I am currently running one battery pack in the front of my vehicle and plan on adding a second one in the rear, at least 12 feet away (as the electrodes fly)
Would there be an issue having long cables on one pack and not the other when connecting the two packs to each other in parallel? Seems like it would create an imbalance in the EV Universe.
As always please explain this to me as if I was a smart eight year old,
G-money
"I don't need to understand how it works, I just need to understand how to make it work!" ~ EV Greg
Yes you want the same resistance drop feeding the HV system, so ideally same length of cable amount of contactors and connections. Otherwise there will be an imbalance, however this will level itself out over the discharge of both pack but can create some weird behaviours.
Making sure the packs are at exactly (or as close as possible) before joining them is key. If not the cells will be happy to provide as much current as they can to equal the voltage.
It depends on the resistance of that additional line, compared to the anyway present resistance in the battery pack.
Let's make an example: You have 12 feet cable length, which is around 4 meters. The cable will go both ways, so in total we have a loop of 8 meters. We assume it is a 25 square millimeter cable (I have no idea, whether this is a good size, but lets just assume it is.) We can calculate the resistance of the loop with a formula: R = 0.018 ohm * mm² / m * L [in meter] / area [in mm²], which gives us around 6 milliohms in our example.
A typical battery pack may have something like 40mOhms to 100mOhms, depending on the temperature and SOC (I have detailled measurements of my Hyundai Ioniq 2018 accu pack, which has 28kWh. No idea whether other packs have much lower resistance.)
If we now compare the resistance of the cable (6mOhms) with the resistance of the battery (40mOhms), we see, that the majority of the resistance is given by the battery, and the cable only has a small part.
My conclusion would be: This setup is fine. You can even use thinner cable and tolerate higher resistance without harm. It becomes only relevant if high current is flowing, so high-power-charging and full-acceleration. In all other use cases, the two packs will perfectly balance, even with a thin cable.
Gregski wrote: ↑Sat Sep 30, 2023 11:21 am
.....
Would there be an issue having long cables on one pack and not the other when connecting the two packs to each other in parallel? Seems like it would create an imbalance in the EV Universe.
.....
Why not connect modules in parallel in the front box and go about the cables from each pair to another pair until you parallel modules in rear box as well. This way you will keep parallel wires as short as possible. you will have to get creative with BMS though, but you will use two BMS anyways no?
I actually tried like you suggest with our Ampera car and i found stray currents going back and forth between packs even when car is stopped. It may be because both packs are diferent. viewtopic.php?t=884&hilit=range+extender
Uhi22's example calculation is a good starting point. If you're using 50mm2 cables for a higher performance application you probably also have a pack with less internal resistance so the ratio remains something like that.
Continuing from it, what happens is that when you cruise, you're taking about 50A. The batteries will settle to a voltage difference of about 50A * 0.006 Ohm = 0.3V. This divided for 96 cells is about 3 mV/cell. That corresponds roughly to 0.3% of capacity.
This means when you stop, one battery will start discharging itself into the other one, about 0.15% each, to balance each other. The initial current will be 0.3V / (40+40) mOhm = 3.75 A. That equals to a short slow charge session between the packs, every time you stop and leave the contactors closed.
This will also happen if the packs are of different age or different type. The more difference in internal resistance, the worse the effect.
It probably won't shorten the lifespan of the batteries much, but it will happen and it's better to avoid effects like that.
I recommend, like arber333, paralleling at the local level and serializing at the car level. Only if you can't reasonably do it or you get other considerable design benefits otherwise, go with the parallel packs.
Having different group of cells separated by long cables will not be an issue but you need to ensure the cables can hold the additionnal current due to extra voltage drop. As a conservetive measure, you need to ensure that each group can handle the total current needed for the powertrain.
Then if you have a higher resistance to reach some cells compare to others, main impact will be for you that you will reach minimum voltage before discharging all cells and so decrease your usable energy at high power. But if you have a power map decreasing the power at low voltage, you can neglect this effect.
One last point, if that for safety reason it is better to have a BMS strategy monitoring the deltaV between all cell to ensure they all behave the same and show no difference comming from a potential defect. With two separate group, this strategy should be applied on each group separatly.
