I am adding a 2v, 25ah battery to my car to provide additional voltage for a 120amp electric supercharger. How do I keep it charged? Could I add a resistor, potentiometer and diode to drop the car’s charging voltage to the 2.37v-2.47v range needed?

  It's possible that there's something important that I don't know about electric superchargers, that if I did know, would make your question seem sensible.  However, it seems to me that there are several things about your question that don't make sense at all; and it seems to me that perhaps you are trying to undertake a project that is sufficiently far beyond your existing knowledge and skill that you ought not do it.

  What jumps out at me most is that you wish to use a 2-volt battery to power this thing.  If it were meant to be used as a supercharger in a car, then surely it would have been designed to run directly off of the car's own 12-volt electrical system.  What the 2-volt battery tells me is that your “supercharger” is probably an electrical fan of some sort that is meant for some entirely different application, and is probably not at all suited for use as an automotive supercharger.

  A supercharger shouldn't need a battery of its own to power it.  As it is only going to be running when the engine itself is running, it ought to run directly off of the car's electrical system.  I will point out that a device that pulls 120 amps, running off of a battery with a capacity of 25 amp-hours, is going to drain that battery in 12½ minutes.

  As far as what, specifically, you are asking; it appears that you want to build a DC-to-DC converter, but you don't have nearly enough knowledge about electronics to do so.

  I think you're in over your head, but you might find the following to be useful:  http://en.wikipedia.org/wiki/DC_to_DC_converter   Based on the parts you say you want to use, I imagine that you probably think you are going to build a fairly simple linear voltage divider.  For this application, I think that would be a very bad idea.  For the amount of current your “supercharger” wants to use, and for the amount of current available in a typical automotive system, I expect your voltage divider will be taking a lot more power from the car's electrical system than it will actually deliver to the “supercharger”; with the excess being discarded as heat.  If you're lucky, it will be the components you've used to build this divider that will burn up and burn out, rather than more expensive and difficult-to-replace components in the car itself.

Thank you for your answer. Here is the additional information you requested.

The supercharger is already installed and operating off the vehicle's 12v battery. The supercharger is wired so it only runs at wide open throttle. Plus the unit only runs infrequently, for 15 second or less bursts (you can go a long way at WOT for 15 seconds). However due the 120amp load, the battery's voltage drops to 12.65 volts while in use. I need at least 13.88 volts to get the rated 25,000rpm out of the supercharger, hence the 2v battery wired in series to increase the voltage to about 14.5v under load. The superchargers output pressure increases geometricaly with voltage ie. 1 volt = 4000+rpm increase in compressor speed. Right now I'm only seeing a dyno measured 6whp gain from the unit, primarily due to the low voltage. If the voltage were to be raised to 14v, I can expect the hp increase to more than double, according to the supercharger's manufacturer, assuming that the a/f ratio were to lean out a little (it's at 12.5:1 now, target a'f ratio is 13.7-13.9.)

I believe I could get at least 15-20 1/4 mile runs out of the supplemental 2v battery before it needs to be charged - this would typically take about a month on my street vehicle. A slow trickle charge would be sufficient to replentish the battery, due to the supercharger's low duty cycle. This could be accomplished by an 110v a/c powered battery charger, a solar pannel or the vehicles alternator. What would you recommend?

Using a set of relays you could switch the 2v booster battery in and out as needed. When it is 'in' it would be in series and give you the 14v you are looking for, and when out it could be connected to some type of charge controler and recharge from your main battery.

I attached a small scheamtic that would work. It uses a 'DPDT' Relay to control the switching of the boost battery in and out, and two high power solenoids to do the actual boost battery switching. (Ford starter selenoids would work, the ones that use to be mounded on the side of the wheel fender).

Basicly the DPDT relay will connect the boost battery to a charger when it is not in use, and when you want 'boost' it 1st disconnects the boost battery from the charger, and then connectes it via the selonoids in series with the car battery to the blower.

The charge controler would depend on what type of boost battery you are using, and how long you want it to take to recharge. It could be as simple as a power resister, or some complex electronic charger.

If a simple trickle charger, perhaps a 70 ohm 20 Watt power resister would work. Should keep the trickle current down low enough. Adding a zener diode at the same voltage as the battery's rated 'float charge' voltage might be a good idea to protect the boost battery.

I have not shown fuses, etc. You should consider putting in fuses and/or fusable links, esp to the main feed from the car battery.

Hope this helps.

Thanks for everyone's input.

Yes, I have dual e-rams. I've been using them for some time now and they work well on my 2.4l engine. I decided to use a small 3v battery charger (200mw). It is such a slow rate of charge that I'm not worried about the voltage being too high while charging. I just leave it on overnight once or twice a month to keep it topped up. This small amount of extra voltage really makes a huge difference to the supercharger's performance. I'm sure that if I was only using 1 e-ram (drawing 60 amps)it wouldn't need the extra battery. When I replace the car's 12v battery I'll probably go with an Optima Red Top or Odyssey battery, which have very low internal resistance, so it can discharge it at a high rate without losing too much voltage.

What kind of electric supercharger did you get, the "ERAM" thing? Thanks