# Simple Battery Tester

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#### Bobii

##### New Member
Hi, I am new to this forum and I am new to electronics as well, but i have a project to do for a club that i'm in. So I have to make a simple 12V 10Ah battery tester that when the battery reaches less than 10.5V it turns off a simple 1.5V battery powered Clock, also this circuit has to be able to drain the battery in a relatively short time.

So as far as my almost non-existent knowledge takes me is that i have to use a 150W light bulb to drain the battery quickly, a diode to cut the voltage when its below 10.5V and some sort of transistor to be used as a switch to turn off the (1.5V battery powered) clock. That's it however i don't know how to pick the right resistors, diodes or transistors, so please help me, I would be really grateful. Thanks in advance.

#### MrAl

##### Well-Known Member
Hi,

You would be better off using a comparator to test for the 10.5 volt point as they are more accurate than discrete circuits.
You could then turn on/off a transistor or relay.
If you want a 150 watt load at 12v you'll have to find one or make one yourself, as a regular 120vac bulb may not be able to do it.

#### MikeMl

##### Well-Known Member
Hi Bobii,

you didn't put in a Location so that we can advise you properly...

this question has come up before, several times. Here are my previous posts: (click here)
Circuit is a TL431 which serves both as a precision voltage reference and an adjustable comparator that can switch either a relay or big PFET high-side switch.

The load for discharging a SLA battery at 150W @ a nominal 12V requires that it draws a current of I=P/E = 150/12 = 12.5A. That could be accomplished by a combination of 1156 or 1157 automotive lamps. These are cheap, and readily available. The power to the lamps will vary a bit as the battery discharges from 12.5V down to 10.5V

How precise do you have to be on maintaining the load to be exactly 150W as the voltage drops? Building a voltage-independent constant-current or constant-power load can be done, but is much more complicated than just using the lamps as the discharging load.

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#### Bobii

##### New Member
Thanx for the fast response,

Hi Bobii,

you didn't put in a Location so that we can advise you properly...

this question has come up before, several times. Here are my previous posts: (click here)
Circuit is a TL431 which serves both as a precision voltage reference and an adjustable comparator that can switch either a relay or big PFET high-side switch.

The load for discharging a SLA battery at 150W @ a nominal 12V requires that it draws a current of I=P/E = 150/12 = 12.5A. That could be accomplished by a combination of 1156 or 1157 automotive lamps. These are cheap, and readily available. The power to the lamps will vary a bit as the battery discharges from 12.5V down to 10.5V

How precise do you have to be on maintaining the load to be exactly 150W as the voltage drops? Building a voltage-independent constant-current or constant-power load can be done, but is much more complicated than just using the lamps as the discharging load.
The load doesn't have to be exactly 150W or precise, in fact looking at the equation i think, i should use a 120W bulb, Also I live in Macedonia, and I'll probably buy the components from my local hobby shop Loging.

Also could you suggest me a Software that I can use to generate a pcb layout, I'll post a design when i figure out how to use voltage regulators and PFETs. Thanx again for your explanation.

#### MikeMl

##### Well-Known Member
...The load doesn't have to be exactly 150W or precise, in fact looking at the equation i think, i should use a 120W bulb...
Where are you going to get a 12V 120Wbulb? A house bulb will draw a tiny fraction of its Wattage when powered with 12V. The reason I linked the automotive lamps is because they should be readily available, cheap, and you can parallel as many as you need to get close to 120W...

Also could you suggest me a Software that I can use to generate a pcb layout, I'll post a design when i figure out how to use voltage regulators and PFETs. Thanx again for your explanation.
I wouldn't mess with a pcb for a one-of, simple circuit with as few parts as this one...I would just hand wire it onto one of these...

Can you source a 12Vdc, 30A relay, like one of these (automotive)?

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#### RODALCO

##### Well-Known Member
Use 2 x car head light bulbs a 100 and a 55 Watt in parrallel or get 3 x 50 Watt halogen ceiling lamps. They are cheap and easily available.

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