Why does a 48-volt battery bank make sense?

Before deciding to use a Chevy Volt as a battery source, the plan was already going to make the switch to 48 volts.  We’ve talked about why the RV industry settled in on 12-volt house systems; now we’re going to look at what might be better.

What are some of the considerations when looking at battery voltage?  The higher the voltage, the easier it will be to transfer power between the source (batteries) and loads.  But higher voltage also carries greater risks–arc distances are longer, and higher voltages can drive more current through you if you happen to complete a circuit.  We also have various electrical codes that outline the practices for making connections and running wire (whether or not they’re mandatory in an RV).  Is there a sweet spot?

I would argue that yes, there is a sweet spot at 48 volts.

    • National Electric Code in several sections defines “low voltage” as less than 60V DC.  So we’re still a low voltage system as far as that goes, which has implications for how we run wiring if we choose to follow the code.
    • 48V is relatively common among home standby power and off-grid systems.  This means there’s a good selection of inverters and chargers at the kinds of capacities we’ll be looking for.
    • The number of batteries needed in series to get to 48V is reasonable.  That is, if we wanted to get started with minimum battery investment, 4 12V batteries or 1 48V module would do the job.
    • DC wiring runs should be twisted together to reduce loop area and self inductance.  It’s a lot easier to do that with smaller wire.
    • We can save a lot on wiring, and the smaller wiring will be easier to work with.  We also save on all of the bits for connecting wires, and things like disconnect solenoids don’t have to be rated for nearly as high of currents.  This is a big enough thing that it warrants is own section further down in this post.
    • Perhaps most importantly–even though 48V was decided before it came into the picture–the battery in the Chevrolet Volt can be easily broken down into eight 48V sections, at 2kWh each (1.6kWh usable).

But I already have a 12V system.  Should I really throw that out for these gains?  In my mind, it’s an easy answer in the affirmative.  Making the switch to lithium batteries, and taking advantage of what that offers, is going to require a new charger at minimum.  We’re also going to be installing disconnects to protect the battery, a solar charge controller (which is rated for a maximum current whether at 12, 24, or 48V), and relocate the batteries to a heated and cooled location, which means running new cabling.

On the subject of wire size, suppose we have a 2 kW inverter running at 92% efficiency with 13V from our lead acid batteries (not going to consider that it drops with state of charge here).  Using the wire sizing spreadsheet in a prior post, we see that we need a 1AWG cable for 2 5-foot cable runs between the battery and inverter.  If we do that, we’ll put 1.68% of the power from our batteries into the wiring when running at capacity.

If we jump to the lithium pack, mostly discharged at 41V, the same calculation would put us into 10AWG wire and 1.35% loss.  The price of wire is going to roughly scale with the amount of copper, and there’s 8 times the amount of copper in a #1 wire.  10AWG wire is also one-third the diameter, which makes a lot of other things easier.

We’re not going to use #10 wire though.  The inverter we’re using has a 6kW surge rating for 30 seconds.  That’s long enough that we want to address it.  If we step up to #6 wire (which happens to match the inverter manufacturer’s recommendation), we’re still at half the diameter and about a third of the wire cost.  When our battery is charged and operating at 2kW with the larger wire, we’ll lose only 0.37% to the wiring.

As you’ll also see as we get into selecting components, the total system cost with this approach will likely end up less than just the lead acid battery cost for equivalent capacity.  That means we can justify the upgrade on the basis of saving money!

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