Partial pack Dc/Dc converter?

[Daveturpin]

I’m in the process of finalizing the install and wiring routing in my Beetle. Trying to figure out the 12v system.

I’m not finding a DC/DC converter that matches my pack, which is 90s1p (288-378v) total. I do see generic converters that would work with single module 18s1p (58-76v) or two modules…

Now my spidey sense is telling me that I shouldn’t tap off between modules unless I could also guarantee balance between the rest of the modules.

What do y’all do in this kind of situation? Is it okay to take the 12v conversion off a single module and assume back flow will keep the modules balanced, or could I tap between ALL the modules via a power diode system, or do I absolutely need a converter that will work with my total pack voltage?

[barracuda816]

I would say unbalanced load is a no no. But as far as I am aware there are plenty of options for your pack range. A Mitsubishi outlander charger with dc-dc built in, for example will work in that range no problem.

I would suggest looking in the OEM options in the wiki.

[Daveturpin]

Let me caveat that I am using a Thunderstruck charger and need a separate DC/DC converter…

I found something with the right specs… but it’s more of a laboratory-style package. Anyone use something like this, possibly in a custom enclosure? (It does say EV is a typical application)

https://www.vicorpower.com/documents/da ... 6F0yzz.pdf

[barracuda816]

Is space/weight at that much of a premium to look for a stand alone? If not, you could just use the DC-DC function and leave the thunderstruck charger to be the charger. Or sell the thunderstruck and have a combined unit. Or look if $$$ is not an issue look at something like this https://www.evwest.com/catalog/product_ ... 9625d791c4

[Daveturpin]

I'll give the OEM units a check.

I just feel like I'm missing something fundamental here. Presumably every single EV conversion needs a DCDC converter. But except for the very pricey EVWest unit, it doesn't seem like aftermarket companies have given it much thought, and where they have, the DCDC converter is like 3x the price of the chargers? The Thunderstruck charger even has a 12v out, but I asked them about it and they say it is just to give 12v to power up the Dilithium BMS or MCU. But in what application would you need to power up the control unit where you don't ALSO have a 12v battery that needs charging?

[tom91]

https://www.thunderstruck-ev.com/elcon- ... erter.html

[Daveturpin]

Maybe I just wasn't looking hard enough... Thanks again, Tom.

[Zieg]

I used a gen 1 Chevy Volt dc/dc which is rated for the voltage and has easily controlled, adjustable output voltage. For me it's also a plus that it's air cooled since I didn't want to deal with running water lines to it. If you are also using a Thunderstruck MCU, it has a setting for it. Just give it pwr/gnd and canbus to the MCU, config the MCU for the Volt type converter, which bus it's on and what you want the output voltage to be. Think I went with 13.5v.

[Aragorn]

FYI 13.5v is undercharging your lead acid, and you will perhaps eventaully run into issues with sulphation. Ideally you want it around 14.1v as a flat value, and certainly not below 13.8. OEM designs will push up towards 15v for a short time, especially on AGM batteries, then will drop back to 14ish after a while.

[Zieg]

Huh, interesting. I'll bump it up then, thanks. I was worried that would over charge it. Been keeping it on a smart tender when not driving so hopefully no damage done, yet.

[Daveturpin]

I used a gen 1 Chevy Volt dc/dc which is rated for the voltage and has easily controlled, adjustable output voltage. For me it's also a plus that it's air cooled since I didn't want to deal with running water lines to it. If you are also using a Thunderstruck MCU, it has a setting for it. Just give it pwr/gnd and canbus to the MCU, config the MCU for the Volt type converter, which bus it's on and what you want the output voltage to be. Think I went with 13.5v.

That is very good to know! I was looking at the Volt converters on eBay. Although I found a used Elcon unit for $200.

It is great that there are options!

Okay next stupid question: Since the converter handles the 12v load (within reason), do y'all get away with motorcycle batteries, or even no 12v battery at all? I understand that the OEMs all use batteries of reasonable size, and the converter only periodically charges them?

[celeron55]

FYI 13.5v is undercharging your lead acid, and you will perhaps eventaully run into issues with sulphation. Ideally you want it around 14.1v as a flat value, and certainly not below 13.8. OEM designs will push up towards 15v for a short time, especially on AGM batteries, then will drop back to 14ish after a while.

You want to have the voltage be temperature dependent.13.5V is fine in a very hot ambient temperature, but if it's -20°C your lead acid battery basically won't charge at all and will freeze and die. And yes, a desulfation cycle is good to have.

[Zieg]

Okay next stupid question: Since the converter handles the 12v load (within reason), do y'all get away with motorcycle batteries, or even no 12v battery at all? I understand that the OEMs all use batteries of reasonable size, and the converter only periodically charges them?

My car was previously powered by a motorcycle engine and I decided to keep using the 14ah motorcycle battery. Remember you need some means of powering up the 12v system, the dcdc converter should be on the switched side of the contactors, so the 12v battery is necessary to bootstrap up to the point where the contactors are closed (and if applicable, the dcdc is told to turn on).

[Bratitude]

Models s dcdc works fine with a 72s pack

[NiHaoMike]

I’m not finding a DC/DC converter that matches my pack, which is 90s1p (288-378v) total.

Just use a switching power supply that's compatible with 240V AC, after rectification the voltage ends up in that range. Server PSUs are cheap on the surplus market and very well built.