morph999 said:
These Thunderskys haven't been out long enough for many people to come forward. And with BMS being forced down everyone's throat, there aren't many people that are willing to take the risk of using them without it. Let me just say that if a BMS tells you that you have a weak cell or the weak cells dies without a BMS, what is the difference? You still have a weak cell. The weak cell will limit the entire pack regardless of a BMS or not.
I'm running my Thunderskys without a BMS and it's just fine. I know one other person who has run his for a year without a BMS and his is fine too.
Anyway, let me say that I think we as EV enthusiasts should be focusing on buying lithium batteries in its purest form in bulk and trying to lower the prices but instead it seems all people want to do is add more circuitry and increase the price of them. We should be buying lithium without any other gadgets attached to them and buy lots of them, get the prices down and get more people into LIFPO4. It's not a matter of gadgetry that it's going to get lifepo4 to the masses, it's a matter of capacity. Buy more capacity than you need and you won't ever have a problem.
You have excellent points, morph, and you certainly have the right to run your pack any way you wish. And I do understand why one might distrust someone with a product to sell. Please, though, if possible, allow for the possibility that folks like Gary and Richard might be spending all their time and money developing a product not because they're looking to line their pockets but because a product is needed and the ones available on the market don't do what they could/should, or don't do it for what most might consider a reasonable cost.
I started out running a 21S pack of Thunder Sky cells with no BMS. I also spend a fair bit of time working with others that do the same. I have just over a year's worth of data collected from the pack while on the road and on the charger. I use all of this to confirm or deny what I read and what I think I know. In my previous 'life' as a military analyst, people could die if I or my team made a mistake. While for most of us lithium isn't life or death, one point that I remember is that bad data is worse than no data. I have data to back-up my current state of understanding and will share it with you if you desire.
You aren't the only person suggesting that a 'weak cell' should be culled from the pack, BMS or no. In order to really look at that, we need to know your definition of 'weak cell'. I'll start with some possibilities in no particular order. If I don't cover your definition, please help me learn something new.
A 'weak cell' could be an otherwise fully functioning cell that just has happens to have the lowest capacity in the pack. In a pack with no BMS, this cell will always be the first to charge (and quickly over charge) and the first to discharge and over discharge. If voltages are kept in line, this cell will likely deliver a full lifespan. With no cell level management this will be the first cell to die. Replacing this cell with another with the same capacity will repeat the process. Replacing this cell with one of higher capacity will simply move the failure to the lowest capacity cell in the stack. It will continually cost money because swapping a cell doesn't solve the problem.
A 'weak cell' could be an otherwise fully functioning cell that just happens to have the highest internal resistance in the pack. This is the one that will have the deepest sag during discharge and will generate the most heat during charge and discharge. If allowed to sag to very low voltages often enough, it will likely have a shorter lifespan than the rest of the pack. If heating is significant, it can affect the other cells close by. Localized heating in a pack can lead to imbalance. The wider the imbalance, with no cell level voltage protection, the more likely cells are to be overcharged or over discharged.
A 'weak cell' could be one that has been damaged thru over charge, over discharge, or over current. A proper BMS keeps this problem from happening. It also allows continued use of the cell (maybe with a mileage or current reduction).
I've said this before, but feel free to visit the VisforVoltage forum and read thru posts in the RMartin and X-Treme maxi-scooter areas. Both of these are filled with other folks running with no BMS. Notice how many of these folks got tired of trying to get replacement cells under warranty and bought replacement cells on their own - and then installed a BMS. Then read thru the EV forums where the car and truck folks run lead-acid. One thing you'll find is the joy from the folks that went from dropping their pack 2-3 times per year to replace batteries to running multiple years straight after installing management components. Everyone is free to have the experience they want - but don't ridicule those that wish to learn from history. :wink:
From a business perspective - a company wants to maximize profit per item sold. A company doesn't do that by including electronics that aren't necessary - unless they can use it as an 'upgrade' - like a car with navigation for a higher price. The Vectrix maxi has a BUNCH of electronics keeping its NiMH pack under control. The EV Chevy S10, Ford Ranger, EV-1, and RAV4s all have pack management - for both the lead-acid and NiMH packs. The hybrids all have BMS systems, as do the various Prius plug-in modules available. The 2011 X-Treme maxi scooters will have BMSs from the factory. The earlier model year scooters weren't shipped with 'naked' packs because they didn't NEED a BMS - they were shipped with no BMS because there wasn't a device available that worked well enough (including the OEM BMS from Thunder Sky). And we're back full-circle to the real benefit and value of Gary and Richard's work. Assembling a pack is very easy. It's finding a way to manage the pack that 's keeping car companies using NiMH - they're already got BMS that works for those and they'll hang on to their systems as long as they can.
From an end-user prospective - what's the price of lithium? Some consider purchase price to be the be-all and end-all. In the large format cells, they're buying 1C capable cells from Hipower instead of the 3C cells they should be using because it 'saves' $1000 on their 100S pack. Others hear 'price' but think 'value'. As in - it's better to pay the $200 necessary for the minimal BMS necessary to get 2000 cycles and 10 years from the pack rather than replacing 2 cells each year. Using round numbers, a 100 series pack at $100 per cell with $200 of added management costs $10,200. The same pack without management and two cells per year costs $12,000. This is why the lowest initial 'cost' might be the most expensive option over time. Others that have a stash of Dewalt packs in the back closet and a cousin that can restock the closet when it's empty will probably make another choice. :wink:
From the attached Intersil paper: "Small differences in the self-discharge rates of individual Li-Ion cells create state-of-charge mismatches in series-connected Li-Ion battery packs. These mismatches, along with limitations in the allowable Li-Ion cell voltages, result in a loss of pack capacity over time. By using cell-balancing techniques, the useable capacity of the Li-Ion pack increases, improving both the system run time and the life of the battery pack."
From the Thunder Sky Battery Manual: "If assembled LFP battery pack are not used with BMS it will not get fire or burn, however, if this kind of battery has been charged and discharged for long time without BMS, the single cell may be overcharged or over-dischared and even the cell voltage will rise to 5V or 10V, and meanwhile discharge voltage of some cell come down to 1.5V or 0V. At this condition the LFP battery pack will not smoke or burn as LCP battery, however those cells that are damaged by overcharging or over-discharging cannot be used any more. So it is necessary to use BMS with any kind of battery."
From the attached Delphi paper: "Lithium based battery technologies offer performance advantages over traditional battery technologies at the cost of increased monitoring and controls overhead. The additional complexity of the hybrid drive system increases the management burden for accurate state of charge prediction, state of health prediction, cell balancing, and charge management."
Andy
