Dealing with higher voltages, batteries, inverters, chargers

Forgive me if this is the wrong place, this topic is going to be a bit unusual from the norm here.

I am bouncing around the idea of getting a prius hybrid car and adding a secondary battery pack and onboard charger to supplement the stock battery and increase the fuel economy. The kits go for 9-12 grand and want to do it cheaper.

Well, the car uses from what I can tell around 240 volts. From one website it seems the voltage varies from 213-260. I know to use batteries to give this voltage would take quite a few in the deep cycle kind and hundreds if little cells are used.

The idea I have is loosely based and reversed from the idea to use a prius as a UPS system. Basically in that idea online, someone used some diodes to hooked the cars battery in parallel to the battery of a commercial 240 volt UPS system. It has trays of 12 volt SLAs in the UPS system and the prius battery is hooked in parallel to it with diodes keeping the UPS battery power from going back into the cars battery. When needed, the car kicks on and recharges the cars battery and power goes to the UPS system.

My idea is to use a 240 volt inverter, put a bridge on it to the car battery input so its DC. THen use the needed configurations of batteries to power the inverter.

Next would be charging the batteries. I know some inverters double as a battery charger too.

What do you guys think? The supplement packs are rated from 9-15 kilowatts and are lithum ion batteries. I was thinking of using lead acid batteries.

I don't know about the Prius, but it is probably similar in operation to the Honda hybrids, which I have extensive knowledge of (in terms of factory service).

The Hondas use a pack of 120 NiMH D-cells (144 volts). There are limitations in the PCM's firmware so you can't simply add on batteries and get better fuel economy. (On the engineering brief, the battery pack was chosen for its weight, where adding batts presents a diminishing return unless the rest of the charging system is so upgraded.)

If you are looking at a kit then there are probably workarounds for Toyota's "Hybrid Synergy Drive" system, wich is an integrated system. Either way, do you realize how much gasoline 10-12 grand will get you?!

BTW, you have probably seen this but I'll mention it anyway. All of the 100% electric racecars I've seen use control components by Zilla.

I guess what I am looking at, is there a cheaper way to supplement the stock battery in a hybrid car with an add on battery that is remotely charged?

I dont know about the specs on a prius, but I think from the website I read on using it as a UPS system its around 240 volts.

The idea was to convert that battery voltage to 240 and supplement or charge the stock battery. Dont most ups systems run off their battery while keeping it topped off?

Yeah, 10-12 g's for the kit wont be cost effective at this point and time, reason I want to entertain this idea. 5-10 lead acid group 24 batteries at 64 bucks each, a couple hundred for an inverter and a few more or a charger, maybe 2 grand topps.

Of course if it allows me to drive in the city without the ICE turning over, I could save a lot just recharging the aux battery pack daily.