# Thread: Tank battery again - improvement to zero-volt-drop solution

1. ## Tank battery again - improvement to zero-volt-drop solution

Hi,

After much thought, I'd like to discuss an improvement to the "zero volt drop" solution proposed by Ricky327.
Context :
1) The "classical" tank battery setup is to have a diode allowing current to flow from the main car battery to the tank battery, but not back to the main battery when cranking.
2) The goal of Ricky327's proposal was to avoid the voltage drop you get when you put a diode between the main and backup batteries by replacing the diode with a resistor, because this drops prevents the tank battery from fully charging. However, as he explains, the resistor has to be carefully chosen to fulfill multiple purposes, among which :
- it has to be big to limit the current sent back to the main battery when cranking
- it has to be small because each and every Ampere that is going to recharge the tank battery will go through it, dissipating power uselessly

My proposal would be to combine both, putting a resistor in parallel with a diode. This way, when strong currents are needed to recharge the tank battey, they flow through the diode (low dissipation). Once the tank battery reaches Vcc - Vd (The voltage of the alternator minus the diode voltage drop), no current flows through the diode anymore, but the resistor still lets the current reach the tank battery until voltages are slowly becoming equal.
This way, the resistor value can be high (220 Ohm for example) as it's out of the way for most of the charge. It only lets the "last drop" reach the backup battery.

I guess someone now thinks "Yeah, but the resistor has a third purpose : to limit the charging current to the tank battery to avoid damaging it with high currents when it is fully discharged".

Right, but nothing prevents you from placing a second resistor R2 behind the resistor/diode pair. This time, it will be chosen as low as possible to limit high currents without dissipating too much. For example, a 6.8 Ohm resistor could be OK.

So the final circuit would be as attached below. All comments are welcome of course...

2. Big hassle, and I'm not sure it'll work the way you think... What about a simple relay normally open that's closed when engine's on ? No power loss, just a 12.5 v zener diode( may be a resistor or two, and a transistor) between the +12v and the relay coil.

Engine's OFF, main batt is roughly 13v, zener eats 12.5 +0.7v => nothing (or not enough juice) in the transistor gate, relay's open, carpc's running from tank batt.

Engine's ON, main batt's (and alternator) give 14.4v, zener still eats 13.3 v, that's 1.1v in the transistor, relay's closed, and tank batt's is wired in // with main batt.

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