Jump to content

Cheap LiFePO4 BMS?


jetzi

Featured Posts

Yes, I've changed my mind and I agree with you. The alarm by itself is not sufficient, there are any number of reasons you might miss the alarm even if you liveaboard (someone else on your boat doesn't know what it means, or you pop out to a shop or to do a lock, or you are listening to loud TV, or you sleep through it). Disconnect on a per-cell basis should be considered essential and the minimum requirement for a BMS. I think the temperature cutoff could be considered optional though, especially if you keep your batteries in the cabin and are aware of the limitation.

Link to comment
Share on other sites

34 minutes ago, MtB said:

 

I disagree. 

 

1) I leave my fridge turned on when I'm not on the boat. This alone makes a "low cell" disconnect mandatory. Agreed the alarm perhaps falls into the 'nice to have' category but I think without the advance warning alarm, one only has to be left sitting in the dark unexpectedly once or twice, to begin to see it as essential.

 

2) Separate channels for load disconnect and charge disconnect are essential too. If you get say, a high cell disconnect you NEED the load circuit to stay connected or how will you discharge the battery to a safe level? Similarly with low voltage disconnect. You NEED the charging devices to remain connected to correct the low SoC.

 


In the end I didn’t go for separate load and charge disconnects. I took the view that both over and under voltage events were extremely unlikely and certainly not part of normal operation. I suppose a low voltage event is probably human error, a high voltage event is equipment fault, so the former is far more likely. My charging devices both have awareness of individual cell voltages.

 

In the event of a low voltage disconnect the BMS disables all its interfaces, display etc and goes to sleep, using well under a milliamp. When ready to charge, you start the engine - alternator is unpowered since leisure batteries are disconnected. You press a button on the BMS, the first thing it does is close the relay, which powers the alternator controller, then the BMS waits a few seconds before starting to check cell voltage again, meanwhile the alternator is now putting in 100A or so and the cell voltage is back up. 

Edited by nicknorman
Link to comment
Share on other sites

11 minutes ago, nicknorman said:

In the end I didn’t go for separate load and charge disconnects. I took the view that both over and under voltage events were extremely unlikely and certainly not part of normal operation. I suppose a low voltage event is probably human error, a high voltage event is equipment fault, so the former is far more likely. My charging devices both have awareness of individual cell voltages.

 

But if you do have separate load and charge disconnects, the system can be self-healing which is nice.

 

But yes I think that was a big shift in perspective from me in the journey of the last 86 pages, seeing the disconnects as emergencies and having the charge sources themselves be smart enough to know whether they should be charging the battery (at least on a whole-battery voltage level).

Link to comment
Share on other sites

3 minutes ago, jetzi said:

 

But if you do have separate load and charge disconnects, the system can be self-healing which is nice.

 

But yes I think that was a big shift in perspective from me in the journey of the last 86 pages, seeing the disconnects as emergencies and having the charge sources themselves be smart enough to know whether they should be charging the battery (at least on a whole-battery voltage level).

I can see that, for example, if you had solar, separate charge and load disconnects would be a good idea. But for our setup and usage, I couldn’t see any advantage.

  • Greenie 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.