dnmun
1 PW
it is fun to be one with your cells, but with a BMS it is even easier. all you have to do is raise the charger voltage enuff for the BMS to fill every cell. you can even let it balance itself, as it is intended to do.
How does a BMS board ACTUALLY work?
- Thread starter Peter Sternersson
- Start date
Tommy L
100 kW
dnmun said:
I thought the BMS will shut the charger off once once cell reaches a certain voltage.
Is there balancing bms boards or they are all suppose to be balancing bms boards?
If so, why do people have out of wack packs? Bad chargers?
Tommy L sends... \\m//
dnmun
1 PW
that's what balancing is. when the high cell reaches the 3.9V level the BMS will shut off the charger while the shunt transistor drains the charge from that high cell until the high cell drops down to about 3.8V, then it turns on and pushes a little more charge into ALL the cells, then shuts off on the HVC of that high cell while drains the charge off the high cell, and repeat over and over.
when the pack gets to where it is balanced, then the time spent in this phase drops to almost nothing and the entire pack will end up balancing at full charge each time, together, or within a few minutes of each other.
the LVC detection is just there to prevent the pack from being accidentally discharged to zero because it disconnects from the battery when the voltage on any cell drops below the 2.1 or 2V LVC.
the LVC is not there to tell you when to stop, it is there to protect the battery.
when the pack gets to where it is balanced, then the time spent in this phase drops to almost nothing and the entire pack will end up balancing at full charge each time, together, or within a few minutes of each other.
the LVC detection is just there to prevent the pack from being accidentally discharged to zero because it disconnects from the battery when the voltage on any cell drops below the 2.1 or 2V LVC.
the LVC is not there to tell you when to stop, it is there to protect the battery.
dogman dan
1 PW
Sounds like maybe that one cell could stand some more charging. Sounds to me like you could have gone a bit further before that one low cell stopped you.
But Dnmum is correct, that if fully charged, you should have seen closer to 15 ah before the bms shut the pack down.
At this point, the worst it can be is you need to buy one extra cell if the bad one fails early. The rest of the pack sounds fine.
Keep leaving it charging overnight.
But Dnmum is correct, that if fully charged, you should have seen closer to 15 ah before the bms shut the pack down.
At this point, the worst it can be is you need to buy one extra cell if the bad one fails early. The rest of the pack sounds fine.
Keep leaving it charging overnight.
Tommy L
100 kW
my concern is that one cell dropped to 2.9x
I'm sure it recovered in voltage before he checked it. Yes? No?
LiFePO4 drops fast after 3.0v. The load tests I've done on my headways shows that
the minute they are just over their available watt hours, they drop in voltage FAST!
If one of his was 2.9x and he checked it after some recovery time, it could have been down much lower under
load.
I get full AH rating out of my headways 38120HP at 2c and 4c right to 3.0 volts and there is about another .5ah there before they drop below 3.0 volts.
On my 12s headway 12s1p 8ah if all cells are balanced, I can take her down to 36v under a 4c load (32 amps constant @ 36v) for 15 minutes +
and a 2c load (16 amps constant @36v) for 30 minutes + and not one cell will drop below 3.0v during it's entire discharge.
dnmun....... Cool on the bms. I thought that they'd just use bleed resistors on cells above 3.7-3.8v.
Tommy L sends.... \\m//
I'm sure it recovered in voltage before he checked it. Yes? No?
LiFePO4 drops fast after 3.0v. The load tests I've done on my headways shows that
the minute they are just over their available watt hours, they drop in voltage FAST!
If one of his was 2.9x and he checked it after some recovery time, it could have been down much lower under
load.
I get full AH rating out of my headways 38120HP at 2c and 4c right to 3.0 volts and there is about another .5ah there before they drop below 3.0 volts.
On my 12s headway 12s1p 8ah if all cells are balanced, I can take her down to 36v under a 4c load (32 amps constant @ 36v) for 15 minutes +
and a 2c load (16 amps constant @36v) for 30 minutes + and not one cell will drop below 3.0v during it's entire discharge.
dnmun....... Cool on the bms. I thought that they'd just use bleed resistors on cells above 3.7-3.8v.
Tommy L sends.... \\m//
Peter Sternersson
100 W
Wow, lots of writing while I was sleeping I see
The limit wasn´t reached at 12Ah, it was me that was on the limit, the limit of freezing to death so I called it a sucess
The low cell is of course not desirable, but I have hopes that it might recover. And if it doesn´t I´ll replace it.
dnmun, I wrote some time ago that
"All cells came up to around 3,7V while on the charger.
