Thread: hardwiring Labtop DC-DC converter directly to battery. ie bypass any switching relay

Originally Posted by kdl

... a small 50mA drain on it would theoretically be an ineffectual drain on the battery.

Not so.
Even Optima did an example of a 17mA drain on one of their ~70AH batteries. (IMO purely academic - and Optima would have failed way before that... LOL!)

50mA is more than 2 LEDs, and even one LED can be considered a problem (viz 17mA example).

Vehicles with ~50mA alarm drains have problems after a few days.
Not to mention their shorter battery life.

How do tools like the CarNetix CNX-P1900 with built-in micro controller work then? assuming a micro along with all other circuitry must drain a few dozen mA's.

This is kind of a big dissapointment. It changes my setup boottime from 3 seconds to ~45. Thats a huge change. From standby to playing music is 5 seconds, faster then my pioneer headunit!


What about adding a 100-200ma solar trickle charger to the setup, tuck it away on the roof or something?

You'd have to ask others with Carnetix etc supplies. Maybe they don't leave their vehicle unstarted or uncharged for longer than one week? And most probably use a dual battery set up.

I deal more with things that have ~10uA as a sleep/standby current, way less than a battery's self-discharge current.


You can try the 50mA, your calcs are probably right.
I'm just saying that your battery won't live as long. With 20 days of sulfation build up (probably being most significant after ~7 days)...
Though a good run after should blow away the sulfates (unless hardened - eg, sulfates older than ~7 days), it's not a desirable situation. And one day your main cranker may simply collapse...


Better with a timed converter that connects when the netbook's battery has discharged, or though to be near discharged. But they too have a limited number of cycles.
Or a second battery for independence form the main, and cheaper replacement ($25 for 7AH etc), though then a battery isolator is required (which can simply be a $3 relay)



Solar chargers... well...
I reckon too expensive, and IMO they MUST have a regulator unless the battery's self discharge current or loads are guaranteed to exceed the solar output. (Do NOT want solar charging to exceed ~14.5V!!)

I like the idea of a timed circuit with a set number of mAh it will expend before simply letting the PC battery discharge. I know that I could setup the PC to auto hibernate when the battery reaches critical levels.

but the circuits that I would need to monitor this setup will further increase the strain on the car battery. I do plan on buying a 9500mAh battery replacment for the netbook which will more then double what I have now, and It already last 1-2days in sleep mode.

So what I was thinking was I could let the netbook run on its own battery down to ~%10 or so THEN hibernate. Figuring It will last in standby for a 2-4 days before It will turn off into hibernation. I use my car a lot more then that so it will likely never get down that low, seeing as the battery will change back up to %80+ within a reasonably short car ride/

I use an eeepc and leave it in standby on the battery. It is a 2 year old battery that was used regularly when I used it as my daily PC around the house. Thus far, it has never gotten below 90%. I drive daily though and have a 30 minute commute. In the unlikely event I park it long enough to drop to 10%, it will hibernate. If I ever park it for a few days when I go away, I just hibernate or shut it down.

I definitely wouldn't recommend leaving the charger running on the battery.

Oh, weird thought. If you were up for the idea of the solar charger, you could use a relay with both normally open and closed. Then you could have input from battery when switched on and on solar when switched off. Spark might be right about price. I know that when bench testing, my charger was very specific about voltage. If I didn't have enough, it would cause the net book to convulse.

Originally Posted by kdl

but the circuits that I would need to monitor this setup will further increase the strain on the car battery.

No they won't.

Why would anyone add a power saving feature that uses more power that it saves?

I'm not saying that people don't (love those little mains AC energy savers eh?), or that people use standard 555 timers that use 10mA on standby (1,000 times the current of a PIC etc) or other stupid circuits etc.
But a true power saver is a power saver.


Chargers are ok left on batteries PROVIDED they have a cut-off or "drop to float" (voltage & current) upon the battery reaching full charge. (But how do they determine that..? IE - "intelligent" chargers.) [Hence the need for solar regulators - even for small panels.]


I'd go for a 2nd battery unless a low voltage cutout was installed, though I might add that anyhow if the 2nd battery is expensive.
The 2nd battery (thru an isolator) ensures cranking battery reserves. (Or rather, ensures the secondary battery and load does not discharge the main battery.)
Many use a 2nd battery the same as the main, hence main battery [I]redundancy/I]. Unfortunately the 2nd often has to be an AGM type if internal etc (cabin, trunk/boot), and that's overkill (as is an AGM for cranking; in general), but a 7AH or larger will often do as an emergency cranker.



Automotive relays typically use 250mA (~50R (Ohms)) but relays can be less , eg 80mA for 160R. But that is still high.
MOSFETs are negligible (uA of not nA), and transistor can be negligible (uA - mA).

I was looking at a few solid state transistor relays and it is good to know that use such a small amount of current.

I also stumbled on the watchdog feature in many micro controllers which allows the micro to run its code, which takes a few micro second, then completely power down for a set amount of time before resuming and running the code again. Now this time off can be set for any amount of time an uses only uA in the off mode.

So say the micro will run for 8 hours "always on" with watchdog you can set it to be off for 2 seconds after it performs its duties. So say we set watchdog for 2 seconds off and the code takes a relatively long 10mS to run; 2S/10mS = 200 . Using the watchdog timer you could increase battery life from 8 hours to ~1600 hours or over 2 months!

Now obviously this method is no good for any application that requires a very accurate and precise monitoring, but with something thats job is to monitor an ignition or control a set of relays a 2 second delay i very tolerable.

I believe the watchdog also has a wake from sleep input monitoring mode, basically if a lines digital logic changes it will wake or sleep the micro. Its something to think about. This paired with a few low uA solid state relays would work perfectly in a carPC setup/

I sometimes forget I'm on a forum that isn't afraid of diodes, transistors & uPCs! I can skip preaching to the converted....


I've been looking at the PICAXE 08M2. Finally a PIC with enough program space etc, simple to use, and only 8 pins. (With SMD (surface mount( versions available). All for ~$3 plus the "overhead" to learn its language, though copy & paste & simple edits is usually enough for common applications. Oh - plus a $20-$30 USB converter if you don;t have a serial port.


But for an 8-pin chip plus its 5V regulated supply & 2 resistors & socket for programming (as with all PICAXEs) plus the "interface circuitry" that is common to any electronic solution, it's IMO the way to go.
Change delays or voltage thresholds or even the combinational logic that triggers an event or events - just reprogram - no trimpots or circuit changes or added chips.


You might find that a simple $2 MOSFET can be the relay. It's simple of it's a grounding relay (open collector) though +ve switching seems to be as simple. And with on resistances (Rds) as low as mili-Ohms, voltage drops aren't a problem.


In fact, there is your "proper" lighting regulator - a slid state switch that only passes current when the "average" voltage dips below 14.2-14.4V etc.
And there too is your headlight or other dimmer (eg - LEDs) if wanting to reduce power etc.
The latter applications would use the PICAXEs PWM output(s).


The sky is the limit. Even serial RAM can be added for datalogging or extra tables etc (eg, EFI or electronic ignition).

sounds very similar to the ATmega328 (arduino uno). Is the clock internal on the PICaxe.

The 8pin through-hole setup is really nice though. is it programmed in C? also is it loaded with kiel/? or does it have its own interface for programming




-- went looking at the AXE myself. Seems like a really nice little chip. cheap and has its own programming environment. unfortunately the cable to program this thing cost more then an entire UNO board