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Looking for advice on DC-DC converters

2448 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.

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There has been a lot of discussion here...
The following is from experience with my EV conversion.
A standard proper DC-DC converter normally has a maximum current output of 30-40Amps.
Do not feed this into a small AGM battery as it will cook it. A small SLA/AGM battery will only accept a few amps.
I got around this by inserting a power diode in between the DC-DC converter (putting out around14Volts) and a 30Amp/Hr SLA/AGM battery (with a maximum charge current of around 9Amps).
This works because the DC-DC converter thinks the battery is fully charged when it sees the "battery" voltage reach the 14V, while the actual battery voltage is 14V less the 0.6V drop across the diode.
This worked for me for over 10 years when I recently replaced the 30Amp/Hr SLA/AGM with a lithium (LiFePo4) 20Amp/Hr battery.
This also works very well and I have only seen a maximum charge current of around 10Amps which is the recommended for this battery.
When I say "proper DC-DC converter", I am suggesting NOT to use a power supply that has an AC input (like Meanwell).
These do not last long in an EV conversion (I had one for a while before is was dead), because the input circuitry can not handle the current surges that are around when the car is accelerating. There is a work around for that, but that is another story.
If you don't want to mess around with diodes etc, keep the original lead acid battery, that came with the car, that can handle the high currents that these DC-DC converters are capable of.
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