Thread: Using a deep cycle battery?

I know this topic was posted a while ago now, but I thought I would post a reply to help out anyone considering something similar. Rather than creating expensive psu circuits to power your system, you could have a small lead acid battery (say 1.2Ah) to take over powering the system while the car is cranking. I admit this may not be the best way to power the computer as you may still have problems when trying to run it when the car is turned off. Having said that, it is a cheap solution if you aren't bothered about using the player for long amounts of time when the car is off. Here it goes...

Firstly create a simple power supply circuit using cheap linear step-down regulators to get the voltages you need. Make sure you buy the very low dropout type or the voltage drop across the regulators will be too big. You then need a good circuit to charge your battery. Wiring the battery directly to the car electrical system is NOT the solution, especially with a small battery. The second battery may draw very large amounts of current, and with a smaller battery it can easily be damaged by a charging current above C/10. This basically means that the maximum charging current for the battery is the Ah capacity divided by 10. In the case of a 1.2Ah battery it shouldn't be charged at more then 120mA. Therefore we need a special charging circuit. There are many schematics on the internet for lead acid chargers, and if anyone is interested I can provide some URL's. You want to look for something that switches to trickle charge when the battery is full. Once the battery is fully charged it will start to give off heat, which can eventually lead to the chemicals inside the battery boiling off and reducing the life of the cells. Put a diode before the charging circuit to ensure it doesn't feed back into the cars electrical system.

As well as wiring the charger to your battery you should also wire your psu to it, putting the positive from the battery through a diode before it reaches the power supply. Finally also wire the cars electrical supply into the psu, again through a diode on the positive side. These diodes will let any current bypass the battery when the car engine is on, so it can be left to charge. It also prevents any large currents feeding into the battery that are not coming from the charging circuit.

If there is any interest I will be happy to draw out a diagram, as what I have written above may be a little hard to understand without actually seeing an example.
Best regards,
Matthew

Electronicsuk, I am very interested in this setup. What if I add a larger starting battery with higher amp hours? In my case, I would not even need linear regs or anything because I am using the epia built in DC-DC power supply, which requires 12v at 4.58 amps. Even my LCD screen requires 12v. The only problem now would be the charging. Don't batteries need a higher voltage, such as 14v, to recharge a battery?

Thanks,
Null

The idea with this system is to charge the battery while the car is running. Considering the alternator is giving out around 13.8v this is fine for a 12v battery charging circuit. You wouldn't want to charge the second battery when the car is off anyway as you would run down the main battery. How many Ah of power are you considering for your battery? Bear in mind that when the car is running the secondary battery will not be in use. The only time it is used is when the car is cranking or the engine is off. If you don't plan to use your system with the car off for longer than about an hour then a 10Ah battery would be fine. If you tell me what sort of capacity battery you are planning to use I can try and design a circuit around that. It would be easier if the battery was under 12Ah but I could probbably come up with something for a larger battery.

Also if you are using a large battery you wont want it to start charging instantly, as you will already be trying to start the car. If you want to use a large battery I can also include a timer circuit to start the charging 30-40 seconds after the car has started.
Best regards,
Matthew

Just get a battery isolator. It will take care of the charging of each battery seperate and prevent one from draining the other when the engine is not running. Keep the mp3 player on the second battery so the battery and the isolator can filter out the surges, spikes, drops, etc in the rest of the car's electrical system. I have a sesitive ampmeter on my old truck and can watch the needle bounce every time I turn something electric on and off.

I have drawn out two circuits for this purpose that should work. It may be useful to people considering this type of system. The circuit at the top of the image does not have any form of timer and will start once the ignition is turned on. The circuit at the bottom of the image included a delay circuit (using a 555 timer) and will switch on about 30 seconds after the ignition is turned on. You may not need the timer circuit and if you want to make life simpler I suggest the first circuit. On the other hand you want to try and put as little load as possible on your battery while starting, so the delay circuit could be worth the effort. It all depends on your personal opinion.

I decided not to have a trickle charge function on the charger circuit as it would only make it more complicated. It would probbably be a good idea for smaller batteries but isn't really needed for anything above 6Ah. Besides, the circuit only charges the battery while the car is turned on which means the battery will get regular breaks from charging. The LM317T regulator is designed for a maximum current of 1.5A, so that is the maximum charging current you can have. The 1.2 Ohm charge current resistor limits the charging current to 1A but this can easily be changed by altering the resistor. If you want charging currents higher than 1.5A you will need a different regulator. There are other versions of the LM317 available which give out a maximum of 5A. Don't forget if you change the regulator/charge current resistor to get more current you will also need a higher wattage resistor and a more powerful D2.

The relay in the circuit is switching the current of the charger and power supply so you need to make sure it is rated at a suitable current. For the circuit I have drawn the relay would need to be rated at above 6Amps for use with a 4.5A power supply. If you are using the timer circuit the relay coil MUST NOT draw above 200mA as this is the maximum output current of the 555. I shouldn't worry about this too much as there are very few relays that would use that much current. Even car headlight relays would probbably be fine.

The charge current resistor MUST be rated at a minimum of 2W as it is limiting the charge current to the battery and can have up to 1Amp flowing through it. There are notes on the schematic relating to power ratings of the diodes in the circuit.

You may want to add a power switch for your PSU at the output of the circuit. If your power supply does not draw any current at all while it is turned off then there is no need. If you are powering an ATX motherboard the PSU may be constantly supplying a small amount of current to the board for features such a wake on LAN and the soft touch power button.

I think I have mentioned everything there. If you have any problems with the circuit or have spotted any mistakes please point them out. I will be happy to help.
Best regards,
Matthew