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Looking for advice on on DC-DC converters

1498 Views 40 Replies 10 Participants Last post by  evric
My '76 Citicar is roadworthy again, and now I'm in the doldrums of ironing out some of the 'nice to haves' to make it more usable. Since getting the new traction battery setup ironed out I've been using a large 12V lead acid battery that came with the car to run the lights/contactors/etc. and have been charging it separately from the traction battery. I'm looking for recommendations on a setup for charging the 12V battery off of the main battery. I have some ideas and a conceptual understanding of how this should work, but was wondering if there's any 'standard' or best practice ways to do this. My main pack is 2 modules from a 2018 leaf in series, a ~48V nominal pack with around ~9kWh for those curious.
I'm not too worried about the setup optimizing for efficiency, I have more capacity/range right now than I'd reasonably use anyway.

Thanks!
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Couple of examples from a quick search on AMZ :

First one is limited to 55V, may be a bit too low for this application (needs to handle up to 58.8V for 14S).
Second one can take up to 100V. I am sure there are many others out there.
 

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Discussion Starter · #9 ·
This is all you need! is only $30 and is normally used in golf carts.

I actually picked one of these up, but the Ebay listing specifically said not to use it to charge batteries. I assume because there's no current limiting so the 12V battery would just eat current until the converter let the magic smoke out. If there's an easy way to limit current I wouldn't mind using that.
 

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I did not see anywhere in the listing that it can not be used to charge batteries, I assume they mean it can not be used as a charger.
your 12v battery will taper down the amps when is charged and not consume energy from the dc/dc if not needed. so this should be perfectly fine.
 

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A DC to DC supplies 12.0V to satisfy your car accessories and will kill your lead acid battery over time.

A battery charger keeps the lead acid battery and the car accessories happy.

The term "DC to DC" is used in the context of EV OEM components, but it is NOT the same as your Alibaba DC to DC converter....they are battery chargers.
 

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Would you mind expanding on this a bit? Electrically speaking I'm not sure I understand the difference between a DC-DC converter and a battery charger
Lead-Acid battery charger has multiple stages in the charge cycle with different voltages to maximize the life of the battery. A solar charge controller will have the same capability.
 

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well, all 12v batteries from all electric cars charge with the DC/DC converter. is basically an alternator, and most ICE cars, don't regulate the alternator charge to the battery, just like a dc/dc, so while you are right about the stages on a lead acid charger, that does not explain how all other cars do it.
I have build several ev conversions and worked on Tesla's, even installed tesla's DC/DC converters on my EV conversions, and non of them do anything about regulating the charge to the 12v. never had a problem.
I also think it is an overkill to do anything about it. but if it makes you feel better, then you could install a solar charge controller, but then the dc to dc can only be feeding the solar charge controller, and all accessories will run directly from the battery that is being charged by the solar controller and the dc/dc.
 

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Teslas go through 12V batteries like $hit through a goose -- bad example of how it's done.

My GMC goes dead regularly. One run of a few dozen miles on the alternator and the battery is rejuvinated for weeks.

Whether the accessories run off the charger or the battery is a matter of bean counting. Put a black box around it and it all runs off the HV battery.
 

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Teslas go through 12V batteries like $hit through a goose -- bad example of how it's done.

My GMC goes dead regularly. One run of a few dozen miles on the alternator and the battery is rejuvinated for weeks.

Whether the accessories run off the charger or the battery is a matter of bean counting. Put a black box around it and it all runs off the HV battery.
Teslas batteries do only last 2 years, so yea, pretty shitty, but are you really gonna go thru all that trouble so that the battery can last an extra year? if you are lucky !?
I think you are right in everything you said, but my point here is that is not worth going thru all that trouble for another year on the battery.
 

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What trouble?

Box A

Box B

Pick the one that cares for the battery up front is my point, and it's not Tesla, and it's not a xV-12V dc-dc off Alibaba.

Yes, you'll power 12V stuff, but a battery is ~$120, which is 4000 miles' worth of electricity 😛

My car batteries last 5-7 years. The Bolt EV 12V battery is a champ at 3 years old right now. The GMC regularly discharges below 8V (lack of use and vampire power...I run the truck once every month or two) and is on its 3rd year, also - but its battery was procured used.

The last battery in the frickin' Model X lasted a bit over a year.
 

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What trouble?

Box A

Box B

Pick the one that cares for the battery up front is my point, and it's not Tesla, and it's not a xV-12V dc-dc off Alibaba.

Yes, you'll power 12V stuff, but a battery is ~$120, which is 4000 miles' worth of electricity 😛

My car batteries last 5-7 years. The Bolt EV 12V battery is a champ at 3 years old right now. The GMC regularly discharges below 8V (lack of use and vampire power...I run the truck once every month or two) and is on its 3rd year, also - but its battery was procured used.

The last battery in the frickin' Model X lasted a bit over a year.
So, how do you propose he connects everything, and what are the parts he needs to buy in order to get this done? could you post some links on products and how to connect them properly?
 
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