wiring SLA battery as standby (S3) power

**~docfx~** · 10-19-2006, 04:38 AM

Hi all,

This is my car PC power setup. The blue bits are what i want to add. I basically want to add a spare battery to keep my PC alive when the car is parked and in S3 Standby. At the moment when the the ignition is off the PSU has no power at all, but with this setup I'm hoping that when the ignition is off the carpc power will be isolated from the cars power and thus keep the RAM alive and not drain my car battery.

My Questions is - I'm thinking I will need to put a rectifier diode in the red circle to prevent the cars electrical system from using the SLA battery power while the ignition is on? Does this sound right? Will it work? If so what rating diode will i need?

Thanks

**~docfx~** · 10-19-2006, 07:54 AM

Maybe I should just give it a go. I've go this huge 6A/1000V diode ... maybe i'll try that... any elelctronics gurus out there??

**freenettech** · 10-19-2006, 08:50 AM

Well that should work, but if your only using a small lead acid battery like from a UPS your cars charging system is going to make that battery puff up like the pillsbury dough boy. Your going to need current limited charger of sorts to keep the charge rate at whatever the battery wants.

Also your going to need to check the datasheet of the battery to be sure that it's going to give enough power over whatever period of time you need it to keep your PC alive.

Edit: I dont know what power supply you have, but 6A is about what my opus 150 draws, so you would be maxing that diode out which could lead to failure because of power spikes.

**strohj** · 10-19-2006, 09:20 AM

just a side note.. do you know how small the power draw is during S3 standby ?

its less than leaving a radar detector plugged in....

its less than a aftermarket alarm...

its negligent in the grand scheme of things.... if you are really worried.. set your pc to go into S1 (hibernate) after 3-5 days of S3 standby

**~docfx~** · 10-19-2006, 09:41 AM

strohj - i thought hibernate was S4 ... anyways ... S3 actually consumes more than you think. I calculated that my atx board uses 0.4A when in S3. Admitidly most car batteries are usually somewhere near 100Ah but they need all that current to crank. I'd rather be safe than sorry.

freenettech - I've got a 300w DC-DC ... bit bigger than your opus eyh .. lol ... yeah but I've got a 20Ah 12v sealed lead acid battery. So you recon it would need a proper charging circuit? mmm...

**freenettech** · 10-19-2006, 10:25 AM

erm, yeah S1 uses considerable more power than S3, S1 all the fans and stuff still run, S3 is only powering RAM and USB. S4 would be hibernate, which still powers the USB on some boards unless you turn it off.

0.4A sounds about right for S3 power draw. I can leave my car easy for 4 days in S3 and still starts right up, granted my battery is only about a year old. But it does just fine and thats with upwards of 6 USB devices.

I just did a quick search on google and came up with

http://lib.store.yahoo.net/lib/zbattery/hr22-12.pdf

You see for this size battery they say to charge it at 300mA for cycle use yeah you can charge it at a more rapid rate, but even it's max charge rate is 6A which your car is going to give it more than that.

**~docfx~** · 10-19-2006, 11:25 AM

thanks for the info freenettech. I've come across a couple of circuits that could be used to make sure the SLA is charged propery. But im thinking it might be easier to just try running on the car battery alone.

I've heard that some ppl have used a 'tank circuit' to do the whole dual battery thing. The thread below did that with a SLA but its only rated at 0.8Ah??? could i modify that to suit my needs? sorry ... im more of a relay person ... dont know much about anything else .. hehe

http://mp3car.com/vbulletin/showthread.php?t=20359

Thanks

**~docfx~** · 10-20-2006, 01:33 AM

I think i've worked it out...

I need to use a resistor to limit the current going to the SLA battery and it shouldnt be damaged by the cars powerful electrical system.

If i have a 20Ah SLA battery, with a max chargeing current of 0.3CA (x12v = 3.6A), what size resistor would i need?

**freenettech** · 10-20-2006, 08:09 AM

i'm gonna dig back through my notes, but for my tank circuit I think had calculated the resistor at 2ohms for 300ma.

http://www.schursastrophotography.co...chargers1.html

There is a lot of good info in that page at least to get you started.

Well they were using the tank circuits to help keep the computer powered on while the car was cranking, not really to keep it powered while off, thats why they were using such a small battery.

**Jammer** · 10-20-2006, 11:58 AM

Originally Posted by ~docfx~

Hi all,

This is my car PC power setup. The blue bits are what i want to add. I basically want to add a spare battery to keep my PC alive when the car is parked and in S3 Standby. At the moment when the the ignition is off the PSU has no power at all, but with this setup I'm hoping that when the ignition is off the carpc power will be isolated from the cars power and thus keep the RAM alive and not drain my car battery.

My Questions is - I'm thinking I will need to put a rectifier diode in the red circle to prevent the cars electrical system from using the SLA battery power while the ignition is on? Does this sound right? Will it work? If so what rating diode will i need?

Thanks

go out and buy a battery isolator.