Does regen improve any over stock "new" battery state? Regen seems to be limited by reduced Hx values, so I wonder if the converse is true? That's one of my biggest complaints is the super weak regen ability of the Leaf. Almost all of my driving is city driving, so I'm often having to stop for traffic and traffic lights anyways, my efficiency would go up significantly if I could utilize more regen, but the Leaf is just too timid. Compared especially to my Prius which gives you full ~20-30kW of regen at any speed, until the tiny battery is full anyways. Perhaps a second motor attached to the rear wheels plus a dedicated controller for "external" regen would be the only option for improving it significantly?arnis wrote:What happens with SOC, SOH and Hx values if two batteries are connected simultaneously?
Well, SOC will be wrong. It will drop like it would according to wasted energy. But then it will stop
dropping at 3-4 bars left. And then it keeps that for long. Until it starts dropping again.
SOH value will recalculate up and up. Until 103%. Which is maximum. Even though actually, there
is more kWh of energy available as there are actually two slightly worn batteries.
Hx value will recalculate to realistic value. It appears, that Hx value is the resistance of vehicle's
battery pack. 100% is factory default value. In case of two batteries, it will skyrocket. But it will be
around the average of two separate batteries (so in case of two new batteries, it will be 200%).
Agreed that an external secondary battery shouldn't affect main battery numbers because those are measured off the main battery's BMS, not the motor controller or battery charger. Basically the BMS will just think you're apparently going downhill because you're using less kWh's per mile out of the main battery.davewill wrote:I'm pretty pretty sure that none of those values are going to change as they are computed and measured by the BMS, which is inside the LEAF battery pack. The car will probably just start thinking you are doing a bang up job on driving economy.
I'm skeptical that charging of an add-on can be done using the LEAF's internal charger. I find it more likely that the secondary will need it's own charger. For one thing, without that you would have no way to deal with charging if the two packs ever fell out of sync voltage-wise.
You might be able to wire it to the same inlet port like the BRUSA integrations that were done, although it will be a little trickier dealing with the case where the secondary needs to charge, but the primary is full and doesn't start the EVSE.
I would be very cautious of using a second battery with a voltage curve drastically different than that of the Leaf's battery; basically the current out of the secondary battery will be very different depending on what the state of charge of the 2 batteries are at. I've done a couple such experiments on the small scale, putting a high capacity power cell in parallel with a lower capacity higher internal resistance cell with a different discharge curve, and while the experiment was done at high rate of constant discharge, it did show that the power cell (akin to the Leaf battery) had most of the load for the first half of discharge, but the lower capacity "auxiliary" battery took a majority of the load toward the end of the discharge (details here: https://secondlifestorage.com/t-Differe ... Experiment)IssacZachary wrote:I've contemplated doing a second battery with LiFePO4 cells. They have a narrower voltage range than the Leaf's Li ion cells, which could actually be advantageous. That way the main battery would always charge up to a higher voltage and would likely discharge to a lower voltage than the auxiliary battery. That way the Arduino system would always start with the auxiliary battery disconnected, and after a while of driving the main bus would always end up matching the auxiliary battery's voltage. Then the auxiliary battery would connect until it's depleted. Then it would be disconnected and leave the main battery to finish discharging. Charging would be the same, only in reverse.
Yes, the voltage monitoring is assumed, I think. But if you can't plug in right away for some reason, or the Arduino decides to crash, and you do end up charging the main battery without charging the secondary, you would have no way to get the secondary charged short of draining the main down to the volatage level of the secondary. In an extreme case, it could even be impossible to get them in sync again just by draining the main. If the idea is get this reasonably foolproof, it needs a better plan than just assuming it won't happen.IssacZachary wrote:... The solution is to put an Arduino system that measures the main bus and the auxiliary battery's voltage and connects the auxiliary traction battery only when both voltages are the same. If you get home and plug in right away both battery's should have the same voltage and both begin charging unless one got a dead cell or loose connection or something in the second battery, which you wouldn't want to charge anyway. ...
True. The LiFePO's would have to be able to handle perhaps full current. This may be possible if the auxiliary battery is also around 24kWh or more, or made of high discharge cells. Of course the cables have to match the current.jkenny23 wrote:I would be very cautious of using a second battery with a voltage curve drastically different than that of the Leaf's battery; basically the current out of the secondary battery will be very different depending on what the state of charge of the 2 batteries are at. I've done a couple such experiments on the small scale, putting a high capacity power cell in parallel with a lower capacity higher internal resistance cell with a different discharge curve, and while the experiment was done at high rate of constant discharge, it did show that the power cell (akin to the Leaf battery) had most of the load for the first half of discharge, but the lower capacity "auxiliary" battery took a majority of the load toward the end of the discharge (details here: https://secondlifestorage.com/t-Differe ... Experiment)
Just don't discharge below 0%. Or disconnect the auxiliary battery when it gets down to 10%. If the auxiliary battery is small enough you may not need to even worry about mismatched voltages after it's installed.davewill wrote:IssacZachary wrote:...In an extreme case... the idea is get this reasonably foolproof...
Thanks for the incouragement! (Really! I'm not being sarcastic.)mux wrote:Also, out of curiosity, IsaacZachary, why would you want to use LiFePO4s in automotive?
He's describing a second pack in parallel. I agree you don't need that kind of monitoring if you're inside the main pack's contactor and the cells are never disconnected from one another, but if that's not the case, then something has to verify that it's safe to close the secondary pack's contactor.mux wrote:As long as you wire entire packs in parallel, there should never be an issue with charging and discharging. I don't see how it's in any way safe to do hot-swapping (or hot-switching, I guess). What if your code glitches? What if your arduino's voltage sensing drifts? What if a cosmic ray hits your microcontroller? These kinds of things are done in automotive hardware and battery switching, but they require intrinsically safe coding practices and hardware redundancy. If you do it wrong, what you get is something close to short circuit currents running between packs.