What do i need to convert car battery dc voltage to a useable 15v 8 amp dc current?

I saw just what you need 55 years ago $2.99 plus shipping and handling 20amp-hourr 2.2 volts lead acid deep cycle battery. 12.8 volts plus 2.2 volts = 15.00000 volts plus or minus 0.6 volts. Is that close enough? Seriously, I asked at my favorite battery store and they had zero 2.2 volt or a bit morerechargeableble batteries in stock, but I'm quite sure they can order for you. Likely about $29.90 now: inflation you know. 20 amp hours or more is likely best, otherwise it will produce 2.0 volts instead of 2.3 volts at 8 amps Alternately you can buy one lithium ion cell 3.5 volts at a few amp hours that will supply 8 amps for about 1/2 hour, but it is much smaller and lighter, but about as expensive. 12.5 volts plus 3.5 volts = 16 volts, plus or minus 1/2 volt. Is that close enough? You should not run your car battery much below 12.5 volts, unless it is near the end of it's useful life. A new car battery will measure about 13 volts at no load;12.9 volts at 8 amps when fully charged. What does yours measure right after you drive a while? You can buy a 16 volt 50 watt zener for about $15?, but it will fail at 4 amps, even with a very good heat sink. Zeners have 5% tolerance, so the zener voltage could be as low as 15.2 volts. If you start the car without disconnecting the zener, you likely killed the zener, otherwise it should hold the voltage under 16.8 volts which is the high end of zener tolerance. Otherwise you can buy a 15 volt zener and put a 0.6 volt 10 amp diode in series if the tolerance is on the low side. The 50 watt zeners may have low availability as you can buy a regulator and a power transistor for about the same money which can handle 8 or 10 amps in this application. I'd tell you more, but I would be guessing. Neil