LFP cell behavior

Horsefly

Photon Sorcerer
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Dec 12, 2020
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906
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Denver, mostly
The thing is it has been a blanket absolute statement made here many times. This information could confuse people that are starting on there learning curve. Besides as long as he uses a BMS that uses relays instead of FET control. And as long as the relays are appropriate, I see no issue with this.

"It is an inexpensive Chinese BMS from LLT. However, I did check the cell voltages reported by the bms against a Fluke dvm and they were within a mV or two. The bms controls charge directly, and discharge using a relay configuration."


I would be more worried about "It is an inexpensive Chinese BMS" You mileage may vary .
An absolute blanket statement that "a BMS is not a charge controller" is 100% correct. No confusion. Doesn't matter how much you spend on a BMS, it still isn't a charge controller. Not sure what the point is of talking about relays vs MOSFETs. A BMS with relays is no more of a charge controller than a BMS with MOSFETs is.
 

bdserv

Solar Enthusiast
Joined
Apr 5, 2021
Messages
35
The thing is it has been a blanket absolute statement made here many times. This information could confuse people that are starting on there learning curve. Besides as long as he uses a BMS that uses relays instead of FET control. And as long as the relays are appropriate, I see no issue with this.

"It is an inexpensive Chinese BMS from LLT. However, I did check the cell voltages reported by the bms against a Fluke dvm and they were within a mV or two. The bms controls charge directly, and discharge using a relay configuration."


I would be more worried about "It is an inexpensive Chinese BMS" You mileage may vary .

There is nothing inherently unreliable about the use of MOSFETs in power control, assuming proper design and application.

Beyond semantics, is there a reason why people are so adamant about not relying on the BMS to protect against over voltage, but at the same time are fine with relying totally on the BMS to protect against under voltage?
 

HRTKD

Boondocker
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Apr 24, 2020
Messages
5,957
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Somewhere South of Denver
There is nothing inherently unreliable about the use of MOSFETs in power control, assuming proper design and application.

Beyond semantics, is there a reason why people are so adamant about not relying on the BMS to protect against over voltage, but at the same time are fine with relying totally on the BMS to protect against under voltage?

We aren't too keen on the BMS stopping discharge either. The loads (usually an inverter) should be programmed to stop drawing a load when the battery hits a certain voltage, long before the BMS has to cut off discharge.
 

Sojourner1

Itinerant
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Sep 30, 2019
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581
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USA
He's using this in a golf cart so no inverter just pure dc power till there's none.
 

HRTKD

Boondocker
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Apr 24, 2020
Messages
5,957
Location
Somewhere South of Denver
He's using this in a golf cart so no inverter just pure dc power till there's none.

Yeah, I figured that was the case. With a less sophisticated system (no programmable elements), it sounds like it is up to the BMS to enforce the low voltage disconnect. Not much you can do about it unless the golf cart has some smarts built into the system.
 

Sojourner1

Itinerant
Joined
Sep 30, 2019
Messages
581
Location
USA
There is nothing inherently unreliable about the use of MOSFETs in power control, assuming proper design and application.

Beyond semantics, is there a reason why people are so adamant about not relying on the BMS to protect against over voltage, but at the same time are fine with relying totally on the BMS to protect against under voltage?
I suppose it's like the emergency brakes in your car, you could use them to stop the car but why not save them till theres an emergency. Bottom line though your build use it the way you want, we can only suggest something maybe you didn't think about or realize. ;)
 
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