I would like to be able to have an automotive "powerpack" in my trunk connected to the car electrical in such a way that it would stay topped off and require *no* maintenance 2 or 3 times a year. Am disappointed that most powerpacks do not seem to have this feature (but if anyone knows of one that does please advise).

So if I were to fashion my own using an extra battery, what would the circuit need to look like? I wonder if I could get mostly there by using passives like just a rectifier diode and power resistor as a "trickle charger"?

I could achieve basically what I want using off the shelf components by kludging
inverter -> trickle charger -> extrabattery -> jumper cables. But it seems rather, well, "kludgey".

(The context is out of town daughters who will not remember to charge their power packs 2 or 3 times a year. Of course I would never and have never forgotten to do so myself, yeah right)

Thanks for input!
John

 

You need to provide more info on the "powerpack".
If it is another 12V battery then a simple rectifier diode is all you need.

 

You need to provide more info on the "powerpack".
If it is another 12V battery then a simple rectifier diode is all you need.

The one I bought most recently is Wagan Power Dome 2354 http://www.wagan.com/index.php/products/power-supplies/power-domes/2354-power-dome-400w.html

But type "jump starter" into amazon.com for the whole genre of what I'm calling a "powerpack". Amazons category is called "Jump Starters, Battery Chargers & Portable Power".

Thanks!
John

 

Thanks. The backup battery is a 12V/18Ah SLAB (sealed lead acid battery).
A 12V SLAB needs about 14.4V to charge and 13.8V for maintenance charge.
A rectifier diode in series with the 12 volt accessory outlet will reduce the voltage (typically about 14.5V) by about 0.7V bringing it down to 13.8V which is just right.

 

Woukd not use a plain rectifier diode connected the car's electrical for 2 reasons:
1. It will get the thread shut down. Read the t.o.s., no automotive "modifications" (touching any of the car's wires) is allowed.
2. In the event you ever use the power pack and deplete the battery to any appreciable degree, as soon as you start the car and it starts charging the pack, the current through the diode might be too high and might blow the diode and/or overstress the car's alternator.

Would use one of the cheap constant current buck/boost modules on amazon/ebay, plugged into the car's cigarette lighter outlet.

 

A thinking outside the box solution to this would be a small and cheap (5w or so) solar panel that you could mount under the rear windshield and run a wire into the trunk. It would keep the battery topped off into perpetuity, and you are not dealing with any hardwiring to the vehicle.

 

Woukd not use a plain rectifier diode connected the car's electrical for 2 reasons:
1. It will get the thread shut down. Read the t.o.s., no automotive "modifications" (touching any of the car's wires) is allowed.
...

Really? If the TS puts the diode external to a cigar lighter receptacle that is already in the car?

My new Subaru has three 12A rated, fused cigar lighter plugs scattered about. One in the dash, one in the jockey box between the seats, and one in the tailgate region. I would feel free to talk about anything I can "plug in" to those receptacles on this forum. If not, maybe we should all look for a new forum...

 

...
2. In the event you ever use the power pack and deplete the battery to any appreciable degree, as soon as you start the car and it starts charging the pack, the current through the diode might be too high and might blow the diode and/or overstress the car's alternator....

No way that Small SLA is going to tax the car's alternator. If the SLA is totally discharged, you may blow the ~10A fuse that powers the car's cigar plug socket. Solution is to put a self-resetting thermal Fuse or a PTC power thermistor in series with the rectifier.

 

Woukd not use a plain rectifier diode connected the car's electrical for 2 reasons:
1. It will get the thread shut down. Read the t.o.s., no automotive "modifications" (touching any of the car's wires) is allowed.
2. In the event you ever use the power pack and deplete the battery to any appreciable degree, as soon as you start the car and it starts charging the pack, the current through the diode might be too high and might blow the diode and/or overstress the car's alternator.

Would use one of the cheap constant current buck/boost modules on amazon/ebay, plugged into the car's cigarette lighter outlet.

1. I have a "key-on-only" accessory "cigarette lighter" outlet in the trunk that I would be plugging my circuit into.
2. That's why I was thinking a power resistor in series? Say six ohms would limit the current to 2 amps if the pack was dead, but provide a 100-200 ma when it was around 12v?

Thanks!
John

 

1. I have a "key-on-only" accessory "cigarette lighter" outlet in the trunk that I would be plugging my circuit into.
2. That's why I was thinking a power resistor in series? Say six ohms would limit the current to 2 amps if the pack was dead, but provide a 100-200 ma when it was around 12v?

Thanks!
John

A 6ohm resistor with 2A flowing through it will drop 12V across it. 12v across it * 2A through it = 24W of power dissipated in it. Typical resistors you see on a PCB are 1/8W or 1/4W. You would need an expensive resistor; would be cheaper to wind your own resistance wire heater coil. Because that's what it would be; a space heater. Despite being criminally inefficient, it would be a hazard that you would have to make sure is placed in an area where it won't catch anything on fire. Maybe a light bulb is a better component than a resistor; you won't get your specific value that you want, but it would be safer and cheaper.

