Transient response of Lead Acid batteries

Interesting question.

I think the problem is that the battery would also be acting like a zenner diode and clamping the power supply, so that it can't follow the peak voltages the way a capacitor would. With a correctly matched/designed transformer it might work.

I shall curl up in a corner and watch this thread...pouncing out on it if it wiggles.

Del

Well I certainly wouldn't recommend using a battery in place of a capacitor when supplied by either a half wave or full wave rectifier. Batteries don't like that sort of thing.

If you are talking about old car batteries, why don't you just power your load directly off the battery and then recharge the battery using a standard (and cheap) battery charger. You are still going to have a problem with getting 6V out of a 12V car battery (unless you are using something like a 6V motor cycle battery or similar).

What is the application?

Where do you live that you can get serviceable batteries for free? Around here, a dead one is worth at least $6 US.

Anyway, I've done what you suggest. It works, but there are three major problems:

1) You need a good, heavy duty battery (oops, there goes the cost savings);

2) You need to vent the battery and the housing area since you often get gas build-up (if you've never seen a large lead-acid battery explode, it ain't pretty);

3) You need to live with a slower response time. Although a battery has capacitance, it is always mostly in the electolyte chemicals rather than on the plates, so that slows things.

4) (Being an engineer, I can always turn 3 answers into 4) The zener effect noted earlier that you see with a battery means a much shorter life than you'd like.

Sorry but I think it's a bad idea in the way that I understand your initial remark.

If you only have a full wave rectifier, then during the "low" potential portion of each cycle, the battery will also try to backfeed the transformer side of the equation.

If your load is not sufficient to keep the battery voltage at/below its nominal, there will be outgassing, then eventually the battery will (try to) charge to peak waveform voltage rather than rms and again you have problems. (Self destruct mode)

We needed a relatively large power supply for the test lab here (for a test with a relatively low duty cycle) and so we put a lead/acid battery onto a regulated power supply so that we could also support the necessary inrush currents for the load.

Even with very good regulation and close monitoring we fried many good batteries. In hindsight I strongly suspect it was the difference between stored energy (chemical) versus stored potential (charge) that was the issue.

Yes, you could use the battery in that way, but you need to add something in series to limit the charging current. What you are describing is essentially a charger permanently attached to your battery, even when you are using it to run a load. Putting the bridge output directly onto the battery will result in a BIG charge current when the voltage peaks from the rectifier go above the battery's voltage - because there is nothing to limit it! (except for the winding resistance of the transformer, etc). Something is going to go up in smoke unless you limit it.

There's nothing wrong with attaching the battery to the bridge output, it cannot pass current back through the transformer through the bridge. (You did remember to connect it in the right polarity, right? )

I don't know what they use in commercial chargers but I'd suggest you add a series resistor to make sure the charge current is limited to something reasonable for your battery. (actually, why not use a commercial charger - it's almost the same thing). I guess you could get fancy and add some type of transistorized circuit to limit the current too. Why not try a kit design, your local electronics store should be able to provide there.

Most battery are good. I want add a bit. A battery needs ripple free voltage for charging. Hence in most battery chargers designers use capacitor & inductor combination to reduce ripple at out put DC. Hence if you use a lead acid battery to reduce ripple in place of a capacitor, it will work but battery life reduces drastically. Like a capacitor, battery can not bear varying voltages. over charging due more voltage than battery can bear, will also damage a battery soon.

I disagree that a battery needs ripple-free voltage for charging. In fact, most cheap commercial chargers that I have looked at use a half-bridge rectifier that provide more of a pulsed output than a constant voltage. As the battery voltage reaches the RMS of the pulses, the charging current tapers off. Even in some higher-end chargers I have seen, one can see the "pulses" on an a DC ammeter when the battery approaches full charge, and the manufacturer's literature assures one that this is a normal indication that the battery is fully charged (which, however, is not necessarily true...)

Some points to ponder,

A battery: A mechanical chemical electron pump(when charged) consisting of an inductor capacitor parallel and series resistance. The values of these depend on the type of battery eg Nicad, lead acid, gel, GAM, Lion etc. What Ah rating?

The transformer/ PSU: In lead acid batteries a usual 1.2v per cell is required for charging. If this voltage is not regulated and allowed to rise the battery is most likely to to suffer damage. Furthermore, the current would ideally have to be controlled as the inrush current to a flat battery can cause destruction of the battery not to mention fire/explosion/burns.

50A 6V: For how long? What kind of duty cycle? All will depend on the charge cct in question as its is NOT advisable to store a flat battery for any amount of time. Once the battery voltage starts to fall the load on the transformer will start to rise and we hope it is able to cope?

Transient response: Again depending on the type of battery. i.e a automotive lead acid has a faster response the a deep cycle. A low ESR (electrical series resistance)capacitor would discharge quite rapidly especially if connected in parallel.

My opinion: Try to avoid using a battery as the primary storage device. It is thwart with dangers. A good, safe, well designed PSU is an invaluable tool for life, it is usually the first piece of equipment one obtains or builds if in the electronic profession. If on a budget start with one capacitor then add on later. Be cautious, as 50 A will burn wire/circuits or why in uncontrolled situations.

Hope this helps.

I have a question about this idea. If you use a car battery, why would it not be just like if it was still in the car charging. Car alternators charge with a strong ripple current constantly. Why would this thread's usage be any different? It sounds to me like the use Pu238 wants is essentially the same as the usage a battery would see in a car. In this case it would just be a 6 volt car.

Hello Pu238,

0,25V_pp ripple where? On load? If so what is R_load? If not, it impossible that the ripple will be 0,25V_pp but 2,5V_pp and the accu will be overcharged and stayed at the voltage of 8,5V. (BTW accepting an accu as a large capacitor is true).

Transient response can not be found while the circuit is open.

regards

All depends on your application, but, if you use the battery the way you describe you will get over-charging problems if the unit is not monitored carefully. Good battery chargers are almost all now switched mode power supplies that give 2.4V /cell at "boost" or "Bulk" charge and reduce to a float mode of 2.25V /cell. (these are for 20ºC and should be reduced by approx 5 mV/ºC above 25ºC ) These figures vary a little between manufacturers but they are all in the same area.

Below are the charge curves for a modern charger. If your charger is a simple transformer rectifier circuit you will find that the charge current curve will be like section B below but the fact that the peak of the wave form will be 1.414 x the RMS voltage will mean that the battery will, if left connected, become overcharged, gas, and fail. I make a sizeable part of my income from replaceing batteries overcharged by old, unregulated, chargers.

In conclusion, Use a capacitor !!

(If this graph doesn't show goto

See the Mastervolt users manual PDF page 21

http://www.mastervolt.com/download.php?id=2819