magudaman said:
Russell:
Once a battery reaches the charger voltage there will be no current flow, so no letting a LiFePO4 cell sit on a charger at 3.70V won't damage it.
Well I pretty sure this is not true. The battery's nominal voltage is 3.2v so if the current is zero that is where it will naturally sits. It takes some level of current to push the battery up to 3.7v. Most lithium battery chemistries have a cut off amperage of C/20 maybe it was C/25 during their charge. You can not keep a float charger on a lithium battery.
There was quite a bit of good info here when I was looking for a decent reference: http://www.mpoweruk.com/chargers.htm
Constant-current Constant-voltage controlled charge system. Used for charging Lithium batteries which are vulnerable to damage if the upper voltage limit is exceeded. Special precautions are needed to ensure the battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit.
The charge voltage rises rapidly to the cell upper voltage limit and is subsequently maintained at that level. As the charge approaches completion the current decreases to a trickle charge. Cut off occurs when a predetermined minimum current point, which indicates a full charge, has been reached. Used for Lithium and SLA batteries
The BMS really need to turn on much sooner at say 95% DOD and balance to the lowest cell instead of shaving off the highest cells at the very high peak of the charge.
Well it is the truth. First of all when talking about charging batteries you have to understand that each chemistry has different characteristics and charging requirements however there are also basic electrical principles too which you need to understand. With respect to liFePO4 cells the generally accepted "fully charged" voltage is 3.65V/cell however they are 99%+ full at around 3.40V.
When you start charging a LiFePO4 battery your charger's voltage will adjust to a level which maintains a constant current until the battery is about 95% full at which point it switches to constant voltage (again this is for LiFePO4, other chemistry profiles are different), for a 16 series pack this can be anywhere from 57.6V to over 61V depending on who supplied the charger. When in this constant voltage mode the current continually falls while the last 5% of the charge is supplied to the battery. Most chargers are adjusted to show a "green" fully charged condition when the current reaches a predetermined setpoint which again varies but as an example let's say it's 0.200A. Now some chargers may turn off but none of mine do, instead if you leave most connected the current will continue to fall towards zero as the battery voltage continues to slowly rise to match the charger's output voltage.
When I'm balancing my pack I leave the charger connected until the current reaches about 0.040A (40mA) as indicated by an in-line wattmeter. At this point the average cell voltage will be approximately 3.70V (this will vary depending on the absolute charger voltage). My BMS will balance to 3.60V so at this point the shunt resistors on it are a little warm as they are trying to bleed the excess voltage from the cells to get them to 3.60V, however since the charger is still connected they can't. If I would leave the charger connected that little 40mA or so of current will continue to flow, the cells will remain at about 3.70V and the BMS shunt resistors will constantly be trying to bleed the cells (assuming of course a "bleeding" type BMS) so they will continue to generate a little heat. When I finally get around to removing the charger the BMS will finally be able to drain the cells to the balance point so when I check the individual cells voltages in 15-20 minutes they all read 3.60V+/- 0.01V. None of this will hurt the battery or BMS.
One way I could damage the BMS is to overwhelm the shunt resistors. On my BMS the shunt resistors are tiny so if I cranked the charger voltage too high I could burn them up. Assuming the BMS can handle it however raising the voltage to 3.8V/cell like Ping does can result in quicker balancing though it also risks more BMS cell overvoltage trips which usually occurs at about 3.90V.
Once again what I have said above is for LiFePO4 with a bleeding BMS, other chemistries will differ. For example when I charged LiMn Bosch Fatpacks the switchover from CC to CV would occur when the battery was 80% full with the CV phase lasting much longer as the current decreased toward zero. I set the charger for 41.3V for the 10 cell pack and when the charger reached the terminal voltage the current would drop to essentially zero. I have to stress this; when the battery reached the terminal voltage of 41.30V there was NO further current flow. I could have left the charger connected forever with its output at 41.30V and the battery at 41.30V and there would be no problem because there would be no further current flow between the charger and battery and NO trickle charging going on.
With Lithium you DO have to ensure that no cell goes too high, in the case of my Fatpacks with no BMS I simply used a charge voltage of 4.13V/cell instead of the normal for most lithium 4.20V. The best way to keep cells from going too high while charging them in series is to be sure the cells are matched to begin with then balance them and keep them balanced. LiFePO4 is so darn safe because they are also much more tolerant of overcharge than other li-ion cells. For example I looked up the specs for my pouch type cells and the vendor specs a 4.2V maximum. If you have a BMS it will generally fault and open the charge circuit at around 3.9V thus keeping the cell from ever reaching this maximum.
-R
