Doing a PHEV DIY system.

What voltage is the battery you are buying? Do you have any other specs like capacity or chemistry?

If it is lower voltage then you need to have some piece of electronics that change step it up to the correct voltage.

If it is similar or higher then you can try some sort of contactor based solution.

EAA's wiki page regarding Prius PHEV
Prius PHEV - EAA-PHEV

 

So that is going to be a challenge.

You'll have to find a source for a dc/dc converter.

The converter will have to be able to take your input voltage (96v) and step it up to the much higher voltage that the car needs. Let's call it 250v. It'd be nice if the output and voltage and amperage could be adjustable.

 

So, for a gen2 Prius you're in the wrong section.

Enginer.us makes exactly what you want, but that will obviously cost some money. DC charging is pretty straightforward but you need some safeguards. Basically you can hook up any DC power source that has a higher voltage than your battery (nominal 201.6 Volt) and it will charge. You need some safeguards to prevent it from overcharging, but if you keep the battery at a high enough voltage, the Prius will think it does not need to charge and draws current from it.
But you need to keep Imagerius-Pack Voltage vs temperature.gif - EAA-PHEV in mind and look at the below the 20%, so significantly above 201.6 Volt.

 

The thing is: the greater the voltage difference (between battery and charger), the higher the charging current. But I don't think you can raise the voltage unlimited without adverse effects (like: put 1000V on a 200V battery).
If you have a DC-DC converter that puts out a fixed voltage than you only have to choose one with a proper output. 1.5V per NiMh cell would be the absolute limit (making a charging voltage of 168 x 1.5 = 252V) but it would be better to not go beyond 1.45V per cell (243.6 total), indeed the image I linked to shows about 225V for a fully charged pack (about 1.34V per cell), but that will be at rest.

So I would say that the 250V ericbecky suggests is the maximum you would want to have, 220V would probably do the trick because that's about 5% DOD (95% full) for the Prius battery and if you keep it at that, I would think the Prius will decide not to charge the battery even more.
And yes, such DC-DC converters are not cheap because they are not common.

 

No more regen when breaking when close to 100%

 

True. I don't know how enginer solves this, but you will need to stay below the SOC where the Prius will start using the ICE to burn off energy.

 

Just 400 miles away: 2012, 59k miles, only $10k 2012 Toyota Prius Plug-in Advanced For Sale In Simi Valley | Cars.com

 

if you have to ask, you don't want to know.

 

How about some fuses between the two batteries.

A contactor to separate the two batteries.

Some monitoring system so you cannot see what the lithium pack is doing. SOC, current, voltage. Also a way to balance all the cells in the lithium pack.

Some sort of protection or alarm to let you know when you are at the upper and lower voltage limits. If you have a system that can automatically disconnect the two batteries when limits are met that'd be nice. Else you can do it manually.

Can be done for $5,000 or less I suppose.

What is your budget?

 

I should have mentioned....
I wouldn't recommend using Lipo.
Hopefully they are LiFePo4. Not as volatile.

And also they need to be able handle the continuous current you plan to be running. Some have very low rates continuous. Make sure you leave some safety margin or else bad things will happen.