# Circuit for backup battery connection with 12/24vDC vehicle battery.

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#### pimh123

##### New Member
Hi guys,
I've a vehicle tracking modem which is going to connect in a vehicle has 12v/24vDC battery. I just need to connect a backup battery(12vDC) to modem for uninterrupt power supply i.e if 12vDC/24vDC battery goes off, want to switch over to backup battery. Also the backup battery should recharge with 12vdc/24vdc vehicle battery supply. Input power supply of the modem is 12v-24vDC and 2Amps. Is there any circuit? I googled but cant find the circuit for my electrical specs.

Thanks

#### ronsimpson

##### Well-Known Member
Input power supply of the modem is 12v-24vDC and 2Amps.
12V=15V and 24V=30V When the engine is running you will have about 30V on a 24V system. Can your modem work with that?
I just need to connect a backup battery
What kind of battery? Car battery?
backup battery(12vDC)
So you want to use a 12V back up battery in a 12V and 24V system?

In the 24V system can you get to the 12V point?

#### pimh123

##### New Member
Yes. It will work with 30v. We used LM2576 regulator which accept upto 40v.
It is Li-ion rechargeable-batt. http://goo.gl/BZAAi1
Yes. My modem input voltage range is 12v to 24vdc.

#### MikeMl

##### Well-Known Member
Sounds like two Schottky rectifiers, cathodes together to the modem positive lead, one anode to the vehicle battery, the other anode to the Li-on. With this connection, power should be drawn from the Li-on only if the vehicle battery is removed.

It will draw power from the vehicle battery even when parked, provided that the voltage of the vehicle battery is at least a few tens of mV higher than the Li-on battery. The C/100 curve on the 12 Volt Lead Acid Battery State of Charge (SOC) vs. Voltage while under discharge plot on page 68 of this pdf shows what voltage the vehicle battery will have while parked.

Making the Li-on automatically charge from the vehicle battery is entirely another kettle of fish...

What is the mean current into the tracker? Is it going to kill the car's starting battery in a week? month?

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#### dr pepper

##### Well-Known Member
You can buy relays that switch over when the engine is running.
2 amps is a fair amount of juice, the average car batter would only last 1/2 a day or so.
Maybe something as simple as a relay would do your job, vehicle battery disconnected, relay drops out and tracker is powered from backup battery.
Might be worth having a look at a chandlers, you can get marine battery chargers and power systems designed to charge more than one battery independently and various other interesting stuff.

#### zahwi

##### Member
Hi guys,
I've a vehicle tracking modem which is going to connect in a vehicle has 12v/24vDC battery. I just need to connect a backup battery(12vDC) to modem for uninterrupt power supply i.e if 12vDC/24vDC battery goes off, want to switch over to backup battery. Also the backup battery should recharge with 12vdc/24vdc vehicle battery supply. Input power supply of the modem is 12v-24vDC and 2Amps. Is there any circuit? I googled but cant find the circuit for my electrical specs.

Thanks
For a charger you need DC to DC boost converter. Look at this post: http://www.electro-tech-online.com/threads/boost-converter-using-3843.136015/
or http://martin-jones.com/2013/02/15/150w-boost-converter-schematic/