12v 7ah Battery Load Tester

[Shey M]

Hi everyone,

Im struggeling to find a load tester that will enable me to do a maintenance test on a 12v 7ah battery (the same batteries that some motorbikes, UPS, and alarm panels use)

Basically what I want to do is test that it can supply the 12v, at 1(or just under) amps, for 6 or so hours. (I guess this is what they should be able to do!)

I was using a load tester that clipped on and a "testing" LED illuminated, and after approx 20-30 seconds it would go out and a red "fail" LED would illuminate if it dropped under 12 volts, but I had to give it back and I completely forgot to copy the circuitry so I could build my own. Unfortunately I cant see me ever getting my hands on that particular load tester again... (boss was a d*ck haha)

I was hoping to get my hands on a circuit that tests this. with the pass/fail LED's, and also hopefully i can add in a small LCD voltmeter, which will hold the lowest voltage it reached in the timeframe..

Hopefully someone can help locate, or maybe even design one for me?

I know its a long shot, but its worth a try.


Thanks so much in advance.
Shey

 

[tpetar]

Basically what I want to do is test that it can supply the 12v, at 1(or just under) amps, for 6 or so hours. (I guess this is what they should be able to do!)

Capacity of 7Ah is given for C/10 that means 0,7A for 10h.

You can monitor internal resistance of battery, that will be usefull.

 

[BradtheRad]

The schematic below is a low-battery alarm. One image shows normal battery voltage, the other shows low.



A 1A load at 12V will be about 13 ohms. It will generate almost 12 watts of heat.

Here are various resistive loads. Radio Shack does (or did) carry 10W resistors.

 

[vedagarbhamsandeep]

Load = ? (say X) for 6 hours

Power Consumption = 6X Watt-Hour

Battery = 12V, 7AH

So,

(6X) Watt-Hour / 12V = 7AH

X = 7 x 12 / 6H = 14W (Ideal Load)

I suppose you need to build a load tester with 12V, 1A .. Am I right ?

 

[tpetar]

No, my friend ideal load for battery is no load. :smile:
If must have load then pulsed current or load in intervals is ideal.

Capacity of 7Ah is given by manufacturer, and this capacity is true for discharging for 10h.

That means max current to get whole 7Ah is 0,7A, or 0,7A*12V (nominal) = around 8,4W

and like additional to this you can search complete battery datasheet for charging discharging from manufacturer. Manufacturers make tests and measurements and they make graph.

 

[vedagarbhamsandeep]

You have considered 10hrs operation, thus the load is 8.4W. When the operation time decreases to 6hrs (as sheym mentions in the 1st thread), the load is increased to 14W.
Basically its the same calculation done by both of us in different ways.

 

[tpetar]

Yes but then you cant get 7Ah from battery, you will get much lower energy from it, and whole calc is changed and fall in water. :wink:

 

[vedagarbhamsandeep]

Yes but then you cant get 7Ah from battery, you will get much lower energy from it, and whole calc is changed and fall in water. :wink:

But battery will give you 12V, 7AH regardless of the load.

Say for example,
1. If the load draws a current of 1A, it operates for 7 hours and battery drains and
2. If the load draws a current of 7A, it operates for 1 hour only and battery drains.

 

[tpetar]

But battery will give you 12V, 7AH regardless of the load.

Say for example,
1. If the load draws a current of 1A, it operates for 7 hours and battery drains and
2. If the load draws a current of 7A, it operates for 1 hour only and battery drains.

No my friend battery capacity is declared by manufacturer and its based on minimum 10h discharge time. Just look battery datasheets. Best is to see manufacturers battery discharging graphs.

People often means battery 55Ah can give 55A for 1h, or 7Ah can give 7A for 1h what is wrong. Math is simple thing on paper, but in life things are small different (if you dont include all parameters). :smile:

If you want to get 7Ah from battery current should be around 0,6A-0,7A for continous in 10h time. Or you can have pulsed current this is different calc.

This is described with Peukert's law.

People very often forget that batteries are chemical things, and they need some time to produce power. They are little factories with productions based on potential inside them, when you charge or discharge them you change that potential inside. There is no hidden 7Ah inside battery on standby, battery is factory with capacity of production of 7Ah in 10h.

You cant invade in factory like some Japanese boss and require production of 15000 cars per month (Gung Ho '86). :smile: If you want 15K per month get bigger factory, with bigger area inside, this will ensure capability to store more potential inside for production.


I attach three relevant manufacturers datasheets examples :