Battery/AC-DC power for multiple circuits...would this work?

[revans]

Ok so somewhere in my mad ravings I came up with this idea for simply controlling the power for my portable-boombox-project-in-progress and was wondering if it would work as I thought. For details sake you can find info about the amp here: www.41hz.com/main.aspx?pageID=118 and the charging circuit here: 12V powered, 12 V battery charger
Basically, I want to be able to power the amplifier either by 12v battery or 12v ac-dc adapter and also to be able to charge the battery using the same adaptor (hence the 12v powered 12v battery charging circuit).
As far as I can see, this achieves that as it allows me to power the amp when batteries are required, and then when wall power is available, I can play and charge at the same time, or only play if I choose so.
The dc socket is the type that has a terminal that breaks when an adapter is plugged in.
The dpdt switch connects the battery to either the charging circuit or the dc jack and I would likely use a centre off switch so it would be play-off-charge.
By the way, the charging circuit has diodes to stop wrong current flow, so I've just pointed that out with a diode symbol.
On the schematic (I really would hesitate to call it that ) I've written what the result is of each combination of battery/ac-dc and switch.
So, pretty much it would be good if I could get peoples' opinions on whether this would work, or perhaps improvements to it (if I could achieve what I outlined above without using a switch that would be great)

**broken link removed**

 

[audioguru]

Why do you have such a huge and heavy battery?
The stereo amplifier has an output of only 6W into 8 ohms or 9W into 4 ohms at clipping per channel. Music is not at full blast all the time, its average is 10% of the max power so the total average power is 1.2W with 8 ohm speakers or 1.8W with 4 ohm speakers. Then the average battery current is only 100mA or 150mA.
A charge on your huge battery will last for 40 hours or 26 hours with music playing all the time.

Instead if you use 10 AA Ni-MH cells a charge would last 20 hours or 13 hours with music playing all the time.

 

[revans]

Can you recharge Nimh cells using a 12v AC-DC adapter? As far as I know if I used nimh cells I'd need a separate battery charger to charge them (which I don't want). So unless you have a circuit like this: 12V powered, 12 V battery charger for nimh cells then I'd prefer to keep at it with the SLA. Though I did notice the auxiliary output for the charging circuit provides 20v to charge 12v 4a Nicd packs, which could work too I guess. Could using nicd be a better idea?

 

[audioguru]

Ni-Cad batteries are obsolete because the Cadmium is a deadly poison. Ni-MH batteries replace them and have 4 times the capacity.

Each cell averages 1.3V over most of its discharge so 10 cells give 13V.
Each cell is 1.4V to 1.5V when fully charged.
AA size Ni-MH cells have a 2500mAh capacity and are charged overnight at 250mA.

 

[revans]

I think 10 cells may be pushing it as the limit of the amp is 14.5VDC so it could be too much when fully charged. However, 10 were alright, how would I go about charging it using a 12v AC-DC adapter?

 

[audioguru]

You can use 9 cells if you want. The fully charged voltage will be from 12.6V to 13.5V. Almost the same as your big and heavy lead-acid battery.

 

[revans]

Yeah to be honest nimh seems like a better option, except .... how can I charge it with a 12V AC-DC adapter? What sort of current and voltage must be provided?

 

[revans]

OK I think I'll sort out what type of battery to use later; but for now, could I please get some feedback on my "schematic" from someone who knows what they're doing (i.e. not me)

 

[audioguru]

Your "schematic" has nothing limiting the charging current to the battery.
It has nothing that disconnects the battery when it is fully charged.
It has nothing to prevent re-charging (and damaging) a battery that is already charged.
A battery charger IC will fix all of the above problems.

 

[revans]

Umm actually it does (unless I'm more confused than I thought).
Th pretty green box which says "12v powered, 12 sla battery charger" is this: 12V powered, 12 V battery charger which I'm pretty sure does what you outlined just before (except for disconnecting the battery, which is what the on-off-o dpdt switch is for)

 

[audioguru]

The battery charger circuit blasts the max output current of the voltage regulator IC which is typically 2.1A. The current is too high for charging a little battery. When the battery becomes over-charged then it will get extremely hot at such a high current and become damaged. The Ni-MH battery needs a battery charger IC that shuts off charging when it detects a full charge. A series power resistor could be used but then you must turn off the charger when you guess the battery is fully charged.

 

[revans]

When you say the charger circuit blasts out 2.1A, are you talking about the auxiliary nicd output or the main sla output?

And just so we're on the same level here, I'm not talking about using the circuit with a nimh battery, but a lead acid battery (I think you were talking about using a nimh with the nicd auxiliary output?)

 

[audioguru]

The main SLA output of the battery charger is the 2.1A that is typical from the 15V regulator IC. Your 5Ah battery is spec'd for a 0.5A charging current.

 

[revans]

So you're saying this circuit doesn't really work properly? The guy who made it apparently has used it for years without any problem. Is there any way to drop the current down to an appropriate level?

 

[revans]

Thanks to audioguru for his enlightenment but... I sort of still need an answer on what I actually started this thread about (yeah we got a bit off topic ) So if for a second we assume that the "12v powered 12v charging circuit" actually provides the appropriate current and will actually work for charging an sla battery... could anyone please just tell me if everything will work as I think it will. Thanks. (by the way I hope I don't come across as overly demanding but I really would like to sort out the "design" of my project)

 

[audioguru]

The battery charger circuit has errors:
1) A sealed lead-acid battery should be charged at 1/10th its current rating.
2) The 7815 voltage regulator is rated at a current of 1A but its typical max output current is 2.1A.
3) The 1N4004 diodes are rated for no more than 1A continuous current.

It is not wize to operate parts at currents higher than their max allowed current.

You don't need the voltage doubler parts of the voltage regulator circuit.

 

[revans]

Ok, so if the charging circuit is got it wrong, would you please be able to explain what I can do to rectify the circuit so it gives an appropriate output to charge a 5aH SLA battery?

 

[revans]

Hold Up!: I just re-read through the details of the charger circuit on its website (12V powered, 12 V battery charger) and happened to see this:

The doubler has power resistors built in, which limit the charging current.

and this...

The maximum charge current will be roughly 400mA.

Also I went and emailed the creator of the circuit (who apparently has been using the circuit no problem with many types of sla over the years) and he said it definitely doesn't put out a charging current of 2.1A but something more like 0.4A

@ audioguru: I think you might be wrong... are you sure you haven't missed anything in the circuit that would mean it actually does provide the correct current?

 

[audioguru]

The circuit has two 10 ohms in parallel that I didn't see. Their total is 5 ohms.They limit the charging current. Good.

 

[revans]

Yay! Finally that's cleared up Anyway though, back on topic, would how I've linked everything together in M$ Paint masterpiece work as I have envisaged?