I have a computer that runs on 12v from two parallel lead acid batteries. I need to be able to swap the battery banks for fresh ones every now and then without ever losing power to my equipment.

I can do this by replacing one battery at a time, that's no problem, however I'm curious about best practices when swapping large batteries that's powering sensitive equipment. Should I be worried about spikes when connecting the wires? Should I be using something like ferrite beads or diodes to protect my stuff?


  • \$\begingroup\$ possible duplicate of Charging marine battery while powering equipment \$\endgroup\$ – Kellenjb Aug 25 '11 at 18:28
  • \$\begingroup\$ @Kellenjb - I'm in favor of leaving this one open; the other question has some concern with charging, in addition to battery swapping, whereas this one is focused on hot-swapping batteries. \$\endgroup\$ – JustJeff Aug 25 '11 at 22:30
  • \$\begingroup\$ @JustJeff Well one or the other needs to be closed. \$\endgroup\$ – Kellenjb Aug 26 '11 at 1:16

What is the equipment apart from a computer?

It should not be a vast issue if the load is not disconnected at any time.

You should have spike protection already present sufficient to meet anything liable to arise. Substantial capacitance on battery lead. Small caps at regulator inputs. Maybe an inductive filter in battery lead. MOVs or similar on power lines do no harm.

The exceedingly cautious may consider using a variable resistor or electronic equivalent to change batteries. This should be overkill if all is designed well.

  • Connect 2nd battery via a resistor that would drop about say 3V at full load current.

    Reduce resistor to zero.

    Hard connect 2nd battery.

    Add resistor set at 0 ohms from V+ to 1st battery +

    Break 1st battery direct connection.

    Increase resistor to max.

    Remove resistor

  • \$\begingroup\$ The additional equipment is a microcontroller and some very small DC motors and a signal amplifier. Load will lie somewhere around 5 - 7 amps. All my equipment is rated for car battery usage. \$\endgroup\$ – Mattias Aug 25 '11 at 16:33

Use wire-or diodes. That is simple and safe. As @Russell McMahon suggested, caps after there. MOVs are not a bad idea.

0-ohm resistor isn't a great idea since the batteries will be at different (open-circuit) voltage.


I can do this by replacing one battery at a time, that's no problem

Yes, that's a good idea. Another alternative is to connect both of the new batteries, and then disconnect the old ones. Since the batteries are in parallel, it doesn't matter if you add a new battery (assuming they are in good health), since the supply voltage will be roughly equal.

Should I be worried about spikes when connecting the wires?

Since you are connecting it in parallel, most likely not. Since there's a load on the from the computer, it is possible that the supply voltage raises slightly (due to the new battery being able to supply more current with less voltage sag), but it should never exceed the rated voltage of the battery. With these 12V batteries, I would not worry.

Should I be using something like ferrite beads or diods to protect my stuff?

If some of the equipment causes lots of switching noise (causing voltage spikes or dips), you might want to include a diode to ensure the voltage drop across the supply is always 12V. However, in most cases a battery can act as a diode itself, so you are already technically protecting yourself via the battery.


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