Re: [EVDL] Temperature rating (and Blowing them up!)
From: Mike Nickerson via EV I think the problem happens when the constant current charger doesn't stop when the cells are full. The cell can't store the energy, but it has to go somewhere. They are sealed, so it can't just bubble off water like lead-acid cells do. The energy breaks down the electrolyte and builds pressure until the cell finally blows. Once full, I don't think it matters how large or small the current is. It will cause damage and it is additive. Jan Steinman via EV wrote: I'm not saying you're wrong. But as an electrical engineer, no one has explained it to me in a way I can understand yet. No working cell should see more than a few hundred milliwatts in the scenario cited. This is roughly the energy dissipated by your left ear. (Well, maybe you left hand, unless you're exerting yourself.) The energy dissipation alone is not enough to cause the damage shown. Heat merely accelerates the whole process. If the power level, or heat dissipating ability of the cell keep the temperature down, these destructive side effects still occur -- they just take longer. Overcharging can destroy a cell in an hour at high temperatures. Or in a few days at room temperature. As Cor mentioned, are these cells actually sealed? With no safety vent? I guess I can imagine an explosion in such a case, but still, basic thermodynamics don't support that. Pressure from disassociation should lead to re-combination elsewhere, due to the limits of the power supplied. Cells that size should be able to dissipate low tens of watts safely. Energy out must not exceed energy in. They have a safety vent; but it if opens, the cell is ruined. Gas (which means water) is lost. The vents do not reseal, so they continue to lose water every time it is charged. The remaining electrolyte gets stronger and stronger as water is lost, and starts corroding things inside. Before long, the cell fails completely. There are recombination reactions, but they don't work very well in nimh cells. They *can't* work well, because the hydrogen is in fact one of the active elements. Nimh cells are really nickel-hydrogen cells, with a spongy metal plate to adsorb the hydrogen so the pressure inside doesn't get too high. If you keep charging a fully-charged cell, you keep disassociating water to produce hydrogen and oxygen. There is no place for it to go, except to increase the pressure inside. Does that help? -- Sometimes when you innovate, you make mistakes. It is best to admit them quickly and get on with improving your other innovations. (Steve Jobs) -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
From: Mike Nickerson via EV ev@lists.evdl.org I think the problem happens when the constant current charger doesn't stop when the cells are full. The cell can't store the energy, but it has to go somewhere. They are sealed, so it can't just bubble off water like lead-acid cells do. The energy breaks down the electrolyte and builds pressure until the cell finally blows. Once full, I don't think it matters how large or small the current is. It will cause damage and it is additive. I'm not saying you're wrong. But as an electrical engineer, no one has explained it to me in a way I can understand yet. No working cell should see more than a few hundred milliwatts in the scenario cited. This is roughly the energy dissipated by your left ear. (Well, maybe you left hand, unless you're exerting yourself.) The energy dissipation alone is not enough to cause the damage shown. As Cor mentioned, are these cells actually sealed? With no safety vent? I guess I can imagine an explosion in such a case, but still, basic thermodynamics don't support that. Pressure from disassociation should lead to re-combination elsewhere, due to the limits of the power supplied. Cells that size should be able to dissipate low tens of watts safely. Energy out must not exceed energy in. Being influenceable is the key to influencing others. -- Stephen R. Covey Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
I believe the problem is that he was sending 75W into a closed box without fan running and the resulting overcharge at high temp (all power is converted to gas and heat) and high pressure, destroyed his pack. BTW, I would expect the modules to have a safety vent. Did that not work? Or did you disable that, Robert? I too have charged a (Classic) with a trickle charge, but had the pack completely opened, so that may have added to the cooling - and I charged for shorter time (but not that much shorter) so I could have gotten the same results - I did feel that the pack had warmed up noticeably towards the end. Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: cwa...@proxim.comPrivate: http://www.cvandewater.info Skype: cor_van_de_water XoIP: +31877841130 Tel: +1 408 383 7626Tel: +91 (040)23117400 x203 -Original Message- From: EV on behalf of Jan Steinman via EV Sent: Wed 4/1/2015 9:33 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Temperature rating (and Blowing them up!) From: Robert Bruninga via EV ev@lists.evdl.org That's 300 mA at 250 VDC. The voltage doesn't really matter, does it? If you're charging with a constant-current source, all but one of the cells could short, and the last cell would still see no more than 360 mW. If you were charging constant-voltage, and you had a cell short, then that would cause the other cells to receive more current, which could cause them to short, etc. until you burn up the string. But it shouldn't happen on a correctly-operating constant-current charger, no? Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) -- next part -- A non-text attachment was scrubbed... Name: not available Type: application/ms-tnef Size: 3773 bytes Desc: not available URL: http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150401/e6c70579/attachment.bin ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
