Battery based tank circuit (tested) - Page 9

PASware · 04-08-2004, 06:28 AM

Quote: Originally Posted by zvi

here are pics:

what kind of a 15Ohm resistor is that? I mean: why such a big one?

zvi · 04-08-2004, 06:34 AM

Quote: Originally Posted by PASware

what kind of a 15Ohm resistor is that? I mean: why such a big one?

well it is just 5 watt resistor, I don't know why is it big, I think it is normal size

Fry · 04-08-2004, 07:14 AM

Hi,

the PW-70 does not need regulated power input (even its better if you have it e.g. with the ITPS). If the voltage goes over 15V volts, the PW is switched off. Works without problems.
The calculation of the 15Ohm resistor is wrong. The current of 0,9A is only reached, if the resistor would be directly connected to ground.
If e.g. a 12V backup battery is connected to the 14V power line, the voltage dropout over the resistor is only 2(!)V,which leads to a current of 0,133A.
0,9A is reached by a resistor of 2,2Ohm and would generate 1,8W power lost. Gets a little bit hot I think.

I only use a shottky diode (and a relay) to separate the backup battery and the CarPC from the car power line.

Ricky327 · 04-08-2004, 08:26 AM

Port20,

the tank bat is to small and that is why it needs the extra diode and a charging resistor

Yes thats true, the original concept was only to survive the crank for a few minutes and not to run the PC for any extented period of time...thats why the smallest workable battery is used. If a diode is not used then a much much bigger backup battery is needed for it to resist the high current drain from the starter motor. The idea was to implement a design so it can >>> survive the crank, small circuit, as cheap as possible, reliable and able to work with other setup. If you look at the original post it did say it was only meant for a laptop...but can be modified to work with higher current.

Its true charging the capacitor through a diode is not much of a problem. But the biggest problem of the capacitor is the discharge curve. In a simple terms, the capacitor discharge rate is very sudden and then slow down in time. A battey disharge cuve is flatter until it reach a certain point. This tell us that the battery is more suitable for powering a PC. I have already calculated how long a 1F capacitor is able to power a PC, well it turn out that its survival is less than 1 sec :

http://www.mp3car.com/vbulletin/power-supplies/18537-battery-powered-tank-circuit.html

I have already played around with the capacitor trying to survive an inverter based PSU...its almost impossible.

mst your setup does not isolate the tank battery from feeding back to the main battery if it has a small current rating it will burn up if it is 18 ah or more not being isolated reduces how well it works an example is if you have a large amp for sub's when they hit the power surge will drain power from the tank battery to feed the amp

No, not true, the most that can ever be drained out of that backup battery by the car is through the 15 ohms resistor. Thats 12v/15 = 800mA...this is the worst case...this is shorting it to the ground. 800mA for 2-3 secs is nothing to a 9Ah battery. Even in this worst possible case a 5W resistor is happy with 800mA.

Again as I said in the original post, this resistor have 3 purposes :

(1) is to limit the charging current of the backup battery.
(2) is to limit the current being drained out of the backup battery by the car.
(3) is to be able to charge the backup battery with full voltage,
the classic diode design charging voltage is 0.7V less than the one I posted. The one I posted ensure that the full 13.8V is able to reach the backup battery but current limited.

As ZVI pointed out it is better to use multiple lower current rating than a single high current rating diode. The voltage drop on the smaller diode is almost always lower.

Thanks for giving us feedback, I know the thread is 9 pages long now but Im sure this has been discussed before in the thread somewhere.

Ricky327 · 04-08-2004, 08:40 AM

The calculation of the 15Ohm resistor is wrong. The current of 0,9A is only reached, if the resistor would be directly connected to ground.

Yes that is wrong in away...but the original calculation is based on worst possible case,
the power rating of the resistor can of course be reduced. But I though Ill just leave it as it is.

It is possible that his backup battery is so flat, its draining the maximum current that the resistor is effectively grounded. It is possible for a battery to have an internal short circuit, faulty battery perhaps?

It is also possible that the main battery is so flat that when you crank the engine, the starter motor cannot get any power from anywhere and it decide to drain the backup battery instead (at very low resistance).

Yes true its very unlikely but grounding that resistor can happen

EDIT :

I only use a shottky diode (and a relay) to separate the backup battery and the CarPC from the car power line.

