# [EVDL] Li-Ion cheap balancing



## EVDL List (Jul 27, 2007)

Mark,

A zener as a balancer is a dangerous idea. First, it won't balance 
the individual cells within your 4-cell module. Second, your zeners 
will have to tell your charger to back off when they activate. 
Otherwise, you'll be catching zeners on fire as each battery reaches 
14.6V and then starts to transfer current to the zener shunt. Unless 
your charger backs off to a safe rate, your entire pack current will 
flow through the zener, which means lots of watts of heat.

Lithium iron phosphate (Valence/A123) likes to be charged to 3.6 or 
3.65V/cell. When the current tapers off, you remove the voltage 
(although somewhere I heard you can float charge them at a lower 
voltage?). Holding it there longer won't equalize them, it will just 
shorten their cycle life.

-Ben

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## EVDL List (Jul 27, 2007)

I'm pretty sure you can hold the Lithium batteries at full charge
indefinitely without damage. I don't think you need to ever terminate the
charge. At least this is what Valence said about their batteries.

Mark:

I recommend looking into building your own "beefy zeners". Use an adjustable
shunt regulator with a resistor divider. Instead of having the adjustable
regulator take the load current, have it take the base current of a PNP
darlington transistor that dissipates power in the PNP and a power resistor.

This solves three problems: you can use one per cell, giving much better
balancing. Also, the adjustable shunt reference is much more accurate than a
Zener. Finally, it's much easier to deal with power dissipation in power
transistors and resistors, allowing you to shunt more current.

Here's a circuit diagram (view with a fixed width font):

_________________________________
| | |
| | |
| | / |
| _|< |
| | / | Q1 |
| _____|< |\ |
| | | \ |
> | |\ | |
>R1 | \____| ___|___
> __|__/ | ___
| / / \ | _______
|_____/ \ U1 > ___
| /_____\ >R3 |
> | > |
>R2 | | |
> | | |
| | | |
|_______|___________|___________|

For example, you could use U1= LM385, Q1 = TIP105, R1 = 100k, R2 = 287k, R3
= 0.5 ohm 30W to bypass almost 8 amps and draw very low quiescent current.
You might need to add a compensation capacitor.

Note, I haven't actually built this circuit to test it, nor do I have a
Lithium pack (yet).

-Morgan LaMoore



> Ben Apollonio <[email protected]> wrote:
> 
> > Mark,
> >
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > Here's a circuit diagram (view with a fixed width font):
> >
> Victor's clampers use this exact method, expanded somewhat:
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > I'm pretty sure you can hold the Lithium batteries at full charge
> > indefinitely without damage. I don't think you need to ever terminate the
> > charge. At least this is what Valence said about their batteries.
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > I'm pretty sure you can hold the Lithium batteries at full charge
> > indefinitely without damage. I don't think you need to ever terminate
> > the charge. At least this is what Valence said about their batteries.
> ...


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