Newbie needs help with a project

[adriaand]

Hi guys

My plan is to have a 12v battery that is connected to a 3500rpm 1A motor. Also supply some 6 white LED's & 4 red LED's. From that same 12v battery I need to drop the voltage to I think 6v (the batt eliminator for servos ) it uses 4 AA batteries. Sorry for the stupid questions but I don't want to mess this up.

If its possible I want some help with a charger... I want to charge the battery for a cigarette lighter in the car.
Is this possible?

Thanks in advance.

 

[MikeMl]

To determine if it is possible to charge a secondary 12V battery in your car, you will have to measure the voltage at your car's cigar lighter (engine running above fast idle). If you get >14.2V (most likely what your car's voltage regulator is set to), you stand a chance of charging a 12V SLA. A 12V SLA requires about 14.7V under charge, but it will charge slowly at 14.2V.

Why do you need 6V?

 

[adriaand]

It is a 12v SLA battery. For the servo's. I'm busy building a remote control boat and the motor runs off the battery. I want to run the servos of the same battery. And the I want to run 10 LED's off it as well. Is it possible? Will the battery still last long?

 

[MikeMl]

Do the servos run on 6V? If so, use a switching buck converter to get from 12V to 6V. An SMPS can be up 90% efficient, while a simpler linear regulator will be less than 50% efficient.

Why not use a 6V battery to operate the boat? You care less about efficiency when charging the battery (in the car, where you have a 50+A alternator).

LEDs can work on 6V as well as 12V.

 

[adriaand]

Jus bear with ma a second... Do I understand right... The 6v will charge quicker than the 12v?
What about the motor,will it still work with a 6v battery?
What is an SMPS?
Switching buck converter?
Do I just run the LED's with resistors?
Yes the servo's pack uses 4AA batteries that I want to elliminate.

 

[MikeMl]

I dont think we are communicating?

You have a 12V Motor, but 6V servos?

 

[adriaand]

Yes a 12v motor but the servos work with 6volt.

 

[MikeMl]

Ok, then how much current does the 12V motor draw? How much current do the 6V servos draw? We'll worry about the LEDs later.

 

[adriaand]

The motor is 1A. The servos I don't have an idea. There's nothing.in the spec sheet. The 4 batteries last quite long but is 4 2700mA batteries. Its a bait boat to carry bait out for fishing.

 

[MikeMl]

Ok, then here is my proposal: Run the motor on unregulated 12V directly from a 12V SLA. At 1A, to a first approximation, a 10Ahr (Amp-hour) battery will run the motor ~10hrs, less if you steal some power for servos and LEDs. Use a linear regulator, like a LM7806 or an LM317 with appropriate resistors to reduce the 12V to 6V. The servos likely draw a low enough current that using a switcher is not worth the effort. Look up the forward voltage of the LEDs, and add enough of them in series to add up to ~9V. Calculate a resistor that will drop 12V to 9V (3V drop) at the current you are going to be operating the LEDs at. Do the test in the running car to see what the charge rate will be.

 

[sebbeb]

The regulator would actually be a LM7806 to have the needed 6V. It can take up to 1A wich I think will be more than enough to run servos.

Seb

 

[adriaand]

Thank you very much for your help. Just a random question.
How can I test a valve regulated lead acid battery 12v SLA to see if it still holds its charge?
Where can I get schematic for the regulator?

 

[MikeMl]

Thank you very much for your help. Just a random question.
How can I test a valve regulated lead acid battery 12v SLA to see if it still holds its charge?

Read about care and feeding of SLA batteries

I would get a standard automotive brake light lamp (usually rated 1.6A)
Charge the battery per the link, above.
Connect the lamp, note the time
Time number of hours it takes the battery terminal voltage to reach 11.5V
Multiply hours by 1.6A; that will be the battery capacity in A-hr. Compare that to what is printed on the battery.
Recharge the battery. SLAs should NEVER be left sitting in a discharged state.

Where can I get schematic for the regulator?

Fig 10 on this data sheet

 

[adriaand]

You are my hero for the day. Thank you

 

[BigB9k]

Couldn't you just use 2- 6v batterys?
use one battery to power the servos, and chain them together to get your 12v?

I know exactly what your trying to do, ans I have been thinking about this for my RC submarine project.

 

[adriaand]

The plan is to have as less extra weight in the boat. And if I can weg away with one battery then it will be great. Means more space for bait.

 

[BigB9k]

They come in all kinds of different sizes. A 12v battery is what 8x4? a 6v is probably only 4x4. Same space, and probably the same weight.

 

[SABorn]

The 2 x 6V battery suggestion is a bad idea as one battery will discharge more than the other causing BIG problems.

Over discharging SLA batteries will result in a very short battery life.

As Mike said recharge after use or the battery will result in a short life if stored flat.

It would pay you to buy a good mains charger to suit your SLA battery that is suited to the battery being left on charge 24/7, and when at home recharge the battery fully off mains power and store it on charge as this will extend the battery life.

You can get away with 90% charges in the field for periodic use but always return the battery to full charge when able.

The last 10% of charge to a battery can take some time as the amp absorption drops off as full charge is approached.

Be kind to your battery and it will serve you well.

Pete.

 

[adriaand]

Thank you. Quite some usefull info.
Another question... If I use a voltage regulator form 12v to 6v. How do I know that the amperage is not to high?
The servos work on max 185mA. So if I am suppling 6V 1A to the servos won't it damage the servos cause the amps are to high?

 

[MikeMl]

...
Another question... If I use a voltage regulator form 12v to 6v. How do I know that the amperage is not to high?
The servos work on max 185mA. So if I am suppling 6V 1A to the servos won't it damage the servos cause the amps are to high?

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