NiMH Battery Pack Charger

[CPW]

One of my hobbies is driving/modifying electric radio-controlled cars and trucks. One of the biggest problems, though, is that any good NiMH chargers are 50-100 bucks. So I had the idea to design my own charger. I have not started yet, because I need some help.

First of all, what is the concept of charging NiMH batteries? Say you have a battery pack that is 7.2 volts at 1100 mah. What voltage and current would you charge the battery pack at?

Also, how would I design the charger? Do you simply put x number of volts through the battery? How does the charger know when its done? Is it when the battery can put out the voltage its rated at?

I would like to make the charger to have adjustable settings for different batteries, too.

Any schematics would be helpful.

Thanks in advance for the help.

BTW, I am planning on using a microcontroller to control the voltages and stuff.

[audioguru]

Hi CPW,
Battery manufacturers such as Energizer have good tutorials about their Ni-Cad and Ni-MH rechargable batteries. They show the temperature and pressure rise when the battery is nearly fully charged and also show the slight voltage drop that occurs and that battery charger circuits look for. The tutorials show backup temperature sensors and timers to avoid an explosion if the voltage-detection circuit goes wrong.

Nearly all semiconductor manufacturers have battery charger ICs. Maxim have some very good ones. 

[pebe]

This is a very good link to batteries.

http://www.batteryuniversity.com/parttwo-33.htm

[ante]

Hi CPW,

What are the charging data for the pack in question? There are two important things to consider here, maximum allowed charge current and the way to cut of charging e.g. peak detecting or temperature. If we can establish this we are half way!

[CPW]

Actually, I don't have a specific pack I want to charge. I want the charger to be compatible with different batteries.

It may sound like a dumb question, but how would you figure out the max. allowed charge current for a battery?

As for the way of cutting off charging, I want to use peak detecting.

Thanks for the help.

[audioguru]

I want the charger to be compatible with different batteries.

... how would you figure out the max. allowed charge current for a battery?

For their Ni-MH cells, Energizer recommends a fast charge at a current no higher than 1C for best cell life.

As for the way of cutting off charging, I want to use peak detecting.

For reliability, Energizer recommends a 3-stage fast-charge routine and detecting the cell's temperature rise to cut back the charging current when it has reached 90% of full charge, with a voltage peak and timer as a backup. Then a 0.1C charge current until the cell is fully-charged then a very small trickle current.

Thanks for the help.

[CPW]

Thanks for the info. One thing I don't understand, though. What does 1C mean?

As for the 3-stage charging routine, how does it work? Also, where can I get some info on what temperature to stop the charging at, etc.?

Thanks.

[audioguru]

One thing I don't understand, though. What does 1C mean?

It is the mA/hr rating of the battery. 0.1C is a charging current that is 1/10th as much.

As for the 3-stage charging routine, how does it work? Also, where can I get some info on what temperature to stop the charging at, etc.?

Go to www.energizer.com and click on Tech Info near the top of the page. On the next page in the select box, choose Ni-MH Rechargable. The next page has a list of datasheets for their Ni-MH cells with an excellent tutorial about them. The tutorial describes the recommended charging procedure and has graphs of temperatures, pressures and voltages.

[CPW]

Thank you so much! I read through the part on charging the batteries and its starting to make more sense.

One thing they didn't make completely clear is whether or not you can determine overcharge by the voltage output. I would like to use this method if it will work, so can you give me some help on how I would achieve this? They said you can't use absolute voltage values, so you would have to detect a certain voltage increase, right?

I would like to make this a fast charger, 30 minutes or less if possible. Will the three-stage charging routine work for charging that fast? Keep in mind that most of the batteries I want to charge will be 7.2V and around 1500 - 3000 mah. I would like adjustable voltage and current rates, though. I want the charger to have a wide range of values to choose from, like 6 - 8.4 (maybe even 9.6) volts and 600 - 3000 mah, even though most of the batteries won't be that low or high, probably somewhere in the middle.

Thanks again!

[audioguru]

Hi CPW,
Energizer does not recommend a quick charge quicker than 1 hour so that the battery cells will last longer without experiencing overcharge. They say that the voltage peak for Ni-MH cells is much less than for Ni-Cads and a circuit can easily miss it, also it occurs too late when overcharging causes very high temperature and pressure.

Chargers that are not very quick use a constant-current circuit and a timer. With constant current then the voltage doesn't matter and a single cell or many cells in series will have the same current. I don't know how they deal with terminating early a charge on a cell or cells that are already fully charged.

[ante]

Hi CPW,

This is peak detect charger I built many years ago, it has adjustable current up to 5 or 6A. You can feed it with 10 – 16 VDC (I used it on the car starter battery) depending on how many cells you are going to charge. Remember to put Q1 on a good heatsink and set the desired current initially and don’t change the setting during the charge. The top rail is positive and the power is applied at the left side and the pack at the right hand side. Some cells are built for fast charge and will survive 3C but always follow the recommendations issued by the manufacturer. 

