• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Battery charging

Status
Not open for further replies.

Jack.Straw

Member
Hello. I've been building these little battery powered practice amplifiers. I was using switched DC jacks to take the battery out of the loop when a power cord is plugged in. However, i want to move to rechargeable batteries.

I have 10.6v Ni-Cad battery packs. Last year a kind gentleman on these forums posted a constant current charging schematic specifically for my needs. I built that circuit and it works well. However, i'm confused on how to integrate it into the design in such a way that the batteries will charge while the amp is plugged in and in use.

Should i wire the DC jack to the battery charging circuit, which then outputs to the battery and then to the amp, with a current limiter between the battery and amp circuit?

Thanks for your time!
-Jack

 

crutschow

Well-Known Member
Most Helpful Member
You can just connect the charger to the battery in parallel with the amp. You don't need an added current limiter.

But the battery won't charge with the amp on unless the amp requires less current than the charger can deliver.
 

Jack.Straw

Member
You can just connect the charger to the battery in parallel with the amp. You don't need an added current limiter.

But the battery won't charge with the amp on unless the amp requires less current than the charger can deliver.


Thanks! Is this what you mean? If the charger boosts the voltage up 6 volts higher than the battery, won't those 6 extra volts feed directly into the amp (when on) also? I would be fine with the battery only charging when the amp is turned off, but i'm worried about the voltage when the amp is on & plugged in. Please forgive my ignorance, i'm still getting a grasp on some things that most people here probably consider the basics :)
 

crutschow

Well-Known Member
Most Helpful Member
That's the proper connection.

The battery acts sort of like a zener diode and will keep the voltage from going above about 1.4V/cell or 12.6V for your battery when charging (assuming the 50mA charger current limit is ≤ 0.1C rate, where C is the battery capacity in mAH). I wouldn't think that would harm your amp(?).
 

audioguru

Well-Known Member
Most Helpful Member
Your fairly complicated circuit is a battery over-charger. Instead you need a simple battery charger IC and a lower battery voltage.

A power supply voltage more than 9V simply makes the little LM386 get hot. It still has a max output of only 1/2W.
 

Jack.Straw

Member
Well, I could break these nicad packs apart and build my own 9.6 volt packs using the 1.2 volt batteries inside, if needed. I understand that i'm not getting more volume by using the 10.8 volt packs as they are, but i thought the increased battery time would be nice. Does the increased heat damage the LM386 or decrease the sound quality? Could i just use a small heat sink instead?

Also, if you have any recommendations on a simple IC charger? I've found quite a few by searching, but i'm not sure what's good and what's not..

Thanks for your help!!
-Jack
 

audioguru

Well-Known Member
Most Helpful Member
A battery with a higher voltage does not increase the battery operating time.
The increased heat from a higher voltage is simply wasted. With fewer battery cells the amplifier will sound the same and the battery will last as long but the entire battery is smaller and lighter.

An LM386 is a small low power amplifier IC that is not supposed to get hot and does not have a metal tab for a heatsink to be attached.

Use a simple battery charger IC circuit. It does everything that is required to quickly and safely charge a battery.

Ni-Cad is obsolete, has a low capacity and self-discharges in 1 month. Ni-MH cells are not toxic and have a capacity that is 4 to 5 times more. The new ones hold a charge for 6 months.
 

Jack.Straw

Member
Got it, thanks! I was trying to use these NiCD batteries because i got them free. Sounds like i should go with a 9v Ni-MH. When you say "Use a simple battery charger IC circuit", what does the IC stand for? Integrated circuit?
 

audioguru

Well-Known Member
Most Helpful Member
A 9V Ni-MH battery is too small to power a 1/2W amplifier for more than an hour. At continuous full power output a charge on the the little battery will last 15 minutes. Six AA Ni-MH cells make 9V and a charge will last for a week or two.

Yes, an IC is an integrated circuit. A circuit made with a battery charger IC is simple and works properly.
 

k7elp60

Active Member
The L386 chip is rated for 12 volts. Think that .6 volts will burn it up?
Jack,
There are different versions of the LM386. The lm386-N4 is rated up to 18 volts. I have a couple in my stock that I will not be using and I would be glad to send them to you. That way you could continue to use your existing packs.

I was the gentleman the gave you the circuit for the charger you posted. At the time it was designed to do the job. As I remember your Ni-cad packs were 9 cells, and the 12V input was not enough to really fully charge them as they need pretty close to 13.5 volts.

In my opinion charging Ni-Cads or Nickle Metal Hydride battery's at capacity X .095 current limit, works fine. I personally have charged both types over the years without problems.
Ned
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
Energizer Battery company says that charging Ni-Cad cells at 0.1 of their capacity is fine for 2 years continuously.
They recommend to switch to 0.04 times the capacity when Ni-MH cells are fully charged.

Panasonic Battery company says the same.

Both recommend using a battery charger IC.
 

tytower

Banned
Energizer Battery company says that charging Ni-Cad cells at 0.1 of their capacity is fine for 2 years continuously.
They recommend to switch to 0.04 times the capacity when Ni-MH cells are fully charged.
Panasonic Battery company says the same.
Both recommend using a battery charger IC.
Now here's the rub. Will they tell you the truth? Why should they? They sell batteries ,the more the merrier . Cars are wired negative ground which rusts them more quickly because the electron emitting terminal is allowed to weep to ground. That is the better way, the car coys say.

