Yum! Tank Circuits!

**kmccann** · 04-16-2005, 06:17 AM

I have decided that a tank circuit would be the best way for me to power my carputer and I am mixing, matching and modifing other peoples ideas from this great collection of threads. I think that I would like to use a second car battery as opposed to the SLA battery that most people seem to be working with due to the fact that I have one and I have the space for it. I do, however, have a question about my setup. All of the schematics I am looking at (the good ones at least) have a resistor between the main battery and the backup battery to limit the charging current to 1/10 or 1/4 (depends on whos schematic) of the SLA battery's Ah rating. So, I am thinking that as long as my backup battery had the same Ah rating as my main battery, I would be OK without the resistor, thus limiting my tank circuit to two diodes and some wire. Am I correct to assume that this would work without frying anything?

My circuit is pretty much like Ricky327's (Battery based tank circuit (tested)) minus the resistor and replace the SLA battery with a match of the main battery. Ricky seems to know his ****, so I figured that would be a good place for me to start.

Thanks!

**Curiosity** · 04-16-2005, 08:00 AM

You need the resistor for charging, but also for when cranking. At that point current flows back through the resistor to the rest of the car, so it needs to be high enough that it doesn't drain it too quickly. Just wire there would make the diodes useless.

**zootjeff** · 04-16-2005, 01:24 PM

Originally Posted by Curiosity

You need the resistor for charging, but also for when cranking. At that point current flows back through the resistor to the rest of the car, so it needs to be high enough that it doesn't drain it too quickly. Just wire there would make the diodes useless.

You don't need to two diodes if your using a 7AH battery.

Ricky said that "with the classic tank circuit, the battery never sees full voltage"

A better solution would be to put the resistor in parallel with the diode. Because with his method, the computer never sees the full voltage of the tank battery or the car battery.

**Curiosity** · 04-16-2005, 02:57 PM

Ricky did another design with a relay that shorts the diodes so it gets 0V drop, switching it between the diodes when cranking. That was a cool idea. After all the different things I've gone through, I've now changed to just using this strange diode that drops .25V at about 7A and goes up to .0V with no load. I really like the simplicity of it, and have no heat sink on it even.

**kmccann** · 04-16-2005, 03:47 PM

Originally Posted by Curiosity

You need the resistor for charging, but also for when cranking. At that point current flows back through the resistor to the rest of the car, so it needs to be high enough that it doesn't drain it too quickly. Just wire there would make the diodes useless.

What if I were to put another diode where the resistor was so that current would only flow from the main battery to the backup battery and from the backup battery to the carputer?

**Curiosity** · 04-16-2005, 04:20 PM

Jeff, I think I understand what you're saying. With a resistor and diode in parallel it would see full voltage when the current is low (as the battery reaches full capacity it trickels and tapers off)?

kmccann, same problem as what Jeff said. It won't fully charge because a diode drops the voltage. I know it's dangerous to directly connect 2 SLA batteries together because one could short and burn up both of them. And with 2 full size I think it's better to use a battery isolator (expensive) because they both could require quite a bit of current to charge. Plus the isolator would let you run the PC without draining both batts which would make that full size 2nd batt useful.

**Arathranar** · 04-16-2005, 04:25 PM

Originally Posted by Curiosity

Ricky did another design with a relay that shorts the diodes so it gets 0V drop, switching it between the diodes when cranking. That was a cool idea. After all the different things I've gone through, I've now changed to just using this strange diode that drops .25V at about 7A and goes up to .0V with no load. I really like the simplicity of it, and have no heat sink on it even.

I use a diode with similar specs. I have a huge heat sink on it which does get warm (it will eventually be in a small enclosed space with next to no airflow). 0.25Vx7A = 1.75W. That has to go somewhere or that diode will still get pretty toasty.

**Curiosity** · 04-16-2005, 04:47 PM

Mine's in a small enclosed space too and still not getting really hot. Maybe someone can understand more from the datasheet:

http://www.st.com/stonline/books/pdf/docs/5512.pdf

From what I understand, more than one in parallel would balance out and decrease the drop even more.

**Arathranar** · 04-16-2005, 05:10 PM

Originally Posted by Curiosity

Mine's in a small enclosed space too and still not getting really hot. Maybe someone can understand more from the datasheet:

http://www.st.com/stonline/books/pdf/docs/5512.pdf

From what I understand, more than one in parallel would balance out and decrease the drop even more.

Maybe - but not definitely. The better one(s) can dominate and end up taking most of the current anyway. It depends how well they match each other in performance whether paralleling them up is worth it. As long as each could take all the current itself, it's not dangerous to try. It's only risky if you are trying to spread the load to avoid hitting a particular diode's limit.

**Curiosity** · 04-16-2005, 07:14 PM

Sure, with normal diodes it picks the path of least resistance, but these are a little different. The more current the more resistance, so one definitely can't dominate. I've never seen a diode that works like this before.