Backup battery circuit in car

Thread Starter


Joined Aug 16, 2020
Hi all, need advice on backup battery circuit in car(please see attached diagram for the circuit).
What I want to do is to install a backup battery in my car for dashcam so
1) when ignition is off(like parking overnight) the relay is disconnected so the dashcam will only draw power from the backup LFP battery
2) when ignition is on and engine(alternator) is running, relay is triggered so the main car circuit will charge the backup LFP battery
3) when ignition is on and starting the engine, the diode should prevent any current being drawn from the backup LFP battery to the main circuit.
I would like to have your advice on the diagram and please let me know if you see any problems, thanks a lot!



Joined Oct 29, 2013
What you need is called a battery isolator. They've been around for ages and allow an accessory battery to be charged when the motor is running, but prevents the accessories from running down the main battery. They're available on Amazon or just about any automotive store.


Joined Dec 24, 2019
A vehicle's alternator is made to recharge a lead acid battery, not a LiFePo battery. LiFePo batteries require 14.4 to 14.6V to reach a full charge and a well rated alternator in good condition puts out about 14.8V at, usually, a good deal above idle. Using a battery isolator may help but would depend on the actual voltage it passes on to the LiFePo battery as it will likely be, at least, slightly lower than alternator voltage. With the circuit you show, you will have at least a 0.7V drop accross your diode leaving only 13.7-13.9V to charge the LFP battery. Best case, your battery will never reach a full charge which may be fine for your application. Worst case, the battery never charges at all and/or could possibly cause damage to the cells.

I would look into a LFP battery charger that runs off of a 12V nominal supply voltage and use the relay to switch it off and on. Just make sure the charger is rated at high enough output to power the cam and charge the battery at the same time.


Joined Nov 6, 2012
All you need are a Diode and a Current Limiting Resistor, or some Light Bulbs.
Size the Resistor as if it was connected directly to ground, to set your maximum Charging Current,
then make sure the Diode and the Resistor can withstand the amount of Charging Current you would like.
Why do you need a 30aH Battery for a Camera ?????, it would probably run that Camera for a month.
A LifePo, or a LiPo, either one, have an extremely low internal Resistance, so if you run it all the way dead,
it will pull so much Charging Current, when the Engine is started, that it may actually catch on fire.
The Resistor limits the Charging Current, and the Diode insures that nothing in the Car can drain it.
Not knowing the Specifications of your Battery,
an assumption must be made as to its maximum safe Charging Current,
a safe assumption is usually "1-C", in other words 1 X 30aH = 30 Amps for a little more than an hour,
to go from Complete Discharge, to fully Charged.
12V / 30A = 0.4 Ohms, 30 Amps X 12V = 360 Watts of heat dissipation out of the Resistor,
so 30 Amps is not really practical, so lets try 40 Ohms,
12V / 40 Ohms = 0.3 Amps, 0.3A X 12V = 3.6 Watts, so a 5 Watt, 40 Ohm Resistor would be fine,
it will get quite hot a the beginning of the charge, ( from dead),
but this setup would take 10 Hours to completely charge a "dead" Battery.

Instead of a Resistor, a much better idea would be to wire 4-Automotive Tail Light Bulbs, (p/n 1156),
in Parallel, to protect against Over-Current.
this would also give you a quick visual indicator as to the Charge State of the Battery,
if the Battery is extremely Discharged, the Bulbs will light-up,
and they will also limit the Charge Current to around ~4-Amps.
If the Battery is only, lets say, half Discharged, the Bubs may not ever light-up.
Do you plan on running the Battery completely dead ????
This is very bad for a LifePo, and absolutely destructive to a LiPo Battery.

You have a 4-S, or a 4-Cell Battery,
connecting it directly to an automotive Alternator,
at ~14, to ~14.5V will only Charge the Battery to around 80 to 85% of its full capacity,
the Diode is going to drop that even further,
but this is great news for the Life Expectancy of the Battery,
and will reduce the Peak Charging Current.
However, you should definitely avoid discharging the Battery below ~10 volts, (2.5V per Cell),
as this will seriously shorten the life expectancy of the Battery.

What are you doing regarding "Cell Balancing" ???
Is this a Hobbyist Model Battery ??? ( they come with a "Cell Balancing Plug").
Does it have a "Battery Management Module" built-in to the Battery ???
( this is a very valuable item, and increases the cost of the Battery quite a bit ).

Another thing that you can do is to just skip all this, and just connect the battery to
an Accessory Fuse that is only Hot with the Ignition "ON",
and unless your main Starting Battery is almost dead,
the LifePo will not be drained by a significant amount while Starting the engine.
But, if your Main Starting Battery is actually almost dead,
the ~30 Amp Accessory Fuse that you have used to charge the LifePo,
will blow when you try to start the engine.
DO NOT alter the Factory Fuse Ratings in your car.
You will have to verify what other Accessories are powered by the Circuit that you choose to connect to,
because the LifePo will keep power available to those items with the Key "off".

Why are you using a LifePo in the first place ???
Why have you chosen one that is so big ???
A better match would be to get the smallest Lead-Acid, Deep-Cycle, Battery that you can find, much cheaper too.
Actually, better than that, get a Camera that doesn't suck so much power, 30aH is ridiculous for a Camera.
Or, simply run 2 identical, standard Starting Batteries in parallel, and be done with the problem.


Joined Nov 18, 2012
Your best bet is a lead acid deep cycle battery serving your camera through a continuous duty magnetic switch. No voltage loss and nothing to worry about.


Joined Nov 6, 2012
Your best bet is a lead acid deep cycle battery serving your camera through a continuous duty magnetic switch. No voltage loss and nothing to worry about.
A "Deep-Cycle" Battery, and a "Starting" Battery, can not be connected in parallel for long periods.
One will ultimately drain the other.
Paralleled Batteries MUST BE identical to each other, or problems will ensue.
So now you're back to a "Battery Isolator" which will drop as much as
1 volt off of the Charging Voltage from the Alternator, to each Battery.
On some cars you can get around this Voltage Drop with an Alternator re-wiring trick.