Please do post your diagram. I heard about just using relays and people have connected it in all sort of ways. I havnt really seen a diagram and an explation on how it works...which I heard does work well for some poeple.

Exchanging idea is good

bsc · 05-31-2004, 05:58 AM

I may be mistaken but I thought lead batteries are charged by constant voltage as opposed to NiCds/NiMh being charged with constant current.

The lead batteries internal resistance limits the charging current, and no charging resistor should be needed as long as the voltage is within limits (14.4V)?

Ricky327 · 05-31-2004, 06:55 AM

bsc,

Thas is correct, an SLA battery is supposed to find its own charging current. In theory theres no need to limit the current going in. I have seen design where a small SLA battery is just connected directly.

But having read the YUASA datasheet they do recomend to limit the charging current. It is also possible for the battery to develop a fault and have an internal short circuit Thats when a directly connected battery will cause problem on the charging circuit.

Bedside that, the limiting resistor was put there mainly to limit the current being drained by the starter motor. Why not use a diode to isolate? because I didnt want any voltage drop on charging...this way the backup battery get the full charging voltage from the alternator.

Looking at the circuit you can substitute nicad/nihm battery directly.

I hope that help anyway

...maybe you have something elses that I missed?

LESLIEx317537 · 05-31-2004, 09:05 AM

I thought I would be doing a battery backup for sure.

Then I poped in my Stinger 1 Farad Cap and cranked my engine and it suvives cranks no problem.

I'm using a Key Power.

the_dude · 06-13-2004, 11:17 PM

I just got my parts from digikey and the mbr3045 diode(or shotkey rectifier) is a little different than what I thought. It has 3 leads instead of 2 like most diodes, so how should I hook up the leads according to the diagram on the first page?

- Kyle

phc · 06-14-2004, 12:48 AM

Look at the data sheet. It should show you the connections.

There are a few versions of this part.

If its a TO-220 (MBR3045WT), it looks like there are 2 diodes in the package, pin 1 is the Anode and 2 is the Common Cathode.

Anode ----|>|----- Cathode
Pin 1 ______________ Pin 2

So connect your car battery to pin 1, connect your 2nd battery to pin 3, connect resistor between 1 & 3, pin 2 (and body tab) provides your system power.

the_dude · 06-14-2004, 03:09 AM

Actually, it's this one:
http://rocky.digikey.com/WebLib/Inte...035WT-45WT.pdf

a TO-247 package

phc · 06-15-2004, 03:30 AM

Quote: Originally Posted by the_dude

Actually, it's this one:
http://rocky.digikey.com/WebLib/Inte...035WT-45WT.pdf

a TO-247 package

Same deal, its 2 diodes in 1 package. 1 and 3 are anodes, 2 is common cathode for both diodes.

the_dude · 06-15-2004, 04:01 AM

alright thanks, the moment of truth(crank) will come tommorrow.

- Kyle

MikeH · 06-15-2004, 08:00 AM

Kyle,

Although I've not addressed "tank" circuits, I've put together this article (look under the Power section) on some other things to keep in mind when trying to survive cranking. Don't forget wire size and the condition of your battery when you do your testing!

Good luck!

Asphyxsia · 06-26-2004, 09:51 AM

I am currently in the process of designing my system and was lucky enough to stumble across an old UPS that is claimed to be good for 255watts.
I am planning to hook the battery in the UPS up in paralell with the car battery for charging purposes. The UPS will only be connected to the car battery when the key is in the ON position and will have a manual turn-on switch for when i want it on otherwise. I will also be running a second relay in-line with the first, which disconnects the car battery from the UPS whenever the key is in the START position. This can be taken from the solenoid on any vehicle's starter, and can therfore be used in any car.
Here is a rough diagram:

The UPS handles all voltage regulating and due to the fact that it has a built in 7Ah lead acid battery, i should get no reboot on cranking.

If you cant get a UPS, just use a similarly sized lead acid battery (7Ah is plenty) and then hook your carputer straight to that as if it were your normal car battery.

Note: Must apologise for poor diagram, drew it on a laptop touchpad.
Also Note: Use fuses! Circuit breakers are even better if you can get them. And always fuse at both ends in case a short develops mid wire...