[time-nuts] Battery chargers (was: HP 5065A battery pack)

Attila Kinali attila at kinali.ch
Sat Nov 21 11:03:39 EST 2015

On Sat, 21 Nov 2015 11:25:31 +0100
Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Lacking the charging controller in mine, what would be a good approach 
> to go about and build one?
> Need to replace the battery setup in my XSRM setup too, the packs I 
> installed is now dead.

NiCd chargers are fairly simple. There are two ways to charge
them: 1/10C slow charge and >0.5C fast charge.

Slow charge works by simply applying a current of 1/10C (ie, if
you have a 1000mAh battery, apply ~100mA) with maximum voltage
that is at least 1-2V higher than the expected charge end voltage.
Done that, you wait for 12h and switch off.
This is way was standard in the 80s and 90s for most applications
and you can see it still in places where component complexity/price
is important. Of course it's not the best way to charge your batteries
as it is basically uncontrolled. And it's damn slow.

Fast charge works in a similar manner: you apply current (below
the max rating of the battery cell) and wait. But now you have
to track the battery voltage and/or temperature in order to
switch off, otherwise you risk destroying the cells, or even
causing a fire. Most of the controllers that use the deta-V method,
meaning they track the change of the battery voltage over time and
when the voltage starts to go down again (a curious effect of the
Ni* chemistry), they switch off. 
An alternative way that was quite often used by RC model builders
was to measure the battery temperature, which would rise suddenly
around the point when the battery is full. There was also quite a
few controllers that were based on delta-V, but used the temperature
as an additional clue, in order not to fry anything.

If you choose NiMH batteries, then the delta-V method is mandatory,
as they are very sensitive to overcharge. Also the delta-V is less
pronounced than with NiCd, which makes it harder to detect reliably.
That's why pulsed charger came up in the mid 90s. These stop the
charging process for a couple of ms, to let the cell stabilize and
then measure the voltage. The better ones even employed a small
discharge pulse to get the chemistry to settle faster.
This pulsed method got later adopted by the (better) LiIon battery
chargers a couple of years later.

A quick review of a few current NiCd/NiMH charger chips shows,
that the pulsed method seems to be either gone out of fashion,
or not worth mentioning in the datasheets anymore.

A nice thing about NiCd and NiMH batteries is, that they are bretty
indestructible (unless overcharged or deep decharged). The cells
are also kind of self-balancing, meaning you can build stacks of
batteries and the electro-chemistry will see that the batteries
will get all about the same charge. Which enabled quite long stacks
of over 10 cells. Li* based cells do not have that and are much
more sensitive to overcharge, which results in the chargers for
these either having coplex balancing circuits or charge each cell

As for the original question, if you want build a NiCd/NiMH charger,
i would just get one of the chips from Ti/Linear/Maxim. These allow
chargers with very few components and do everything you need to.
Costs between 2 and 8USD or so. Alternatively, you can see whether
you can find anything already soldered on ebay/aliexpress.
(considering that you can get an iMAX B6 NiCd/NiMH/LiPoly charger
for less than 20USD including shipping...)

Alternatively, if you have an uC board (Arduino or the like) with an
ADC that gives you >10ENOB and a power transistor lying around,
you can build a charger in a long afternoon on a breadboard.

> Tempted to think in terms of LiFePO.

Be carefull with that. As explained above, Li* are no drop-in replacement
for NiCd expecting circuits. The charger is quite considerably more
complicated. Depending on how much the designers of old relied on
the high series resistance of the battery pack, this might also cause
you trouble when using modern chemistry batteries. Replacing NiCd
by NiMH is almost always safe, if the charger is accordingly modified
to be more sensitive on switch-off.

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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