Charging one battery from another as an alternative to parallel packs.

[Jacobsmess]

Hi, just spit balling ideas here and wondering if there is any reason you couldn't or shouldn't charge one pack from another in motion.
Basically, I was thinking of options about increasing capacity, and using parallel packs, the suggestion is that you try to match pack performance/chemistry as best possible. I was wondering, instead, could you potentially have a 2nd pack installed that would charge the main pack via a DCDC or perhaps some OBC (or gen2 Prius inverter).

In my case, I could potentially have a separate secondary battery that is temporarily fixed, allowing for weight savings when unused but better range when required. This would open up battery pack options quite a bit as it would allow the use of any battery pack rather than requiring another ≈50kWh pack.

Thanks

[johu]

If you're savvy it's easy to rig this up by extracting the buck/boost stage from a Prius Gen2 inverter. As an invariant the additional pack must always have a higher voltage than the installed pack so it can sit at the boost side of the converter.
That said for the rare use case it might be easier to just charge both batteries to full before a long trip and then simply parallel them

[Jacobsmess]

Thanks., provided the nominal voltage of the secondary pack was the same as the main pack and the main pack was in a discharged state the secondary pack (provided it was fully charged) would always be on the higher side of things.

Also the main thinking behind this approach would be perhaps a more appropriate use of mismatched capacity packs.

[uhi22]

What about using two buck-boost converters back-to-back, to allow both batteries have any voltage, and the software can select in which direction the current flows. Would allow to use any pack as "range extender", no matter of the nominal voltage or capacity or state-of-charge.

[Jacobsmess]

What about using two buck-boost converters back-to-back.

Definitely an interesting approach, so the buck boosters would work in tandem to act as a battery balancer?

[P.S.Mangelsdorf]

I was actually thinking a lot about this recently, and I'm glad someone made a thread! My thought was to use a larger pack in a race trailer/tow rig for charging at the track, and was trying to figure out a safe way to do that, (without the inefficiencies of going battery to inverter to ac charger to onboard charger to battery). Prius inverters are plentiful in a junkyard near me, so I may need to look at using those!

[Aragorn]

What happens if you feed DC into the AC input of a typical OBC (say a telsa Gen3 or one of the MG units)? Many switch mode PSU's i believe will happily accept DC on their input?

[Jacobsmess]

Many switch mode PSU's i believe will happily accept DC on their input?

I'm by no means an expert but recall Johu suggesting that is the case

[uhi22]

As I understand, the OBCs are step-up-converters, they are able to make a power-factor-correction, following the sinus input voltage of e.g. 0 to 330V and boost it to e.g. 400V battery voltage. Not sure, because our nominal voltage of 230V AC are ~320V peak voltage, and my empty battery has 300V, so it would also need to step-down. Hm.

[Aragorn]

My understanding of big smps is that they will have a PFC stage to boost the internal DC bus up to a voltage higher than is required, then they buck down from there to the desired output voltage?