[jwiger]

Don't forget that the 12v battery will also be a load on the converter.

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[brian_]

I don't see how the battery is a load. It gets charged, and it returns that charge, averaging near zero. The whole point of a battery is that the power source (alternator, or in this case DC-to-DC converter) doesn't need to provide exactly what is being used at all times, only the average.

 

[brian_]

I have put together a list of the cars current 12v systems including those that have been removed and estimated the total sum of the fuses.

In the linked page, you note:

Obviously the alternator could not keep up with all the fuse ratings, anyone know what rule of thumb they use for rated use vs. rated fuse?

That's not the only factor. Even if you listed the maximum current for each circuit, the total would still be higher than the peak total current, and far higher than the average... because everything is not 'on' at the same time.

For a hint of the peak total current, look for larger fuses or fusible links elsewhere, which feed that fuse panel. A service manual should have wiring diagrams showing those major fuses.

 

[jwiger]

I don't see how the battery is a load.

The charge and discharge rates will be different than a starting battery on an ICE. However, and this is assuming you're using a lead-acid 12v battery, your charging system (aka your DC-DC converter) still has to sit at a voltage higher than the batteries nominal voltage (of 12.6v) to get it recharged. Usually around 14 volts. Depending on how low the battery was it could take a few dozen amps initially, then tapper off to a float charge of one amp or so. At which point the battery is acting as a buffer for the load on the charging system, but lead acid is still a bit of bottom-less pit. You'll have to dial your DC-DC converter to a voltage higher than the battery's nominal voltage, and the charge feed never drops to zero amps.



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[brian_]

The charge and discharge rates will be different than a starting battery on an ICE.

Yes, but not much, once you're driving...

• The engine starting battery discharges massively for a couple of seconds, then spends the rest of the time charging back up (a small fraction of the battery capacity) and smoothing out power demands.
• The EV auxiliary battery discharges slowly over the time between uses, then once the vehicle is running spends the rest of the time charging back up (a small fraction of the battery capacity) and smoothing out power demands.

Yes, the EV auxiliary battery should probably be a deep-discharge design rather than a starting design, but a common AGM battery would be suitable.

However, and this is assuming you're using a lead-acid 12v battery, your charging system (aka your DC-DC converter) still has to sit at a voltage higher than the batteries nominal voltage (of 12.6v) to get it recharged. Usually around 14 volts. Depending on how low the battery was it could take a few dozen amps initially, then tapper off to a float charge of one amp or so. At which point the battery is acting as a buffer for the load on the charging system, but lead acid is still a bit of bottom-less pit. You'll have to dial your DC-DC converter to a voltage higher than the battery's nominal voltage, and the charge feed never drops to zero amps.

All great information, well described. Perhaps I should have been more clear in my comment...

The battery does not need to a load in addition to the others; it is a load when there is spare capacity, if charging is needed. If everything else is running, there's no need to be charging the battery. Even if the converter has a current limit well below the worst-case peak load - and so it doesn't maintain a charging-level voltage in that condition - that's not a problem; the battery will provide the needed current until the load drops and the converter can keep up, and recharge the battery.

Also, while I don't know if people are doing this with their DC-DC converters, it can be run like a modern battery charger, rather than just floating at a fixed compromise voltage. Even the charging system of an ordinary modern car changes the alternator output voltage to suit conditions, and my guess is that production cars with DC-DC converters instead of alternators are doing the same.

That also suggests a guide to converter sizing: what do Toyota and others find suitable? (Keeping in mind that typical hybrids use a 12V battery the same way that an EV does).

 

[Duncan]

When I first put my car on the road I did not bother with a DC-DC - I simply charged the 12v auxiliary battery at the same time as I charged the main battery

Worked fine for a couple of years

Then I upgraded my main battery and decided to go for a cheap DC-DC

I had an old laptop power supply - which I used as a DC-DC - it was 14v which was a bit too much for my old headways 4S auxilary battery - so I used a diode to drop the voltage to about 13v - GREAT
when I have the headlamps on my battery voltage does drop - but as soon as I switch them off I'm back up again

bit flakey but it works

 

[Yabert]

Personnally, I think the best DC-DC converter (low cost / high quality wise) are the Meanwell power supplies.
http://www.trcelectronics.com/12-volt-power-supplies-chassis-5.shtml
Many models can accept up to 370V DC and some can take up to 430V DC.

 

[zepol_wube]

Maybe I could approach this a different way. Tell me what you are using and under what conditions?

Vehicle, year, original engine alternator rating. i.e. VW Golf 2003, 90 amps
Pack voltage full, empty? i.e. 396-355v
DC-DC convertor, model, watt rating, input v, output v? Issues? Would you buy again?

Is your vacuum pump, steering pump, A/C, water pump... running off the HV side, or the 12V side of your system? i.e. Vacuum-LV, Steering-HV, A/C-HV, Water-LV, Heater Core-HV, Heater pump-LV.