Battery based tank circuit (tested)

Someone might have already done this before but anyway I havent seen it on this board so I thought Ill post it :)
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Alot of poeple here seems to have problem fully charging the tank battery. The standard tank circuit uses a diode typically the MBR6045. Because of the voltage drop of the diode, the charging voltage is always 0.6-0.7V lower than it should be...13.8V. With this standard setup the backup battery never get fully charge and may not function properly during crank.

On the diagram I have added a limiting resistor and an extra diode. The limiting resistor is directly connected to the main battery, this ensure that the backup battery get the full voltage from the main battery.

The typical charging current for a sealed lead acid should be 1/10th of the Ah rating. It is not neccessary to have this limiting resistor in a normal setup since the battery will find its own charging current. Having read the datasheet from Yuasa it says that the charging current can be limited to 1/4 of the Ah rating. Using a 0.8Ah battery a 68 Ohms 3 Watt is found to be suitable.

This resistor is very important in this circuit :

-To limit the charging current to 1/4 of the Ah rating.
-To unsure that the back battery get the full voltage of the main battery.
-To limit the current being drawn from the back up battery during cranking.

The additional diode is there so that the PC PSU cannot drain the charging current going into the backup battery therefore maintaining the full charging voltage.

The diode on the top ensure that the backup battery never get drained during cranking...but only through the limiting resistor. Since the current is limited a heavy drain cannot occur on the backup battery during cranking.

I have tested the circuit and it does work as expected :)

Compaq Evo N400C
Targus DC-DC PSU

Engine off = 12.1V
Engine cranking 10.1V
Engine running 13.5V

PSU cut off = 11.0V
PSU/Laptop curent drain = 2.7A

The circuit was tested with a 1N5822, this diode have a drop of 0.525 at 3A. It is better to use 1N5820 as labelled on the diagram and this only drop by 0.475V. So an even better results can be expected with the 1N5820.

With the test, hardly any heat is generated on any components. The only time alot of heat can be expected is when the backup battery is fully drained but then the 3W resistor should cope with it.

The backup battery was delibrately drained down such as the PC die completely. This cut out happened when the backup is supplying 11.2V. The backup battery is then charged for 3 mins by running the engine. A recrank is made, and yup again it survive a crank after only being recharged up for 3 mins :)

I havent tested how long the backup battery will last if it was charged up for 1 hour of driving as I didnt have the time. Im expecting like 3-4 mins if the battery was fully charged up. But 15 seconds of crank survival should be more than enough.


The maximum current can be drain with this circuit is only 3A as limited by the diode. I dont have a more powerful diode to test it out :( A bigger backup battery can also be used together with a recalculated limiting resistor value and suitable wattage.

For some reason the standard tank circuit uses MBR6045, its better to use MBR3045 or something with a lower voltage drop. MBR6045 drop by 0.7V while the MBR3045 drop by 0.5V

Even with this improved setup I dont think its good enough for ITPS. The only way around the ITPS is to add a relay that short out the diode so no voltage is ever lost. Even with that zero voltage drop circuit, the ITPS minimum operating voltage of 13.3V is still pushing it to the limit.



Ill use a higher rating diode next time on a bigger PC...If anyone wanna try first please do so and report back :)

i'm not using anything but a relay to connect to the tanks connection... Bad?

i'm not using anything but a relay to contro the tanks connection... Bad?

Have you done it? how is it exactly connected?

sorry, was using on screen keyboard so i didn't want to type much or fix my mistakes.

I've got a 4 gauge wire going from the posative batery terminal under thood to a relay, then to the posative terminal on the tank battery. the relay is wired such that it only connects the tank to the car battery when the accessory wire off the ignition is powered (so it disconects the battery completely durring crank).

Yes, i've done it, and i haven't had any problems thus far

Im still trying to understand your circuit :confused:

If you dont crank your engine and your backup run down then your PC will do a reboot?

I dont think your setup is bad....Im just trying to avoid the relay :)

Just a question,maybe it has been answered before,if so I am sorry,me bad:A Epia board with laptop hard drive,does it need 12 volt to stay on during a crank (the screen I am sure yes)but the board?
Would it be possible to make the "tank circuit"with a relay(as you Ricky327 have done in another tread"zero voltage drop" tank circuit)with a voltage sensor so when the main battery circuit drops to 12 or 13 volt the carputer only gets current from the second battery?I have a voltmeter (and am amp.meter)in my car and I often see the voltage drop to 12 volt (zero discharge on the amp.meter)when I stop for a red light,car idling,,brake light on,radio and amplifier,heater on,maybe lights also.In that case if I use a ITPS me carputer would cut if it cannot get the current direct from the second battery.
Well that makes two questions

Heisann :)

A Epia board with laptop hard drive,does it need 12 volt to stay on during a crank

The laptop HDD doesnt require a 12V so yes it should be fine. Im not so sure about the board though.


Would it be possible to make the "tank circuit"with a relay(as you Ricky327 have done in another tread"zero voltage drop" tank circuit)with a voltage sensor.

Yes just a matter of switching the relay over to use either one of the battery when the main one goes below the threshold level. Alot of people think the diodes are not needed in that circuit, but the reason for that is to ensure that the voltage are being supplied all the time. A complete loss of supply occur when the relay swithes over from main to backup battery...although this happen very quicky but its enough to crash the machine... a big capacitor may be needed to get around this. The problem with that circuit is a loss of 0.6V during the relay switching over. I havent tested that circuit at all but I think you get the idea how it works?

So you want your circuit continually switching between the main and backup battery according to the voltage level of your main...even when driving? yes it is possible but using a relay is not good idea.

ITPS is just not easy to get it to work in a car.

Yes
I think the easiest will be to wait until Maxim sends the Max787's,build the sproggy 3 that I have already bought and later sell the PW-70 and forget ITPS and tank circuits;
I understand that the sproggy will survive crank?

I understand that the sproggy will survive crank?

From what poeple say, yes they do. I dont really know because by looking at the diagram the 12V supply cannot survive. Those who said it survive the crank probably didnt use the 12V anyway...correct me if im wrong here.

Yes building a tank circuit do get kinda messy. But its mainly aimed for those who cant use the type of PSU that can survive the crank. So build the sproggy if you think that will work best for you.

Correct if I am wrong but the Sproggy will always put out the 5 volt (even if the car's voltage is 8 volt)so it should be possible to"bump" the some of the 5 volt to 12 volt?
At least for the motherboard and the screen,the hard drive (laptop) does not need 12 volt?