3 Stage Battery Charger

[PeterDove]

Hi All,

I hope I am not going to get lynched for starting a new thread but I wanted to present my first attempt at a 3 stage battery charger. The input is 12V DC at 300mA. I am charging 8 x 1.2V Nimh

The curcuit uses two op amps, an LM35DZ along with some resistors and a couple of NPN transistors.

Here is stage one - the temperature is only 15degC, the voltage of the batteries measures 7.7 volts ( undercharged ) so it is getting near the full amount of amps. The 9V battery symbol is put there just to represent some kind of load that the 8 batteries would represent - V2 is meant to be the measurement of the current voltage of the 8 batts.

**broken link removed**

Stage Two - Batteries are now showing 10.45V which is 1.3V per battery - at this point the compator causes the flow through Q2 to be reduced and now the batteries ( which are 2000maH ) to charge at rate of 150mAh ish

**broken link removed**

Stage Three - the batteries temperature hits over 35degC and that causes Q1 to turn off too further reducing the current to a trickle.

**broken link removed**

Any comments, improvements or suggestion are welcome.

Peter

 

[Nigel Goodwin]

The entire idea is pretty bizzare? (and wouldn't work very well), you need to study how to charge NiCd and NiMh.

First thing you need is to provide current limiting to the batteries, preferably a constant current charging source. Then you should monitor the voltage across the batteries, looking for a small DROP in voltage - this occurs when they are fully charged, and you then switch to trickle charging. As a further safety measure you monitor the battery temperature, this increases once they are fuly charged, and again such an increase should switch the charger off leaving a trickle current to them.

 

[PeterDove]

The entire idea is pretty bizzare? (and wouldn't work very well), you need to study how to charge NiCd and NiMh.

First thing you need is to provide current limiting to the batteries, preferably a constant current charging source. Then you should monitor the voltage across the batteries, looking for a small DROP in voltage - this occurs when they are fully charged, and you then switch to trickle charging. As a further safety measure you monitor the battery temperature, this increases once they are fuly charged, and again such an increase should switch the charger off leaving a trickle current to them.

I think I mentioned that the charging source was 12V 300mA

FYI I have looked at the battery charging manual provided by energiser and they recommend a thermal based system. Although mine isnt exactly as theres I think I can change it around - however I think my system would be even safer as the stage at which thermal is used is right at then end to make it trickle charge. Below is the quote from the manual from Energiser

"Three-Stage—Here a fast charge restores approximately 90 percent of the
discharged capacity, an intermediate timed charge completes the charge and restores
full capacity, then a maintenance charge provides a continuous trickle current to
balance the cells and compensate for self-discharge. The fast charge (with currents in
the 1C range) is typically switched to the intermediate charge using a temperaturesensing
technique, which triggers at the onset of overcharge. The intermediate
charge normally consists of a 0.1C charge for a timed duration selected based on
battery pack configuration. This intermediate-charge replaces the need to fast-charge
deeply into the overcharge region to ensure that the cell has received a full charge.
Three-step charging, such as illustrated in Figure 20, requires greater charger
complexity (to incorporate a second switch point and third charge rate), but reduces
cell exposure to life-limiting overcharge."

 

[PeterDove]

The entire idea is pretty bizzare? (and wouldn't work very well), you need to study how to charge NiCd and NiMh.

First thing you need is to provide current limiting to the batteries, preferably a constant current charging source. Then you should monitor the voltage across the batteries, looking for a small DROP in voltage - this occurs when they are fully charged, and you then switch to trickle charging. As a further safety measure you monitor the battery temperature, this increases once they are fuly charged, and again such an increase should switch the charger off leaving a trickle current to them.

It occurred to me whilst rereading the manual which I quoted to you that my system simply needs a little switching around to conform to their suggestion - I'll give it a whirl later today.

 

[audioguru]

Eight Ni-MH cells will be about 11.2V when fully charged. With only a 12V supply, the max output voltage of most opamps is +10.8V and the output transistor has a base-emitter drop of 0.8V so the max charging voltage will be only +10.0V and the battery will never be fully charged.

How is the 12V source going to limit its current to 300mA? By smoking?
Actually since its voltage is too low then the battery won't draw much current anyway unless it is flat or is shorted. Then the 12V supply will make lots of smoke.

