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Slow-charging secondary battery in parallel

Discussion in 'Gen 2 Prius Technical Discussion' started by landspeed, Mar 8, 2019.

  1. landspeed

    landspeed Active Member

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    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!
     
    #1 landspeed, Mar 8, 2019
  2. bisco

    bisco cookie crumbler

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    when are you being re deployed?:p
     
    #2 bisco, Mar 8, 2019
  3. landspeed

    landspeed Active Member

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    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)
     
    #3 landspeed, Mar 9, 2019
  4. R-P

    R-P Active Member

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    If you can get your hands on a buck CC CV power supply that works with 250V, then you could do charge the second battery from the first one. But I looked for one and haven't yet found anything that will do this: needed/wanted it for charging the HV battery.

    A buck-boost would be even better, but a buck converter set to e.g. 240V and 1A maximum current will charge the secundary battery with 1A max. as long as the first battery is above 242V or so. You should switch this thing ON so it only works when the engine is ON.

    But between theory and practise there's a big difference...

    I have used a 0.47 ohm resistor between two carbatteries before, but that was to charge the main battery from a solar/mains charged secondary battery. The 0.47 ohm resistor made sure there was never more than ~3A running through this wire.
    You *could* do this with a hefty 10 ohm resistor and a diode for the Prius battery. The second battery would obviously never be charged to the level of the primary battery,

    I wouldn't, but you could.

    There are 168 cells in a Prius pack. If you depleted it to 1V per cell while the car wasn't running, you'd have 168V at the secondary battery pack. If the charging voltage after starting/braking would run as high as (260V, but lets round up to) 268V, there would be a 100V difference over the resistor. So there would be 10A running through it. And it would be dissipating 1000W...
     
    #4 R-P, Mar 10, 2019
  5. landspeed

    landspeed Active Member

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    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!
     
    #5 landspeed, Mar 11, 2019
  6. bisco

    bisco cookie crumbler

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    after reading your other thread, i'm not counting you out (y)
     
    #6 bisco, Mar 11, 2019
  7. R-P

    R-P Active Member

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    I've since read your plans for hacking the monitoring batterycomputer and its software, so I realise now I may have kept things overly simple for your level of expertise/understanding :ROFLMAO:
     
    #7 R-P, Mar 12, 2019
  8. landspeed

    landspeed Active Member

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    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!
     
    #8 landspeed, Mar 12, 2019
    edthefox5 likes this.
  9. R-P

    R-P Active Member

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    Any decent 110-240VAC charger will indeed work, as the first thing that does is rectify the AC voltage to a DC voltage (~150V for 110VAC up to 340VDC for a 240VAC voltage).
    But many such powersupplies have a pretty poor design if the origin is questionable, where the primary side and the secondary side are not properly isolated from eachother (most commonly used isolation is simply distance, and sometimes there is less than 1mm between primary and secondary side...)

    One big difference between AC and DC is that AC voltage drops to zero volts 100 times per second (50Hz) or even 120 times per second (60Hz) for you Yanks (Landspeed is from NZ I think, but it is mainly a US forum I recon.)
    And the fact that DC does NOT do this, makes it way more dangerous in e.g. arcing: if an arc is produced in AC, it is quenched 100 times per second (the voltage drops to zero, so what is there to maintain???), but in DC, it can exist and maintain itself for seconds or longer.

    This may not be relevant, but keep it in the back of your head.
     
    #9 R-P, Mar 12, 2019
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