Need a battery charger schematic

[Jack.Straw]

I've just finished building a small battery powered amplifier. It's currently powered by a 9v battery, but the circuit can handle up to 16 volts. I want to power it at 12 volts using rechargeable batteries, & today i scored a bunch of NiCd batteries on freecycle.com. The batteries are Sanyo Cadinca Extra KR-600AE 1.2v 600mAh (Standard Charge 14-16 Hours at 60am). I've wired 10 of them in series to make a 12 volt battery pack.

Now i need a safe way to recharge them. I have a 12v adapter for home use, and a cigarette lighter adapter for use on the road, so I would like to build a circuit that will use a 12v supply to charge the batteries up to a safe level, and then trickle. Can anyone here recommend a schematic?

Thanks so much for your time,
-Jack

 

[k7elp60]

I've just finished building a small battery powered amplifier. It's currently powered by a 9v battery, but the circuit can handle up to 16 volts. I want to power it at 12 volts using rechargeable batteries, & today i scored a bunch of NiCd batteries on freecycle.com. The batteries are Sanyo Cadinca Extra KR-600AE 1.2v 600mAh (Standard Charge 14-16 Hours at 60am). I've wired 10 of them in series to make a 12 volt battery pack.

Now i need a safe way to recharge them. I have a 12v adapter for home use, and a cigarette lighter adapter for use on the road, so I would like to build a circuit that will use a 12v supply to charge the batteries up to a safe level, and then trickle. Can anyone here recommend a schematic?

Thanks so much for your time,
-Jack

In order to charge them in series the charger will have to put out close to 15V, as each cell will be fully charged when they are near 1.5 volts per cell.
How are you connecting the cells in series? Are you using some type of a battery holders? I hope you are not soldering wires to the idividual cells, as doing so can damage the seals.

 

[Jack.Straw]

In order to charge them in series the charger will have to put out close to 15V, as each cell will be fully charged when they are near 1.5 volts per cell.

How are you connecting the cells in series? Are you using some type of a battery holders? I hope you are not soldering wires to the idividual cells, as doing so can damage the seals.

Ahhh, I see... thanks for the reply. These batteries were previously in a packs of their own - sets of 9 maiking 10.8v packs. I broke open a couple backs to make my 12v source, and yes i'm afraid i did solder them together (oops). So... if i were to just use one of the 10.8v packs as-is (which would save me the bother of building a holder) you're saying I will need a 13.5v source to recharge? By the same logic, if i want this thing to be able to be charged by my car's cigarette lighter, i'm limited to a 8v battery pack?

 

[k7elp60]

Ahhh, I see... thanks for the reply. These batteries were previously in a packs of their own - sets of 9 maiking 10.8v packs. I broke open a couple backs to make my 12v source, and yes i'm afraid i did solder them together (oops). So... if i were to just use one of the 10.8v packs as-is (which would save me the bother of building a holder) you're saying I will need a 13.5v source to recharge? By the same logic, if i want this thing to be able to be charged by my car's cigarette lighter, i'm limited to a 8v battery pack?

I'm afraid it's not that simple. The recommended way to charge Ni-Cad batteries is with a contant currrent. If they are charged at room temperature(about 72 degrees F) @ .095 X capacity (in this case charge current would be about 57mA)they will not overcharge and you don't have to remove them from the charger after 12-14 hours. If you are going to charge them in another way there is still a voltage drop across the charger module. So to charge them from a automobile adaptor , the charger may need as much as 16 to 17 volts. There are circuits that will boost the car voltage enough to charge them. The reason I mentioned temperature is the fact that NiCads are somewhat sensitive to temperature and they can warm up both during charging and discharging, and if the become too warm they can explode.
I would be happy to help you with circuits for a charger for both home and auto. Please tell me your skill level on building electronic circuits.

 

[Jack.Straw]

I'm afraid it's not that simple. The recommended way to charge Ni-Cad batteries is with a contant currrent. If they are charged at room temperature(about 72 degrees F) @ .095 X capacity (in this case charge current would be about 57mA)they will not overcharge and you don't have to remove them from the charger after 12-14 hours. If you are going to charge them in another way there is still a voltage drop across the charger module. So to charge them from a automobile adaptor , the charger may need as much as 16 to 17 volts. There are circuits that will boost the car voltage enough to charge them. The reason I mentioned temperature is the fact that NiCads are somewhat sensitive to temperature and they can warm up both during charging and discharging, and if the become too warm they can explode.
I would be happy to help you with circuits for a charger for both home and auto. Please tell me your skill level on building electronic circuits.

Thanks k7elp60, I appreciate your help! I would say my skill level with building electrical circuits is still rather novice, but i'm not intimidated. I have only made 3 things. The first thing was a simple color organ kit, and that was enough for me to get a basic understanding. Since then i've built a JFET based pre-amp for the upright bass i made, and the portable solid-state amplifier that this thread concerns.

