my Black And decker 12v charger likes to stop charging the TS-LP20AH at around 3.65 -3.75v per cell...I think it tests to see how much more current the cells can handle and then it somehow knows to cut back on current and it never goes over 3.75v from my experience
What gets everybody into trouble is that flat LiFePO4 discharge curve. You can no longer measure the voltage(s) and determine the SOC. The only accurate way is to measure the actual amp-hours going in and out on each individual cell. That is why the GEM battery monitor lied. It used pack voltage to determine SOC which is not valid for lithium.
What you've discovered goes along with my BMS design. I was only going to clamp the cells that are in the over voltage zone. As the clamps turn on I was going to throttle back the charger. When 10% of the clamps are on I was going to shut down the charger. Running it longer just wastes energy with little gain in range. My clamps are going to be semiconductors not resistors. If the switch element should fail the individual fuse blows this would be detected and shut down the charge.
I still think that balancing could make some sense, but it should work in different way: there should be a possibility to initially charge only the battery that has lowest voltage, to the level of the next one. Then charge the whole pack to 3,75. What do you think?
Your explanations are very clear. The whole shunt regulator concept is flawed.
I imagine batteries as glass burettes that you fill up with water, but they have slightly different diameter, so to reach the same level you have to put different amount of water to them. Instead, you can just put the same amount of water to each of them and not think about water level.
I agree. My experience tells me that as long as your C rates reasonable and you stay within the upper and lower limits, the cells will tend to cycle closer together over time, not farther apart.
This has been observed going back to high performance LiPo powered RCs several years ago. LiFePO4 is even more forgiving.
The flat curve of LiFe PO4 means that the butter zone is between 3V and 3.3~3.4V generally (assuming no surface charge).
My chinese battery supplier told me point blank that they tried using the shunt system and they burned up so they moved to the balance charger setup instead.
I also agree that 4.25V is stupidly high and completely unnecessary. Mine (not TS) will typically be limited to 3.75 by the charger.
As for saying that you need dynamic balancing for charging AND discharging....... they are nuts! Not to mention the price of them. Unless you have a bad cell they will self balance over time.
We actually have a charger FROM Thundersky designed specifically for 24 cells and 72 volts. It charges to 87 volts, which is 3.625 volts per cell - pretty close to the 3.65-3.75 vdc range I've been advocating.
The disconnect is the concept that you balance at the end where you are putting in 10 amps charging, and unbalance the end where you are pumping 300 amps discharging.
At high currents, the cells on the downhill side are driven to destruction in seconds.
my Black And decker 12v charger likes to stop charging the TS-LP20AH at around 3.65 -3.75v per cell...I think it tests to see how much more current the cells can handle and then it somehow knows to cut back on current and it never goes over 3.75v from my experience
cypher954 2 years ago
LOL, the alloy wheels on the golf car are the same as the ones on the mini.
not my taste at all btw.
zeblezeb 2 years ago
What gets everybody into trouble is that flat LiFePO4 discharge curve. You can no longer measure the voltage(s) and determine the SOC. The only accurate way is to measure the actual amp-hours going in and out on each individual cell. That is why the GEM battery monitor lied. It used pack voltage to determine SOC which is not valid for lithium.
randycarter2001 2 years ago
What you've discovered goes along with my BMS design. I was only going to clamp the cells that are in the over voltage zone. As the clamps turn on I was going to throttle back the charger. When 10% of the clamps are on I was going to shut down the charger. Running it longer just wastes energy with little gain in range. My clamps are going to be semiconductors not resistors. If the switch element should fail the individual fuse blows this would be detected and shut down the charge.
randycarter2001 2 years ago
I still think that balancing could make some sense, but it should work in different way: there should be a possibility to initially charge only the battery that has lowest voltage, to the level of the next one. Then charge the whole pack to 3,75. What do you think?
mho69 2 years ago
Your explanations are very clear. The whole shunt regulator concept is flawed.
I imagine batteries as glass burettes that you fill up with water, but they have slightly different diameter, so to reach the same level you have to put different amount of water to them. Instead, you can just put the same amount of water to each of them and not think about water level.
mho69 2 years ago
I agree. My experience tells me that as long as your C rates reasonable and you stay within the upper and lower limits, the cells will tend to cycle closer together over time, not farther apart.
This has been observed going back to high performance LiPo powered RCs several years ago. LiFePO4 is even more forgiving.
The flat curve of LiFe PO4 means that the butter zone is between 3V and 3.3~3.4V generally (assuming no surface charge).
Stay close to that and just run it.
grndiesel 2 years ago
My chinese battery supplier told me point blank that they tried using the shunt system and they burned up so they moved to the balance charger setup instead.
I also agree that 4.25V is stupidly high and completely unnecessary. Mine (not TS) will typically be limited to 3.75 by the charger.
As for saying that you need dynamic balancing for charging AND discharging....... they are nuts! Not to mention the price of them. Unless you have a bad cell they will self balance over time.
grndiesel 2 years ago
We actually have a charger FROM Thundersky designed specifically for 24 cells and 72 volts. It charges to 87 volts, which is 3.625 volts per cell - pretty close to the 3.65-3.75 vdc range I've been advocating.
The disconnect is the concept that you balance at the end where you are putting in 10 amps charging, and unbalance the end where you are pumping 300 amps discharging.
At high currents, the cells on the downhill side are driven to destruction in seconds.
marionrickard 2 years ago