100 Ah and 200Ah LiFePo in parallel- why twice the current?

doenertier12doenertier12 Registered Users, Users Awaiting Email Confirmation Posts: 1
edited June 2019 in New Battery Technologies #1

If I connect 2 Batteries in parallel, one with 100Ah and one with 200Ah, why does the 200Ah charge and discharge  with twice the current, if they have nearly the same internal resistance? I thought the internal resistance was the only current limiting factor. If the 100Ah would have an internal resistance of half the size it would make sense to me but it has nearly the same. I thought maybe the 200Ah needs twice the electrons in order to reach the same voltage but I am not sure if this is the right explanation.

Comments

  • mcgivormcgivor Solar Expert Posts: 3,426 ✭✭✭✭✭
    The internal resistance is governed by the electrolyte space between the plates, both of which would be the same.The surface area of the plates however would be double in the 200Ah cell, so it has the ability to move more electrons.
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  • dehvdehv Registered Users Posts: 5
    You shouldn't put different size batteries in parallel because you will end up stressing out the smaller one - the larger one makes the voltage look lower to the CC but you'll actually be cooking the smaller one. 
  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    I am not sure I would have a heart attack with a 100 and 200 AH Li Ion batteries in parallel. If they are of similar construction.

    If they are different types of batteries (say the 100 AH is rated for higher surge current), then you may be getting into issues with charging and discharging.

    If you have a DC Current Clamp DMM (or similar)--And can measure that the current flow (charging and discharging) is roughly 2/3 rds to the 200 AH battery and 1/3rd to the 100 AH battery--That is about all you can ask for.

    https://www.amazon.com/gp/product/B019CY4FB4 (mid priced AC/DC Current Clamp Meter).

    You do run into some issues if "something goes wrong" (say 200 AH battery cable "opens" and you try to pull full current from the 100 AH battery--The excessive current could damage the smaller battery.

    Where you would get into real trouble is if you put the two batteries in series (i.e., 12 volt + 12 volt in series = 24 volt battery bus). The dissimilar AH cells could be a mess.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LiFePo4batteryLiFePo4battery Registered Users Posts: 5 ✭✭
    You can't put different capacity batteries in parallel. They have different voltage while you are using them. they will charging together. Lower capacity one will be charged by the higher one. in this way by time goes by, the higher capacity one will be damaged very quickly. If I am right, they are both a completely battery pack with independt BMS.

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  • AmpsterAmpster Registered Users Posts: 143 ✭✭✭
    Physics says they will have exactly the same voltage when in parallel. Do you mean State of Charge?
    There will be eddy currents between the cells as they charge and discharge. I am not sure why the higher capacity battery will be damaged. Please explain?

    I assume you are talking about Lithium batteries? 
  • BB.BB. Super Moderators, Administrators Posts: 30,634 admin
    I too wonder why two different capacity (LiFePO4?) batteries in parallel is an issue as described.... If same brand/chemistry/construction, then each battery will be at the same voltage and accept/provide current in proportion to its active area/chemistry.

    I can make lots of arguments about paralleling cells/batteries and why that can be problematic for things like safety, adding more points of failure, etc... But the whole Li Ion battery community of manufacturers/engineers have made it their lot in life to parallel multiple smaller cells in parallel (sometimes massively parallel) monoblock batteries (albeit with the appropriate monitoring, cutoff switches, etc.--Which is another complexity in itself).

    How does one string of cells not see the the other XX parallel strings of cells as anything but a smaller AH capacity in parallel with a much larger AH capacity monoblock battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,814 ✭✭✭✭✭
    Batteries in parallel, need to have same internal resistance.   Generally, battery capacity affects internal resistance, so unless the loads and recharge are very small, it won't work well.
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  • Dave AngeliniDave Angelini Solar Expert Posts: 5,221 ✭✭✭✭✭
    I think you guys are missing the advantage of parallel battery systems each with their own BMS. They do not need the same internal resistance. I have a guy with 20 batteries in parallel on a couple xw+. Each battery shows as its own device screen. Below is one device of the 20. The other advantage is they can be added to with more at any time and different capacity batteries are fine with their own BMS.

