# how to work with car battery

Discussion in 'General Electronics Chat' started by indianhits, Mar 25, 2011.

1. ### indianhits Thread Starter Active Member

Jul 25, 2009
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hello guys i have a small doubt my battery says it can supply 220Ah.Now suppose if i have a circuit which consumes 180mA.Now i have like 1.2Ah.

Again if i have other circuit which takes lets say 100mA then i have 785mAh left right?

2. ### SgtWookie Expert

Jul 17, 2007
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How did you come up with the 1.2Ah?

Car batteries should not be used for deep-cycle applications, as they will quickly fall apart inside. The plates are very thin compared to deep-cycle batteries.

I don't know where you are coming up with these numbers.

If you discharge a lead-acid battery to 12.5v @ 25°C, plate sulfation begins.
If you routinely discharge a lead-acid battery below 70% of full charge, it will have a relatively short life.

I didn't answer your question, but you seem to be wanting to try something that will result in damage to your battery.

3. ### indianhits Thread Starter Active Member

Jul 25, 2009
86
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oh sorry i really messed it up i have very poor knowledge of batteries.Normally we see battery rated as 7Ah,220Ah...etc.so, as far as i know it says like it can supply 7 amps per hour right?then what will happen if i connect a circuit which uses like 180mA to the battery then how does the calculation go?

Again what happens if i have 2 circuits which needs 180mA and 100mA respectively(which should run with battery)??

and it also says it can give 12V right so in reality how much voltage can it give??

4. ### SgtWookie Expert

Jul 17, 2007
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http://www.1st-optima-batteries.com/amp_hours.asp

7AH is the rating for current draw over a 20 hour period, or 7/20 = 350mA. At the end of 20 hours, a 7AH battery with a constant 350mA load will be discharged to ~10.5v @ 25°C/77°F; considered 100% discharged. Doing such a thing to an automotive battery will just about guarantee that you will have a ruined battery.

220AH / 20 = a constant 11 Ampere load for 20 hours before the battery is discharged to ~10.5V.

However, you are probably looking at the "CCA" rating, and not the AH rating. They are very different. CCA is "cold cranking amps"; how much current the battery can put out before dropping below a given voltage in 30 seconds.

Military batteries for trucks, like the 6TMF battery, are VERY large and heavy (72 lbs, or 32.7kg), and are rated for 120AH. A typical automotive battery might be rated for 20AH to 50AH.

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5. ### wenn32 Member

Nov 9, 2010
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i checked the battery it is written as 12v,7Ah that's it!

so its like for 7Ah battery we must installing a circuit which is less than 350mA right??

i heard we must use only 60% of battery so its like (7Ah X 60%).is the calculation correct??

Another question when we design a circuit we must keep the voltage,power and impedance in mind right??

Thanks for the help!

Last edited: Mar 26, 2011
6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,759
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AMP and HOURS

They are two different things.

7 Ah can be had several ways.

7 amps in 1 hour
3.5 amps in 2 hours
.7 amps in 10 hours
.35 amps in 20 hours

Are you with me on the mathematical breakdown? Any combination of time and current which equals the battery rating.

To prevent damage to the battery with cyclical use, simply divide your answer by two.

7 amps in 1/2 hour
3.5 amps in 1 hour
.7 amps in 5 hours
.35 amps in 10 hours

Now you have a maximum discharge rate which will leave your battery only 50%(or so) discharged. NEVER exceed this discharge rate, or the battery will suffer internal damage and its life will be much much shorter.

For recharging you can put energy back in much faster than you take it out. Most deep cycle batteries will allow for a 2C recharge(meaning 2 times the capacity rating of the battery)

If you can discharge at 7 amps for 30 minutes in a 7aH battery then you could charge at 14 amps maximum.

Again, it is BETTER to not push a battery to such extremes of charging current and into deep discharges, but they can be used this way for many cycles. They will last much longer if they are used in a more gentle manner.

AS FOR THE ORIGINAL QUESTION ON CAR BATTERIES

They CANNOT be discharged more than 5 or 10% and then must be IMMEDIATELY recharged. The plates are very thin and there are lots of them packed inside each cell. It is built this way to provide extremely high current discharges for short durations of just a few SECONDS. Not minutes. Not hours. Seconds. As in 5 seconds, or 10 seconds. If you try to use a CAR STARTING BATTERY as a power source for a circuit, be prepared to replace it within the week, or even sooner. They will fall to pieces on the inside, and the debris will gather at the bottom of the battery and short out the plates in the cell.

