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> 18v Batery Cell Balancer.., 5 x 3.6v cells..
gremlinsa
Posted: July 13, 2017 09:38 pm
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So a while back i got a box of old rechargeable 3.6V cells (forget what type) and built a replacement 5 cell battery pack for a cheep portable/cordless drill..

It's been working pretty well, until last week when i placed it on charge, and 30 min later it was DEAD...

Upon opening i found that one of the cells was dead (0.0v), however when checking the rest of the cells i found that they were very far off from balanced.. Highest cell 3.85V and lowest at 3.4V..

However when looking for simple cell balancers, everything i could find is either micro controlled or very active (kicking in during charging and discharging to keep cells balanced), or else require disconnecting from the battery once the cells are balanced, to prevent discharging while standing idle..

I've been thinking of adding this simple circuit into my cellpack (while replacing the dead cell) to help balance the cells during charging, and preventing any one cell from overvoltage.

user posted image

The trick is that the charger is a simple wallwart with current limiting that could deliver upto 22V when the battery pack is full..

The Zenners would be 3.6V (or perhaps 3.9's that i also have in stock), thus limiting the full voltage across the pack to 18V (or 19.5), and the resistors are there just to limit the shunting current on each cell... thinking around 18 - 20 ohm each which is ~100ma per cell shunted (ave charging current from wallwart)

Of course the resistors cant be too large else the V drop across them will again allow the voltage across the cells to climb too high..

This should also be a circuit that can be left permanently connected to the cells because if the voltage drops to just below 3.6V per cell, the shunting will stop.. thus the circuit should be passive, and only kick in at times when individual cells exceed the 3.6V.

Is there anything i'm overlooking, or over thinking ??


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Sch3mat1c
Posted: July 14, 2017 12:55 pm
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Wow, Photobucket is seriously fucked up today, I guess. (On a related note, people still use it anymore?) The embed thumbnail works, but I click it and it shows a nag pic? From their own site?!

Looks like a good opportunity to switch to Imgur or something like that. Seamless embedding, never any nag pics.

Anyway,
If what you've shown is just a zener stack, then no: zeners under 5V are terrifically "soft". About as soft as a stack of (forward biased) diodes, and about as much tempco.

So, no, it's not something you can use during charging, because you'll never get an accurate voltage limit, nor is it something you can leave connected because the leakage current is huge (~mA). sad.gif

You can do this with TL431, or preferably TLV431 which consumes less bias current (0.08mA vs. 1mA). Neither is big enough to handle full charging current, so you need a transistor to boost the current capacity, and probably a heatsink for the transistor. (This gives a pack that responds much like good old NiCd or gel cell: keep charging and it just gets hot, it doesn't really overcharge.)

But I'd still recommend putting the balance circuit on the charger somehow. The battery connector in a laptop (and lots of other things powered by similar batteries) has extra pins for this (I think; or, at least, they could've been designed this way..), so the balancing is done in the charger rather than the pack.

I think newer power tools put the battery management circuitry (including balancing) inside the pack. Kind of expensive, but saves on pins.

I suppose if you're modding an existing pack, you don't have the luxury of adding pins. You probably don't have the luxury of adding a bunch of power transistors and a heatsink either, though, huh? unsure.gif

Tim


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gremlinsa
Posted: July 14, 2017 08:44 pm
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Yeah... I'm modding an existing pack, although i could add connectors or a switch in some spare space...

However the datasheet's show a leakage of 5uA not 1mA for the zeeners, which if i worked it out correctly will bleed the 1000mAh cells down in about 8000 days...

Hell I use it almost every weekend, and put it on charge for ~5 hours so as not to overcharge it because of the 'dumb' wallwart charger.. (A diode, two resistors and a LED)... Allot of help that did, i still killed a cell...


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Sch3mat1c
Posted: July 14, 2017 10:58 pm
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Read the datasheet carefully. See what voltage the define leakage at? wink.gif

You're asking to go from 1uA to >> 1mA in tenths of a volt. Even 6V+ zeners don't do that. Only an active circuit can (like a TLV431).

Tim


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gremlinsa
Posted: July 15, 2017 11:07 pm
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Ahhh

Thanks Tim .... Knocking some sense back into this old fart.... 1uA at 1V.. The cells are dead by that point...

Think i'm gona then have to work on something a little more active..

OK so... 1 comparative op-amp per cell, driving a transistor load, Allot more than i was expecting to put in, but i do have the space inside the battery compartment....

Hmmm need to workout a supply, voltage bridge and cell voltage taps all in the uA level...

Think I might put a separate charge port on the pack, and not use the charge docking station.. That way I can power the balancer circuit only when charging and use a diode to charge the cells but block them from powering the circuit..

working on a new schematic now..


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Nothing40
Posted: July 16, 2017 04:05 am
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3.6V cells suggest Lithium type.. You could look at the DW01 protection chip meant for Lithium Ion cells,but IIRC that also requires two MOSFET's.
https://cdn.sparkfun.com/assets/learn_tutor...taSheet_V10.pdf

Or maybe something similar to what's used on super capacitor banks,though they're usually about 2.5V max,so you might need to fiddle the set point a bit. I think the '431 is commonly used on super cap banks a lot also. Typically a '431 (or similar voltage reference ),A transistor (mosfet), and some load/shunt resistors.

I seem to recall another chip for doing this sort of thing,but I can't remember the part number now. (8205-S,perhaps?) I also remember seeing a few fancier ones that were meant for 3-6S packs.

There are even pre-made protection boards on Ebay,if you look around a bit. I was going to get a couple for some 3S packs I have,but never got around to it. Some of the smaller ones aren't able to handle a lot of current though.
http://www.ebay.com/itm/3-4-5-6-13S-11-1-4...d-/282429061673



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MacFromOK
Posted: July 16, 2017 09:02 pm
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How about using a voltage regulator (LM317?) to each cell? huh.gif


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