# Batteries and Battery Chargers

#### prodigyaj

Joined Dec 11, 2007
48
I have a 9.6V 2.2Ah Ni-Mh Batteries ! I plan to make the charging circuit on my own. I have known that a good charger must have the following properties:

1 - An Ideal charger must have supply constant currrent and should have a current limiter

2 - The constant current should be a minimum of 0.1 * battery Ah (Ideal)or maximum of 0.4* battery Ah(Maximum limit).

3 - The charging potential should be equal to battery potential ( maximum of +10% of battery potential )

any other parameters i need to consider ? and any links to good charging circuits ?

#### John Luciani

Joined Apr 3, 2007
477
Linear Technology and Maxim make battery charger ICs. You should be able to
find some application notes at both websites.

(* jcl *)

#### prodigyaj

Joined Dec 11, 2007
48
thank you for your replies !
A query i would want to put forward ?
I have a 9.6V 2.2Ah Ni-Mh batteries(brand new ones) !
Currently if i check the voltage of the batteries voltage is 6.5V
I have a 750mA 9V power source ! will my batteris charge ? if not till 9.6V but till 9V ?
Can some one give me a detailed behaviour with current voltage parameters on my batteries ?

#### SgtWookie

Joined Jul 17, 2007
22,210
If your power supply is rated for 750mA @ 9V, then it will attempt to charge your batteries at a 750mA rate until they explode from overheating due to the excessive current across the batteries' internal resistance, unless just by sheer luck they happen to charge to nearly 9V before they explode, spraying highly alkaline chemicals and metal fragments everywhere.

I do not know what the optimum temperature is for charging NiMH batteries. However, if you get them too hot, bang!

Check out the ICs that have already been recommended; they are highly efficient and will charge your batteries safely and quickly.

Another approach would be to use an LM317 or similar in a constant current configuration. However, that IC is a linear regulator and will expend energy in waste heat, rather than using it to do useful work.

You have not provided a part number for your batteries.

However, if you go to the manufacturer's website, they could supply you with that information - and they're actually paid to do so.

#### prodigyaj

Joined Dec 11, 2007
48
but the batteries rated at 2.2Amps cant it handle a current of 750mA ?

#### SgtWookie

Joined Jul 17, 2007
22,210
Why don't you try it and let us know how it worked out for you?

#### prodigyaj

Joined Dec 11, 2007
48
was that sarcastic or a serious suggestion ? anyways i promise i will let you know !!

#### thingmaker3

Joined May 16, 2005
5,084
Wear full-face protection. You'll be glad you did. Just because the batteries can source a given current safely does not mean they won't be damaged by overcharging.

#### Audioguru

Joined Dec 20, 2007
11,249
Nope.
The 750mA power supply will overheat, not the battery.
The battery overheats when it is overcharged that won't happen with the supply voltage that is too low.

Ni-MH battery cells don't explode, they hiss and maybe leak a little fluid from their safety vents if they are drastically overcharged.

Joined Jan 10, 2006
614
NiMH batteries CAN be fast charged at a high current, and in fact prefer it as opposed to NiCd batteries. The problem of coarse, is detecting when the batteries have reached a full charge state, otherwise YES, they will go bang... and leak lots corrosive gunk (voice of experiance). Most commercial chargers use either Delta T or Delta V (or both) sensing methods, using sensors to detect either a sharp rise in Temperature, or a slight "Knee" in the terminal voltage. I am guessing these batteries are either old Power tool batteries or similar, in which case they may already have a temperature sensor built inside the pack..(how many terminals are on the pack..?). The Original charger probably would have used this sensor to detect when the battery was close to full charge, and switched from Fast charge mode, to Trickle charge mode for final top up.
Your 9 volt 750mA power supply wont charge these cells. If all cells are in good condition and can be charged to 9.6 v, the charge current will just drop off as the battery approaches 9 volts, and the battery will stop accepting charge. You need more voltage. A simple approach to current control would just be to put a resistor in series with this higher charge voltage, or you could make a simple current regulator from an LM317.
Low charge currents are indeed safer for prolonged charging of these batteries, but can reduce the long term life due to Dendrite formation. Fast charging is my recomendation, but full charge sensing MUST be used to avoid a catastropy.

#### prodigyaj

Joined Dec 11, 2007
48
thanks a lot for giving CONSIDERATE replies i just laid my hands on 130mA 11.3 volts charger ! i guess that should do the trick ?

#### SgtWookie

Joined Jul 17, 2007
22,210
thanks a lot for giving CONSIDERATE replies i just laid my hands on 130mA 11.3 volts charger ! i guess that should do the trick?
You need a plan to prevent overcharging; whether through charging over a period of time, or sensing the voltage on the battery.

A constant 130mA rate sounds reasonable, but you need to taper off the charge rate when the battery nears being fully charged.

The ICs previously recommended have circuitry in them to take care of such things. I suggest that you find the datasheets for them and read up, along with the applications notes. Even though you may not use the circuits, it will give you an insight as to how they work.

And if you believe that my previous reply was inconsiderate - well, it made you start to consider things, didn't it?

This world isn't safe, and in your prior thread regarding your stepper drivers, you seemed bound and determined to destroy your project. I did make a small mistake early on (by suggesting increasing Vsense to 0.8-something Ohms), but as the thread evolved that was corrected. Yet you kept insisting on taking a path that would've defeated the point of the entire circuit.

The best suggestions have already been made, yet you continue to introduce obliques. It is no wonder that replies to your threads are becoming further and fewer between.

#### Audioguru

Joined Dec 20, 2007
11,249
Is the 11.3V "charger" current-limited at 130mA or is that its max allowed current?
If it doesn't have a current limiting circuit then this charger will overheat when trying to charge a dead battery.

Is the 11.3V regulated or will it rise as the charging current drops as the battery charges?
When the 9.6V battery is fully charged then its voltage will be about 11.2V.
If the charger's voltage rises then the battery will become overcharged and warm which reduces its life.