     Not many want less ;)

    The screen below shows the BTS for this one battery. It can show the overall state of charge also. If there were a battery failure only the bad battery would be lost.



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  • AmpsterAmpster Registered Users Posts: 143 ✭✭✭
    edited September 2019 #10
    I think you guys are missing the advantage of parallel battery systems each with their own BMS. They do not need the same internal resistance. I have a guy with 20 batteries in parallel on a couple xw+. Each battery shows as its own device screen. Below is one device of the 20. The other advantage is they can be added to with more at any time and different capacity batteries are fine with their own BMS.

     .........
    There may indeed be advantages, but at $1,000 a kWh for those fully contained systems like SimpliPhi, there are less expensive alternatives. I would not design a LFP system to use different capacity batteries but as noted above the risks are smaller than with Lead Acid.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,221 ✭✭✭✭✭
    There is really not any risk as each is its own system. In multiple battery systems, a battery can be turned off for months, probably over a year and used as an expensive spare. The price is really an individual choice. There are many folks in very remote places that the price is just one of the issues and may be down the list. 

    They are not inexpensive. 

     I do not use Simplyphi because it does not communicate/network with the power system. A few other reasons also.
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  • AmpsterAmpster Registered Users Posts: 143 ✭✭✭
    And there is not any risk with one BMS and some LFPs at $400/ kWhr. I am sure price is down the list for some. Changing the pack size depending on the season could be an intriguing option but how does that compare to using lees of the pack? For a hobbyist like me price is high on the list. 
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,221 ✭✭✭✭✭
    The risk in a one BMS system is that it has new ways to lose power that were never there in a simple lead acid battery.

    That is the part I do not like on any single BMS system. It improved some battery characteristics at the expense of some new ways to lose power.
    That may not matter to a hobbiest as much as someone in a snowstorm or a hurricane 100 miles from town or on a boat on the ocean.

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
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  • AmpsterAmpster Registered Users Posts: 143 ✭✭✭
    New ways to lose power is a new one to me. LOL  Are you saying that is not an issue with multiple BMSs?
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    A BMS is a component not present in a typical LA system, and if it doesn't exist, it can't fail.  If it does, it can (and will eventually) fail.

    Seems to me, with multiple BMSs treated as discrete inputs, the probability of "lights out" because of BMS failure diminishes as the number of such discrete inputs increases. 
    Off-grid.  
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  • AmpsterAmpster Registered Users Posts: 143 ✭✭✭
    edited September 2019 #16
    If that were a cost effective strategy I would have thought the EV manufacturers would have done that. That may be a good argument for these enclosed systems like Simplify in an off grid scenerio. I am also sure as one scales up the the kind of battery systems that are replacing peaker plants multiple BMS, Inverter and packs are fairly common. That does appear to be the trend with complete systems like Tesla and others who offer modular scalable solution for grid tie.

    For my grid tie hybrid I prefer to keep it simple. Mine is also designed so that failure of the BMS does not bring down the pack. My biggest risk is not hurricanes or snowstorms, it is PG&E power outages. 
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,221 ✭✭✭✭✭
    It does exist with the EV market ! It is very cost effective. It is called a Tow truck.

    A guy down in the valley did video of all the tow trucks coming up Interstate 5 with Teslas from LA to factory repair.
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  • ElectricLoveElectricLove Registered Users Posts: 11 ✭✭
    Ampster said:
    I think you guys are missing the advantage of parallel battery systems each with their own BMS. They do not need the same internal resistance. I have a guy with 20 batteries in parallel on a couple xw+. Each battery shows as its own device screen. Below is one device of the 20. The other advantage is they can be added to with more at any time and different capacity batteries are fine with their own BMS.

     .........
    There may indeed be advantages, but at $1,000 a kWh for those fully contained systems like SimpliPhi, there are less expensive alternatives. I would not design a LFP system to use different capacity batteries but as noted above the risks are smaller than with Lead Acid.
    This is an easy one to answer...  Say no to overpriced systems like SimpliPhi.  Say yes to self-contained (internal-BMS) LFP batteries at half the per kWh price from companies like Ohmmu (www.ohmmu.com/group31).  Unless I'm missing something on what makes SimpliPhi worth it?
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