How charging batteries is done?

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MacIntoshCZ

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Imagine constant current nimh charger, more than one slot

1) AC to DC SMPS output 3v3/5V?
2) If its constant current than transistor is regulated by op amp (variable resistance) or for each slot there is buck converter with current and voltage feedback.
* if current is regulated by transistor than charger is damn unefficient.
If its charged with pulse current than it cant be called continous, its average current

How its done in standart battery chargers from wallmart?
thansk
 
I use an OPUS charger that does 4 cells ( a mix of 4 AA or AAA) or a mix of (two C and D) with adapters. It has a display for each battery and mutiple selction of charging currents, Some test modes for capacity, some charging modes including complete discharge frst. Voltage and mAh. It's not too expensive. i.e about $40.00 USD. It can do Li-ion, Ni-Cd and N-mH without telling it the battery type. it's not too expensive.
This one does not do battery packs.

There are specific charging IC's.

Each type of battery has a different charging profile. Packs have battery balancing circuits.
 
MacIntoshCZ said:
How its done in standart battery chargers from wallmart?
Click to expand...

In general, VERY, VERY poorly - cheap chargers usually charge the batteries very poorly, and offer little or no protection to them.

The picture in post #3 is obviously a much more complicated (and presumably better) charger than the cheap ones.
 
MacIntoshCZ said:
How its done in standart battery chargers from wallmart?
Click to expand...
The simplest method is just a voltage regulator plus a resistor to feed each cell slot.

The current drops off as the cell approaches full charge voltage.
As long as it's less than 1/10th the cell capacity rating when the cell reaches full charge, it's OK as a "14 Hour" charger. If less than 1/20th capacity (eg. 100mA for a 2000mAH cell) it can be left on for long periods.

Fast chargers must monitor the cell voltage and drop down to a trickle charge once the cell is full, or near so.
 
How voltage monitoring is done? It needs to turn off charging, wait a minute and then measure voltage on battery i guess?
 
The battery is relatively low impedance, so the voltage can be measured moderately accurately while on charge.
If it reaches the voltage limit the current must be reduced anyway.
 
No. The charge voltage across a NiMH battery will rise during charging, and then start to fall when it's fully charged. Monitor the charge voltage for this inflection point.

The temperature of the battery will quickly rise when it's fully charged.

In the early days of NiMH batteries, the company I worked at was using a battery pack with 8 D cells. What we didn't know at the time was that the manufacturer was having difficulty making D cells – a large number of them experienced early failures. With 8 cells in the packs, failure was too common and customers unhappy. The task fell to me to develop a test/exercising station for the battery packs. As I recall, we exercised six or eight packs at a time through about 4 charge/discharge cycles in 24 hours. I looked at a lot of charge/discharge curves until the manufacturer got their act in order.

 
rjenkinsgb said:
The battery is relatively low impedance, so the voltage can be measured moderately accurately while on charge.
If it reaches the voltage limit the current must be reduced anyway.
Click to expand...
I tried that but voltages change by .3 volts when voltage source is disconected.
If i need to charge battery fully then i need to check for dV drop. What if i want to charge battery on exact voltage? Then i need to check voltage by turning off charging?
 
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MacIntoshCZ said:
How voltage monitoring is done? It needs to turn off charging, wait a minute and then measure voltage on battery i guess?
Click to expand...

No, simply monitor it while charging - as visitor also mentioned, it's normal to monitor the temperature as well.

All the batteries we change are in packs with current fuses, thermal fuses, and thermistors, and charged correctly by specific charger IC's (usually the LTC4011) - yet they still have a pretty short life span.
 
MacIntoshCZ said:
If i need to charge battery fully then i need to check for dV drop. What if i want to charge battery on exact voltage? Then i need to check voltage by turning off charging?
Click to expand...
You should just reduce the current down to a safe trickle level once the voltage passes around 1.45V (or if the cells start to warm up noticeably).

The voltage drop only really matters if you are trying to fast charge right to the limit, which is not normally required.
Note from Visitor's graph that the drop off only happens after the nominal 100% charge point.
Reducing the charge current earlier keeps the temperature and pressures down.
 
MacIntoshCZ said:
DIY ok, But this is will be really inefficient, when he is sensing charging current with 1R8 resistor.
Battery has impendace around ohms.
Click to expand...

The resistor is also used for discharging the battery as well - hence it's value. It also allows a high enough voltage drop not to need opamps.
 
Its easier, no doubt about it. But not "represantitive" ( funny when i say that ) . Also such high resistance limit maximum current to under 1A. Btw thanks anyway
 
MacIntoshCZ said:
Its easier, no doubt about it. But not "represantitive" ( funny when i say that ) . Also such high resistance limit maximum current to under 1A. Btw thanks anyway
Click to expand...

You don't need to copy it 'as is' but it's a good starting point, and gives you an idea of the kind of things you need.
 
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