I only see 2 options. 1) lots of batteries and jumper cables. 2) generator and invertor.

 

Pumping energy into a dead battery is a bad idea. Disconnect it.

 

A lower amperage, below 5A, charger has one advantage: In many vehicles you can get power to the battery straight through the cigarette lighter port. Of course it's slower than direct connection to the battery with higher amperage, and there are definitely cars where it would not work.

Ideally you let the battery charge overnight before trying to mess with anything else.

Which brings me to the basic problem. Your client probably needs a new battery no matter what, since a deep discharge is said to cut a battery's life in half. What good is a freshly programmed fob if they can't start the car and get on their way? It may be a pain to swipe a battery in the field, but that may be the most effective solution in the long run. Then you don't need to worry about charging time.

 

A lower amperage, below 5A, charger has one advantage: In many vehicles you can get power to the battery straight through the cigarette lighter port.

The problem with that is, some cars disconnect the cigarette lighter port when the engine is off.

 

A lower amperage, below 5A, charger has one advantage: In many vehicles you can get power to the battery straight through the cigarette lighter port. Of course it's slower than direct connection to the battery with higher amperage, and there are definitely cars where it would not work.

On my 1990 BMW E34, the rear cigarette lighter had a "decoupling diode". And the ignition switch needed to be in the run position...

EDIT: But the front cigarette lighter is always enabled.

 

A lower amperage, below 5A, charger has one advantage: In many vehicles you can get power to the battery straight through the cigarette lighter port. Of course it's slower than direct connection to the battery with higher amperage, and there are definitely cars where it would not work.

Ideally you let the battery charge overnight before trying to mess with anything else.

Which brings me to the basic problem. Your client probably needs a new battery no matter what, since a deep discharge is said to cut a battery's life in half. What good is a freshly programmed fob if they can't start the car and get on their way? It may be a pain to swipe a battery in the field, but that may be the most effective solution in the long run. Then you don't need to worry about charging time.

A lower amperage, below 5A, charger has one advantage: In many vehicles you can get power to the battery straight through the cigarette lighter port. Of course it's slower than direct connection to the battery with higher amperage, and there are definitely cars where it would not work.

Ideally you let the battery charge overnight before trying to mess with anything else.

Which brings me to the basic problem. Your client probably needs a new battery no matter what, since a deep discharge is said to cut a battery's life in half. What good is a freshly programmed fob if they can't start the car and get on their way? It may be a pain to swipe a battery in the field, but that may be the most effective solution in the long run. Then you don't need to worry about charging time.

Thank you so much for giving me some of your time.

They almost always need a new battery, but that is definitely not something I can provide, for many reasons. A large percentage of time I am leaving them with a working key/FOB for a vehicle that won't start for soooo many various reasons (battery, starter, seized engine or no engine, no steering wheel, flood vehicle full of mold and so many other things I have seen). My job is to make them a working key/FOB, over the decades I have learned not to be concerned with "fixing" other problems. If I put in a brand new fully charged battery, that is not as good as disconnecting their battery and hooking up a small external AC powered "charger/power supply" that delivers around 13 to 14 volts DC with very little amperage. Or if I disconnect one of their cables and hook up a jump box to supply power during the programming that also works well in most, if not all of the faster programming procedures.

Hooking up a fully charged car battery directly to a fully discharged battery has many problems (I do know disconnecting their dead battery eliminates many problems). Amperage draw between the good and bad battery is a huge problem for me (possibly the only real problem), including but not limited to... quickly lowering the good batteries voltage to an unusable voltage in a matter of minutes. Also the dead battery could have a "short" of varying degrees through sulfation. In both those scenarios I would want to limit the amps that bad battery can pull from a good battery.

I know 00 cables will deliver more amps between the dead battery and the fully charged battery than lets say 18 gauge wire, but that is not a good solution either way, both could/would be dangerous in their own way. It would be helpful to have something "like" a potentiometer that could safely limit the amount of amps the dead battery could pull from the good battery, but keep the voltage available in a better range (I am not sure if something like that is off-the-shelf available). I need very little amps (2-10? amps) during the programming, but I do need a pretty stable voltage above 12 volts. I understand just putting a resistor/potentiometer between the two is not a solution. I know the circuit needs to be more involved than just putting in a resistor/potentiometer to be safe and/or efficient.

If I had a way to limit the amount of amperage the dead battery pulls from the good battery that would be very helpful. I am not looking to start the car or have lights or radio on. If I could limit the draw to around 2-10 amps that would be helpful. I don't know what would be the minimum amperage needed to program the key/FOB. I need volts to be high enough but very little amps.

I can supply some AC power to the vehicle, but I would like to keep the AC watts needed to be as low as possible.

Thank you all for bearing with me, I understand it can be a bit difficult to follow what and how I put my thoughts into words.

 

If the dead battery is in the circuit, the only way to lower the current it will draw is to lower the voltage. You cannot set both the voltage and the current to any given load. Limiting the current to your dead battery will drop the voltage below the 12V you need no matter where the current comes from.

 

If the dead battery is in the circuit, the only way to lower the current it will draw is to lower the voltage. You cannot set both the voltage and the current to any given load. Limiting the current to your dead battery will drop the voltage below the 12V you need no matter where the current comes from.

Forgive me please if this is way off or if I am not understanding. I know what I will be asking will be incredibly basic to everyone here in this forum.

I think what you are saying is the dead battery will require a certain amperage in order to get it up to or above 12V as it is seen in the cars "circuit"? If the amperage is too low, the voltage will decrease, in the "circuit" of the car?

I really appreciate you having this patience with me. I understand you and several here are probably quite frustrated with me and my problem.

I have been making keys for several decades and power in the field has been a pretty big issue and it is only becoming a bigger problem each year.

 

I think what you are saying is the dead battery will require a certain amperage in order to get it up to or above 12V as it is seen in the cars "circuit"? If the amperage is too low, the voltage will decrease, in the "circuit" of the car?

Exactly. If the dead battery has 12V across it, it will be pulling some (unknown) current. You cannot make it pull less current and still have 12V across it. The current will probably vary over the time you have powered. And, if the battery is not charging, all the power going into it turns into heat.

Edited to add: Think of the dead battery as a large load in parallel with the car circuitry.

 

Yes, I think disconnecting the battery - status unknown - would be a wise first step towards getting the rest of your chore done as quickly and certainly as possible. As you've said, fixing their battery problem is not your concern. From the point of view of your chore, it's nothing but a nuisance. With it out of the picture, you can control the system voltage yourself with less than 5A of supply, maybe less than 1-2A.

 

Modern cars can suck a lot of juice just sitting there with the ignition on and the engine off. There's DC-DC converters designed for RVs that allow one battery to charge another, and I've seen an old booster pack (Schumacher or Motomaster, I think) that used that trick to allow a small lead-acid battery transfer its juice into a car battery, but you'll still need a substantial deep-cycle battery to provide that juice. Alternatively, there's small Honda generators with 12V charging outputs. Or, I guess if you arrived by car or truck, leave it idling and feed the other car through jumper cables; mount a big Powerpole receptacle in the front grille to make hooking up more convenient.