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!