What is the story with battery supplies?

What makes a good battery supply in a preamp?

What are good cells to use ?

When you place cells in series the overall mAH capacity of the pack will be equal to that of the lowest rated cell, correct?

How many mAH are necessary? I think there are going to be 4 opamps in my preamp. 2 BUF04's and 2 THS6012's. supply current for the buf04's is about 7mA each, and about 12mA each for the 6012s. So about 50mA including LED's and the output currents (these are minimal I am thinking due to the very high impedances they are driving).

My digikey catalot lists all the rechargeable 9V batteries at 150mAH. So 2 9V batteries would last about 3 hours? That is a bit too short. I know in the RC Car realm they have some pretty hefty batteries. 3000mAH at 8.4V with 7 C cells capable of delivering 30 Amps for 6 minutes?. Costing about 60-80 dollars per pack, but that would give me about 60 hours of play time right? This option costs about the same as an xformer + bridge + capacitors or about 150 dollars. It isn't including what the charger will cost and would only provide 8V rails vs 15V. A better option might be AA batteries. Digikey sells 1500mAH NiMH AA batteries for about 3.25 each. Compared to 9 dollars for the 3000mAH C cells. This works out to about 50 dollars + charger instead of 150+ charger, unless matched cells are important. The AA's would have me recharging once every couple weeks, C's about once / month, and 9V's once every other day or so.

They make a big deal about matched cells in RC cars. How important is it to match the cells ? What is involved in the matching process if I wanted to do it myself.

How do the chargers work? I have read the "low cost battery charger" application note when looking at the datasheets for the LM317 voltage regulator. Is that basically all there is to one of them? If I know what the full charge voltage of each battery pack is(I can see where matched voltage cells would help a great deal in this area), I am thinking it would be pretty simple to make a circuit to shut the charger off when it gets there.

How is a battery supply going to affect the grounding of the system? Is there a good chance I am going to run into ground loop problems since the "ground" in my leach amp might be at a different potential than the "ground" in the preamp?

ideas? comments?

Thanks

jt
 
Battery Power Supply and Grounding

If you use two batteries to create a +/- power supply with a 0V
reference. . . you should be okay. . . 0V is zero. If anything, you
are going to reduce the chance of ground loops with battery
power supplies. It is more expensive, but you get 100% true
DC rails!

I would consider 19V cordless power tool batteries. . .high amp hour
ratings. . . high +/- voltage rails.
 
Re: Battery Power Supply and Grounding

gni said:
If you use two batteries to create a +/- power supply with a 0V
reference. . . you should be okay. . . 0V is zero.
If anything, you
are going to reduce the chance of ground loops with battery
power supplies.
It is more expensive, but you get 100% true
DC rails!

I would consider 19V cordless power tool batteries. . .high amp hour
ratings. . . high +/- voltage rails.


Large Batteries are good supply for power amplifiers.
No mains ripple! High currents delivery capacity!
Even Nelson Pass has built power amps for Battery use, in the past.

If it wasn't for the charging issues --- it would be perfect!

For example two +24VDC truck batteries.
With 4 ohms speakers, +-20 volt peak gives 50 Watt RMS.


lineup :cool: has had battery amp ideas, too
 
I use two 12V, 1.3Ah gel-cell batteries to form a +/- 12V supplies to power my preamp. I have zero noise, no ground problems, and 40+ hours (I've never ran it very far down) of continuous use between charges. The batteries run $8.95 each at batterymart.com and I use a 24V wheelchair/scooter charger for the monthly charging. The charger was $35 on eBay and though it is about 10X the charging current needed (3A charger designed to charge 30Ah batteries), because it is only used monthly, the batteries can handle the cyclic nature of the charges. I've been using this setup and my preamp for years with flawless, noise free performance. I highly recommend trying this and if the rest of your preamp circuity and fabrication is well implemented, you may never go back to a line-powered preamp again.
 
Re: Battery Power Supply and Grounding

jteef said:
When you place cells in series the overall mAH capacity of the pack will be equal to that of the lowest rated cell, correct?

In theory - but only on the first discharge. You should NEVER do this as it will be impossible to perform proper charge-discharge cycles without damaging some of the cells in some manner. Some are more resistant to it t han others, but the end result will be the same, just a matter of time.


How many mAH are necessary? So about 50mA including LED's and the output currents (these are minimal I am thinking due to the very high impedances they are driving).

In theory, supply current times operating hours you wish to have between charging. Practise is different, most cells cannot use their whole capacity without degradation of the power supply, cell, or durability. Good rule of thumb: consumption x operating hours (max) x 2.


