Extra Battery, How to Integrate with 24kWh Traction Battery?

[davewill]

But, there is a continuous measuring of both pack and cell pair voltages. It’s just done in the vehicle. The BMS is monitoring that, and sending that amp request every 100ms.

Sounds like a fun project that you have started. One thing to understand in any of the various DC fast charge standards is that the vehicle ALWAYS controls the charge rate. The charger is a dumb box, doing what it is told, within the programmed limits.

So my takeaway from this is that in this parallel battery scenario, the BMS for the main batt is going to request a certain amperage based on the state of the main battery, and the add-on is going to take some of that, slowing the charge rate. The upshot being that the whole charging process will go slower that it theoretically could if additional amperage for the add-on were requested. This will be tempered by the amount of time the charger spends at it's maximum output where we couldn't charge any faster anyway.

[mux]

That's all. I'm currently reading up on the spec, as I'm currently designing a CHAdeMO charger to install at home, using a bunch of spare batteries as a high-power buffer.

Sounds like a fun project that you have started. One thing to understand in any of the various DC fast charge standards is that the vehicle ALWAYS controls the charge rate. The charger is a dumb box, doing what it is told, within the programmed limits.

Yes, this is very much what attracted me to this idea for a project. It's a lab power supply! It's a welding inverter with CAN! It's just such a useful thing to have and right up my alley (I'm a power electronics design engineer by trade). I've got some even better ideas that can probably enable a CHAdeMO charger for specific vehicle types with no dc/dc converter on a shoestring budget. Anyway, that's a story for another time.

More ontopic: I expect my CAN spoofer hardware to arrive in about 2ish weeks. I've finished reverse engineering my extender battery CAN protocol, which enables me to read their cell voltages, balance the pack and gauge remaining capacity. I'm also hoping to procure a few CAN current sensors. All of this should combine to form a system that shows the correct remaining range in the GoM and report the correct remaining charge time etc.. I should also be able to precisely calculate how much of the juice is coming from the main pack and from the extender. If and when I get all of that done, I think this will be pretty much exactly what people would want from a battery extender. This will all get tutorialized.

[goldbrick]

So my takeaway from this is that in this parallel battery scenario, the BMS for the main batt is going to request a certain amperage based on the state of the main battery, and the add-on is going to take some of that, slowing the charge rate. The upshot being that the whole charging process will go slower that it theoretically could if additional amperage for the add-on were requested. This will be tempered by the amount of time the charger spends at it's maximum output where we couldn't charge any faster anyway.

To the (original) BMS, the additional battery should just look like a power transmission or efficiency loss. In other words, can the BMS tell the difference between a high resistance cord and the additional battery? Eg, the BMS requests X amps, monitors the cell voltages, sees they are at a certain state, requests X+Y amps, repeats until the cell voltage feedback reaches the target. The BMS can't tell whether power is being lost in the cable or to the add-on battery, it only sees what effect the amperage request is having on the cells it is measuring and adjusts the amperage request accordingly. Of course, there may be some sanity checking limits on the expected behavior that the BMS will not exceed (I hope so!) but it's possible that if the current draw from the additional battery is small enough relative to the normal range of operation that the BMS will charge at full speed (assuming no other limits).

[IssacZachary]

So my takeaway from this is that in this parallel battery scenario, the BMS for the main batt is going to request a certain amperage based on the state of the main battery, and the add-on is going to take some of that, slowing the charge rate. The upshot being that the whole charging process will go slower that it theoretically could if additional amperage for the add-on were requested. This will be tempered by the amount of time the charger spends at it's maximum output where we couldn't charge any faster anyway.

To the (original) BMS, the additional battery should just look like a power transmission or efficiency loss. In other words, can the BMS tell the difference between a high resistance cord and the additional battery? Eg, the BMS requests X amps, monitors the cell voltages, sees they are at a certain state, requests X+Y amps, repeats until the cell voltage feedback reaches the target. The BMS can't tell whether power is being lost in the cable or to the add-on battery, it only sees what effect the amperage request is having on the cells it is measuring and adjusts the amperage request accordingly. Of course, there may be some sanity checking limits on the expected behavior that the BMS will not exceed (I hope so!) but it's possible that if the current draw from the additional battery is small enough relative to the normal range of operation that the BMS will charge at full speed (assuming no other limits).

