4S BMS on Aliexpress

[Will Prowse]

Gee whiz that is a large balance cable connection considering it is 4S

Just found this a few minutes ago

 

[Ponderosa Pete]

Somewhere on this forum (I can't find it again but I did capture part of it because it seemed significant at the time) you said, "Considering the likelihood of over voltage situation from most high quality mppt, and the chance of matched LiFePO4 cells going out of balance is rare (and BMS will correct for cell drift over time), and that LiFePO4 can be over charged to 4.2v per cell before electrolyte degradation... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely. We have been doing it this way for years, but people still want to use a BMS."

I am building an 8 cell (parallel and series from your video) 12v system from China and wonder if I need a BMS?

 

[Steve_S]

IMO and everyone will likely say the same thing, NO BMS' is begging for Trouble and an Open Invitation for Murphy's Laws to be applied to you. In otherwords, extremely foolish to risk expensive battery cells without using a BMS. Essentially, consider that the total investment can by trashed to recycle with ONE screw up.

LFP can NEVER be charged to 4.2V, whoever told you that shlock was / is a fool.

BTW: This is NOT a thread to talk about YOUR battery, start a separate thread to ask questions and discuss it.

 

[Gazoo]

Somewhere on this forum (I can't find it again but I did capture part of it because it seemed significant at the time) you said, "Considering the likelihood of over voltage situation from most high quality mppt, and the chance of matched LiFePO4 cells going out of balance is rare (and BMS will correct for cell drift over time), and that LiFePO4 can be over charged to 4.2v per cell before electrolyte degradation... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely. We have been doing it this way for years, but people still want to use a BMS."

I am building an 8 cell (parallel and series from your video) 12v system from China and wonder if I need a BMS?

This isn't the proper thread to post this but here goes. @Will Prowse was referring to using a separate port BMS with matched cells. He never said not to use a BMS. If using a separate port BMS he is saying it's ok to bypass the BMS's charging port and charge the battery directly if using matched cells. But still use the BMS for low voltage protection. Since the majority of us are ordering mis-matched cells, it's best to use both the LVD and HVD of a separate port BMS or a common port BMS.

I don't know when electrolyte degradation starts due to too much voltage but I would be very cautious here. Will may have been referring to some other type of LFE cells. As I recall, there have been bloated cells when overcharged to 4.2 volts.

Will goes on to say:
"Considering the likelihood of over voltage situation from most high quality mppt, and the chance of matched LiFePO4 cells going out of balance is rare (and BMS will correct for cell drift over time), and that LiFePO4 can be over charged to 4.2v per cell before electrolyte degradation... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely. We have been doing it this way for years, but people still want to use a BMS.

I would say use BMS for loads, and not for chargers. If you have mismatched cells, and some hit a higher voltage at high SOC quicker than others, drop the upper limit voltage of your controller. 14.0-14.2v is a safe charging voltage that can give full capacity with LiFePO4 12v."

BMS common port vs seperate port

Can someone explain the difference? When would you want one or the other? Benefits?

diysolarforum.com

 

[Will Prowse]

Yeah, but I think most people who are inexperienced should use a bms regardless.

I still charge raw cells to manual absorption voltage all the time. I watch the voltage. I ran systems like this 24/7, but I ensure they were top balanced and matched. Just because you can safely do it, doesn't mean you should. Stick to using a bms.

 

[Will Prowse]

Somewhere on this forum (I can't find it again but I did capture part of it because it seemed significant at the time) you said, "Considering the likelihood of over voltage situation from most high quality mppt, and the chance of matched LiFePO4 cells going out of balance is rare (and BMS will correct for cell drift over time), and that LiFePO4 can be over charged to 4.2v per cell before electrolyte degradation... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely. We have been doing it this way for years, but people still want to use a BMS."

I am building an 8 cell (parallel and series from your video) 12v system from China and wonder if I need a BMS?

No, use a bms. In context on that thread, and in practice for some, you can go without a bms (I still do). But everyone should use a bms.

 

[Ponderosa Pete]

Understood. Sorry for posting here.