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Battery capacity tester

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BioniC187

Member
Hey all,

Just something quick i saw 2 days ago. My car battery ran flat. After boosting it, I thought a cell is blown, so I went to my local battery center for a second opinion.
So procedure says he must first test my battery, so he got out his fancy meter, and tested. He then proceeded to tell me my battery had 32% of charge in it and that's too low. Afterwards i expected my battery to be drained, but it wasn't. His tester did not run the battery flat!

So, how do these testers work without discharging the battery? I would like to build my own if possible;)
 

cowboybob

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Most Helpful Member
The simplest testers, in essence, put a dead short across the battery terminals which allows the tester to observe the discharge response of the battery while doing that.

While this occurs, it is, of course, seriously discharging the battery, but only for a brief time during the test. It is the easiest way to test battery "life".

The speed at which the voltage level is reduced while the short is in place gives a relative "battery condition" indication, sometimes shown on the voltmeter as a percentage or green (good), orange (fair) or red (bad) battery condition estimates.

You could build your own, but you might note that the short element, during these tests, gets "red" hot and is a tad dangerous...

The voltage meter is really just a low level (50 to 100uA) DC ammeter that is calibrated to reveal the voltage across the short.
 
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NorthGuy

Well-Known Member
The speed at which the voltage level is reduced while the short is in place gives a relative "battery condition" indication.
If you place a dead short on something, according to the Ohm's law, the voltage accross the dead short will be, obviously, zero.
 

MikeMl

Well-Known Member
Most Helpful Member
Not quite a dead short, but close.


I have one of the Harbor Freight ones. It does a good job of evaluating if a lead-acid starting battery is worth putting back in the vehicle...

The Carbon pile is effectively a rehostat that can be adjusted to create a load on the battery that approximates the cranking current of the starter motor. If a battery can supply 500A for 15 seconds, it has enough capacity to be used in the vehicle.
 

JimB

Super Moderator
Most Helpful Member
Whilst we tend to think of a "Dead Short" as zero Ohms, which is a mathematical and physical impossibility, at a practical level we should probably define a dead short as a connection which has a resistance which is very much less than the normal resistance of the circuit elements.

JimB
 

cowboybob

Well-Known Member
Most Helpful Member
If you place a dead short on something, according to the Ohm's law, the voltage accross the dead short will be, obviously, zero.
Depends, I guess, on the definition of "dead short", in this context. The length of the "short" conductor (whatever its composition) used for this type of battery tester is, for all intents and purposes, the equivalent of what I would define as a dead short (extremely low resistance).

Strictly speacking, any conductor, however short in length, will exhibit a degree of resistance at room temperature and, as a result, will have a measurable voltage across it when energized.

Unless, of course, it happened to be at 0 degrees K.
 

NorthGuy

Well-Known Member
Depends, I guess, on the definition of "dead short", in this context. The length of the "short" conductor (whatever its composition) used for this type of battery tester is, for all intents and purposes, the equivalent of what I would define as a dead short (extremely low resistance).
What is the resistance of the "dead short" you're talking about?
 

audioguru

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You go to a guy who WANTS to sell you a new battery for a battery test?
Do you believe him? Of course not! He want to sell you a new battery.

I got my oil changed and it came with a free 25 points inspection. They said I need a new battery (and brakes and tires and tranny oil).
I said thanks for changing the oil but I am in a hurry and I will get the other things later.
That was 3 years ago and the battery, brakes, tires and tranny oil still work perfectly.

I test my AA Ni-MH battery cells by shorting them with my DVM set to measure Amps. A good cell measures 8A and an old worn out cell measures a few hundred mA.
The test leads and probes measure about 0.1 ohms and the meter shunt is about 0.02 ohms so the load is 0.12 ohms. A good cell is 1.0V at 8A and its internal resistance is 0.03 ohms.
 

cowboybob

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NorthGuy,

A "dead short" is a colloquialism.

In this case, a dead short has no specific value, in the context of the OP's query. It does, however, carry the implied resistance value of the length of conductor that is used as the load being placed on the tested battery.

Given that a car battery might have a capacity of, say, 500 CCAs (at 12VDC), were this entire source placed across a load of 1 ohm, we would see 6,000 watts of power be expended. To the battery, this is a dead short but, as stated, it is actually 1 ohm.

So, take your pick. I might refer you the defintion of "dead slow"...
 

crutschow

Well-Known Member
Most Helpful Member
A "dead short" is a colloquialism.

In this case, a dead short has no specific value, in the context of the OP's query. It does, however, carry the implied resistance value of the length of conductor that is used as the load being placed on the tested battery.

Given that a car battery might have a capacity of, say, 500 CCAs (at 12VDC), were this entire source placed across a load of 1 ohm, we would see 6,000 watts of power be expended. To the battery, this is a dead short but, as stated, it is actually 1 ohm.

So, take your pick. I might refer you the defintion of "dead slow"...
12V across 1 ohm is only 12 amps or 144 watts. For 500 amps the resistance must be much smaller than that.

I would say a "dead short" is a resistance small enough to conduct all the current a source can deliver with a low (undefined but probably less than a volt) drop. So what would be dead short for a car battery would be much lower resistance than a dead short for an AA battery.
 

BioniC187

Member
Ok. So ultimately, there is no way to measure available capacity without discharging a battery in some way. So that fancy meter did a discharge for a small amount of time, and was able to determine capacity of the battery based on some other parameters as well.

Thanks people!

Regards
Devan
 
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