermal fuse in this application as the rest of the equipment is very high value. Why did they not use Li-Ion ? Try shipping Li-Ion around the world these days.... paperwork nightmare.Whilst I agree that in RC scenarios, fuses are not used. This is just a 'Racing' pack that is used in a non RC application. The cells do not contain any 'special sauce' for RC use only. The packs output cables are silicon insulated but do not look like motor drive type high current cables. Maximum discharge from the cells is stated as over 7A though.Interestingly I have tested the replacement identical VAPEX battery packs and they are failing the capacity tests so they are not impressing me at the moment. 1300mAh capacity at 0.38C draw from a stated 2600mAh pack is not good. I just tested some nice Panasonic HHR cells and they perform well so I will be building new packs using those. Its a PITA as they were intended for another project and building packs of this size is a faff I could do without.Fraser

author: SeanB
author_rank: Super Contributor
timestamp: May 15, 2016, 02:12:15 pm
content: A polyfuse instead of a single use fuse would be good, simply use a 5A one, and spot weld it in the pack at the end where it can be in the gap between cells. Put 2 one time thermal fuses in the pack, spot welded into the cells near the middle, so the sensing is at around 1/3 of the way through, so 2 cells in from the end. Making a symmetric cell assembly means a simple jig to fit the fuses, using a small silicone sleeve over the one lead and an outer sleeve over the fuse, or use a radial lead fuse even though it has a lower current rating.

author: Fraser
author_rank: Super Contributor
timestamp: May 15, 2016, 02:38:37 pm
content: SeanB,Thanks for the excellent advice, as ever.Heard back from the owners of the equipment today. They said to do whatever is needed to make the battery packs safe for shipping by air and charging in the field. I shall adopt a belt and braces approach to the battery build and purchase a better charger. The units came with some cheap universal charger 'thing' containing terrible build quality and I do not trust it at all. The owner has had several chargers fail to date but thankfully none in a manner that would overcharge the battery.Fraser

author: amyk
author_rank: Super Contributor
timestamp: May 15, 2016, 03:34:16 pm
content: I remember reading somewhere that NiMH cells should have a pressure disconnect so when they get hot and pressure builds up inside, they automatically stop charging.

author: SeanB
author_rank: Super Contributor
timestamp: May 15, 2016, 05:20:15 pm
content: No, they have a vent so they will dump hot corrosive alkaline electroyte rather than building up pressure till the case or end crimp ruptures. Works for one operation, then it will be suspect.I did rejuvenate a few NiCd cells by uncasing them out of the steel shell and wrapping with lacing twine to keep the powder together, then putting in a glass jar ( small one a little taller than the cell) and connecting some wire to the terminal strips, then filling the jar to cover the cell with NaOH solution ( no KOH at hand, but plenty of drain cleaner flakes at home) and they would work somewhat again. They all failed eventually as the wires corroded away. Did seal the top with heavy plastic sheet held down with some more string to keep evaporation down.