JK-BMS says 3.317V per cell but only 3% SoC

[sunshine_eggo]

I'm unclear why Andy moves protection back to 3.65V after he sets the "full charge" to be equal to 3.451V.

>3.65V is where LFP can be damaged in terms of reduced cycle life. This is where you want protection.

If 3.451V is 100% full, isn't that where you want to stop charging?

Yes, but you do that with the equipment, not the BMS.

To charge conservatively don't you want to keep protection kicking in around 3.45V? Or 3.55V?

You NEVER use the BMS for routine cut-offs. You set the BMS to the meaningful protection levels (3.65V/2.5V) and set your hardware to operate INSIDE those limits.

I thought pushing the cells to 3.65V regularly wasn't so good for their longevity.

Most cell manufacturers give a 2.5V-3.65V range (with 0.05C tail current). If you operate according to the datasheet specifications, you will get quoted cycle life (on average). If you use the batteries less aggressively, it is generally accepted that you will experience improved cycle life.

Don't overthink it. Typical settings:

BMS protection at 3.65/2.5V
Fast charge to 3.55-3.65V with 15-30 minute absorption and/or 0.05C tail current - useful for generator charging, and you want to actually have 0 absorption if you can with generator unless you NEED true 100%.
Slow charge to 3.45V with one to two hours of absorption - may experience improved cycle life.

Worth noting that many "dumb" BMS have protection values at 3.75V. The server rack type batteries initially were about 3.9V.

LFP can safely be charged to 4.20V, but you get very very little capacity above 3.65V, and you definitely reduce cycle life.

These cells aren't tender snowflakes that will melt if you look at them funny. They are pretty rugged and will perform well if not abused.

 

[The Terrible Triplett]

Had to google this. "Xiaoxiang Smart BMS is a battery management system for electric vehicles."
So this app works with JK-BMS and you're saying it's better than using the stock JK-BMS app?

Apologies. I read JK and acted on JBD BMS. My bad. Sorry.

That Reset Capacity is a pretty cool feature on the Xiaoxiang App, the full free version.

 

[kolek]

You set the BMS to the meaningful protection levels (3.65V/2.5V) and set your hardware to operate INSIDE those limits.

Sorry for the noob question, but by "hardware," in my case that means you want your AiO inverter to start and stop the charging, not the BMS, right?

 

[kolek]

These cells aren't tender snowflakes that will melt if you look at them funny.

What if the owner/operator is a tender snowflake that melts if you look at them funny?
Oh sorry, nevermind... guess that is irrelevant to the settings.

 

[kolek]

Don't overthink it.

But overthinking everything is how I overcompensate for growing up fat.

 

[sunshine_eggo]

Sorry for the noob question, but by "hardware," in my case that means you want your AiO inverter to start and stop the charging, not the BMS, right?

Yes. Absorption to whatever voltage you like, below 3.65V/cell

What if the owner/operator is a tender snowflake that melts if you look at them funny?
Oh sorry, nevermind... guess that is irrelevant to the settings.

Be brave!

But overthinking everything is how I overcompensate for growing up fat.

Well... I grew up thin, but I'm growing old fat. If you're thin now, you're doing it right. Keep it up.

 

[kolek]

My JK setting presently View attachment 213737View attachment 213738

Matched except for the calibration numbers and MOS OTP and MOS OTPR which cannot be changed apparently. Thanks a lot @Skypower !

 

[kolek]

I calibrated voltage already but not current. What's the best technique for calibrating current? I have a clamp meter. Something like disconnect PV, disconnect load, then set it to 0?

 

[Skypower]

I'm unclear why Andy moves protection back to 3.65V after he sets the "full charge" to be equal to 3.451V.

If 3.451V is 100% full, isn't that where you want to stop charging?

To charge conservatively don't you want to keep protection kicking in around 3.45V? Or 3.55V? I thought pushing the cells to 3.65V regularly wasn't so good for their longevity.

You never want to be using the protection of your bms on a regular basis. That’s your last line of defense. When it’s triggered it’s opening your charge side FETs of your bms and if the value is left there it may do this a dozen times or more each cycle. No Bueno! It’s best to set your absorption/bulk voltage maximum voltage by the inverter. That way at full charge some cells will go over, and with a small amount of time come down when the balancer has done it’s job. Say for instance you haven’t gotten to full charge for a week because of bad weather and your delta will probably be around 60mv or so, and the highest cell might be 30mv over the average, that’s only 3.48 volts. No big deal, it’s not going to be there long with 2 amps of balance power and it’s WAY below the 3.65 vpc max.

 

[Skypower]

I calibrated voltage already but not current. What's the best technique for calibrating current? I have a clamp meter. Something like disconnect PV, disconnect load, then set it to 0?

You need a very stable charge current to do that no loads kicking on or off and a highly accurate clamp meter. I’d suggest a 20 to 30 amp charge load.