2 componet simple charge regulator lifepo4

pgt400

pgt400

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I was searching on the TS battery yahoo forum and see were some are using simple charge regulators across thier TS cells. The device consists of a white LED and TIP 105,125 PNP darlington transistor. Apparently a white LED has a consistant 2.5v drop and the TIP105 a 1.2v drop during conduction (total 3.7v). Connected emitter to + , Base to LED anode, LED cath to - and collector to -. As the voltage nears 3.7 volts the Darlington will switch on thru the led and base and shunt the current. Very simple. They do warn againist using this with high AH cells where higher charging currents could cause the transistor to overheat. Anyone try these?View attachment simple lifepo4 clamp.JPG
 
Shunting directly across the battery is a bad idea, the 10 ohm resistor is NOT optional, neither is heatsinking. This design will also have issues with tempco - the voltages will vary depending on the individual characteristics of the darlington and the LED, the ambient temperature and also the age of the components.

Also - if you test the voltage drop across a white LED - it changes with the current across it - and they range from 2.3-2.8V forward voltage from the same batch. Urk.

This is the main reason why programmable voltage references were made - its just not accurate enough.

Also - someone tested this out...

"Posted: Wed Mar 04, 2009 7:14 am Post subject: [ThunderSky] Re: Inexpensive balancing BMS recommendations p

--------------------------------------------------------------------------------

Jan:

I did some further experiments with alarming results. It appears that the darlingtons avalanche at some point with no real recovery on a battery system.

On a lab power supply, I rigged up the Darlington and white LED. At about 3.85 it starts to conduct and at 4.00 volts it goes to a about 1.5 amps. At that point, the amperage counts up, the voltage counts down. But as the voltage comes down, the amperage continues up.

I took 3 TIP105s and a white LED and made a board with about 7 square inches of heavy copper foil as a heat sink with holes for the battery terminals.

On the bench, it exhibited the same behavior. I thought well, maybe on the battery the battery voltage will keep this in control.

I mounted the board on the battery, put on a charger, and charged it until the voltage reached about 4.00 volts. Then I shut off the power supply and disconnected it.

The voltage fell steadily but slowly. The current kept climbing. Then the board melted. Finally the fire started.

I managed to get a hot bolt off before burning down the garage.

The LED isn't the problem per se. The voltage across the LED was constant the whole time.

But the darlingtons, even with a pretty good amount of heat sink, and three of them in parallel, hit some sort of temperature point and there is no getting back with disconnecting it.

On the lab supply, I was able to back off to 2 volts or even 0 and that would allow it to recover.

I can't do that with a battery.

We need some sort of controllable silicon device that when we shut it off, it shuts off. "

http://www.convertthefuture.com/bbs/viewtopic.php?t=22889&start=15&sid=5a48500837e06a68543e35efb9187727
 
No problems why not, just HEATSINK and use a resistor to limit the power dissipation - the main killer of this is thermal runaway - limit your dissipation and your transistors wont go north on you.

Only issue with this design is the tempco - at least they will all balance to the same(ish) voltage given the same ambient temperatures.
 
ever try a voltblocher?

www.voltblocher.com

the kit is cheap and works for bigger cells. I'm using 24 of them. There's a pot so that you can adjust the voltage. This is important since it needs to be tuned carefully to the battery, charger and takes into consideration variation of the individual components.
 
Cute - and cheap! Small one looks like a comparator + mosfet + resistor - don't know how he does them so cheaply.
 
You can't parallel BJTs like you can with MOSFETs. MOSFETs have a positive temp coefficient to Rdson - as one heats up, its resistance increases and current diverts to the others. It's a nice self-balancing effect. Unfortunately, BJTs have the opposite behavior. When one heats up it draws more current, heats up more, draws more current... pop! :shock:

This circuit seems like a bad idea on several levels.
 
Even worse, MOSFETs can't be paralleled except in hard switching applications.
Their tempco is negative when biased linearly (i.e., low gate voltages). Check out the datasheet for the popular IRFB4110 at http://www.irf.com/product-info/datasheets/data/irfb4110pbf.pdf. In figure 3 on page 3, Typical Transfer Characteristics, you can see that for gate voltages under about 5.2V, the hottest of any paralleled MOSFETs will start drawing more current and will get even hotter, drawing even more current, etc. Very soon after that you're dodging molten copper droplets and epoxy fragments. :mrgreen:

The only paralleled MOSFETs that will even begin to cooperate when biased at lower gate voltages are linear MOSFETs. But they are very expensive and typically have very low current ratings and very high on-state resistances compared to most large-cased (TO-220, TO-246, TO-264, etc.) switching MOSFETs.
 
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