Battery based tank circuit (tested) - Page 4

Ricky327 · 01-07-2004, 07:48 AM

In your original post (schematic), shouldn't your top diode (from main battery/alternator) be on the OTHER SIDE (left side) of the current limit resistor? If not, won't current begin flowing from your backup battery to the main battery during cranking, limited only by the resistor?

Hehe, I knew someone will mention this.

As I said on the original post, the current that can be drained out of the backup battery by the starter motor is limited by the 68Ohms resistor. I have tested this in the worst possible case by shorting out the connection where the main battery goes...of course whithout the main battery. The terminals are shorted as if a realy heavy drain is made by the starter motor, the maximum current that get drain out of the backup in this extreme condition is = 12.5/68 = 0.18A, assumingg the voltage across the backup is 12.5v. Of course this extreme drain will never happen as starter motor only drop the main battery by 10.1V in my case. So in normal crank (12.5-10.1) / 68 = 0.035A is drained out of the backup. This current is nothing compared to 2.7A drained by my laptop.

The resistor was placed there deliberately so that the backup do get the full charging voltage from the main battery. As I said before, original tank circuit will never charge your backup fully unless your alternator put out more than 14V to compesate for the voltage lost caused by the diode.

I hope that clearer now

I shall post the update soon how long the battery last with a current drain of 2.7A

EDIT :

A 0.035A additional drain out of the backup battery for 2-3 second during crank is almost nothing really.

Ricky327 · 01-07-2004, 07:59 AM

Just for curiosities sake, how big a battery are we talking about? Voltage, Ampherage and anything else I need to know. there are alot of batteries for sale on eBay...maybe my next project?

The battery on test is the smallest I can find...thats 12V 0.7Ah as labelled on the diagram. This should last a few minutes at 2.7Ah drain.

If you want a heavier load then use something like 2Ah or more, of course the limiting resistor is also lowered and will cause more drain on your back up when the starter kick in. But then this higher drain is compensated by the bigger Ah rating of your battery.

Bear in mind, the pupose of this circuit is to survive a crank and not to extend the running time of your PC. So even a batttery that can only last for 5mins is far more than enough as you only need to hold the voltage for 2-3 seconds during crank.

robiewp · 01-07-2004, 10:12 AM

or a "tank circuit" could be used for extended carputer use. the same designs should work

MikeH · 01-07-2004, 11:02 AM

Ricky,

I'm going to give your circuit a try. I'll let you know how it comes out. Here's what I plan to implement . Let me know if you see any major flaws in the approach.

thx

Ricky327 · 01-07-2004, 03:55 PM

MikeH,

I dont know much about the radioshack DC-DC converter, The one I used is the targus one and can work right down to 11V before cutting out. Your setup look fine, yes I know some poeple here might say its complicated by I guess I can understand why you did it that way.

But ill recommend you use a higher Ah rating battery and recalculate the resistor value. Use a diode with the lowest voltage drop you can find with the right current rating of course.

I know 0.32V drop do exist :

http://uk.farnell.com/jsp/search/sea...0000~026006000

Take a pick, I know the lowest voltage drop are kinda expensive. So you have to decide on this one.

I have done a test on the backup battery being fully charged, it did last about 3mins at 2.7A drain. I dont know your total drain but Ill expect it to be more thats why I recomend you use a bigger battery.

Good luck and let us know what happen

bertybassett · 01-12-2004, 06:45 PM

rikcy I looking to power my custom made regulator 10v-18v. I've been given a free 38ah sealed battery do you think this is to powerfully for using as a spare battery?

my car Pc is only using about 40-45w

Ricky327 · 01-13-2004, 04:50 AM

The big battery would work with this circuit, but its overkill just for crank survival. The one on the circuit diagram is only 0.8Ah...and a 2.8Ah battery should be more than enough to survive a crank for most PC.

The limiting resistor needs to be recalculated to a lower value and a higher wattage otherwise your large battery will take forever to charge up. The diodes need looking in to as well, the one I stated above can only handle up to 3A. Getting a smaller battery is not that expensive, less hassle in installation.

As I said before that circuit was designed for low powered equipment so you cant just use those components I listed. Its main function is surviving the crank and not for extending the PC running power...run your engine if you want this until you run out of fuel

If you do want to use the second battery as an extra running power for your PC then another approach might be a better solutions.

OPPS I missed your last sentence...

