# Filtering Car Battery into clean 12v DC high amps

#### jjmalove

Joined Jul 6, 2016
51
Hello,

I'm interested in building a circuit (or purchasing something COTS?) that will do the following:

Attach to a car's battery (9-14.4Vish, 50Ah) and clean it to a steady 12V DC line to run a lot of various electronics. The circuit will need to be able to supply a lot of amps, 20+. It is acceptable that this will drain the battery quite fast. How can I make this resilient to the cranking when turning the car on (drops Voltage down as low as 6V sometimes) and also resilient to voltage spikes (40+V on occasion in a live car).

The best I have been able to find/come up with are regulators, but they cap out around 5A from what I can tell. Any ideas?

Second question/scenario:
What happens if I take a second car battery and tie it in parallel with the car's original battery? Will it naturally recharge correctly, just slower, from the alternator? If I did this to help get enough amps/clean dc would it help/hurt/make no difference? Even more batteries?

Thanks!

#### Papabravo

Joined Feb 24, 2006
14,876
What you want is almost an oxymoron. At any given terminal voltage, a battery is just about as clean as you can make it. In order to make a constant 12V DC you need to use a buck-boost regulator in order to supply the high currents at a fixed voltage. Any other method would generate far to much heat. As a switching regulator there will be some switching noise on the output. You takes yer money, and you takes yer chances.

#### tcmtech

Joined Nov 4, 2013
2,867
If you're draining your starting battery down to 9 volts you are doing some degree of damage to it. 10.5 volts is a good stopping point that represents around 95% discharge.

Also, if it's pulling down to ~6 volts a the terminals while cranking either you have way too small of battery or it's already damaged.

If it was me I would recommend using a battery isolator and dual batteries and have the second battery that runs the main load a good deep cycle type.

Lastly, us knowing what the load is would help a lot too.

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#### jjmalove

Joined Jul 6, 2016
51
Sorry I misspoke a little. I am aware that a battery by itself is "clean". So to take a car battery (not hooked to a car) and give a nice steady 12V DC independent of the battery's charge that can handle high amps, I could use something like this and be basically good to go:

http://www.digikey.com/product-deta...ons-inc/UWE-12-10-Q12PB-C/811-3061-ND/5170625
This sound correct?

However, my bigger concern is how do I do this while actually siphoning off a live car's battery.
@tcmtech I must be mistaken then. So a full charge car battery is 14.4V, and dead around 10V, this accurate? I thought it cranked as low as 6 but is that incorrect? How low does it go when you start a car usually? Also sorry what do you mean by load id?

@Papabravo We were looking at a DC to DC converter. Siphon off car battery into this:
http://www.digikey.com/product-detail/en/cui-inc/VHK150W-Q24-S12/102-2273-ND/2162727
Then output side of it into our electronics that can potentially pull 10 Amps at 12V DC.

Does this look like it would work? You mentioned the buck regulator but from what I could find quickly they all were too low amps? Something I'm missing?

#### Papabravo

Joined Feb 24, 2006
14,876
I thought you wanted a regulator that would output +12V regardless of the battery level. If battery is below +12V then you need a boost, and if it is above you need a buck regulator. There is a topology that will do both and the current is only limited by the external pass device.

https://en.wikipedia.org/wiki/Buck–boost_converter

What is always true in these kinds of device is that Power Out is always less than Power In. In some cases -- much less. It is those some cases you want to avoid in driving a high power load. In addition you must find a way to dissipate the difference between input power and output power. Nothing comes for free.

Notice the heat sink on the Digi-Key device. 150 Watts output assuming 85% efficiency means approximately 176 Watts input. That 26 watts has to go somewhere. If the efficiency is better than 85% that's good for you, but if it is less -- well that's not so good. I see they claim 90%, which is pretty good. Notice the derating curve for ambient temperatures above 50°C. It also needs a substantial airflow as the ambient goes up to maintain it's power rating.

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#### ian field

Joined Oct 27, 2012
6,539
What you want is almost an oxymoron. At any given terminal voltage, a battery is just about as clean as you can make it. In order to make a constant 12V DC you need to use a buck-boost regulator in order to supply the high currents at a fixed voltage. Any other method would generate far to much heat. As a switching regulator there will be some switching noise on the output. You takes yer money, and you takes yer chances.
A car battery is about as clean as it gets - until you hook it up to the car's electrical system.

To answer one of the questions by the TS: 60Ah is a fairly common rating for a modest size car battery. Its worth bearing in mind that the specified Ah rating is not at a 1C discharge rate - you only get the equivalent total energy at 10 or 20 hour discharge rate. If you draw 20A from a 60Ah battery - you won't get 3 hours.

#### crutschow

Joined Mar 14, 2008
26,057
What devices do you want to operate from this supply?

#### tcmtech

Joined Nov 4, 2013
2,867
However, my bigger concern is how do I do this while actually siphoning off a live car's battery.
@tcmtech I must be mistaken then. So a full charge car battery is 14.4V, and dead around 10V, this accurate? I thought it cranked as low as 6 but is that incorrect? How low does it go when you start a car usually? Also sorry what do you mean by load id?

A good battery fully charged will sit between 12.6 and 13.2 volts. Charging is usually done around 14.4 volts.

Cranking voltage drop I don't like to see things go under 10 volts at the battery.

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#### jjmalove

Joined Jul 6, 2016
51
I'll try to answer all the questions, thanks for everything so far and in advance.

@Papabravo
We're going away from the tapping into the car battery idea and are just going to have the system use its own 12V car battery source next to it for prototyping. We'll work on integrating it in parallel with a car's battery later. So at this point I believe this should do everything I need:
http://www.digikey.com/product-deta...ons-inc/UWE-12-10-Q12PB-C/811-3061-ND/5170625

Am I correct that whether the battery is at 100% charge or 5% charge, this will output a solid 12V DC output that can be pushed as high as 10 Amps? It will generate a good chunk of heat that we will need to deal with correct at that threshold correct?

@ian field
Is there a formula for determining how long we could push a battery? So lets say the device is pulling 10 Amps at 12V, or 10aH on a 60aH Battery. You're saying we won't get 6 hours, but less. How much less? Rough estimate is fine our goal at the moment is two hours at 10 Amps on a 50aH or so battery. This requirement has some wiggle room.

@crustchow
I'm limited what I am able to say due to the nature of the project. We are taking currently a dedicated 12V Battery, but in the future both a dedicated 12V Battery and a car's on-board battery, filtering them into usable steady 12V DC, then powering a mixture of single board computers, radios, and other various low power dc devices. There is also a circuit which will take the steady 12V DC and drop it down to 5V for a few of the devices, but that is simple at this point. The burst amp load should max at 10A.

@tcmtech
Thanks for the battery info. Descriptions of load is listed a few lines back.

#### Papabravo

Joined Feb 24, 2006
14,876
As has already been pointed out you dont wan't the battery voltage to drop as low as 9VDC, unless you want to constantly replace them. The batteries you use should not be automotive, but Deep Cycle Marine Batteries.