Old BMS General Thread

[boelle]

hi

I'm new on this thread, but already got a few ideas and questions

1. if using 6 Volt cells can the voltage simply be divided by 2 in a simple resistor network and adjust cutin and cutout accordingly?

2. instead of burning energy in the load resistor why not leed the energy towards the weakest cell in the chain? i know you can pay for finished modules that do this for you but why not add it to the slave modules? some extra heavy wires might be needed...

3. anyone that know the charger in a Saxo? how to tell it to stop charging?

4. what are the difference in charging a NICD cell and Li-fepo4 cell And lead acid? Nicd are used in the saxo but it can be told to charge lead acid.. are any of the 2 close enough to be used with li-fepo4 ?

sorry for my bad English, have driven a taxi for 9 hours don't help it much either


/bo

[retepsnikrep]

hi

I'm new on this thread, but already got a few ideas and questions

1. if using 6 Volt cells can the voltage simply be divided by 2 in a simple resistor network and adjust cutin and cutout accordingly?

Possibly but the pic is only rated to 5.5v max, any potential divider you use on the input has to be able to cope with the load current reqd by the circuit. The 250ma load resistor would have to be external to the pd.

2. instead of burning energy in the load resistor why not leed the energy towards the weakest cell in the chain? i know you can pay for finished modules that do this for you but why not add it to the slave modules? some extra heavy wires might be needed...

The amount of energy wasted is miniscule compared to the battery capacity, to divert energy to weaker cells requires an isolated dc-dc converter or something similar, neither cheap or easy, but be my guest and have a go.

3. anyone that know the charger in a Saxo? how to tell it to stop charging?

Can't help I've no idea.

4. what are the difference in charging a NICD cell and Li-fepo4 cell And lead acid? Nicd are used in the saxo but it can be told to charge lead acid.. are any of the 2 close enough to be used with li-fepo4 ?

Certainly a lead acid charger could be used with li-fepo4 it depends on the type and charge curve etc and wether it can be controlled by the BMS. The Zivan NG3 IUa charge curve for low capacity lead acid cells is OK for li-fepo4.

sorry for my bad English, have driven a taxi for 9 hours don't help it much either


/bo[/quote]

[boelle]

Hi Peter

and thanks for answers... had some big time trouble getting to the BVS site this morning

1. is there any point in burning more watts in the balance resistor? one could build a liquid coolling system for them all and mount a radiator at the front somewhere....

the external balance i had in mind was something like this:

http://www.powercharge.dk/link.asp?menu=16&submenu=34

sadly i have yet to find this as DIY

2. i dont remember if i read correct somewhere.... but what is the max capacity cell i can use with this system?


off to the city shopping bikini with the girlfriend... have a meal and home again starting to draw some pcb

[retepsnikrep]

In my experience with two different lots of Lithium cells they stay in balance very well and minimal bypassing is needed, which makes most of the ideas about saving that energy a waste of time and effort.

I would say upto 200A cells should be OK with the BMS as is. The boards were designed to fit most cells upto 200ah with a bit of dremel work or filing of the holes/slots.

[boelle]

hm.. then i must have read wrong, thought i saw a place that the limit of the picaxe was 65a cells

EDIT: Found this http://www.geocities.com/sorefeets/bala ... matics.htm

[retepsnikrep]

The software running on the Master board can be configured for upto 650Ah cells, I have it set for 65A cells max at present as that increases the resolution of the AH counting. The Picaxe basic 16bit variables limit calculations to integer only max of 65335.

If you use the slave boards in analog mode without a master board there is no limit on capacity.

[boelle]

wow... i do plan to go at 400 ah cells... they cost but they fit the range i want in a saxo

at the moment i have started to draw the PCB... a task since i want to do it SMD...

so far i have the opto's in place (4 pins right? ) and the picaxe... so far i have to find the smd counterpart for so92

EDIT: it's hard to locate a counterpart for the bc337 and lm385

[boelle]

now i only miss D1, Q1, R2, JP1 and JP2

EDIT: only Q1, D1 and R2..... are 3watt availble in SMD ?

[retepsnikrep]

The lm385 is available as an smd.

The bc337 is any standard low frequency npn transistor capable of handling about 500ma. Needs gain of about 50-100

JP2 is not reqd so can be omitted.

JP1 is included to act as a hard reset in the event of the pic locking up. It avoids having to take the board off the cell to cut the power! JP1 can also be reqd to initiate a program download depending on the pic speed set in the software.

R1 can be a higher value if you are happy with a lower balancing capability or a lower value if you wish to burn off more current.

If you get a SMD version of the board built please release the files for the benfit of the others on this thread. Will you do this? Please post your schematic etc so we/i can look at your layout in case any obvious errors creep in.

Are you building an Analog or Digital version? I recommend the digital version layout which can also operate in a pseudo analog mode with different software. I using the digital layout in analog mode at present prior to my final push to complete the project over the summer.

I do need to do it as well as one of my cells is starting to show signs of high IR and I need to find out which one!!!

[boelle]

of course i will release it.. but i use the pcb123 software since i can get a price for a complete and ready board as i draw it

only missing a few bits left and then i have to find part numbers from digikey as sunstone.com only use them

it's the digital version i do.. but i figure i could do a few mods and have the analog version also