You still need to upgrade your diode to handle more than 3A. A 5A diode should be OK for your application. Find a diode with the lowest voltage drop on the link I posted above

cotwild · 01-28-2004, 02:51 AM

Quote: Originally Posted by mst

A few months ago I was nearly finished my setup for my "still under renovation" Golf GTI 1983 with a 16v engine with a laptop 266 Mhz, mainly for GPS,then I had the (mis)fortune to read about the Via Epia motherboards on the net and later I found the MP3car forum,it has been quite an expensive meeting,the laptop is dished,so is the car radio and it will take quiet a few months before the car is finished;but the carputer can do a lot more then an old laptop

i can feel with you..

i planed to use a 400Mhz IBM Thinkpad.. now i'll have to buy a 1Ghz VIA EPIA board!

LESLIEx317537 · 02-15-2004, 12:42 PM

I'm going to try a mbr6045 diode. It's good 50amps and 800amps surge.

Can I use a 5.1Ah Ni-Cd battery for this application? I got one from TheSpecialist that he's selling on eBay.

Also will I need the resistor setup since I have a Ni-Cd battery or can I just use the 1 diode to make sure the current is always going one way?

How will the car know when to stop charging the Ni-Cd? Can it overcharge it and leak?

MikeHunt79 · 02-15-2004, 05:57 PM

Umm, you need to look into battery chemistry really... I think that lead acid cannot be overcharged but Ni-cad can, but I'm not sure... I'm sure google knows

If I were you, I'd get a small lead acid battery.

phc · 02-15-2004, 06:06 PM

Quote: Originally Posted by MikeHunt79

Umm, you need to look into battery chemistry really... I think that lead acid cannot be overcharged but Ni-cad can, but I'm not sure... I'm sure google knows

If I were you, I'd get a small lead acid battery.

Right, Lead acids can be charged at constant voltage, as long as the voltage does not go too high. For car batteries, its around 13.8 - 14V or so. For sealed lead acid, you also must limit the current, or they will bloat and possibly explode.

For Ni-Cd or NiMH or Lithium ion, or Lithium polymer, you must be very careful to limit both the voltage and current.

For this application, I recommend Hawker Sealed Lead Acid - with a 12 V pack, you don't need to limit the current and the max charge voltage is 15V. They make a 2.5V - 6V mono-bloc.

LESLIEx317537 · 02-15-2004, 07:43 PM

OK, so I went to RadioShack and saw they had a 12 volt sealed lead acid 5Ah battery for 24 buks.

Now how can I limit the current to be 1.5amps charge which the box states?

Or should I get that Hawker SLA battery? How many amps does that charge at? Thanks. I am trying to get this tank built.

LESLIEx317537 · 02-15-2004, 08:58 PM

I just came up with a idea I think.

I need to get a relay and learn how to hook it up. Also I need to make a diagram.

LESLIEx317537 · 02-16-2004, 10:26 AM

Forget about running the relay off the accesory line, casue when you crank most cars accesory drops out. - Thats why HU's sometimes go off and back on.

Run the relay off the PC PS 12 volt molex. When you crank, no drop in that line casue the backup battery is connected to the main line whenever the relay sees the 12 volts on the PC PS Molex line.

This way the crank survival should work for everyone. Easy to make one for people too.

Just get a box, add a battery, put a diode in, and add a molex connector to hook up to the relay. Now we know everyones carpc's Molex is the same so during crank, they all should act the same tehrefor crank survival shoudl be the same for everyone. No matter what setup you have as long as your diode can handle the foward voltage.

Now examine my setup, I wish I had a diagram, but tell me what's wrong? I think it shouold owrk for my Ni-Cd 5.1 amp bbatery since i got a 20 amp fuse on the line, the Ni-Cd will never try to charge at a rate greater than 20amps, which is a 4C charge rate. 4C charge rate is ok for a Ni-Cd. It won't blow it up. If the battery is fully drained out its 5100mAh, then the battery should take 15 mins to charge at a 4C rate. So I shouldn't need a resistor for a Ni-Cd setup correct? Oyeah, the Ni-Cd's won't blow up right, they will only leak air out of the vents the abttery makers installed on the back and front of the abtts. Right? Now, I don'e reccomend running Lithium Ions in the car, they like Dynamite, but I would run Lithium Polymers since they just marshemelow and puff up like stay puff.

LESLIEx317537 · 02-17-2004, 09:30 PM

Quote: Originally Posted by phc

For this application, I recommend Hawker Sealed Lead Acid - with a 12 V pack, you don't need to limit the current and the max charge voltage is 15V. They make a 2.5V - 6V mono-bloc.

Did you mean you don't need to limit the voltage?