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NI-mh battery charger

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markhao

New Member
hi all, this is my first thread. i'm about to make a ni-mh 6volts 1800 mAh battery charger, can anyone give me some schematics without using any microcontrollers? :D

cheers!
 

audioguru

Well-Known Member
Most Helpful Member
Hi again Mark,
I answered on the other website. (Use a battery charger IC).
 

markhao

New Member
Hi again Mark,
I answered on the other website. (Use a battery charger IC).

hmm, thanks sir audioguru... but, what specific number sir? sorry for my question :D


Or just buy a battery charger, they're pretty cheap.
yup, cheap, but, some of them are not reliable, i had seen some but, it contains pair of diode and some LED.. but, i have many scraps here, so i came up with my idea that i will build one from scrap. :D

does anyone here have some schematics? so it will be easy for me to build one, by having some references

cheers!
 

Blueteeth

Well-Known Member
I've designed four different chargers so far, and sorry to just re-enforce what these guys have said but....a dedicated IC will save you a lot of headaches. They are sods to work out exactly when charge is complete.....requiring one to monitor temperature and voltage.

If you wish to charge very slowly, then you could use a timer and a low constant current source (<100mA) but its not that good for your batteries.

Anything for a charge time of less than 8-10 hours and you'll need a form of cut-off, usually, in order: change in temp, change in voltage, max temp or max voltage. Even the 10-bit ADC in your average micro controller can have trouble spotting the slight drop in voltage when they are fully charged....so its almost standard to have several methods, with different criteria, to determine charge termination.

I emailed this guy several times about charging, no microcontrollers, but he knows his stuff: Electronics - USB Powered AA NiMH and NiCd Battery Charger - Stefan Vorkoetter

Blueteeth
 

markhao

New Member
@fsmyth

bro, thanks for the good link there.. it contains lots of circuits for different chargers...

@blueteeth

I emailed this guy several times about charging, no microcontrollers, but he knows his stuff: Electronics - USB Powered AA NiMH and NiCd Battery Charger - Stefan Vorkoetter
wow, this one is good, i can directly replace the usb port with my 7805... though, usb port supplies 500mA, 7805 can supply for about 1.5A... so my charging time will be not that long...

from that link you gave, here's the schematic:


can i replace LM393 with any dual op-amp like lm358? or quad op-amp like lm324?

i think this one's good for 2battery in series, about 2.4volts... well, how can i transfor this circuit, because i'm gonna use this one with 6volts charger ???

sorry for my late reply guys... i'm a little bit buzy the past days.. thanks..


cheers!
 

Boncuk

New Member
Use a MAX712. (1 to 16 series cells), also already stated in another forum.
 

MrAl

Well-Known Member
Most Helpful Member
Hi Mark,

NiMH chargers are rather tricky beasts, requiring accurate voltage measurement
to get the end of charge right. I see however that you went with temperature
measurement for the end-of-charge termination. This is a good idea because
that's the easiest thing you can measure and delta V is a little hard to measure.

You can also measure delta T (T for temperature) with a little more circuitry.

There are a few things you have to be careful about when using temperature
as a termination parameter. That is, usually thermistors are used to measure
the temperature and although in theory this works pretty well, in practice the
problems that come up include:
1. Thermistor becomes inadvertently detached from the battery cell, causing
the circuit to charge the cell forever until it burns up.
2. Thermistor goes bad and also charges forever and burns up the cell.

Because of these problems, manufacturers started adding a second circuit to
*all* of their chargers no matter what termination method they use, and this
is a simple "Timer". The timer is set for the maximum time it should take to
charge a cell based on its total capacity and the charge current level.
For example, if you charge at 1 amp and you have a 2000mAh cell, then the
max time to charge is 2 hours times 1.1, or 2.2 hours. The timer is set for
this time and if the circuit is still charging after this time period the timer
shuts down the circuit so charging stops. This is an added safety measure
to make sure nothing really harmful happens.
Another technique in addition to the timer is to simply double the measurement
circuit. To do this, you simply use another circuit to monitor the same cell(s)
and allow that one to trip the charge termination also, just in case the first
circuit fails. Sometimes this second circuit even uses a separate power supply.

One last note...
If you use an LM358 you should include a diode on the output to drop the
voltage a little, or a base to emitter resistor (maybe 1k). This is to make sure
the transistor can shut off properly as the LM358 output voltage does not
go up to the full Vcc supply voltage and so may keep the transistor on when
it is supposed to be turned off.


Good luck with it.
 
Last edited:

Blueteeth

Well-Known Member
wow, this one is good, i can directly replace the usb port with my 7805... though, usb port supplies 500mA, 7805 can supply for about 1.5A... so my charging time will be not that long...
Not entirely true :) what that circuit is is a constant current source, which is started 'trigged' by the insertion of batteries, and turned off by the temperature of the batteries.
The time taken to chage batteries depends on two things: 1) battery capacity, 2) charge current.

In the above circuit the charge current is determined by the transistor gain and its base resistor. The comparator pulls the base of the transistor low via the resistor, so the current through the resistor is, according to the designer, 5.2mA. He says the gain of the transistor is 90, so..90 * 5.2 = 468mA.
Now a USB port has a limit of 500mA, so he chose the above value to be well under that, but if you were to use a 7805, sure it can supply 1A (max!!). But the circuit would still draw 468mA. Lowering the value of the base resistor would increase the current flowing through the transistor...or...pick a PNP power transistor with higher gain, say 150. (780mA) this would charge standard 2000mAH batteries in about 3 hours.

That said the comparators output has limited current capability, so if you google 'constant current source' you will find an alternative circuit. You could also use a LM317 as a constant current source! Better regulation, more current capability but you would need a small signal transistor to turn it on/off.

can i replace LM393 with any dual op-amp like lm358? or quad op-amp like lm324?
I don't see why not for this circuit, just beware that the inputs/outputs of many opamps cannot go within a volt or two of the supply rails.

i think this one's good for 2battery in series, about 2.4volts... well, how can i transfor this circuit, because i'm gonna use this one with 6volts charger ???
Use a higher voltage power supply :)

When charging with constant current the voltage across the batteries is determined BY the batteries. However, if they reach the same voltage as your power supply, no current will flow. So, for 6 volts....you'll want a bit of headroom, so try a 9 volt supply, or a 7809.

The threshold of the thermistor is determined by a voltage divider with the power supply, so you should be able to use the same part values for all the resistors/caps.

If I were you, I would measure it and test it completely before letting it lose on batteries. Because the original circuit was designed to cut off charge for a charge current of 468mA. If you charge at a higher current, the temperature threshold may change, but its still a great failsafe providing the thermistor is in contact with the batteries directly.

Blueteeth.
 
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