Real Life Car Battery Experiment

BMWPower06

I studied mechanical engineering in college, so electrical engineering isn't my strongest suit. Basically I have a 12V car battery that I need to discharge to 70%, how can I do this?

I know that I can monitor the state of charge by determining the open circuit voltage after ~24 hours of non-use of the battery. So a fully charged battery would be 12.6V after 24 of no usage. Now, obviously a fully discharged battery will not read 0V or will it? If that is the case then I can just do 12.6V*(0.7) and that would give me the voltage I would need to have for a 70% charge state.

However, I have read on numerous places online that a fully discharged battery is actually about 11V, but I don't know how or why...

Any battery experts out there can you help me figure this one out? Thanks!

mfb

Batteries are not linear - if 70% of the initial capacity is left, the voltage will be higher than 70% (probably something like >90%).
So the first question is: what do you mean with "discharge to 70%"? Voltage, or stored energy?
Same effect. If the battery has 11V without load, there is nearly no energy left. As soon as you connect any load, the voltage drops to ~0.

BMWPower06

by 70% I am referring to stored energy (I believe). Basically, I know that the alternator will charge the battery to 100%, I need the battery to be at 70% prior to starting the car and having the alternator charge it.

Also, how do you know that there is no charge left if the battery is at 11V with no load. Is there a formula or something that says this? Again, I'm sorry, not an electrical engineer. To me, it would seem that if the battery had a no load voltage of 11V, it would still work...

mfb

Test it. Or try to find technical data for the voltage<->energy relation.

It depends on details of the battery, there is no general way to know this.
You can imagine batteries like a perfect power source in series with a resistor. As the stored energy gets reduced, this resistance increases and the voltage of the power source drops a bit. If you try to get power out of a discharged battery, that high internal resistance limits the possible current, and leads to a large internal voltage drop in the battery, giving a lower output voltage.
This is just a model, but it works well for batteries.

BMWPower06

Thanks for the explanation, I think I understand it a little better now.

As for testing it, I keep coming across these numbers as seen on wikipedia, where 75% charge of a 12V battery shows a no load voltage of 12.35V, however, I cannot determine how they are getting this value. Do you have any ideas?

http://en.wikipedia.org/wiki/Automot...rminal_voltage

Thanks for all of your help!

256bits

wiki has a chart under Terminal Voltage
http://en.wikipedia.org/wiki/Automotive_battery
11.8 volts is a dead battery they say.
As the battery discharges the sufate attaches to the lead plates, reducing the concentration of acid and likely increasing the internal resistance.

berkeman

You should be able to find a datasheet for your battery or a similar one. I used Google Images to search for Lead Acid Battery Discharge Curve, and got lots of hits. Note how the discharge curve depends on the discharge rate...

http://www.yuasaeurope.com/images/up...acteristic.jpg

BMWPower06

ok so if it is universally accepted that 11.8V is a fully discharged battery and 12.6V is a fully charged battery then:
12.6V - 11.8V = 0.8V
0.8V*0.70 = 0.56V Charge
11.8V + 0.56V = 12.36V for a 70% charged battery

Is this acceptable?

berkeman

It's an approximation. You should be using the datasheet curves if you want to be more accurate. Notice how the discharge is non-linear on the curves...

What is your overall goal in this? What level of accuracy do you need?

BMWPower06

My goal is to see the variation of how the alternator/engine will react when dealing with a 100% charged battery vs a 70% charged battery. My experiment for determining the variation in response from the alternator/engine is set up, I just need to figure out how to drop the charge of the battery, which I think I now know.

The battery is an enertec 12V 60Ah 280A DIN battery, if that makes a difference. I believe as long as I can get close to about 70% charge level then it should be okay for this test.