Adjunct battery packs?

[danielsantos]

My 2013 Leaf is down to 8 bars and I'll surely loose another come summer. Having recently sold my house, I finally have the money to either replace/upgrade batteries or buy a new car. I'm excited about the prospect of a 40kWh LiFePO₄ battery pack from EVs Enhanced, but it's not ready yet. Since I was already looking at 48V LiFePO₄ battery packs as a possible solution to my in-car 110V AC supply, I started thinking about larger adjunctive battery packs serving as an supplement to the main battery pack.

I do a fair amount of electrical engineering at my job as a firmware engineer, but I don't do a lot of power electronics and have never actually worked on high voltage circuits, so I'll have a lot to learn in this endeavour. This isn't a particularly efficient idea, because we'll need a current-limiting boost converter to match the voltage of the main battery pack, and then the car's inverter will chop it again with each conversion incurring an efficiency loss, but I believe the idea should work.

(I'll still need to work out how to make sure we don't accidentally charge the main battery pack -- this is an area I'm not that experienced with. But speaking naively, sticking a big fat diode on the main battery would only waste about 0.4% of the total power and dissipate 400W of heat in the very worst case.)

There will need to be a CAN bridge (with computer) in between the main battery pack and the system CAN bus. Adjunct battery pack(s) will need to report their current consumption/expenditure so that all current data can be aggregated and properly reported to the system CAN bus, leaving the rest of the system to see a single battery pack.

Has anybody worked on anything similar to this before? It would be nice to have a $1-2k USD way to reliably extend an old Leaf's range by at least a small amount. Perhaps it could even serve as the sole (if temporary) power source when having a battery pack rebuilt or such. If LiFePO₄ batteries indeed reach $56/kWh this year, then it should be possible to add 12kWh to the trunk (theoretically, 55L of the 679.6L of trunk space) for $672 + hardware, controller, some thermal management, boost converter, maybe a bit of crash safety hardware, etc.

Of course, I would still want to be able to draw from it for other uses (110 inverter, etc.).

 

[Lothsahn]

Muxsan built such a device. The engineering to do it properly and safely while not triggering vehicle codes was considerable. I do not recommend this. If you want one, see if Muxsan has one for sale.

Consider swapping with a 40kWh leaf battery (see Dala's canbus adapter) or if you're really adventurous, with a custom battery made out of CATL cells from China.

 

[mux]

Hi! I have a company specializing in range extender batteries. In short - building it as a range extender battery that also works properly with reverse-engineered car electronics is hard. It took us about 2 years to have a product that wouldn't have people come back all the time (~2018-2020). It takes a company to do this properly.

One of the big issues you're going to have with a small added battery is that you can't really guarantee how the packs are loaded during normal use. For instance, if you have a 24kWh Leaf where basically all your capacity sits between 370-395V, in order to use most of your LFP or NMC pack, you also want to use the 320-370V range (where the other 50%+ of your capacity sits). That means that when you're driving with the batteries at 350V, the extender pack is delivering like 80% of the current. If you're accelerating at 80kW, 64kW comes from that 12kWh extender battery, which will be discharging at 5C+. Also, your cabling will need to be pretty beefy to cope with that, or you'll get such a large voltage excursion on the main battery that the car will throw a fit.

 

[danielsantos]

Muxsan built such a device. The engineering to do it properly and safely while not triggering vehicle codes was considerable. I do not recommend this. If you want one, see if Muxsan has one for sale.

Consider swapping with a 40kWh leaf battery (see Dala's canbus adapter) or if you're really adventurous, with a custom battery made out of CATL cells from China.

Thank you Lothsahn! It's good to know that somebody is already doing this! Even if it's a proprietary, commercial enterprise. I would really like to see a solution out there in the FOSS / open hardware space and would prefer to use LiFePO₄.

Thanks for the lead on CATL. This is the first time I'm actually digging into this type of thing as it's the first time I actually have the money and need to actually do something serious. I suppose the other challenge then is finding a reasonable dealer to buy the batteries from?

I think I want to build a small scale prototype of my concept using a tiny 1kWh battery pack and controller and test that out. All-in-all, I'm feeling like I might want to make a slight pivot in my career. I work on satellite communications firmware (ground stuff) now so I do a fair bit of electrical engineering, though it's mostly software. I might feel more fulfilled working in an area that has an environmental benefit.

Also, I think I'm going to get registered with the local auto auction (I live near Dallas) and keep an eye out for a wrecked Leaf as a means of getting a cheap, used battery pack to transplant. My life is a 2013.

 

[danielsantos]

Hi! I have a company specializing in range extender batteries. In short - building it as a range extender battery that also works properly with reverse-engineered car electronics is hard. It took us about 2 years to have a product that wouldn't have people come back all the time (~2018-2020). It takes a company to do this properly.

One of the big issues you're going to have with a small added battery is that you can't really guarantee how the packs are loaded during normal use. For instance, if you have a 24kWh Leaf where basically all your capacity sits between 370-395V, in order to use most of your LFP or NMC pack, you also want to use the 320-370V range (where the other 50%+ of your capacity sits). That means that when you're driving with the batteries at 350V, the extender pack is delivering like 80% of the current. If you're accelerating at 80kW, 64kW comes from that 12kWh extender battery, which will be discharging at 5C+. Also, your cabling will need to be pretty beefy to cope with that, or you'll get such a large voltage excursion on the main battery that the car will throw a fit.

Hello mux! Thank you for sharing your experience! My concept is slightly different however, though I won't say it's the best idea. My plan is to put a current-limiting, boost power supply between the adjunct battery and the LBC-PDM junction so that I have precise control over both current and voltage. Unfortunately, I have to run for a dr appointment, so will write more later!

 

[mux]

That's definitely a solution to those problems. But doesn't that reintroduce a lot of inefficiency you were trying to avoid?

 

[danielsantos]

That's definitely a solution to those problems. But doesn't that reintroduce a lot of inefficiency you were trying to avoid?

Well, that's what I'm going to examine. I'm researching various circuits and I'll probably set up a SPICE simulation to see how it does.

 

[mux]

You're rolling your own bidirectional DC/DC? I really wouldn't without a whole lot of experience in this field (saying that as a design engineer specialized in electrical power processing). I wouldn't touch this personally, just for the amount of smoke you'll be able to produce with little effort

There's companies that do this kind of stuff and have nice solutions already. Bel Fuse is the highend (we call them the 'rolls royce' of dc/dc converters), but e.g. Deligreen and Dilong in China make/sell decent quality dc/dc converters for automotive, right in the kind of power range you'd need, with CAN control.

 

[EVPowers]

An idea similar to yours was done over a decade ago. ~2011-2012?

The company added-on a modest low voltage pack and stepped it up to high voltage that trickle charged the main pack.

4, 8 or 12 kWh of 48V batteries, a 5kW DC/DC inverter to boost to 360-400V, which is injected onto HV bus

It was not bi-directional.

The add-on pack was tapped into the charger area of the system.

The add-on pack had a separate 48 charger of it's own for refilling the add-on pack.