Or look like an accessory.

You can run your lights, A/C, heater, etc. while charging off of CHAdeMO or EVSE. That too is current diverted from the battery.

[goldbrick]

You can run your lights, A/C, heater, etc. while charging off of CHAdeMO or EVSE. That too is current diverted from the battery.

In principle, yes. The difference is that the car and BMS can be aware that the heater is on while the additional battery is invisible to the BMS except that is an (unaccounted for) power sink.

[IssacZachary]

You can run your lights, A/C, heater, etc. while charging off of CHAdeMO or EVSE. That too is current diverted from the battery.

In principle, yes. The difference is that the car and BMS can be aware that the heater is on while the additional battery is invisible to the BMS except that is an (unaccounted for) power sink.

True. But it would be hard to make a system that can monitor where every last milliamp goes. The car has an idea of what power the accessories are using, but doesn't say "5 amps to A/C, 2 to inverter and 51 to battery, please send 58 amps through CHAdeMO." It's more like, "Some power might be used by accessories. But what voltage is the battery at? Send more or less current according to what the battery is doing."

[jkenny23]

True. But it would be hard to make a system that can monitor where every last milliamp goes. The car has an idea of what power the accessories are using, but doesn't say "5 amps to A/C, 2 to inverter and 51 to battery, please send 58 amps through CHAdeMO." It's more like, "Some power might be used by accessories. But what voltage is the battery at? Send more or less current according to what the battery is doing."

It does make guesses for the energy usage screen on the Nav. Presumably it could do the same when charging. I have to check whether it is actually requesting more power from the DCQC station if the AC/heater is on next time.

Honestly I find it quite surprising that it doesn't see such a large difference in requested power and power into the battery and stop the charge. Fortunate for us, but it's a strange omission; it can sense milliamps flowing through a ground short via GFCI yet 25kW going away into another battery it doesn't know about, no issue!

[arnis]

Yes. If there is more power available from DC station, vehicle will consume it.
When consumers are activated (AC, PTC etc) they all act as one big consumer.
Another battery will act the same. Power is measured by DC station (blue) but
not by battery (green).
Notice me turning on heater in the middle of DC charge cycle (power that goes
into battery continues to drop but current supplied by DC station spikes and
then later on, when cabin heats up, is hardly noticeable (less than 1kW), and at
the moment I did the screenshot I opened door and HVAC cranked up again,
notice 8,4kW battery input and 11,8kW DC station output.

Half of the graph is filled with home charging (last drops) before my departure.
Note that maximum current (120A) was available for few minutes, and then
it started dropping rapidly (first cell reached 4.1V early) - chilly battery (+15*C).

[mux]

Re this entire scenario: yes, it is technically possible for the car to accurately account for where all the current is going. But there is well over 10kW worth of non-charging consumers on the car that do not contain any kind of power measurement devices. Power measurement is hella expensive - CAN current sensors that are accurate enough to do this stuff are $300-400 a pop. Imagine having to put one on the AC, heater, DC/DC, battery PTC heater... that's a massive expense on a car that's already being squeezed as far as budget goes. It's simply not practical, so the manufacturer simply tries to get away with the absolute minimum possible. The only current measurement in the entire car is one shunt in the battery pack and that's it. The rest of the stuff you see on the energy info screen is just guesswork by the infotainment's computer.

Combine this with the inability of the charger itself to accurately account for where the current goes and how much voltage drop there is everywhere, and any kind of closed-loop system that tries to spot issues with excess current draw is going to throw false positives all the time. This is not robust nor, honestly, necessary for safety. The car doesn't explode if it draws excess current, the car explodes if battery temps go haywire. So that's directly measured, and the excess current is just... whatever. The car doesn't care.

[davewill]

And this guy...

Did you really just cherry pick a 7 year old post? I suppose that nobody could learn anything in 7 years? And of course he couldn't hire smart people to help with the development!

mod note: I deleted the post in question. I don't have time to search every thread for such occurrences. Please report posts if necessary..