Re: Use power supply to charge battery

w_tom wrote:

On Jun 22, 11:17 pm, budgie <m...@xxxxxxxxxxx> wrote:
No, I *don't* understand "why some do discharging before recharging"

It is a largely pointless exercise which only eats away at useful cell
cycle life.

We measure cell voltage. It is lower. Does that mean a cell is
fully charged or requires charging? Same number can mean either. As an
engineer, budgie would have seen that chart - same voltage can
indicate two completely different battery states.
WRONG !
WRONG !
WRONG !
The voltage of Lithium rechargeable batteries does not go down at the end of
charging because the batteries are charged using a current-limited constant
voltage charger. At the end of charging, the charger is in the constant
voltage region. When the current drops below a certain threshold (which is
in the data***), then this signifies the end of charge, however
continuing to charge at the proper voltage does not cause a hazard, it only
shortens the cell life slightly. This is why some manufacturers have
suggested that their lithium cells are also suitable for float charging in
backup / UPS applications. See the following:
http://www.electronicsweekly.com/Articles/2006/02/01/37528/Float+charging+lithium+ion+cells.htm

I think that you might be getting confused with NiCd cells where it is
common to detect the end of charge by looking for a drop in the charging
voltage, which can happen in this case because NiCd batteries are charged
in a constant current charger.

Discharging is a
first tests performed by some lithium battery controllers - one of
many algorithms used. If discharging creates a battery voltage
reduction, then cell may be recharged. Apparently budgie is only
familiar too few lithium recharge algorithms.
It is not necessary to do this. If the lithium cell is charged at the
proper voltage, then if it is fully charged then it will draw only a small
current from the charger and the charger will then detect that the cell is
fully charged by the low current that is being drawn.

A battery may measure same voltage when fully charged or when
significantly discharged. Do we charge a fully charged battery? Or
do we first learn whether the battery can accept a charge without
explosion. Some recharging algorithms do this by discharging. How do
we recharge a battery. Some first perform a discharge - in direct
contradiction to what budgie posted.
Lithium cells (unless defective) do not mind being connected to the proper
charging voltage, subject to a current limit, regardless of whether they
are fully charged or discharged (as long as the voltage is not below the
minimum allowable charging voltage - see data ***). Provided the cell
voltage is checked before applying the charging voltage, and provided the
current is limited to the value in the data***, the cell can be safely
charged from ANY initial state of charge, even a fully charged cell can be
float charged, for extended perions, if a small loss of battery life is
acceptable.

Meanwhile, that same chart for lead acid batteries only has one
(highest) voltage for fully charged and a completely different voltage
for discharged.
Just like Lithium cells!

Therefore lead acid (and NiCd) batteries can be
recharged with a regulated power supply.
Lead acid and NiCd cells require totally different charging: Lead acid need
constant voltage, current limited charging, like lithium cells, but the
lithium cells require more accurate voltage setting, and series strings of
lithium cells require individual control of the voltage on each cell
whereas lead-acid batteries with multiple cells can be charged without
monitoring the cells individually. NiCd cells on the other hand should NOT
be charged with a constant voltage, and should be charged with a constant
current instead. Sensing the end of charge for a NiCd cells is relatively
difficult, and failing to switch off the current at the right time can
damage the cell if the charging current is high. On the other hand,
Lithium and Lead acid cells when connected to the correct constant voltage,
will not get overcharged, and will simply draw less and less current of
their own accord as they approach full state of charge.

Of course lead acid batteries and discharing are irrelevant to the
OPs question. He cannot and must not try recharging lithium batteries
with a power supply.
Purest Bollocks.

Consequences by others have demonstrated the
explosive threat. He would not be the first person to have created an
exploding battery by using a power supply.
Many people successfully charge lithium cells with a power supply, including
me. It is wise to consider the possibility of a failure inside the power
supply, such as a shorted pass transistor in the regulator. I have never
had any kind of fire or accident in this process, but even some of the
largest consumer electronics companies occasionally have problems with
battery fires, so I always plan as though the battery might possibly catch
fire, and make sure that in that event, the fire would not be able to
spread in such a way as to create a serious danger.

Discharging is sometimes used to recharge a lithium battery,
obviously, from electrical charts for lithium batteries. Those charts
demonstrate why recharging microcontrollers have sophisticated
algorithms by discharging, monitoring temperature, or other
parameters. IOW the OP should buy a lithium battery charger and not
use a power supply - to avoid threats to human life.

I suggest you read this document, you might learn something:
http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_LiIon_Precautions.pdf

Chris


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