Lead Acid Batteries

Ignore the post above. Sorry!

look at the battery as a DC generator in a box with a series resistor. This is what they call an equivalent circuit. Lead acid batteries have quite a low internal resistance.

The current through this resistance = wasted energy.

The battery also operates a reversible chemical cell, with a nominal voltage of 2.0 volts per cell. The actual voltage of a fully charged battery is 13.6 volts, but for most purposes 2 is OK.

If you charge or draw a high current you get two loss factors.

One is the IR drop through the internal resistance and the other is how far you depart from reversibility. A car craking in cold weather can draw 500 Amps and soon deplete the battery. It will also get hot, = large losses in internal resistance. The discharge will also depart far from reversibility = large losses from that.

So cranking a car will flatten the battery and you will only get ~25% of the stored energy out as engine work. If you put a 1/4 Amp light bulb on it, you will lose very little in heat and the cells will operate close to reversibility, again little lost. On charging, the same is true. try to charge will 400 amps = hot battery and the cells with be non reversible and waste energy dissociating water as well. So quite a bit of your charge will be wasted.

Why do this? Well, lead acid has such a low internal resistance you can draw large starting currents, hopefully the engine starts in 1/2 second

Now searches will enlighten you on this. Remember, slow charge and discharge = max efficiency. This is why they think of the use of capacitors to help batteries with the huge start currents.

Capacitors are an analog of springs. Hard to run a car far on spring winding,but they do have good torque. Capacitors also deplete as charge = 1/2 CV²

c= farads v= volts. and batteries are full chemical reactions, and are often 20 times the capacity of a capacitor at the same voltage. You can make high voltage caps.but then the farads drop. Suffice it to say capacitors can give very high discharge currents again and again for 100,000 times, so they are a good adjunct to batteries in some cases. the so called 'super capacitors' are the ones touted for cars

If I recall correctly, LA batteries are about 70% efficient.

Here is a good link that says between 50% and 92%.....with a .pdf to download:-

http://en.wikipedia.org/wiki/Lead_acid_battery

I hope this helps you further.....