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why do some DC to DC use 13.8 volts and others use 12 - I have a DC to DC that has 12volt output is that OK - its just that my sealed battery got hot when The other DC to DC pumped 13.8 volts into it so I am looking at the 12 volt - will that be a problem???
 

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12V is just a nominal label, usually means "anywhere between 11V and 15V should be tolerated".

Input range to a DCDC should be even wider.

13.8V is a very common OUTPUT, low end of the usual charging range if there is a UPS type GEL or LFP standby/buffer battery involved.

Higher might require charging intelligence, dropping to lower Float after battery is full.

But AGM needs higher volts, each model has its specs on the data sheet.
 

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12.7v is the expected fully-charged [open circuit] voltage of a lead-acid battery at rest. 11.4v is the fully discharged voltage of a lead-acid battery. So normally things need to be able to operate in that range. But as was pointed out, the nature of the lead-acid batteries is such that they're charged at a significantly higher voltage on purpose. Thus if the load is expected to be plugged in when charger is active as well, it needs to tolerate that higher voltage. That being said, If I were setting a variable output [switched] power supply to substitute for a lead-acid battery, I would tune it for 12.7v.
 

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Check the voltage on an ICE cars battery that is running, and then again when it's off.

The "12vdc" components on a typical non-ev run at ~13.8vdc, which includes the voltage that is charging the battery.

On a conversion, low voltage components at 13.8v is basically keeping the voltage 'the same as it was'.

It's unlikely that it would matter, but I would be more comfortable running dc-dc @ 13.8. 13.8v should be less of a concern...

When you say 'hot', do you mean burn-your-skin hot?
 

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High School auto shop exam question was voltage per cell on a fully charged automotive lead acid battery. A: 2.2V.

Mutiply by 6 cells to get battery voltage

Higher voltages push current into the cells. So, your 13.8 will push current into the battery.

Your DC-DC is not a charger, which limits charge current, it's a power supply capable of supplying high current.

If, say, it's a 100A DC-DC, the battery's internal resistance is the only thing limiting that 0.6V (or whatever) difference between full charge of say 13.2V and the DC-DC 13.8V setpoint.

The internal resistance gets toasty as the square of that current. 100A battery charging has 2500 times the power dissipation in the battery as 2 amps.
 

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A DC-to-DC converter which is intended to replace a (nominally) 12 volt battery would logically be set at about 12 volts, so it provides a voltage in the middle of the range that a battery might provide. A DC-to-DC converter which is intended for use with a (nominally) 12 volt battery would be set at a voltage which would keep that battery reasonably charged. Two different applications; two different voltages.

An EV normally has a 12 volt battery, because low-voltage power is needed for controls before the high voltage battery is enabled and the DC-to-DC is started. Unlike the starting battery in a vehicle with an internal combustion engine, the (nominally) 12 volt battery in an EV never sees a high discharge rate, so it never needs to be rapidly recharged, and lead-acid batteries like to stay on a "float" charge. The obvious DC-to-DC setting would then be whatever the battery manufacturer recommends as a "float" voltage, which is typically near the fully-charged open-circuit voltage... common recommendations are from 2.25 V/cell (so 13.5 V in this case) to as high as 2.27 to 2.30 V/cell (13.6 to 13.8 V).
 
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