I know in the RC Car realm they have some pretty hefty batteries. 3000mAH at 8.4V with 7 C cells capable of delivering 30 Amps for 6 minutes?. Costing about 60-80 dollars per pack, but that would give me about 60 hours of play time right?

It would if things were ideal, but they are not.
This kind of NiMH battery is designed for maximum capacity to be used in a relatively short time. One consequence is that their self-discharge is high, and in fact, some may even manifest memory problems (and vastly reduced lifetime) if discharged this way.


They make a big deal about matched cells in RC cars. How important is it to match the cells ? What is involved in the matching process if I wanted to do it myself.

Not important for you - you should not be using these for your preamp anyway. The matching is important to assure fairly equal charge and discharge of the pack, given that both are to be done quickly.


How do the chargers work? I have read the "low cost battery charger" application note when looking at the datasheets for the LM317 voltage regulator. Is that basically all there is to one of them? If I know what the full charge voltage of each battery pack is (I can see where matched voltage cells would help a great deal in this area), I am thinking it would be pretty simple to make a circuit to shut the charger off when it gets there.

No it won't - not if you want long lasting cells. Charging NiMH and especiallly lithium is a rather complex process if it is to be done right (insuring many cycles of charge-discharge). Again, you really don't need this. NiMH is not suitable for your application.
Perhaps the perfect type, assuming weight is not the problem, is a lead gel-cell. These are sealed lead batteries, they come in a variety of sizes, many are used in applications such as alarm installations and computer UPS, because they have good self-discharge characterostics and are very easy to charge. Since it's a 2V per cell structure, the most often found ones are 2, 4, 6 and 12V packs, usually in the several Ah range.
Charging is simple, a current limited constant voltage charger - a simple LM317 and 3 resistors can do it. The charge voltage is 14.3V for a 12V cell, at which the current should be approaching 'zero' (actually, the self discharge current of the cell). For lower voltages, current has to be limited to what your power supply can hancle, normally it;s a few hundred mA for cells of small size.


How is a battery supply going to affect the grounding of the system? Is there a good chance I am going to run into ground loop problems since the "ground" in my leach amp might be at a different potential than the "ground" in the preamp?

You should not have problems, as long as your internal ground (battery) is floating, just like it would with aregular AC power supply. Regarding ground loops, see the answer below.

gni said:
... If anything, you are going to reduce the chance of ground loops with battery power supplies. It is more expensive, but you get 100% true
DC rails!

This is a very misguiding answer! Please do not assign magicalproperties to battery power supplies, there is enough unneeded voodoo about this already!
Using a battery supply will no more remove ground loops than powering a car with a diesel engine insted of gasoline will make it four wheel drive when it's wasn't already.
If there is a ground loop, there is a ground loop - it's a topological 'feature' and unless someone uses a soldering iron, snips and wire rerouting, it's not going to go away. The only difference is that AC rectification currents are possibly not going to be wandering around this loop, instead you are going to get DC current. This may be better, or it may be worse - using a battery supply will not determin this in itself. You still have to do your homework regarding grounds.


I would consider 19V cordless power tool batteries. . .high amp hour
ratings. . . high +/- voltage rails.

Being NiMH type, they have high capacity but high self-discharge as well. Not suitable for a 50mA current consumption. They would be if it was a power amp...
 
Discharge Rate

If I didn't learn something. . . . wow.

I should have stated that by using a pure DC source that you would
have a floating ground (in the power supply and then the signal
ground). True. . .magically the ground loop will not go away with
batteries. . . it just seems that if one were playing electrical
engineer with batteries, one would be careful of the topology also.
You should have 99.9% pure DC and could use small caps instead.

In the end. . .it takes the complex AC mains.. . earth grounding. . .
diodes. . . transformer. . . .and big supply caps part of the circuit
and replaces it with high current 'well behaved' at discharging
batteries.

I do wish to note that there are some power supplies that are
not floating or are pseudo-floating. Ranenote 151 talks about
different ways the power supply ground & signal ground can be
linked with passive or no passive elements to earth ground.

Sometimes 'ground' is earth ground; sometimes 'ground' it the
floating type as mentioned below. Units with a 'ground lift' switch
can link or break the link to earth ground. . . depending on the other
units the first unit is attached to.

It seems easier to design a system that is a floating-ground since
it becomes easier to route wires and not worry so much about
upstream currents in the grounding scheme getting mixed and
pulling in higher currents from the downstream power supply.