Personally I have found that any battery continuously trickle charged lasts a lot less than one which is not. Use it ,don't discharge it below about 30% of its actual fully charged output voltage and charge it when it gets to that point quickly. I mean like a car battery charge it up quickly once the lower threshold is reached and at .3 or so of its rated capacity .

Seems to work well for me and cars have been doing it for many decades. Only the poor quality determines their poor life of about 3 years these days. Never used to be the case back when batteries were rebuilt. I treat my ni-cads and nickle/metal hydride batteries the same .
 

audioguru

Well-Known Member
Most Helpful Member
My math was wrong. Energizer Battery company and Panasonic recommend a trickle charge current for Ni-MH cells of 1/40th the capacity that is 0.025 times.

The allowed rate of 0.1 times the capacity for old Ni-Cad cells is because a Ni-Cad cell has 1/4 or 1/5th the capacity of a modern Ni-MH cell. 500mAh for an old Ni-cad AA cell and 2500mAh for a modern Ni-MH AA cell.
 
Last edited:

Jack.Straw

Member
Jack,
There are different versions of the LM386. The lm386-N4 is rated up to 18 volts. I have a couple in my stock that I will not be using and I would be glad to send them to you. That way you could continue to use your existing packs.

I was the gentleman the gave you the circuit for the charger you posted. At the time it was designed to do the job. As I remember your Ni-cad packs were 9 cells, and the 12V input was not enough to really fully charge them as they need pretty close to 13.5 volts.

In my opinion charging Ni-Cads or Nickle Metal Hydride battery's at capacity X .095 current limit, works fine. I personally have charged both types over the years without problems.
Ned
Ned, I still use the original amp i built using the schematic you designed for me. I couldn't figure out how to wire it in a way that wouldn't send too much voltage to the LM386, so i ended up installing two DC inputs with a toggle switch. That works fine, but is inconvenient and unprofessional. I'm going to build a number of these for other people now, so i'm wanting to do a better job of it this time around. I would simply upgrade to the LM386-N4, but i've already purchased a bulk supply of the LM386's.

I like audioguru's idea of using an IC charger chip because it seems like it might be simpler and more cost effective. I'm trying to keep my costs as low as possible, including my time. What do you think?

Thanks a TON, you guys rock.
-Jack
 

audioguru

Well-Known Member
Most Helpful Member
One of my chargers is made by Energizer Battery Company. It is just a simple timer with current-limiting.
It charges dead 2500mAh AA Ni-MH cells in 6 hours.
It severely overcharges charged 2500mAh AA Ni-MH cells in 6 hours.
It severely overcharges dead 500mAh Ni-Cad AA cells in 6 hours and they get very hot.
It severely cooks charged AA 500mAh Ni-Cad cells in 6 hours.

A battery charger IC detects that a battery is already charged and properly detects when it is fully charged then it shuts off. It also has an input for a temperature probe so that a charging battery doesn't get too hot.

The LM386N-4 amplifier has a max power supply voltage of 18V because it is designed to power a 32 ohm speaker. It can drive a normal 8 ohm speaker when its supply voltage is no more than 9V. Like the other versions of the LM386 it overheats if it continuously drives an 8 ohm speaker at max output with a 12V supply.

There is a guitar amplifier on the internet that bridges two LM386 amplifiers for 3 times more output power. The amplifier ICs get extremely hot.
 

Jack.Straw

Member
I'm looking at 3 different IC chips. Two by Maxim and one by Linear Tech. The Linear Tech one says "No Audible Noise with Ceramic Capacitors" and the other two say nothing about noise, which in an amplifier I think might be a concern? Which of these would you guys choose?

Max712 - QuickView - MAX712, MAX713 NiCd/NiMH Battery Fast-Charge Controllers

Max DS2715 - QuickView - DS2715 NiMH Battery Pack Charge Controller

Linear LTC4009 - Linear Technology - LTC4009 - High Efficiency, Multi-Chemistry Battery Charger
 

audioguru

Well-Known Member
Most Helpful Member
The MAX712 is simple and works by detecting the battery voltage peak when it is fully charged. Use it.
The DS2715 detects when a battery gets too hot then stops the charging. It is difficult to measure the temperature of separate cells.
The LTC4009 is too complicated for a simple battery charger.
 

k7elp60

Active Member
Ned, I still use the original amp i built using the schematic you designed for me. I couldn't figure out how to wire it in a way that wouldn't send too much voltage to the LM386, so i ended up installing two DC inputs with a toggle switch. That works fine, but is inconvenient and unprofessional. I'm going to build a number of these for other people now, so i'm wanting to do a better job of it this time around. I would simply upgrade to the LM386-N4, but i've already purchased a bulk supply of the LM386's.

I like audioguru's idea of using an IC charger chip because it seems like it might be simpler and more cost effective. I'm trying to keep my costs as low as possible, including my time. What do you think?

Thanks a TON, you guys rock.
-Jack
Jack at the time you built the circuit I designed it worked for you. I don't remember all the conditions. The LM386N-4 is obsolete so getting more can be a problem. I would go with a new design, with a number of cells that will work with your version of the LM386.
The part of the circuit that you posted with Q2(a constant current source) was fine for the cell it was designed for. The charge current was close to 50mA. The value of the 24Ω resistor determined the charge current.
There are opinions on how to charge batteries. If I had some nicad or nicklemetal hydride batteries to charge I would use the circuit of Q2 and change the resistor for the charge current.
I also think Audioguru has a valid point.
Ned
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top