[ante]

Oh, I forgot; the input at R5 I made for extra safety. If you like you can put a thermal switch (NO closing @45C) connected between the positive rail and R5 and put it on your pack. In case of a faulty pack or something else going wrong the T-switch will shutdown the charging process only leaving the trickle charge (via R10) on.  ;D

[CPW]

Thanks! That should help me a lot!

If I were to charge a 7.2V pack, what voltage would I supply the circuit with?

My main goal was to use a microcontroller to contol the voltage and current, so how do you think I could modify your circuit to eliminate everything not needed if I use a microcontroller?

[audioguru]

The voltage of a fully charged Ni-Cad cell depends on the manufacturer, the temperature and the phase of the moon. It is about 1.4V per cell, so a 7.2V Ni-Cad battery will measure about 8.4V when fully charged. Therefore the charging circuit must have a higher voltage to allow a voltage drop across the current-limiting parts.

[ante]

If you use anything from 11 to 14 Volts you’ll be fine since the circuit controls the current to your pack. If you go higher Q1 will dissipate more heat and if you go lower the circuit will at some point lose regulation.

If you go for a uP you just need Q1 and a few resistors and not the rest of the circuit.

[jasssam]

IN this web

http://www.elec-micro.com/electronics--circuit--Nicd%20batt%20charger.html

I found circuit batt charger

bye Good day.

[audioguru]

IN this web

http://www.elec-micro.com/electronics--circuit--Nicd%20batt%20charger.html

I found circuit batt charger.

It has a problem with its parts. The schematic has a 5V regulator but the text says an 8V regulator.
The current is limited by light bulbs but it doesn't say what voltage or current.

I think with a 5V regulator then 3.5V/50mA light bulbs from a series string of Christmas tree bulbs would work. For an 8V regulator then a 6.5V/50mA light bulbs should be used.

[ante]

That circuit will not supply a constant current!

[CPW]

Oops, I forgot about this post!

Anyway, I guess I'll use temperature sensing because that seems to be the most reliable way. I still would prefer peak detecting, but if it won't work for what I need then I guess I won't use it.

I am probably going to buy a Mini LST sometime soon, so I want to build a charger before I get it. Here is the battery it uses: http://www.teamlosi.com/Products/Default.aspx?ProdID=LOSB1202. Can someone give me some help on charging batteries like this? Like, where can I find charging info and stuff on this battery?

Thanks everyone!  ;D

[audioguru]

I am probably going to buy a Mini LST sometime soon, so I want to build a charger before I get it. Here is the battery it uses: http://www.teamlosi.com/Products/Default.aspx?ProdID=LOSB1202. Can someone give me some help on charging batteries like this? Like, where can I find charging info and stuff on this battery?

All battery manufacturers have info on their websites. My favorite is www.energizer.com . At the top of the 1st page select Technical Info, then select Ni-MH Rechargable. You will see a link to their excellent tutorial and a datasheet to their AA cells.

[CPW]

I looked all over their website but they don't seem to have any info on the battery.

My favorite is www.energizer.com . At the top of the 1st page select Technical Info, then select Ni-MH Rechargable. You will see a link to their excellent tutorial and a datasheet to their AA cells.

Energizer does have a good site.

[audioguru]

I looked all over their website but they don't seem to have any info on the battery.

The battery that you linked to doesn't have its physical size shown. So we can't know which standard size of Ni-MH cells are used. With a capacity of 1100mA/hrs, the cells could be old AA size because new ones are rated at 2500mA/hrs, or the cells could be AAA size with a very good mA/hr rating for that small size. It is difficult to see if N size cells were used.

[CPW]

Well, I'll figure it out when I get it.

Back to the charger's design. I decided I would still rather use peak detecting (lol, I keep changing my mind). Here is my plan for the charger and the program I will write for the microcontroller that monitors the battery:

First, the charger will begin charging at 1c. Second, there will be a loop that keeps checking the increase in voltage over every half a second or so, and when the increase starts to get larger, it will switch to a timed charge, right? Then third, it will use trickle charge to keep the battery fully charged. This is how the three-stage charge routine works right? Will this technique work?

Also, can someone give me an idea of the circuit I will need. I would like it to be as simple as possible. And remember, I am using a microcontroller to control the current and voltage going through the battery.

Thanks everyone.  ;D

[audioguru]

Before charging begins, a voltage measurement should be made that the battery isn't already charged.
During charging, a voltage measurement should be made to determine if the battery has a shorted cell.

It seems that looking at the datasheet of a battery charger IC from Maxim or another semiconductor manufacturer would allow you to re-invent the wheel without leaving anything out.

[ante]

It seems that looking at the datasheet of a battery charger IC from Maxim or another semiconductor manufacturer would allow you to re-invent the wheel without leaving anything out.

Very smartly put AG!  ;D