Hello folks,

We went thru a whole deal on this NiMH battery charge system.

The most important factor is battery temperature. As the NiMH cell
tops off, battery temp rises
to 45C or so. For the most part, charge level can be determined simply
by the temperature. As
a general rule, charging without monitoring temperature is a good way
to have a fire.

As the cell is approaching full charge, the cell voltage rises to
1.50V per cell. Cut it off there, if you can. The knee is softer on
NiMH and more difficult to read.

... and DON'T trickle-charge NiMH cells, they don't like it. Charge at
a rate of C/5 for good cell life.

Good documentation is hard to find. But Tynergy Batteries has V-I
charts that are accurate.

--Bob A

On Wed, Sep 17, 2008 at 10:35 PM, Apptech <apptech@paradise.net.nz> wrote:
>> Does anyone have a good reference about end of charge voltage for
>> NiMh batteries depending on the temperature ?
>
>> Reading one of Russel's posts I found his idea of using a shunt
>> regulator to control the "termination" of the battery charge great but was
>> not able to find any references anywhere I searched for what would be the
>> best voltage to clamp. My application does not need 100% of the batteries
>> capacity but temperature variation may be big and I need the batteries to
>> last as long as possible. 1.4 volts "fells about right" but I would prefer
>> to find some references to begin testing.
>
> I'm researching this at present - also for niCd.
> It's substantially more complex than one would like it to be.
> It varies somewhat with the specific call, drops with increasing temperature
> and rises with charge rate. if you can stop charging and let it settle or
> even load it slightly then you may get a better idea but I don't have that
> luxury.
>
> I'm notionally setting about 1.35V plus a delta V for charge current so that
> a heavily charging cell will not trip out before a slower charging one.
>
> For a typical cell there is a fairly hard knee point which they will not
> rise above and setting the trip point just above or just below there will
> lead to very different results.
> If you have the luxury of significant spare capacity (and this helps cell
> longevity greatly, then setting the voltage to about 1.35v is liable to be
> good.
>
> Note that the useful voltage also depends on what you do when it is reached.
>
> I am trialing both a series regulator that backs off current when set point
> is reached or a clamp regulator that shunts charge current at this stage.
> The implications for the battery vary. I'm also looking at hysteresis so the
> setpoint is dropped somewhat when reached.
>
> All this sounds rather complex - and it is- but the complexity is largely in
> the design process - the end result will cost almost nothing and be very
> simple. It should end up with current varied reference voltage plus
> hysteresis. No tem[perature compenation yet. For a single cell design I'm
> using the TLV431 1.25 V shunt regulator as reference and for an N cell
> design the TL431.
>
> Total cuoff may not be necessary - if you can muchly reduce charge curremt
> you may get down to "safe infinite period trickle current"  assuming such
> exosts for your cell type and size.
>
>
>
>   Russell
>
>
>
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