Car battery Backup Circuit

[brooksware2000]

Hello all, I am knew to these forums although I have been an Nuts & volts reader for a long time. My question is this; I am looking for a battery backup circuit that I can implement in my car so that my Mini-Itx computer will not shutdown during cranking of the car. I have some batteries from an UPS system. I think the batteries are rated for 7aH. I was thinking of a relay type circuit that will have the backup battery switched in during cranking but switched out while the car is running. The backup battery also needs to be charged from while the car is running.<p>Any help would be appreciated.
Thanks,
Curtis

[Chris Smith]

I used to have two batteries, one for camping, which were isolated circuits for the same reasons you mention, which should also take care of your needs?<p> I placed two standard car batteries in my TOYOTA, and both were fed and kept charged by the alternator. <p>They were kept separate so that cranking would only draw from the main, and camping/accesories would only draw from the other. <p>While diving both were charging, but separate. <p>I had a battery cable and 400 amp solenoid connecting the two in case one went dry, and a chime to remind me only when they were connected.<p> When one is low or dead, click to connect the two but listen to a chime to remind you its connected. Fail safe?<p>This way I can drain one or the other completely and still have the other to start the truck, or run what ever is necessary, and being isolated, both going dead or low is just short of ....you have to “be stupid”.

[myp71]

The alternator can charge two batteries? I have heard anything over 120% the alternator output will damage it?<p>What do you guys think?

[sofaspud]

Seems to me you're on the right track. I don't really have a circuit I can point you to. If your backup battery is 12V/7Ah it shouldn't be a problem finding a recharge circuit.
I think it would be reasonably easy to implement a manual switch that would toggle between the vehicle supply and the backup supply. If automatic switching is desired, then you need a timer circuit that will activate the relay when the key is turned to ON and will hold it for about a minute (to allow time for starting the car), then deactivate to return to vehicle power. Keep in mind that the switching needs to be make-before-break or your computer is likely to reboot. I'd bet there are some cheap common components to implement this, I just don't have them in mind and would have to look them up. I hope this helps at least a little.

[brooksware2000]

How about the two circuits at this link http://www.geocities.com/brooksware2000/index.htm <p>I found them on another forum and would like anyones thoughts on them. Basically they will disconnect the main battery when the ACC line is on and connect to the backup battery only. When the car is started the backup battery is disconnected and the main battery is connect but still charges the backup battery. These circuits looks as though they will work but you do need to limit the amount of current that is applied to the backup battery during charging. You also need to keep voltage from getting back to the main battery.

[Chris Smith]

"The alternator can charge two batteries? I have heard anything over 120% the alternator output will damage it?"
*****************************
Not even? <p>The basics of electronics will teach you that you can charge any amount of batteries, from any source. <p>Alternators can not produce any more than they can produce....<p>Down hill, is the current direction. <p>The diodes/wire size, etc limit the alternator charge [out put] in any direction and unless your discharging the batteries faster than the alternator can handle, it will merely get as hot as it can get, trying to recharge any amount of batteries. <p>It will succeed, or it will run as hot as it can,... trying. <p>End of story.<p>Other then this, it may, or may not shorten the life of the alternator. <p>Mine lasted till this day, 20 years later!

[sofaspud]

I took a look at those circuits. The bottom circuit provides a very basic schematic. Again, if it were me, I would put the relay on a timer rather than energizing and de-energizing it at the same time as the starter. That way I know I won't have any issues with voltage drops when cranking the engine (the original problem), or any of that sort of thing that may crop up.
The upper circuit might be closer to what the finished project would be, but I must admit I don't understand what purpose the 2 diodes serve. Also, I'm no expert on battery recharging but the battery's internal resistance will limit the current at least somewhat. I have a store-bought powerpack that uses a 4Ah battery and it connects directly to the cigarette lighter socket, and of course voltage will only return to the main battery if the backup is at a higher potential. A diode would solve any problem with that but the forward drop may need to be taken into account.
Referring to the bottom schematic, perhaps it is possible to put a diode just to the right of the relay contacts, with the cathode toward the PC. Then, when that rail is at 13.8V, power is supplied by the vehicle. When the rail is below 12V the diode allows power to be supplied by the backup battery and blocks current from flowing to the main battery.

[sofaspud]

...and, if the series diode works, you probably don't need the relay.

[myp71]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Chris Smith:
"The alternator can charge two batteries? I have heard anything over 120% the alternator output will damage it?"
*****************************
Not even? <p>The basics of electronics will teach you that you can charge any amount of batteries, from any source. <p>Alternators can not produce any more than they can produce....<p>Down hill, is the current direction. <p>The diodes/wire size, etc limit the alternator charge [out put] in any direction and unless your discharging the batteries faster than the alternator can handle, it will merely get as hot as it can get, trying to recharge any amount of batteries. <p>It will succeed, or it will run as hot as it can,... trying. <p>End of story.<p>Other then this, it may, or may not shorten the life of the alternator. <p>Mine lasted till this day, 20 years later!<hr></blockquote><p>
I know that Mecp thing is the the worlds best book but that what they say about the 120 % over the output can damage and it's the diodes that get damanged from drawing too much power.Also they say the battery is not the main source of power when the engine is running.Also your horsepower will be affected when you try to charge two batteries.

[dacflyer]

i do it simply like this....your power to your lappy is being interrupted because newer cars have the cigarette lighter plug on a switched circuit....have your UPS batteries in parallel with the ccigarette lighter...this why when you start your car, your UPS batteries will still supply power to you lappy....and once car is running the batteries will then get charged...no control circuitry is necessary...i been using mine like this for 3 yrs now...good lucj

[MrAl]

Hello there,<p>I dont know if i would trust the battery
voltage while the car is being cranked...
it could go very low in winter causing
the comp to freeze up due to low voltage.
You could try it though.<p>If it doesnt work, i'd go with the series
diode. Use a nice size Schottky diode between
the car battery and the aux battery. Run the
comp off of the aux battery. When the car
cranks, the aux battery provides the power
to the comp. When the car runs, the alternator
charges the aux battery. Remember a fuse
in series with the diode too, 10 amp should
do it.<p>Take care,
Al

[brooksware2000]

Thanks to all for the help that you have provided.<p>Just to address the 2 circuits that I posted on the link above. The 2 diodes are there to make sure that not all of the battery power is removed while the relay is switching over. That way there is no possibility for the computer to drop out.<p>I like the idea of the timed control listed above so I suppose a microcontroller setup might be able to achieve this. I am open for suggestions.<p>I want this to be robust so that I don't have to worry about the backup batteries exploding in my trunk.<p>Thanks to all

[sofaspud]

If you go with a microcontroller, then any worries with the battery recharge can be put to rest by employing a second timer circuit. For example (a lot depends on your driving habits), the recharge can be timed to shut off after fifteen minutes of drive time, or set for 2 hours total per day, or 1 hour on & 3 hours off, etc. Good luck with the project.