Slow-charging secondary battery in parallel

I have two spare Prius batteries and am going to replace a cell in one, which will make it better than my actual working battery. I will use a cell of equivalent capacity etc (I will find the cell from the not-so-good spare battery). Once I have done this, I will then swap this 'rebuilt' battery into my car and do some road testing with my EngineLink app, and watch temperatures, block voltages etc, and make sure everything is well balanced.

However, I am wondering if there is a way to connect the battery in parallel, but charge it more slowly? As an example, if my main HV battery was at 80% SoC and the other was at 30%, I guess there would be a huge current flow if I just connected it 'in parallel'. Is there any way to set a secondary hybrid battery to 'trickle charge' from the main battery (one-way only using a suitable diode), and to charge at a rate that won't cause overheating, so that I can use a fan set to an equivalent of 'level 3' to keep the battery cool?

I don't want to make a mini-PHEV or anything like that, but have been thinking of using the battery, which goes up to around 240v, to power things such as laptops etc, possibly even coolers. I am thinking of making a set-up where I can go touring, and having a large supply of HV power, which is 'deep cycle' (to a degree, better than lead-acid) would be helpful.

I know that, if done incorrectly, many things can happen, including those impressive swollen battery photos! But would be keen to have the ability to connect a spare HV battery sometimes as a 'deep cycle' battery (which would only every get deep cycled to 40% SoC). Using the car to charge it would also mean it would reach, max, 80% SoC also.....

Any advice on this? The diode would make the charging one-way, cooling would be taken care of; I don't know of any simple way to slow the charge rate without using resistors, which (with a power output of I(squared)R, might end up getting a little hot!

 

Any time (sort of); I have my full time job, but live in NZ in a large city and would like to explore more of this country, and came across the concept of Prius RVs!

It probably makes more sense to hook in to the main hybrid battery, monitoring the voltage so I don’t over-drain it; would be a lot simple and save a lot of space! I still think that a Prius pack could make a good battery for a full size RV (although an inverter would be needed unless you only use certain devices which don’t actually need AC)

 

It is difficult getting around Ohm's law! :/

I am working on other things (including my day job!), but my idea for this would be to find a way to 'swap' battery packs; so that the 'spare' battery pack, once drained down to 40% SoH, is used as the main battery, when driving around, and eventually, gets close to 80% charge, then (after time to cool!) is disconnected and the original battery is reconnected. This would be challenging, would involve a relay for the HV line(s), and a few relays for whichever I/O from the battery pack can't be shared (CAN-bus lines, and the like, mainly). I have the info from TIS; for now, I think a 12v 'deep cycle' battery, used in a shallow cycle, is much simpler for my purposes!

 

I don't know!! The battery computer (software) is going to be a ridiculous challenge; there are hundreds of thousands of instructions, and the chip used is not documented. Also, the binary isn't even available but that is being worked on. I am using a Prius engine ECU and Camry ECU binary for now (similar system-on-chip but not identical).

I stlll like the idea of a second battery in parallel, to connect 240v AC devices to (I would do careful research and would be aware that AC-DC conversion changes the voltage anyway, to a higher DC voltage). Good laptop chargers should work flawlessly (if they are ones that can do 110-230 volts AC), but if not, they will fail, possibly by exploding, catching fire, or sending mains to the laptop. Apparently, high-quality wall chargers can also work (again, cheap wall chargers electrocute people randomly anyway, so all of this is *not* advised!)

I am slowly working on the emulator that will help with the battery ECU hacking, but it is going to take some time!