I am looking for a good solution to supply a constant 12v to 14v to a car with a dead battery (without disconnecting the battery).

For the purpose of OBD II / computer access work done on vehicle without having access to ac power outlet (such as in the middle of a parking lot).

Power would need to be supplied for up to 3 hours. During that time all things such as lights, radio and fans that can be shut off, will be shut off.

An unmodified jump box can't maintain that level for that amount of time with a dead battery. A battery maintainer works great with an inverter except when the battery is too low. They tend to either not turn on or go into a battery repair mode, where it will cycle on and off.

The way i see it... I need to supply power to the car with a dead battery but limit the amps that can be drawn from a jump box or a "battery maintainer" that just supplies enough constant power to work through the OBD II / computer.

Rob

 

Disconnect the dead battery and connect a charged one.

 

For the purpose of OBD II / computer access work done on vehicle without having access to ac power outlet (such as in the middle of a parking lot).

And why would this need to be done in the middle of a parking lot?

 

That's a great question!

Lost car keys/FOB.

 

That's a great question!

Lost car keys/FOB.

What's that got do do with a dead battery?

Sounds like you are trying to hot wire a car.

 

Sorry, voltage requirements and time required to program a new key/FOB vary from vehicle to vehicle.

Time to program can vary from seconds to hours.

Many keys/FOB's won't successfully program if voltage drops below about 11.2v. Some need to stay above 12.8v for over 2 hours in order to program successfully. If at any moment the vehicle voltage drops below the threshold voltage, the key/FOB will fail to program.

 

I forgot... most vehicles need to be on during the key/FOB programming procedure.

 

If this is for a an going service problem, why not carry a smallspare battery?
There are ones that can be carried in one hand for this purpose, Home Depot etc!

 

A battery seller where I got my replacement car battery kept a small AGM battery with a pair of clip leads attached to it with their tool kit for battery replacement. The tech would clip it on before removing the car battery. This kept ECM setting, radio settings, etc. from being lost during the replacement. It was on the order of a small UPS battery. That's probably all you need.

 

I have 4 AGM car batteries in the van that get charged every night. One big jump box and one really big jump box. Huge 00 jump cables with heavy duty disconnect. An inverter that I can run an extension cord to the vehicle I am working on, at that point I can plug in a battery charger. But power can still be an issue especially in the winter.

Now car batteries can be in so many places (behind tire in wheelwell, under seat, under spare tire) that is not a great option, specially at night in several feet of snow and well below zero working wil winter gloves on.

 

It was on the order of a small UPS battery. That's probably all you need.

See my previous post!

 

Why not just use jumper cables and another car?

 

See my previous post!

In the past I have carried a full size truck battery in the van for exactly that purpose, it does an ok job under many scenarios. One of the problems is that a dead battery can quickly suck a battery down from 12.8 down to a low 11 volts, in as little as 15 to 20 minutes. It is not uncommon to get to a vehicle with less than .1 volt.

My really big jump box does a better job, but it still falls short often enough. That box was over 300 dollars.

In the past I have hooked up as many as 3 car batteries to keep the voltage high enough, but that was only once.

 

Why not just use jumper cables and another car?

Sometimes I am too far away for cables. Cables are good in most scenarios when I can use them.

 

Disconnect the dead battery and connect a charged one.

If the battery is very dead this has been a great solution. I would disconnect one of their battery cables, then connect a jump box to the disconnected cable and the other to either a ground or their other cable, of course it depends on what cable i disconnected positive or negative. The huge bonus to this is their battery does not kill my battery. But it can be a pain to disconnect one of their cables sometimes.

 

I am looking for a good solution to supply a constant 12v to 14v to a car with a dead battery (without disconnecting the battery).

For a weak battery, e.g. not enough charge to start the vehicle, you can connect another 12V battery in parallel.

If the battery is completely dead, why are you concerned about maintaining any dynamic settings? They've already likely been lost.

 

For a weak battery, e.g. not enough charge to start the vehicle, you can connect another 12V battery in parallel.

If the battery is completely dead, why are you concerned about maintaining any dynamic settings? They've already likely been lost.

I'm not concerned about the settings at all.

When people call me it is because they lost all the keys/FOB's to the vehicle. It is not uncommon that the vehicle has been sitting un-driven weeks, months or years. Very dead batteries are very common. Buried in snow is very common.

Key/FOB programming requires the vehicle is in accessory position for anywhere from a few minutes to several hours, during that time the voltage needs to stay above high 11v up to 12.8 volts. If it drops below a certain voltage the key/FOB won't program and you need to start over. The voltage threshold and time for programming varies depending on year, make and model.

Some of the new GM vehicles need to stay above 12.8v for over 2 hours, while in the accessory position.

That is why a maintainer is the best option I know of at this time. But most maintainers need the battery to be above around 2 volts. I have purchased around 9 maintainers and have not found one that works with a battery that has 0 volts, yet.

 

That is why a maintainer is the best option I know of at this time. But most maintainers need the battery to be above around 2 volts. I have purchased around 9 maintainers and have not found one that works with a battery that has 0 volts, yet.

Maintainers don't work with completely dead batteries, and they need AC power. I'd just carry a spare car battery. If the existing battery is too low, just disconnect the negative terminal.

 

Key/FOB programming requires the vehicle is in accessory position for anywhere from a few minutes to several hours, during that time the voltage needs to stay above high 11v up to 12.8 volts. If it drops below a certain voltage the key/FOB won't program and you need to start over. The voltage threshold and time for programming varies depending on year, make and model.

Some of the new GM vehicles need to stay above 12.8v for over 2 hours, while in the accessory position.

Well that's a challenge. Even a good battery may not hold 12.8V over 2 hours if the current draw is as high as the accessory position can be. I listen to the radio in my car and it'll shut down after 30 minutes or so to protect the battery from discharge.

I understand that jumper cables may not be practical. How about a long extension cord? Put an inverter at a good vehicle, giving you 1500W there, plug in the extension cord and run it to a battery charger at the dead battery end?

 

Well that's a challenge. Even a good battery may not hold 12.8V over 2 hours if the current draw is as high as the accessory position can be. I listen to the radio in my car and it'll shut down after 30 minutes or so to protect the battery from discharge.

I understand that jumper cables may not be practical. How about a long extension cord? Put an inverter at a good vehicle, giving you 1500W there, plug in the extension cord and run it to a battery charger at the dead battery end?

You just described one of my best options at this time. It's also the only option where the voltage at the OBD II port increases instead of decreasing over time.

I've had fantastic results running extension cord and battery maintainer, except if the battery is too dead.

I've also had fantastic results plugging in a small 150w inverter to a huge jump box, then a maintainer into the small 150w inverter. But I have the same problem with a battery that is too dead. This option does not drain my jump box nearly as much as hooking box directly to the dead vehicle.

With all the posts here so far, i think this is the path to the best solution. Is there a very dumb 120v ac charger that I could use that would put out a fairly steady 14v to a dead battery? Without overheating? At around 2 to maybe 5 or 10 amps? Everything that can be turned off in the vehicle is always off during the programming. I understand leaving such a device connected to the car battery would overcharge the battery but that would never happen because it would never be connected long enough. I think something a small as a 2 amp force charge would work. The next thing I was going to look for was a 120v 14v dc output laptop battery charger, but I think it would probably overheat.