Low cost BMS

[Erc1727]

whats up post 2 outta 10 so i can pm

[harlequin2]

Go on then!

[DJBecker]

Two design options to consider:

Use fusible link wire for the first 50mm of wire from the cell in place of a traditional fuse. Fusible wire isn't as good as protecting electronics as replaceable fuses, and has a reputation of being a PITA to diagnose if blown, but provides broader protection and a neater installation.

What about using a low voltage (say 2.5V 2W-5W) zener diode in series with the shunt resistor. This will protect the cell against complete discharge if the shunt transistor or controller fails on.

[harlequin2]

I guess I didn't make it clear that the cell module boards bolt directly to the cell terminals so there are no connecting wires. There is a fuse in the form of a bit of skinny track though.
I don't see any need to protect against a shunt switch shorting "on".In the vanishingly unlikely event of such a failure, the cell voltage poll would show a discrepancy.
But thanks for your thoughts.

[ianbartie]

Problems plagued the Volt Blochers (which is why I suspect they disappeared).
They had no form of fuse anywhere & they were permanently across the cells.
One of the two ICs it used was directly connected across the cell without a resistor divider. One particular brand of IC had a nasty habit of failing ON & then draining the battery to death as there was no safeguard to limit battery discharge & no indication it was happening either.
They also generated a LOT of heat from the TIP122 & the 5W resistor when they were shunting.
I still favour a system that disconnects the BMS electrically from the cells when the vehicle is off.
My current BMS has miniature relays on each board that switch off when the car is turned off.

[harlequin2]

Originally Posted by ianbartie

Similar problems plagued the Volt Blochers (which is why I suspect they disappeared).
They had no form of fuse anywhere & they were permanently across the cells.
One of the two ICs it used was directly connected across the cell without a resistor divider. One particular brand of IC had a nasty habit of failing ON & then draining the battery to death as there was no safeguard to limit battery discharge & no indication it was happening either.
They also generated a LOT of heat from the TIP122 & the 5W resistor when they were shunting.
I still favour a system that disconnects the BMS electrically from the cells when the vehicle is off.
My current BMS has miniature relays on each board that switch off when the car is turned off.

Similar problems to what?
This BMS doesn't have any problems.
You can favour whatever you like but just read the title of this thread again. Your relays cost more than my whole board!
The shunt is 6R8 4 watts and made by series/paralleling 4 x 1 watt SMD resistors. The resultant surface area is large enough to keep the resistor temp below 6o deg.
Build one and see!

[ianbartie]

Harlequin2, my apologies for any confusion.
I was in no way refering to your BMS or your designs.
I was refering to the Volt Blocher sawickm mentioned & the 2 design options DJBecker brought up.
One about the fuse & one about about a safeguard to prevent a complete cell discharge. It may be something to think about.
The BMS I refered did not have those features & as a result a number of batteries in the US died. It was even sold by EV stores over there.
Don't get me wrong. I really want your system to be the one that works right.
There are so many that just don't make the grade.

[adamj12b]

Originally Posted by harlequin2

Unfortunately, Adam's charger design is quite illegal in most countries, including this one (New Zealand). I'd be surprised if it was legal in the US, but you never know!

Hey harlequin,

Im already aware of this. The problem with isolation is is you either need a massive line side transformer or a high frequency transformer inside the charger which will greatly reduce output range, or just add to cost to the charger while decreasing its efficiency.

-Adam

[jackbauer]

Likewise. I'd like to see a workable bms solution and hope this one suceeds. I like the idea of the relay disconnecting the modules when the car is turned off.

Regards the charger issue. While I understand the legality issue , I still fail to see the advantage of an isolated charger in a modern well built conversion.

[harlequin2]

Yes, the legality issue is one for each builder to confront personally. However, it is quite possible to design and build an isolated charger that is legal, efficient and reasonable cost. I have worked with a company here that is now producing a 5.4 kw charger programmable for Li and/or sla, can be powered from 3-phase or single phase mains with a PF of .99 and an efficiency of 92%. It can be set up for any output voltage in the range of 40 to 180 volts. I'm using one in my car to charge my 45 cell Li battery and it delivers 15A running from 230v single phase - I only have a 10A rated wall socket ie 2.3 kw input. Retail price is $US1995. It is isolated, legal and complies with all the regulations everywhere!