The amp sounds satisfactory running on a 9v battery, but i've read good things about how it performs at higher voltages... so i was shooting for 12-16v, rechargeable, in the end. However, in light of what i'm learning today... i can be flexible on the voltage. I'd like to be able to recharge this from a cigarette lighter, and heat could be an issue as it gets hot here in the summer. The housing for the amp is a Heineken mini-keg, so there is plenty of room in the amp for a heat sink and/or a small cooling fan if needed. I think i have a couple spare cpu heat-sink/fan combo's around here..

I would also like to be able to recharge using a 120v wall adapter. If i have to build 2 different circuits &/or install 2 separate power jacks, i will.

Many thanks,
-Jack

 

[k7elp60]

I'm working on a design for you that will be one box. It will have three IC's. A LM317t as a preregulator a LT1054 as a voltage pump and a LM317T as a constant current charger. As soon as I get several parts values figgured I'll post the circuit for you.

 

[Jack.Straw]

Awesome, thanks so much!!

 

[k7elp60]

Here is the schematic. Initially set the 1k trim pot to center scale. Then adjust it so that the voltage between point A and the - lead is 6V higher than the full charge voltage of the pack. Any questions please let me know.
Circuit operation: Q1 regulates the input voltage to about 8V, Q3 doubles the voltage,and Q2 supplies a constant current to the NiCad pack. Under load the voltage between point A and the - line is about 4 volts above the full charge voltage of the battery pack.

 

[Jack.Straw]

Thanks!! I do have a couple initial questions. Does this circuit deal with overcharging? If the unit is plugged in too long will the batteries get damaged?

Also, I was making a parts list form your diagram. Does this look right?

Resistors (1/4 watt)
120 ohm
24 ohm
1k trimpot

Capacitors (25v)
10 uF (2)
100 uF

Diodes
1N914 (4)

LM317T (2)
LT1054CP
Small Heatsink


I'm leaving town for a week so I won't be able to try this until I get back. Thank you SO much for giving up your time to help me. I can't wait to give this a try!

 

[k7elp60]

No the 600mA NiCads will not over charge as the current is limited to about 52mA. The parts list looks correct. The trim pot can be a single turn type with a screwdriver adjustment. I have a pretty good inventory of parts, so I breadboarded the circuit and it does work. The input voltage can be anywhere between 12 to 20 volts input.

 

[Jack.Straw]

Sweet!

I want to wire this in such a way that if the battery goes dead I can power the amp directly from the dc jack. If i wire the jack to this recharge circuit and to the amp's input, is there something i can put in between the battery and the amp's input that will prevent power from the dc jack from back-feeding the battery? A sort of one-way device? Or is there a better way?

**broken link removed**

Edit: I am using a stereo phone jack with the negative lead from the battery going to the ring terminal on the jack so that the amp turns on when you plug something in.

 

[audioguru]

Increasing the supply voltage of an LM386 little amp from 9V to 12V does not increase its output much but increases is heating a lot. Look at the datasheet.

The output at clipping with a 9V supply is only 0.45W into 8 ohms and its heating is 0.52W.
The output at clipping with a 12V supply is only 0.53W into 8 ohms and its heating is
0.8W.

Use a little (also 8 pins) TDA2822M amp IC instead. With a 9V supply its output at clipping is 1.1W into 8 ohms and its heating is 1W. The schematic and a pcb design is in its datasheet.
With a 12V supply then it gets too hot.

 

[robotgangsta]

sorry to be out of context but hi audioguru good to see your still on the board after 3 years

 

[Jack.Straw]

Increasing the supply voltage of an LM386 little amp from 9V to 12V does not increase its output much but increases is heating a lot. Look at the datasheet.

The output at clipping with a 9V supply is only 0.45W into 8 ohms and its heating is 0.52W.
The output at clipping with a 12V supply is only 0.53W into 8 ohms and its heating is
0.8W.

Use a little (also 8 pins) TDA2822M amp IC instead. With a 9V supply its output at clipping is 1.1W into 8 ohms and its heating is 1W. The schematic and a pcb design is in its datasheet.
With a 12V supply then it gets too hot.

I'm definitely going to give that chip a try next time. I'm probably going to go with a 10.2v battery pack this time simply because that's what i happen to have.

 

[Jack.Straw]

k7elp60, I finally got some time to get back into this project. The charger circuit works great, thank you SO much!! Sorry it took so long for me to put your generous time/knowledge donation into action... i really really appreciate it.

The charging circuit works perfectly, but i'm still trying to wrap my brain around how to wire it up so that the device can be powered by battery or dc input, and allow for the recharging circuit. A switched dc jack would cut the battery out of the loop when plugged in. Will i need a 2nd dc jack for charging, or is there a way i can make it happen with just 1?

Thanks again!!
-Scott