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I would like to hook up a computer to a 12V battery, and I need a clean 12V input to the computer because only buck regulators will be used to deliver the outputs to the computer, no sinusoidal waves.

The fly in the ointment is that the battery(ies) need to be constantly charged. If I charge them using a typical high power multi-stage charger (sometimes called "smart" chargers), will the charger pollute the DC output from the battery(ies) with sinusoidal waves that would be bad for the computer?

If indeed the charger will pollute the output of the battery(ies) if it is directly connected to them, I can see two possible solutions to this problem. One would be to install some GIGANTIC capacitors between the battery(ies) and the charger to rectify the charging input to the batteries. The other would be to use relays and a double stack of batteries. In other words, I would have not one, but two power reservoirs. The computer(s) would run off of one of the reservoirs, while the other was being charged. Then it would reverse. The computers would switch to the second reservoir and the charger would simultaneously switch to the first reservoir. Would either of these strategies work?

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    \$\begingroup\$ Why is it so essential to get clean DC power? Anyway, there shouldn't be much 60 Hz or 50 Hz pollution getting in from the charger. The charger is, itself, a buck converter, in essence. The battery is, itself, a giant capacitor. Buck converters do introduce ripple voltage, however. It is just not at 50 or 60 Hz. \$\endgroup\$
    – user57037
    Commented Nov 1, 2020 at 18:10
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    \$\begingroup\$ also, nothing in this world (aside from things at 0 K) is noise-free. Everything "pollutes". You need to set actual numbers that limit acceptable noise / ripple or else discussing this makes no sense – voting to close as "too broad". \$\endgroup\$
    – Marcus Müller
    Commented Nov 1, 2020 at 18:29
  • \$\begingroup\$ @MarcusMüller In my case the number is tolerance sufficient for input to a computer motherboard, which is +/- 5% typically. In other words, the output from the battery should not fluctuate rapidly more than 0.6 V. \$\endgroup\$
    – Tyler Durden
    Commented Nov 1, 2020 at 18:45
  • \$\begingroup\$ that spec makes no sense - your computer needs way way better regulation, you need a DC/DC converter to have stable 12V, and anything that deserves the term "converter" will have better than 5% regulation if appropriately designed. \$\endgroup\$
    – Marcus Müller
    Commented Nov 1, 2020 at 18:55
  • \$\begingroup\$ @MarcusMüller The battery doesn't have to be good enough for the computer itself. It just has to be good enough to feed the buck regulator that feeds the computer. \$\endgroup\$
    – Tyler Durden
    Commented Nov 1, 2020 at 19:14

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The battery has low impedance the charger has high impedance, the battery will dominate the voltage waveform.

No battery produces a stable 12V, well a 10S NimH or NiCd battery comes close, possibly close-enough. lithium and lead batteries operate in the 12V ballpark, but do not stay close to 12V over their useful charge level range.

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