Low cost / High performance universal BMS

[nlc]

Sorry but I don't understood the question, what do you mean by bottom balancing of the cell ?

[nlc]

2 days ago I received the PCBs of the preproduction :



I also received all the component, I will try to begin the soldering tomorrow

[Alcibiades Ramos]

For lithium iron phosphate cells it is more important that, when discharging, they don't go under. Their safe bottom is around 2.7 volts. Below that the discharging curve is steep and they are prompt for reversal, in other words they die.
There is unfortunate notorious case with a guy that left his Tesla in a warehouse because he was moving and never charged the cells in that period. He lost the whole pack at U$40K.
Whoever uses lithium has to keep his/her cells between 2.7v and 3.8v at the most. Once you bottom balance your cells you almost don't have to worry about the top end. You just set up the charger to cut off at the whole pack's nominal voltage and cells take care of themselves. You don't bottom balance and they are an accident waiting for a place to happen.

[skeeterb]

OUCH!!!! $40K is a LOT of moola. you could probably convert several cars to EV with that amount of money. If I ever do convert a car to an EV, I plan on using Lithium Iron Phosphate batteries in it. How do you bottom balance your LFePO4 batteries? This could be vital, IF i ever decide to convert, because I have been wanting to convert an ICE to EV for a long time.Those boards look pretty good Nlc, can't wait to see what they would look like with all the components on them.

[Alcibiades Ramos]

Well, with a lot of patience. From what I've learn it seems that after you bring a cell down to 2.7v you need to wait to see where it goes. That is after 24hrs you might find a higher voltage upon measuring. Cells chemical almost organic things. If that is the case you want to discharge the cell again to 2.75 v and wait.
You do the same with the rest of the pack. If you see anyone that has gone under (a BMS measuring bottom voltage) then you pump it up back to 2.75v. Once you see that 24hrs go by and the cells are stable then and only THEN you charge them to specs.
You will see that after several cycles they go down at the same rate which is what you want for them to have a long life.
There is a site evtv me that has a couple of programs dedicated to bottom balancing LiPoFe. Peace.

P.S. I found the link but I'm going to leave spaces instead of the dots since it does not let me post it.

www

[nlc]

Sorry but one more time I don't understand why you talk about bottom "balancing", why "balancing" ?
The low voltage threshold protection is the more important job of the BMS, to shut down discharging if voltage comes below the low voltage threshold.

Maybe the Tesla BMS is not very good and consume power on each cell even when pack is disconnected by the protection ?

My slave boards will consume below 20uA of current on the cell, it's probably lower than the cell auto discharge.

This morning I finished to solder my 25 slave boards preproduction. 450 components and 1300 solder by hand, my eyes took a few hour to recover the far vision !!

[skeeterb]

Ouch man, you must have REALLY been straining your eyes to solder all those tiny components onto the slave boards.

[nlc]

For now my eyes are still good enough, it took me 2h30 to solder the 25 preproduction boards

[nlc]

Just a small question, I do a small survey : what is your opinion about the ideal size/resolution of the tft touch screen for the dashboard ?

[mrbigh]

Originally Posted by nlc

Just a small question, I do a small survey : what is your opinion about the ideal size/resolution of the tft touch screen for the dashboard ?

My personal preference is 7" diag screen. Besides that's what I actually use every single day and is part of the dashboard.



But depending in the quantity of information you are planing to show on the screen and the functionality you want to offer, will be the size of the monitor
I already once suggested and external VGA monitor with a USB touch screen interface