Sorting out power issues

**MattW** · 07-16-2001, 02:13 AM

Okay, I've been planning a car computer for a while and just came across this message board. I wasn't *completely* sure about what I was going to do for power, but my thoughts were to tap the battery and run an inverter to an ATX power supply (I'd *love* to do DC to DC, but that seems to be very expensive, and I'm doing this on a budget!). I'm really not fond of boot times, so I was thinking I could leave it suspended (STR) for days at a time without draining the battery. Then I come here and I read that batteries go dead much quicker than that, so now I have more problems to sort out. My friend tells me he has run his computer and a 15" CRT monitor off a car battery (a smaller call battery, none-the-less) for about four hours (though, a lot of that was idle time). Wouldn't a computer use only a tiny fraction of that power with STR? The only thing that's getting power is the RAM (and then you have whatever loss you get from the inefficiency of the inverter setup). Maybe my friend was doped up on something when he quoted four hours?

So now I'm thinking that I shouldn't run this thing directly off the car's battery. I have an extra car battery that I figured I'd just keep in the trunk in case of an emergency, but now I'm envisioning a setup where it runs off this one battery, which in turn gets charged by the alternator, but doesn't drain power from the main battery. Has anyone successfully implemented a system like this? Is it necessary? What about lead-acid batteries; is it bad to drain them completely? I really don't know a lot about lead-acid batteries.

Has anyone devised a way of checking the battery power from the computer and then shutting down in case it gets too low? I can think of how it could be done, but I don't have the programming skills for it.

**magnetik** · 07-16-2001, 03:00 AM

without a DC-DC and a supported motherboard dont bother with suspend to RAM you will kill the battery. And boot times can be lowered.... I have 22sec boot for my W98 system... by the time I buckle up, check mirrors and back up the driveway thats 40 seconds easy.......

errr the second battery...... I would avoid that....... get a bigger main battery first.....

im sure I will get some different responses to this matter though!

[ 07-16-2001: Message edited by: magnetik ]

**jshrieve** · 07-16-2001, 09:44 AM

Don't bother with suspend to ram if you're using an inverter. An inverter will use half an amp easily when turned on, with nothing plugged into it! It'll drain your battery fast.

If you do get a dc-dc power supply, then suspend to ram may be an option. It depends how frequently you drive. If you drive daily, it should be easy. Suspend-to-ram will draw between 150-460mAh (at 12V), depending on your individual system setup. If you don't drive daily (like me), then you have to think more about extra batteries to provide the extra current you'll need.

Setting up a second battery should be easy, but I wouldn't bother with it, or suspend to RAM.

In the end, you'll find that even a minute bootup time seems like nothing. When you're pulling out of your driveway, stopping at stop signs, etc, a minute is gone in a flash.

ALso, if you plan on controlling your system via an IRman, beware. Often your serial port programs won't work after they resume from STR unless they close and reopen the port.

[ 07-16-2001: Message edited by: jshrieve ]

**Aaron Cake** · 07-16-2001, 11:01 AM

If you are really hard set on using suspend to RAM, a DC-DC is the only way to go.

Adding a second battery is really unnecessary. All you are going to accomplish is adding weight, wiring and complication. And the second battery must be properly secured in case of a collision, so that means buying and installing an approved battery box. It'd stay away from it if I were you...

As far as monitoring the voltage, it would be relativly easy. Just build an op-amp circuit that toggles a line high or something when the voltage drops below 10V or so. This can either activate an ATX style shutdown via the parallel port and custom software, or just hit a relay for an emergency dirty shutdown.

**MattW** · 07-16-2001, 03:05 PM

Cool, some great ideas. If I didn't use STR, I'd just use hibernation (yeay for win2k--not like I'm going to put win98 in a car!), so I guess that's what I'll probably do. Though, I doubt the inverter uses that much extra power, or that it will drain my battery that quickly. I ran (as a test before I went camping, hehe) a fluorescent light (one of the small ones, like half a normal size tube) for thirty hours off my extra, smaller-than-usual car battery. Think STR could use as much power as the light? Either way, I think I am going to go DC to DC after seeing Jeff's power supply. Oh yeah, and another idea I have, is well, I was planning on having a little ethernet connection in the garage, so now I figure I could have a little 12v supply, too, to give it power and keep the battery charged. I want some hose (or something) that's coiled up, you know what I mean? Like a telephone cord, so it stretches (only with bigger loops) and I'd hang it from the ceiling in the garage and you could just plug it in through the window. Anyway, yeah, I'll ditch the second battery idea (but I think I'll keep a charged battery in the trunk for at least a few weeks just in case I drain my battery on accident).

**Falcon62Travis** · 07-16-2001, 03:23 PM

You can use one of these to hold your wires.
Just cut off the ends and thread your cables inside. There should be plenty of room if all you're adding is one CAT5 and two wires for 12v CD.

**jshrieve** · 07-16-2001, 04:41 PM

ATX-style shutdown via the parallel port? Better yet, do an ATX-style shutdown via the power button. You can configure recent motherboards to hibernate/str/turnoff on a power button press.

As for "doubting" whether or not your battery will really drain that fast.. just do the math.

Figure out what your drain will be, then find out the capacity of your battery. Take a percentage of that capacity (you don't want to completely discharge the battery -- you still need a good percentage of power in order to generate enough CCA to start the engine). Divide and you'll see approximately how many hours you can suspend to RAM before needing a recharge.

With DC-DC, STR is quite possible if you're a daily driver. If you frequently go several days without driving, like myself, then you need more capacity.

Hibernate should work fine with a DC-DC PS or an inverter. You can have the power to the inverter completely cut off after the system shuts down and there won't be any significant drain.

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