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Re: [EVDL] Zener regs: Can I get use 'em & still get a full charge?

Zener regs are used to allow charging current to bypass a battery. A 6.8V
zener will indeed start to conduct when your battery voltage gets above
6.8V, but it won't bypass ALL of your charging current. IIRC, Lee's zener
regs are designed to bypass a maximum of about 0.5A of current - a limit
set by the PR2 bulb. The rest of the charging current continues to charge
the battery. And while the voltage across the zener diode is clamped to
6.8V, the voltage across the reg (and the battery) will continue to rise.

The net result on a series string of batteries is that any battery that
reaches 6.8V will have its charging current reduced (in this case by 0.5A)
compared to the rest of the string, giving the rest of the batteries a bit
of time to catch up. This may take many charge cycles if your batteries
are way out of balance.

For example, if during a charge one battery reaches 6.8V 30 minutes before
the rest, it will see 0.5A less charging current during that time,
allowing the other batteries to "catch up" in charge by 0.25AHr (0.5A *
0.5 Hr).

Zener regs aren't particularly useful for flooded batteries as they like a
bit of overcharge. It helps equalize the individual cells in the
batteries. The regs may help keep a pack more in line, but it's nothing
that a good equalization charge every now and then won't do.

I believe zener regs were designed as a "quick and dirty" (read: cheap)
solution to charging AGM batteries - which do NOT tolerate overcharge. By
limiting charge current near the end of charge to below 0.5A, zener regs
will keep full batteries from overcharging (and ruining expensive AGMs)
while the rest of the string finishes charging.

HTH,

-Adrian

Steve Kobb wrote:

>
> There is something about zener regs that I don't understand.
>
> Looking at Lee Hart's design, I see that the higher-voltage zener is
> specified at 6.8v.
>
> If I were to put these regs on my pack, how would I ever get a full
> charge?
>
> USBattery says that I should charge up to a maximum of 7.75v x 26 =
> 201.5v.
>
> If I use zeners to dissipate excess energy, the pack would never get past
> 6.8 x 26 = 176.8v, which is obviously WAY below what the manufacturer
> considers to be a full charge.
>
> Is that correct? Do I have that right?
>
> If I were to select 7.5v zeners, would that affect the other part
> specifications? Would I need differently-sized resistors, or different
> bulbs, for example?
>
> Bottom line: I NEED to use zener regs because some of my batts are WAY
> out
> of balance, but I also want to get a good full charge to take advantage
> of
> my new Elcon PFC4000. How do I do both?
>
> Your comments are welcome.
>
> Cheers,
>
> Steve Kobb
> http://www.myelectrictruck.com
>
>

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