And now after 16h all cells are around 3,57V, except one that was only 3,4V."
I will do some more measuring on the cells this weekend and try to determine if the lower cell will recover or not.
The main purpose with my bike is to take me to and from work, which consumes about 8-10Ah if I hold WOT and pedal like crazy. So for every day use I´ll get by fine for now...
The limit wasn´t reached at 12Ah, it was me that was on the limit, the limit of freezing to death so I called it a sucess
The low cell is of course not desirable, but I have hopes that it might recover. And if it doesn´t I´ll replace it.
dnmun, I wrote some time ago that
"All cells came up to around 3,7V while on the charger.
And now after 16h all cells are around 3,57V, except one that was only 3,4V."
I will do some more measuring on the cells this weekend and try to determine if the lower cell will recover or not.
The main purpose with my bike is to take me to and from work, which consumes about 8-10Ah if I hold WOT and pedal like crazy. So for every day use I´ll get by fine for now...
fivari
10 W
I don't want to break in into this thread, but since my problem is somewhat related:
On the first page of this post, I mentioned a similar problem:
ground to first: 3.45V
all other between 3.7 and 3.8
The BMS is just warm, not hot. It are indeed the shunt resistors. The BMS has two 8 pin connectors, so the ground uses the power wire as connection to the BMS.
I tried again to charge the first cell separately, but it looks as if the energy I put in the first cell is dissipated immediately.
However, after such a treatment of separately charging, I was able to get 35Ah out of the pack being 36Ah nominal. In that case, it was not the first cell that made the BMS cut off.
When I leave the pack for some days, more power is drawn from the first cell. First cell is then at 3.29V whereas the others are at 3.32V. I still guess that the BMS contains a wrong treshold voltage for the first one.
So the question remains: how to check my assumption and if the assumption is correct==> How to correct this? I suppose I have to replace/modify some resistors on the BMS
On the first page of this post, I mentioned a similar problem:
I was suggested to measure the voltage of the cells during charging.
Here we go, near the end of charging:dnmun said:
ground to first: 3.45V
all other between 3.7 and 3.8
The BMS is just warm, not hot. It are indeed the shunt resistors. The BMS has two 8 pin connectors, so the ground uses the power wire as connection to the BMS.
I tried again to charge the first cell separately, but it looks as if the energy I put in the first cell is dissipated immediately.
However, after such a treatment of separately charging, I was able to get 35Ah out of the pack being 36Ah nominal. In that case, it was not the first cell that made the BMS cut off.
When I leave the pack for some days, more power is drawn from the first cell. First cell is then at 3.29V whereas the others are at 3.32V. I still guess that the BMS contains a wrong treshold voltage for the first one.
So the question remains: how to check my assumption and if the assumption is correct==> How to correct this? I suppose I have to replace/modify some resistors on the BMS
Tommy L
100 kW
fivari said:
I've got my Flack Jacket on........ lol
At this point, I like No BMS. I use Cell Logs and Warning Buzzers to tell me whats up with my pack.
I'm looking for a good RC charger and Power Supply. The RC Charger has a built in BMS and will top off all the cells
while bleeding off the cells that climb to a higher voltage first, leveling off the pack
For now, I bulk charge and use Battery Medics in a "Discharge" mode. I set the discharge to be 3.7v. My Battery Medics
do not bleed off a lot of watts, so my pack has to be fairly balanced for this to work. Also, I notice that on one of my Battery Medics,
one of the Resistor bleed circuits is not working. Anyone know how to check/fix this? and/or allow for more Watts to bleed off by adding
resistors?
Not using a BMS gets you on the same frame of mind with what your cells are doing. You become the interface and deal with it accordingly.
There are many hear on ES that do this. Pro's and Con's....yada yada yada and if you ever go LiCo (RC style, non LiFePO4) these are non BMS batteries and
using alarms and meters is what most use. I just got some cell logs.... One of the readout displays each cell as a BAR GRAPH showing
how full each cell is. This is great to watch them under load and see how the pack/each cell performs
Also, I purchase some 8s LED read outs with alarm buzzers. WOW! they are loud! and you can set the voltage on them to alarm at the voltage you
specify
Tommy L sends.... \\m//
Tommy L sends....
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