I think the problem happens when the constant current charger doesn't stop when the cells are full. The cell can't store the energy, but it has to go somewhere. They are sealed, so it can't just bubble off water like lead-acid cells do. The energy breaks down the electrolyte and builds pressure until the cell finally blows. Once full, I don't think it matters how large or small the current is. It will cause damage and it is additive. Mike On April 1, 2015 10:33:42 PM MDT, Jan Steinman via EV ev@lists.evdl.org wrote: From: Robert Bruninga via EV ev@lists.evdl.org That's 300 mA at 250 VDC. The voltage doesn't really matter, does it? If you're charging with a constant-current source, all but one of the cells could short, and the last cell would still see no more than 360 mW. If you were charging constant-voltage, and you had a cell short, then that would cause the other cells to receive more current, which could cause them to short, etc. until you burn up the string. But it shouldn't happen on a correctly-operating constant-current charger, no? Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
From: Robert Bruninga via EVev@lists.evdl.org That's 300 mA at 250 VDC. Jan Steinman via EV wrote: The voltage doesn't really matter, does it? It does indirectly. We're charging Prius packs. Each 6-cell module is very flat, and sandwiched tightly between its neighbors. It has almost no exposed surface area for cooling. So when you apply 350 ma continuously, it is generating about 9v x 0.35a = 3 watts of heat. It turns out that this *is* enough to make its temperature soar. There is also the potential for thermal runaway. This can happen when you use a regulated power supply or smart charger. As the cells heat up, their voltage goes *down*. This fools the charger into think the cells aren't fully charged, so it fails to shut off, and may even *increase* the current. What I know (from personal experience) is that continuously charging nimh cells *will* cause their temperature to climb, even at low rates. If the charger doesn't shut off, this will continue until the cell is destroyed. -- Sometimes when you innovate, you make mistakes. It is best to admit them quickly and get on with improving your other innovations. (Steve Jobs) -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
From: Robert Bruninga via EV ev@lists.evdl.org That's 300 mA at 250 VDC. The voltage doesn't really matter, does it? If you're charging with a constant-current source, all but one of the cells could short, and the last cell would still see no more than 360 mW. If you were charging constant-voltage, and you had a cell short, then that would cause the other cells to receive more current, which could cause them to short, etc. until you burn up the string. But it shouldn't happen on a correctly-operating constant-current charger, no? Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
Same happens with NiMh: See my blown up Prius cells. http://aprs.org/prius/photos/bad/OverC-Left-CellsX.JPG http://aprs.org/prius/photos/bad/OverC-Right-BentX.JPG All I did was p ut a 300 mA trickle on them... and then forgot about it. Until 8 hours later when they began to explode... Bob, WB4APR The cell containment vessel can make a big difference in this temperature limit, however. Cylindrical cells can often hold significant pressure while prismatic and pouch cells can withstand less or perhaps no pressure. ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
From: Robert Bruninga via EV ev@lists.evdl.org Same happens with NiMh: See my blown up Prius cells... All I did was p ut a 300 mA trickle on them... and then forgot about it. Until 8 hours later when they began to explode... Wow, Bob. That is difficult to understand. Across one cell, there should only 360 milliwatts, which a half-watt resistor can safely dissipate! Even if a cell were shorted, the wiring to it could probably dissipate 300 mA without melting. Are you absolutely certain your charger didn't go bonkers on you? It's hard to reconcile 300 mA with the sort of damage pictured. The problems of energy use are not technical but cultural. Americans want speed, acceleration and big cars. -- Pat Murphy Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
Re: [EVDL] Temperature rating (and Blowing them up!)
That's 300 mA at 250 VDC. And it proves exactly what the literature says, You CANNOT trickle charge a NiMh cells beyond 100% and the same holds for Lithium. They overpressure and explode or vent or whatever it takes to relieve the pressure... This is completely different from our 60 year experience with NiCd's which can ALWAYS be trickle charged indefinitely at 0.1C trickle charge. *not so* with NiMh and Lithiums.. Bob -Original Message- From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Jan Steinman via EV Sent: Wednesday, April 01, 2015 5:37 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Temperature rating (and Blowing them up!) From: Robert Bruninga via EV ev@lists.evdl.org Same happens with NiMh: See my blown up Prius cells... All I did was p ut a 300 mA trickle on them... and then forgot about it. Until 8 hours later when they began to explode... Wow, Bob. That is difficult to understand. Across one cell, there should only 360 milliwatts, which a half-watt resistor can safely dissipate! Even if a cell were shorted, the wiring to it could probably dissipate 300 mA without melting. Are you absolutely certain your charger didn't go bonkers on you? It's hard to reconcile 300 mA with the sort of damage pictured. The problems of energy use are not technical but cultural. Americans want speed, acceleration and big cars. -- Pat Murphy Jan Steinman, EcoReality Co-op ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) ___ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
