recumpence said:
Oh no, here we go on the battery arguement!
No arguement, I just think it is important that people reading this get all the facts.
recumpence said:
Gary, you have me there on the cost. However, I have never seen a LiFe pack that is as light as a LiPo pack or as small. From my research, they are typically twice the weight and size of similar LiPo pack. My recumbent pack is 5 pounds exactly with wiring and it is a 48 volt, 10ah pack. Also, A123 cells can put out huge current, but they drop voltage further than good quality LiPo to do it. If I pull 10C out of my LiPo pack at full charge, I only lose 3 volts under load.
First of all, your 12s setup has a nominal voltage of 44.4V, not 48V, and gets charged to 50.4V. A "48V" 16s LiFePO4 pack has a nominal voltage of 52.8V, and gets charged to 58.4V. The reason these 16s LiFePO4 setups are called "48V" is because a 48V SLA setup gets charged to about the same level (2.45V per cell x 24 = 58.8V...), but SLA cells have only a 2.0V nominal voltage, so 48V. Since many LiFePO4 packs end up as drop-in replacements for SLAs, they can use the same charger. There is an added performance boost in that the bike's electronics need to be able to handle the voltage fresh off the charger (i.e. -- 58V...) but SLAs will quickly come down to the nominal voltage, even lower under load. LiFePO4s will start out at the same 58V, and will come down to their nominal voltage pretty quick, but the nominal voltage is a full 4V higher.
As for voltage drops, I just don't see a big difference in 10Ah-size packs. Maybe you are thinking of single "p" configurations, like in RC models? There I would agree a new generation 2500 mAh LiPo pack is going to see less voltage drop under load than a single string a123 pack. That is just not been my experience, however, with the a123-based ebike packs that I've been using, most of which are in either 4p (9.2Ah) or 5p (11.5Ah) configurations. I don't see any more than 3V drop on any of them and that's pulling the same 80-100A loads. Unless the pack is empty, I just don't ever see less than 49-50V. That, by the way, is what your pack is, fresh off the charger.
recumpence said:
One thing I neglected to factor in (in my previous post) is charging. LiPo packs require an expensive charger to be charged safely. That dramatically increases innitial cost. The chargers are also large. So, they would typically not be carried with the bike. Whereas Ping packs typically use a small, lightweight charger that can be carried along easily. That, along with the safety factor, makes Ping packs and other LiFe packs more desireable than LiPo for the majority of riders.
Yes, this is true, but what you are also forgetting about is that the BMS on LiFePO4 packs also include the balancing function. More accurately, what the BMS circuits do is to shunt current off to the next cell so that if a cell is full the next ones will still get current supplied, so that they can "catch up". This way each cell gets a full charge and can do so at its own pace. It is more like having individual chargers for each cell. RC-type balancers typically just drain the high cells down to the level of the lowest. The BMS also contains an individual LVC circuit for each cell. This protects
any cell from being over-discharged. You can use a pack-level LVC function only if cells are closely balanced.
recumpence said:
I will repeat my warning on safety here..........
LiPo packs can be hazardous! I have had one explode when overcharged (bad charger). They are NOT for the beginner or the faint of heart. I have been running them for years, though, and am thoroughly familiar with their behaviour. That is why I run them.
[pulpit]
I wholeheartedly agree. LiPos are very powerful, and if proper precautions are taken, they can work quite well, and can be trouble-free. The problems arise when these precautions are not followed. In the RC world, we've all had this beat into us enough that people treat these packs with the required safe procedures. LiPo use over in this world, however, is fairly new. Worse is the fact most people's experience here with Lithium-based packs has been with the much safer LiFePO4 variety, so there's no built-in "worry factor". One early example I've seen is that some people here are simply using simple fixed voltage power supplies/chargers on these packs, instead of the inherently much safer RC chargers, most of which will at least monitor the voltage of each cell as the pack is charging, and will terminate the charge if any one cell gets too high (above 4.25V...). With LiFePO4-based packs, you can let cells go above the charge limit of about 3.7V and the worst that will happen is that you may eventually lose some capacity if you do this on a regular basis. You let a Lithium-Cobalt-based LiPo cell get above 4.3V and the fireworks will begin.
The only safe way to bulk charge a LiPo pack is with an RC charger that monitors each cell.
I'm sorry if I sound like a broken record, but I feel strongly that we can't be too cautious in this area, and those of us with lots of LiPo experience should not miss a chance to preach the safety aspects of how to handle, use and charge these packs.
[/pulpit] :wink:
-- Gary
oooh there should be some hot videos this season 
