Single cell voltage dropper...and multiples of..a balancer

[NeilP]

I know what I am asking is a balancer unit..but all the balancers I see are digital, with displays, , programable etc etc. Just wondered about a simple minimal component fixed voltage, discharger that you set to a fixed voltage...drops the cell to that voltage then cuts out

I was wondering a very basic circuit..say a reference voltage zener or similar and trimmer, a FET and power resistor, no doubt a few control components, no external power source, powered from single cell.

The idea is a single stand alone circuit that drops just a single cell to a reference voltage. and then if you want a 10 cell balance unit..you make up 10 of them...or use one or two on any cells that are getting a bit high while bulk charging.


I have no theoretical knowledge to knock out a schematic, so if any one could show me a simple circuit that would be great.

My current method is a two pin JST plug, on to a 'Y splitter" one side to a bulb and the other two a celllog with an alarm set to my required voltage. I just unplug the bulb when the Celllog alarm goes off.

 

[dnmun]

easiest thing for you to do is use a circuit like ping did on the old v1 signalab with the 431 voltage reference and make the resistor divider bridge using some 1% parts.

 

[NeilP]

Ok, thanks

So in the schematic as in

http://www.ti.com/product/lm431

Where does the load balancing current flow? through the bridge?
I am guessing that if that is so, it won't be so good for high current if using accurate 1% resistors

Ok, not massive currents, but 2-3 amps?

 

[circuit]

What you are describing is bottom balancing.
Seriously?

 

[NeilP]

Am I?
That was not the intention. I want a circuit, simple with a few a components as possible to turn on and allow current to flow when connected to a cell above a certain voltage, and shut off when hitting a preset voltage.

Probably your reading of my description that makes you think 'bottom balancing' but that is not the intention.

I was envisaging a 'manual balance solution' , with as many of these as you feel you need.

Quite often when using the iCharger if a pack is way out, the charger takes an age to drop voltage of two or three cells. Just want a bit more 'automation' and convienence than my present system of putting a bulb and cel logger across the high cell and listening for celllog alarm and disconnecting the battery. Yes could use celllog alarm output and relay to disconnect bulb but it is all extra bits.

Something like a power transistor and power resistor turned hard on / off at a trim adjustable voltage set point . If all passive components soldered and heat shrunk direct to legs of TO -92 style package, screwed to a heat sink bar.

 

[RLT]

Is something like this:
http://www.batteryspace.com/Smart-LED-Balance-Module-for-3.2V-LFP-Cell-with-250mA-Balance-Current.aspx
in the neighborhood of what you are looking for?

 

[NeilP]

Thanks for trying, but no. That is only low current balancing.

I want a bit more than that, 2-3 Amp minimum with ability to set the discharge voltage within a range of 4.00 to 4.16 volts.

Just want a transistor to turn hard on at a or above a setable voltage, and turn hard off again when the cell voltage drops back to that set point.

I am sure there is a way to do it, but not enough knowledge to design it myself.

 

[thepronghorn]

I have actually been wanting to do something similar recently. This is the best example I have found.

http://scolton.blogspot.com/2009/10/whats-next-charger-balancer.html
http://scolton.blogspot.com/2009/10/last-in-first-out-charger-balancer.html

I don't really care if mine is portable, so I was just going to use an external power supply as the reference voltage for a comparator. I got hung up because I wasn't sure what transistor to use. I could just copy the build above, but I'm not quite sure how the transistor's specs relate to what we're trying to do. Anyways I hope to return to this in a couple weeks, so I look forward to seeing what you come up with.

 

[dnmun]

I have actually been wanting to do something similar recently. This is the best example I have found.

http://scolton.blogspot.com/2009/10/whats-next-charger-balancer.html
http://scolton.blogspot.com/2009/10/last-in-first-out-charger-balancer.html

I don't really care if mine is portable, so I was just going to use an external power supply as the reference voltage for a comparator. I got hung up because I wasn't sure what transistor to use. I could just copy the build above, but I'm not quite sure how the transistor's specs relate to what we're trying to do. Anyways I hope to return to this in a couple weeks, so I look forward to seeing what you come up with.

really bad design. guy doesn't have a clue about real BMSs. it is as though he has never looked at a BMS except the one he made up. a ping v1 is so much better and cheaper.

 

[NeilP]

But with the LM431, is the divider bridge also the load

Does there need to be a coma parrot or reference voltage? I just want a single channel, with a transistor that turns full on at a set voltage, which is adjustable within a small range.
The transistor will then pass the voltage from the cell to a power resistor as load..

Just need a way to set the on point of the POWER transistor, an IRFB 3077 or similar as I have a box of them.

 

[dnmun]

no, the resistor divider network is attached to the cathode. the cathode will remain at 2.495V so for a 3.60V balancing voltage, the divider will have 2.495/3.60 at the cathode. i don't know the current capacity of the IC but i assume around 200-250mW power max.

 

[NeilP]

OK, so straight from the data sheet.



If the single cell is between V+ and ground, how would i get it to drive a FET hard 'on' to draw a load of say 5-10 amps through a power resistor? and then switch off as the cell reaches 4.15 volts, as it discharges?