Topic
Batteries: Sources & Cell Types 3 years 11 months ago #193056
There's a lot of people online who will suggest battery systems for EV conversions that were dreamt up in the early 2000s-2010s that used existing technology at the time. In my opinion, these are largely obsolete and should be ignored in favour of newer technology.
New or Nearly New Cells:
It is entirely possible to source batteries that are new or new-original stock. If you find the right deal, this can be cost effective, but the only way to do this is to keep a close eye on classifieds pages, eBay etc. New batteries imported directly from the manufacturers (usually in China) can be done, but is usually quite costly. The common types of new batteries are listed;
The industry that has spent the most in the last 5-10 years on battery development is the automotive industry and its suppliers, so it is the best place to start to find batteries. The most "bang for your buck" batteries are likely to be second hand Lithium batteries from crashed/wrecked OEM EVs. There is a whole industry that has sprung up around recovering and selling these battery packs, and I'll do my best to outline the ones I know about. There are other EV manufacturers that may be worth looking into, but I've not done this for them, so can't really comment.
Tesla:
These packs are generally regarded as some of the best avaliable - though their price tag matches this. A Tesla battery pack breaks down into smaller modules, which are liquid cooled (there are cooling channels that run inbetween the cells in the battery pack). Tesla modules are built up of thousands of cylindrical cells in series/parallel combinations, and are capable of very high current output.
For example: a Tesla Model S battery module, comprises of 3400mAh cylindrical battery cells in a 6s74p configuration - i.e. Each module is a string of 6 groups in series, where each group is 74 cylindrical cells in parallel. This gives the battery the ability to kick out a lot of current, but the voltage of each individual module (22.2V nominal) is quite low. This means you need quite a few modules in series to get the voltage which many inverters need.
Nissan:
The modules inside a Nissan Leaf's battery are well-liked by some DIY EV builders, as they are in a convenient form-factor and are capable of excellent output power. Unfortunately, Nissan's engineers neglected (and continue to do so) to design in any form of active thermal management in their batteries, so they tend to degrade much more than modules from other EVs. If you can find a Nissan Leaf battery pack that is in excellent health, you may have a winner on your hands - but this is non trivial. Most scrapped Leaf packs tend to be in pretty poor states of health and are unsuitable for 2nd life EV use (they can be used by people building home DIY battery-based energy storage though).
Battery management & charging are worthy of separate threads, so I'll post my thoughts in a separate thread. My opinion is that it's extremely irresponsible to not have an active battery management system when using any form of Lithium battery cells (and Pb-Acid should be avoided for EVs nowadays, despite them tolerating abuse more). A BMS at the very minimum makes sure that the cells aren't overcharged, overdischarged & out of balance. A more advanced BMS will measure the temperature of the batteries, control the battery charger to limit the charge rate, and provide feedback about state-of-charge to the driver.
New or Nearly New Cells:
It is entirely possible to source batteries that are new or new-original stock. If you find the right deal, this can be cost effective, but the only way to do this is to keep a close eye on classifieds pages, eBay etc. New batteries imported directly from the manufacturers (usually in China) can be done, but is usually quite costly. The common types of new batteries are listed;
- Lead Acid: Avoid these, they're old technology, very poor energy density & performance and equally poor longevity. Usually a little cheaper than new Lithiums though
- Lithium Iron Phosphate (LiFePO4): An older Lithium Ion battery technology, these can be a good option. Lower individual cell voltages than other Lithium Chemistries, but are inherently more stable. LiFePO4 batteries need less care and are not likely to catch fire even if heavily abused. Lower energy-density per unit mass & volume than Li-Polymer but far better than Lead-Acid.
- Lithium Polymer: There are a range of chemistry variants here, but these offer the best energy-densities & highest performance. It is necessary to treat LiPoly batteries carefully as they're likely to go up in flames if punctured. Well-designed battery enclosures can mitigate this
The industry that has spent the most in the last 5-10 years on battery development is the automotive industry and its suppliers, so it is the best place to start to find batteries. The most "bang for your buck" batteries are likely to be second hand Lithium batteries from crashed/wrecked OEM EVs. There is a whole industry that has sprung up around recovering and selling these battery packs, and I'll do my best to outline the ones I know about. There are other EV manufacturers that may be worth looking into, but I've not done this for them, so can't really comment.
Tesla:
These packs are generally regarded as some of the best avaliable - though their price tag matches this. A Tesla battery pack breaks down into smaller modules, which are liquid cooled (there are cooling channels that run inbetween the cells in the battery pack). Tesla modules are built up of thousands of cylindrical cells in series/parallel combinations, and are capable of very high current output.
For example: a Tesla Model S battery module, comprises of 3400mAh cylindrical battery cells in a 6s74p configuration - i.e. Each module is a string of 6 groups in series, where each group is 74 cylindrical cells in parallel. This gives the battery the ability to kick out a lot of current, but the voltage of each individual module (22.2V nominal) is quite low. This means you need quite a few modules in series to get the voltage which many inverters need.
Nissan:
The modules inside a Nissan Leaf's battery are well-liked by some DIY EV builders, as they are in a convenient form-factor and are capable of excellent output power. Unfortunately, Nissan's engineers neglected (and continue to do so) to design in any form of active thermal management in their batteries, so they tend to degrade much more than modules from other EVs. If you can find a Nissan Leaf battery pack that is in excellent health, you may have a winner on your hands - but this is non trivial. Most scrapped Leaf packs tend to be in pretty poor states of health and are unsuitable for 2nd life EV use (they can be used by people building home DIY battery-based energy storage though).
Battery management & charging are worthy of separate threads, so I'll post my thoughts in a separate thread. My opinion is that it's extremely irresponsible to not have an active battery management system when using any form of Lithium battery cells (and Pb-Acid should be avoided for EVs nowadays, despite them tolerating abuse more). A BMS at the very minimum makes sure that the cells aren't overcharged, overdischarged & out of balance. A more advanced BMS will measure the temperature of the batteries, control the battery charger to limit the charge rate, and provide feedback about state-of-charge to the driver.
Batteries: Sources & Cell Types 3 years 11 months ago #193072
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