Find a wheel chair battery, they used to be made like car batteries, same size and shape. I used to use one in a river tubing stereo that used an old (this was back in 2000) Rockford Fosgste 75wpc x2 amp and it would play from the time we left until we returned 10 hours later and it was blasted the entire time. Never ran out of juice.
It will be limited to its power output rating so you need to figure out what will work, basically it will be the buffer between your charger and amp like a capacitor would be. I ran that amp with 4 x 4Ω mids , 2 x piezo tweeters and mono 4Ω 12" sub running the amp in tri mode with analog crossover parts. It probably was drawing 300W or more plus it powered all inflators that aired up 4 or more huge fun island floating rafts and 2 man boats for coolers.
Pictures of said tubing tunes.
I paid around $100 for the battery. If I was you I'd just pick up a cheap Chinese amp that has 100wpc x2 and has Bluetooth and AUX inputs, some are just for subs. Make sure you get a power supply that will unleash the most from the amp. Amp cost $4 for a 50wpch x2 + 100w mono sub output has controls for sub volume and boost center, bass and treble controls and an overall volume control. $10 for a 24vDC 20A power supply.. I bought 2 amps and 2 PSUs, im surprised they work as well as they do but I wouldn't rely on them as subwoofer amps, they do make some that are just dedicated sub amps and they make mono amps but you need to check voltage and amps required to run them..
I just looked at prices and those I picked up are priced 2 to 3 times more than 2 years ago but even the highest 300wpc x2 amps are under $30. The problem is finding PSU's that will power them. Some can work with just 5vDC but need high Amp output with lower watts to speakers. They are limited to a min and max input voltage anywhere from 5 to 80 volts and amperage ratings can be from 5 to 60 amps. Make sure to check what PSU will work to get max output and prices can vary. A total watt output of 800W can be reached by PSU's with 80V 10A or 12V 70A and anything between (if allowed by amp), both of those PSU 's are hard to find and cost $! Those I have are 24vDC max and at least 20A to make spec, but can run with lower volts and less amps, you have to check required input and available PSU's to run them.
I also checked out all caps in the cheap power supplies and found the 2 biggest caps (small 400v totalling 130uf) were way low and I replaced them both with 1 big 400v 150uf Ruby cap. The other caps tested right on spec same manufacturer for all caps on the board? This is the cheapest way to get some usable wattage with decent clarity I know of. Ditch the car amps since you need a battery as a buffer and a charger constantly running, anything but a wheel chair battery or a big lipo battery will end up killing the battery quickly, my wheelchair battery was bought used. I'll bet they're less now that people are switching them for new LiOn batteries.
