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Cheap and Easy UV18 Lithium Conversion with some important UV18/26 information

Discussion in 'Diver Propulsion Vehicles' started by tbone1004, Mar 26, 2019.

  1. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    Alright, couple of important notes because Rodney's website is wrong, as is the Tahoe data.
    The below data is taken from the technical specifications from the battery manufacturer. The burn time assumptions are based on ~500w consumption from the motor which is taken from the Tahoe benchmark.
    If we assume the 45 minute on high runtime is correct, the EP16 batteries that were originally installed and recommended only have a capacity of 12.5ah, or 144wh/cell for a total pack capacity of 288wh with a discharge to 9v/cell or 18v for the pack which is where the Venom is set for voltage protection. The ranges quoted by Rodney are BS as most people are aware and are based on the assumption that the EP16 is able to give it's theoretical capacity of ~190wh.

    For reference, the EP26 which is used in the UV26, to 9v, we have maximum wh of around 270wh/cell for a total pack capacity of 540wh. Of note, Tahoe reports the ranges based on a pack capacity of 623wh which is the theoretical capacity of the battery that has not been adjusted for voltage drop. Lead batteries are typically based on a C/20 discharge rate and their capacities are greatly impacted by discharge rates.

    Now. Based on the thread the other day, I contacted batteryspace. We had this discussion a few years ago, but never actually contacted them.
    The battery below is a ~24v, 10ah battery, with a 240wh capacity at a 2c discharge.
    Powerizer LiFePO4 Battery: 24V 10Ah (240Wh, 40A rate) --- Replace SLA 24V 10Ah with 4 times cycle life, lighter weight -- UN 38.3 Passed (DGR)

    Why is this interesting? That battery essentially gives UV26 range in a UV18 with no real changes other than sticking some lead into the body and changing the battery wiring from series to parallel. They have confirmed that they can be used in 2p or 4p configurations which means if you can get them into the UV26 battery sled, you can nearly double the UV26 range *bit more wiring involved with that one though since you have 4 batteries vs 2*.

    Rather important bit. The battery cutoff on the batteries is higher than the venom speed controller if you are using one. Why does this matter? The scooter will hard shut off instead of kick down to a lower speed. Unfortunately the brushed DPV's are using proprietary electronics that we can't program. You can ask Rodney if he can change the cutoff voltage, or if upgrading to the venom, get the one for the Viper/UV40 as the voltage cutoff is different *can't remember what the voltage cutoff is and couldn't find it online, only that the SLA cutoff is 18v*.

    Hope this helps someone out there.
     
  2. Doby45

    Doby45 Do I have something in my teeth?

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  3. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    If you did that, you would have to disconnect them and charge them separately. SUPER idiotic to charge lithium packs in series which is why the Dive X power tool modules require you to take the batteries out. With the ones I linked, you can charge them together
     
  4. Doby45

    Doby45 Do I have something in my teeth?

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    Isn't that what you are doing when you make a Life Pack from these cells?

    LiFePO4 Prismatic Module: 3.2V 40 Ah, 10C Rate (128 wh) --- UN38.3 Passed (DGR)

    Serious question. It would appear that you use 4 of those to make a 12v "pack" and then make a second one with 4 more cells to come up with a 24volt pack. I am sure you do not disconnect each of those packs to charge them. I say this to say, maybe the "idiotic" comment could have been omitted. If you have some wisdom (which you appear to have) then simply impart your wisdom. Thanks.

    PS. Personally I would have no issue charging two packs separately in order to get the burn time you would get from those packs. I think you could easily come up with a charging solution that would allow you to simply disconnect the "connector" between the two batteries and plug into each battery seperately.
     
  5. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    @Doby45 sort of. When you make one pack, the BMS will have leads going to individual cells for balancing which prevents any individual cell from overcharging and has the ability to shut the charger off.
    The 12v packs that you linked have a BMS inside of them, but if you charge them in series, when the first pack says "I'm done", the second pack is getting over twice the voltage that it needs.

    If you want to use those two batteries, just have to make sure that you buy 2x chargers and disconnect them for charging. Will function the same as the power tool module concept from DiveX.
    The concerns are going to be that if one of those packs decides it's done, then you're out cold since the pack voltage will have dropped well below the voltage cutoff in the controller. With a pack that's build in parallel, if one pack dies you still get the capacity in the other.