 

[PeterDove]

Eight Ni-MH cells will be about 11.2V when fully charged. With only a 12V supply, the max output voltage of most opamps is +10.8V and the output transistor has a base-emitter drop of 0.8V so the max charging voltage will be only +10.0V and the battery will never be fully charged.

Thanks for the heads up on that

How is the 12V source going to limit its current to 300mA? By smoking?
Actually since its voltage is too low then the battery won't draw much current anyway unless it is flat or is shorted. Then the 12V supply will make lots of smoke.

I am not sure what you mean here. My DC transformer that I am plugging into the wall gives its output as 12V 300mA. I assume that means the transformer has already limited the current to 300mA

Peter

 

[Hero999]

For a start there's no such thing as a DC transformer, a wall adaptor is an AC transformer with a bridge rectifier and smoothing capacitor on the output.

Secondly 300mA is its current rating, it doesn't mean it's limited to 300mA, it will probably have a short circuit current of 3A in which case it will smoke or maybe not as there probably is a thermal protection system but it probably isn't resetable.

 

[PeterDove]

For a start there's no such thing as a DC transformer, a wall adaptor is an AC transformer with a bridge rectifier and smoothing capacitor on the output.

Secondly 300mA is its current rating, it doesn't mean it's limited to 300mA, it will probably have a short circuit current of 3A in which case it will smoke or maybe not as there probably is a thermal protection system but it probably isn't resetable.

OK, so I take then end of the 2.5mm jack, I stick my positive lead from my Ammeter to the inside of the jack and touch the negative to the outside - voila 300mA... so what does that mean? Is it limited to 300mA cause that what it looks like to my newbie mind.

And thanks for rectifying my terminology so things will go smoother on later output from me..

Peter

 

[Hero999]

Have you actually measured the short circuit current on an Ammeter?

300mA is just the maximum current you can draw from it before it overheats, it like the maximum weight a bridge will take before it collapses.

 

[audioguru]

I assume that means the transformer has already limited the current to 300mA.

Nope. It will smoke if you overload it. 300mA is its max rating and any more current is overload and will cause smoke.
It is probably very simple so it doesn't have a regulator circuit. Then without a load its voltage will be about 16V or more. With a 300mA load it will have 12V and will be warm. With a 600mA load its output voltage will be about 8V or less and it will keep getting hotter and hotter until it breaks.

 

[PeterDove]

Nope. It will smoke if you overload it. 300mA is its max rating and any more current is overload and will cause smoke.
It is probably very simple so it doesn't have a regulator circuit. Then without a load its voltage will be about 16V or more. With a 300mA load it will have 12V and will be warm. With a 600mA load its output voltage will be about 8V or less and it will keep getting hotter and hotter until it breaks.

OK, so I will need a current regulator or somekind. I will look into it - I assumed that because I was looking after the current through transisors which have their own max current throughput that that would suffice.

Peter

 

[audioguru]

The transistor that is connected to the battery doesn't have anything limiting its current if the battery is very low or is shorted. The transistor has a max current and max power rating and if they are exceeded by your circuit then the transistor will smoke.

Only a "smart" IC like a voltage regulator shuts down if its current or temperature are too high. A transistor will just go, "POOF". If the voltage was high enough for the battery then the battery might also go, "POOF".

 

[kchriste]

FYI I have looked at the battery charging manual provided by energiser and they recommend a thermal based system.

You are on the right track with your temperature sensing technique for charge termination. The problem with your current setup is that the batteries will not get fully charged on a Summer's day and will be over charged on a cold day. What you really want to do is have 2 temperature sensors. One monitors ambiant temperature and the other battery temperature and you terminate on the 15 degree difference between the two. That way, assuming the batteries are at ambiant temperature when you put then in the charger, the batteries will charge properly reguardless of the ambiant temperature. It is also good to have an absolute battery temperature setting above which the charger won't attempt to charge the batteries in case you put them in hot.
Here is a link to a PIC16F872 controlled charger I made:
**broken link removed**

 

[justDIY]

check out this chip - BQ2002

it seems to include all the features you're looking for, in a single 8 pin DIP

>Fast charge of nickel cadmium or nickel-metal hydride batteries
>Direct LED output displays charge status
>Fast-charge termination by -ΔV, maximum voltage, maximum temperature, and maximum time

https://focus.ti.com/docs/prod/folders/print/bq2002.html