Go to a store which sells batteries and ask for a battery that can run a trolling motor, or a golf cart. Not a STARTING battery. If the rating tag on the battery has 'CA' or 'CCA' on it then the battery is a starting battery and cannot be used in deep cycle applications.

Last edited: Mar 26, 2011
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7. ### wenn32 Member

Nov 9, 2010
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pretty good explanation!

8. ### SgtWookie Expert

Jul 17, 2007
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Here is a chart that I excerpted from a 12V 7AH AGM lead-acid battery datasheet:

Note that at the end of each discharge plot, the battery is VERY deeply discharged; much lower than what most consider 100% discharged. A battery subjected to that kind of abuse would not last long at all; maybe a few dozen cycles.

Kermit,
The discharge rates and final charge you mentioned are not accurate.
If you go strictly by AH rating over 20 hours, 7/20 = 350mA, the battery will be 100% discharged (by this chart, worse than that) at the end of 20 hours, not 50% discharged.

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9. ### Audioguru New Member

Dec 20, 2007
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The 7Ah battery can supply 7A for 8 minutes before it drops its voltage to below 12V.
It can supply 3.5A for 22 minutes.
It can supply 0.35A for 10 hours.

10. ### indianhits Thread Starter Active Member

Jul 25, 2009
86
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how did you calculate can you please tell me.

11. ### Audioguru New Member

Dec 20, 2007
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I didn't calculate anything. I simply looked at the graph.

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12. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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That is one hundred percent correct.

And I do add that I set 1/2 that as a MAX, and suggest one never exceed it.

Everyone - follow the Sgt. and heed his posting on this subject. But, if you can't remember any other way, take half the rating of the battery and NEVER exceed it, aim much lower, and you will be satisfied with the results of the circuit.

13. ### soda Active Member

Dec 7, 2008
174
13
Hi

Is a 7Ah gel cell (SLA) battery a deep cycle battery?

14. ### SgtWookie Expert

Jul 17, 2007
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Yes, BUT ... the plot I posted is from a datasheet of an AGM VRLA battery.

Gel cells need to be treated more carefully than AGM's. If you charge or discharge a gel cell too rapidly, or overcharge a gel cell, you will cause bubbles to form in the gel that cannot be removed. The bubbles will permanently reduce the capacity of the battery.

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15. ### indianhits Thread Starter Active Member

Jul 25, 2009
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ok you said "before falling below 12v".thanks mr.audioguru & mr.SgtWookie for helping me with the datasheet & explanation.Also thanks to mr.Kermit for easy explanation.stuffs easy but didnt had good teacher.Thanks! Again

16. ### curry87 Member

May 30, 2010
101
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How can you tell when a car battery capacity has dropped to 70% ?

17. ### wenn32 Member

Nov 9, 2010
37
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i would also like to know that

and do we need to add a regulator circuit since it varies depending on capacity right??if yes then is 10v enough?

18. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,759
920
http://www.solar-electric.com/frhdprhy1.html

Its called a 'hydrometer'. and it measures the specific gravity(concentration) of the acid in the battery.

A completely discharged battery will have water in the cells(theoretically). A fully charged battery will have a strong sulfuric acid solution in the cells.(concentration varies among manufacturers) All conditions in between full charge and empty have a corresponding concentration of acid, which can be read with the hydrometer. It is the way BIG battery banks are double checked for proper operation.

Adding acid to a battery is a NO NO. Do not do it. It will not charge the battery, nor deliver more electricity from it. It will destroy the battery.

Acid is added if the battery loses any by accident. Not to increase charge or make a battery 'better'.

19. ### wenn32 Member

Nov 9, 2010
37
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that's a great explanation!

but i wanted to know how you do it with a circuit.
do you compare the voltage levels using comparator??
It is necessary to have a voltage regulator for battery operated circuits right?

Thanks!

20. ### Audioguru New Member

Dec 20, 2007
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Almost all of my battery powered circuits do not have a voltage regulator and they work perfectly.

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