    The first option is certainly much better and more convenient, but if the extra wh are that important to you, then no problem. Before you go down that road though, I would highly recommend reading up on battery management systems in lithium batteries *they're the same with lipo and lifepo4, so no need to look for specific lifepo4 articles*. The BMS is the only thing that keeps your pack from going boom and skirting the rules with them can be problematic. Running is parallel provided the packs are the same voltage is considered a relative non-issue and is common in most big packs, but running packs in series is really not ideal.
     
    Doby45 likes this.
  6. victorzamora

    victorzamora Solo Diver

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    Not quite. The above bolded should read "Voltage, capacity, chemistry, and internal resistance."
     
  7. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    true, was implying running identical packs in parallel, but good to reiterate. Capacity seems to be a grey area, especially with certain light manufacturers....
     
  8. Doby45

    Doby45 Do I have something in my teeth?

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    Now that makes perfect sense both regarding the charging and the dangers of running that pack if one battery decides to dump you are automagically under the voltage cut off of the Venom, which is 18v. Thanks, the extra 5ah is not worth that.
     
  9. Jon Nellis

    Jon Nellis Captain

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    That is not what happens with two BMS protected packs in series. Whether charging or discharging, when one BMS's MOSFET shuts off, all current flow stops. One of the packs will not see the full 24V after the first pack BMS shuts down. Draw the circuit, open one leg and try to explain how the current flows around a BMS in series that is shutdown.

    Since they are in series, if they are not equally charged to begin with, they will never be equally charged, as when the first pack hits shutoff voltage, the second pack stops charging as well. To charge them individually, you just need two chargers in series with the center tap wire between the packs, they do not need to be disconnected. Three wires, and the second charger's 0V (Ground) is floating on the first charger's positive (assuming the charger's DC 0V is not tied to chassis/earth ground).

    The Genesis uses two batteries in series, but we charge them individually. It has worked well for 8 years now. They reason we do that is primarily for regulatory concerns, as some countries consider anything above 48V "high voltage" and others are at 75V for high voltage. The Genesis operates at 60V to reduce current and associated I^2C loses.

    If you have a weak cell in one of two packs in parallel, you will get the same power out of two packs in series with the same single weak cell, if they have the same total capacity and voltage. Here's the math for a hypothetical 24V 800Wh total system capacity.

    Scenario #1 Two 400Wh 24V packs in parallel using 50Wh cells configured 8S. One weak cell in one pack puts out only 40 Wh, which makes that entire battery shut down at 320Wh (40X8), so you get 720Wh total. (320Wh from one and 400Wh from the other)

    Scenario #2 Two 400wh 12V packs in series, using the same 50Wh cells, but configured 4S2P . Again, one cell only puts out 40Wh, but it is paralleled with a 50Wh cell, so you get 90Wh out of that set of cells before it shuts down both batteries. 8 sets of cells outputing 90Wh = 720Wh.

    If you then look at system current requirements, If each cell can only output 10 amps and you need 20 amps to operate. When the first paralleled 24V battery shuts down, you will be trying to pull full system current out of only one half the source, and voltage will sag more, causing capacity to decrease slightly more, due to I^2C losses in the cells. With 12V packs in series, you maintain full current output until cutoff. Charging in series is a slightly more complicated with two packs in series, since you need two chargers, but discharging is more robust.

    Cheers,
    Jon
     
  10. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    @Jon Nellis good call, thanks for clarification. I still think I'd rather run them in parallel if given the choice.

    Fair on the discharge, but these are both rated at ~500w discharge on the individual packs. That's about what the motor is going to consume at that voltage anyway.

    Goal on this was a quick and easy option to keep these dpv's running since the SLA packs are getting more and more expensive and I think the quickest/easiest is to use the original packs I linked in parallel.

    If I was doing this for me, I would likely buy a pair of these
    LiFePO4 Prismatic Battery: 12.8V 20Ah (256Wh, 10C rate) (24.0) - UN38.3 Passed (DGR)
    One of these to charge
    Smart Charger (6 A) for 25.6V ( 8 cells) LiFePO4 Battery Pack (110V only, Standard Female Tamiya plug)
    and one of these for balance
    Protection Circuit Module (PCM ) with Equilibrium Function for 8 cells (25.6V) LiFePO4 Battery Pack at 60A limit

    More wiring, have to mount the PCM, etc. but I think that's probably a bit more robust of an option if you're going to commit to it.
     

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