 

Really? If the TS puts the diode external to a cigar lighter receptacle that is already in the car?

My new Subaru has three 12A rated, fused cigar lighter plugs scattered about. One in the dash, one in the jockey box between the seats, and one in the tailgate region. I would feel free to talk about anything I can "plug in" to those receptacles on this forum. If not, maybe we should all look for a new forum...

I think you misunderstood me. I wasn't saying that devices which plug into a lighter outlet are against the tos. I was saying that devices which modify the car's wiring are against the tos, therefore the discussion should be limited to devices which DO plug into the lighter outlet. Also, i expressed no agreement or support of the tos, only issued a warning based on past observations.

 

The quick and easy way around the TOS here for any automotive add ons is to say you will be using one of the factory supplied power points and the correct factory type connector for what ever power point you are using.

Being you are getting power through a factory supplied power point that is protected by the factory equipped fuses relating to said power point you are using nothing of the actual factory design is being altered.

 

No way that Small SLA is going to tax the car's alternator. If the SLA is totally discharged, you may blow the ~10A fuse that powers the car's cigar plug socket. Solution is to put a self-resetting thermal Fuse or a PTC power thermistor in series with the rectifier.

situations where this power pack would be used (to jump start a car) imply the car's onboard battery will already be a large load on the alternator when the car is started. This load will be in addition to that, and considering a car's own battery alone can be enough load to kill an alternator by itself, IMO it would be prudent to limit the current consumed by this charging device to that which is inline with the "trickle charge" described by OP.

I would use a constant current buck/boost converter from amazon or ebay; $15 for a premade PCB that can be set to deliver 100mA or 200mA or 205mA, whatever is desired, up to 12v or 13v or 13.5v, whatever is desired, without the voltage drop of a diode and without the excess waste heat of resistive current limiting, and without the risk of blowing any fuses or alternators, and without the need for any type of self resetting fuse or breaker (that would likely trip on/off in machine gun fashion if the pack was dead).

 

Given the small AH rating of the battery I see absolutely no reason that any form of current limiting would be needed.

Reason being if anyone did the math behind the relevant wiring and its likely size that would be involved it would be obvious why.

 

A 6ohm resistor with 2A flowing through it will drop 12V across it. 12v across it * 2A through it = 24W of power dissipated in it. Typical resistors you see on a PCB are 1/8W or 1/4W. You would need an expensive resistor; would be cheaper to wind your own resistance wire heater coil. Because that's what it would be; a space heater. Despite being criminally inefficient, it would be a hazard that you would have to make sure is placed in an area where it won't catch anything on fire. Maybe a light bulb is a better component than a resistor; you won't get your specific value that you want, but it would be safer and cheaper.

A "dead pack" means that the open-circuit battery terminal voltage will be about 10.5 to 11.5V, instead of the nominal 12.9 to 13.5V when the pack is fully charged. A Schottky rectifier (50V, 15 to 50A, Mouser has lots) in series with a 0.47Ω to 1Ω wire-wound 10W power resistor would limit the peak charging current to about 3A, and would prevent the battery pack from discharging backwards into the car while the car is parked.

 

If it was me I would just use one of the unused power points at the rear of the vehicle for a power source and be done with it. No worry about diodes amp limiters or alternator drain being the vehicles own wiring will work as a charge rate limiter should the booster ever be drained down too far.

Now for finding an unused power point thats fairly simple. Just get a schematic of the vehicles wiring systems from the local dealership and look for a optional accessory that the vehicle does not have such as a rear window wiper, defroster, heater/AC blower, security system, audio amplifier power point or such and use that as the new power source. Typically to make a non functioning accessory line active all it needs is the correct fuse installed in the right place and at worst a switch/relay that originally activated it added someplace.

Connection wise as far as I know all of those booster packs come with a common cigarette lighter adapters and some sort of built in reverse power feed protection built in being if a vehicle gets its battery ran down while one of these units is plugged in they apparently do not drain down with the rest of the system nor do they back feed when the engine is cranking over either in so if it was me I would just install a cigarette lighter port as a power socket and not worry about the rest of the potential issues unless they ever become an actual reality.

My point is getting power off of the stock wiring harness of most vehicles is surprisingly easy if a small amount of research is done ahead of time.

 

If it was me I would just use one of the unused power points at the rear of the vehicle for a power source and be done with it. No worry about diodes amp limiters or alternator drain being the vehicles own wiring will work as a charge rate limiter should the booster ever be drained down too far.

I wasn't sure what you were getting at back in post #14, but I was hoping it wasn't this.

My dad's mustang got rear-ended once and a tail light wire got pinched between the bumper and the frame. The vehicle's own wiring did indeed work as a current limiter, just like you described. It limited the current through that wire to the direct short, to whatever current is required to melt half an automobile wiring harness. Then, in a show of it's ultimate current limiting capability, it fused open.

 

You are aware that for the longest time the Ford Mustang was just an over priced Pinto with cheap track shoes instead of hippie flip flops right?

What I am referring to is a common and legal way to add accessories to vehicles without breaking the TOS here. What really happens/happened in your vehicle stays in your vehicle.