powermed
10 W
- Joined
- Jun 22, 2008
- Messages
- 80
I'm posting this separately since it might be too far buried in other threads - mostly about Prius failures. There's been quite a bit of mystery and misunderstanding about these amazing cells. It's no coincidence that Toyota has been using them successfully for almost a decade, and decided to stick with them in the new high end Lexus sports car. My information is gleaned from the original published paper (you have to buy it and it's copyrighted of course) and now approximately 3 months and 200 cycles. I commute 10 miles each day, 5 miles round trip. My standard pack is 44.4v (6 modules) on either a 600w Wilderness brushless hub motor or a 1000w mid-engine (through the gears) brushless currie. They should be strongly considered for commutes of 10 miles each way or less.
Each module is 11 x 4.75 x .75 and contains 6 cells in series for a total nominal voltage of 7.2v and 6.5 amps.
These are not typical NiMH chemistry in any way. They can deliver 10C + discharge rate, have very low internal resistance, deliver rated output to over 80% discharge (performance starts to fade), can be safely charged in about an hour, and have a proven life span of thousands of cycles and at least 10 years. They do not go out of balance when used this way. The performance is quite comparable to A123 cells. The pack shown is 6 modules in series with chargers - weight is around 14#.
1. Do not buy anything except recent Gen III cells that have not been sitting around. The cells must have a resting voltage of at least 7.2 when you buy them. Use matched cells - you should probably consider buying an entire battery and taking it apart rather than scooping up individual cells.
2. Do not even bother trying to charge them with a standard commercial NiMH charger. I have found no NiMH charger that can charge them safely - they all miss the delta V.
3. Charge them to 8.3v CV / module (CC if at all possible, but I've found no ill effects from regulated chargers that can deliver up to a rated current of 4.5a). I have not tested higher amps, but the cells by specs should handle 1C charge rates. Measure the charge time to get to 8.3v for your use. I tend to add about 15 minutes after 8.3v - the charger output tends to fall to 1a or less.
Other checks worth doing to make sure your chargers are working properly are listening for subtle crackling sounds at the vent ports (gassing indicating end of charge) and infrared temp checks down the thermal wells looking for a fast delta T.
Notebook chargers that are rated at 16v output will often have 16.65 - 16.68v outputs. If you can find matched ones go for it and charge them in strings of 2 cells - make sure the charger voltages are within .02v of each other. There is a degree of self balancing. Chargers start up warm at initial charge, then cool towards the end of charge.
CRITICAL USE INFORMATION
Treat them like A123's - DO NOT CONTINUE TO USE THEM ONCE PERFORMANCE DROP IS NOTED - recharge before any continued drain on the cells. I have had no trouble doing a full ride, charging, and using them again. Also common has been riding for 30 minutes, charging for a short time, and using them again. I've noticed no heat nor swelling of the packs, I do not believe compression is required for healthy cells that are well treated.
I've made some of these packs for family and local colleagues that are commuting, and have a few extra packs I made from the last Prius battery I bought (I'm selling them on ebay - do a search for 44.4v*). I can make about 3 more if folks are interested: $255 for 29.6v pack, $375 for a 44.4v pack, and $480 for a 59.2v pack (all wired up includes shipping and 4.5a chargers).
I assume the technical types here might want to buy a full prius battery, carve it up etc rather than pay for ready made packs. I'm personally not into the group buy thing but someone here might have the inclination.
Each module is 11 x 4.75 x .75 and contains 6 cells in series for a total nominal voltage of 7.2v and 6.5 amps.
These are not typical NiMH chemistry in any way. They can deliver 10C + discharge rate, have very low internal resistance, deliver rated output to over 80% discharge (performance starts to fade), can be safely charged in about an hour, and have a proven life span of thousands of cycles and at least 10 years. They do not go out of balance when used this way. The performance is quite comparable to A123 cells. The pack shown is 6 modules in series with chargers - weight is around 14#.
1. Do not buy anything except recent Gen III cells that have not been sitting around. The cells must have a resting voltage of at least 7.2 when you buy them. Use matched cells - you should probably consider buying an entire battery and taking it apart rather than scooping up individual cells.
2. Do not even bother trying to charge them with a standard commercial NiMH charger. I have found no NiMH charger that can charge them safely - they all miss the delta V.
3. Charge them to 8.3v CV / module (CC if at all possible, but I've found no ill effects from regulated chargers that can deliver up to a rated current of 4.5a). I have not tested higher amps, but the cells by specs should handle 1C charge rates. Measure the charge time to get to 8.3v for your use. I tend to add about 15 minutes after 8.3v - the charger output tends to fall to 1a or less.
Other checks worth doing to make sure your chargers are working properly are listening for subtle crackling sounds at the vent ports (gassing indicating end of charge) and infrared temp checks down the thermal wells looking for a fast delta T.
Notebook chargers that are rated at 16v output will often have 16.65 - 16.68v outputs. If you can find matched ones go for it and charge them in strings of 2 cells - make sure the charger voltages are within .02v of each other. There is a degree of self balancing. Chargers start up warm at initial charge, then cool towards the end of charge.
CRITICAL USE INFORMATION
Treat them like A123's - DO NOT CONTINUE TO USE THEM ONCE PERFORMANCE DROP IS NOTED - recharge before any continued drain on the cells. I have had no trouble doing a full ride, charging, and using them again. Also common has been riding for 30 minutes, charging for a short time, and using them again. I've noticed no heat nor swelling of the packs, I do not believe compression is required for healthy cells that are well treated.
I've made some of these packs for family and local colleagues that are commuting, and have a few extra packs I made from the last Prius battery I bought (I'm selling them on ebay - do a search for 44.4v*). I can make about 3 more if folks are interested: $255 for 29.6v pack, $375 for a 44.4v pack, and $480 for a 59.2v pack (all wired up includes shipping and 4.5a chargers).
I assume the technical types here might want to buy a full prius battery, carve it up etc rather than pay for ready made packs. I'm personally not into the group buy thing but someone here might have the inclination.