BMS For Dummies...

[Aphers]

OK to show just how new I am to this I have some *really* basic questions about battery protection, to see if I'm understanding this properly.

1- simplest option seems to be use the BMS itself as the cutoff device on both charge and load sides.
2- better is to use the BMS to trigger separate cutoff devices (relays?), one on each side of the battery, sized according to the current they have to carry.
3- perhaps even better is to use smarter relays e.g. VSR that will do their thing without the BMS needing to tell them. On top of this, a smart charge controller or inverter will know when to stop sending/drawing power, adding a further layer of protection.

Does this sound about right? I know there are plenty of ways of configuring things. I'm hoping to install a LFP system but I'm concerned that my budget will not stretch to it, so I need to work out a sort of lowest cost practical system so that I can do a meaningful comparison with the lead-acids I would otherwise be buying.

 

[snoobler]

1. yes.
2. preference based. relays may incur a load penalty as they consume power to activate. More common to use relays when one has higher current requirements than most BMS support.
3. VSR is not a substitute for BMS. Each LFP cell needs to be monitored. That's what the BMS does - LV and HV protection on a CELL basis.

If you can't afford a BMS, you shouldn't do LFP.

In @Craig's signature: BMS! or be a mess!

Plenty of opportunity to destroy your expensive investment by ignoring proper management.

 

[Aphers]

1. yes.
2. preference based. relays may incur a load penalty as they consume power to activate. More common to use relays when one has higher current requirements than most BMS support.
3. VSR is not a substitute for BMS. Each LFP cell needs to be monitored. That's what the BMS does - LV and HV protection on a CELL basis.

If you can't afford a BMS, you shouldn't do LFP.

In @Craig's signature: BMS! or be a mess!

Plenty of opportunity to destroy your expensive investment by ignoring proper management.

Thanks. I didn't list no-BMS as one of the options, I was just thinking of using smart devices that can sense whole-pack voltage, and the BMS would be a second layer of protection, with the benefit of cell-level voltage data.

I would like to run some higher power loads- the biggest single one would be 1600w via an inverter, so in practise that means a 2kw inverter drawing 170A. Apart from that one load, nothing going in or out of the pack would exceed 60A, so does that mean I can spec a BMS that can handle that, and use a relay on the inverter?

 

[snoobler]

Most quality chargers and inverters or inverter/chargers allow for programming of desired operating limits inside of BMS limits.

You would need to spec a 170A+ BMS. For a BMS to work, it must be able to cut the circuit.

Or you use a BMS that only monitors cells and opens/closes relays as needed.

 

[Zil]

The proper way to use BMS is as the last ditch protection of the battery. There should be a means to cut off discharge before the BMS trips. The charging devices should not over charge the battery and the BMS should only trip if one (or more) cell over volts.

 

[Aphers]

The proper way to use BMS is as the last ditch protection of the battery. There should be a means to cut off discharge before the BMS trips. The charging devices should not over charge the battery and the BMS should only trip if one (or more) cell over volts.

Thanks. Just to dlarify, do you mean option 3 above, or option 2?

 

[Zil]

Not your option #1. Option #2; Depending on the BMS, some more expensive ones can do that. Option #3; Most inexpensive BMS need this option to meet my construction specification. But as Snoople said, you must have a BMS with Li battery.

 

[Aphers]

Not your option #1. Option #2; Depending on the BMS, some more expensive ones can do that. Option #3; Most inexpensive BMS need this option to meet my construction specification. But as Snoople said, you must have a BMS with Li battery.

Yup I know that option 1 is a good way to ruin my cells, so that's off the table.
It seems like with a really good system, you could remove the BMS and everything would still work, i.e. it's a completely separate backup protection system.

To play devil's advocate, at what point will we consider cells to be cheap enough that this level of protection is no longer required?

 

[Zil]

The nature of how Li cells behave, each cell must have monitoring on the cell level. One or more cells could discharge lower or charge higher than the other cells in the battery. Monitoring the overall battery voltage will not show the individual cells approaching calamity. If the cells were free I would not risk this.