What am I trying to accomplish?

It’s probably about time to talk about what I’m trying to accomplish with this project, and why I didn’t just replace my aging lead acid batteries.  That would have been easy, but not much of an improvement.

Here’s my wish list so far:

Relocate the batteries, gain storage space, help weight distribution

Currently, the 6 batteries sit in the bottom of one storage bay.  Leaving room for watering the batteries, and with the venting at the top of that bay, it’s effectively full.  The inverter and charger sit in an adjacent bay, that’s also effectively full, as they need space for air to circulate.  With a system of lead acid batteries at 12V, that equipment can’t (according to the RVIA low voltage standard) and shouldn’t be located together.

That’s a pretty inefficient use of space.  I have a storage space under one of the beds that’s not too easy to access without moving a mattress that is well suited to the new battery setup.  It’s a fairly short (and accessible) run between the AC and DC distribution panels.  It also moves about 500lbs of equipment from just a few feet behind the front axle (which is at capacity) to a few feet behind the rear, which has plenty (20,000lbs) of margin.  Freeing up those two bays also gives me some much more useful storage, and the battery compartment may see an air compressor installed.

I’ll end up reducing the total space occupied by system while increasing capacity.

Faster Charging

With any lead acid battery system, getting to a full charge happens in two phases: a bulk charge, where the charger is putting out a fixed power, an absorption phase where voltage is fixed and current declines.  Neither is particularly fast, even with a large charger, as the batteries’ internal resistance and maximum voltage are limiting.

In the case of the Volt, the 45Ah cell packs can handle charging at 10C, or 450 amps.  With 12 cells in series for 48V, that’s almost 22kW, which would be equivalent to over 1500A if it were at 12V.  Think about that–with typical 45A converters, you’d need 35 of them to match that.  What’s more, that rate can be sustained right up to a full charge.

Now, 22kW isn’t going to be easy to find.  That’s almost double what’s available with a 50A/240V RV hookup.  But if we scale back to one of those hookups, and an 80% loading to be safe, a 9.6kW charge rate means that our 12kW battery pack can go from empty to full in an hour and fifteen minutes.  If we were talking about a capacity equivalent to the 6 golf cart batteries in there currently (about 1/4 of a Volt pack), we’d be able to charge in 20 minutes.

When in an RV park, charging that fast isn’t typically necessary.  But when boondocking, and running the generator to charge, it’s a big deal.  Now, instead of having to run the generator for hours, one will do.  That translates into a fairly significant fuel savings.  Using the published data for my generator, I’d be able to fully charge the battery pack with a gallon and a half of diesel.  The same charge using a typical converter and lead acid batteries would take over 10 gallons, with the generator essentially idling.  Even at sub-$2 diesel, that’s a $17 difference.

Significantly Increase Usable Capacity

You can never have too much battery capacity, only too much space and/or weight occupied by batteries.  I was starting with 6 T105-type batteries that are at least 4 years old:

At about 75 pounds each, these batteries weigh 450 pounds before you start including cabling, battery boxes, and mounting.  The Volt battery is a little bit lighter, at just over 400 pounds, but is 12kWh at 80% depth-of-discharge.  That means we’re getting about four times the specific energy in terms of weight.

The AC output of the inverter is also going to be sized to run everything on-board: the air conditioners, electric water heater, microwave, etc.  With the expanded battery capacity, I’m expecting several hours with both air conditioners running, and I hope to easily make it through the night in all but the most extreme temperatures leaving the thermostat in control of the inside temperature without running the generator.

No transfer switching

Even the short time it takes for a transfer switch to operate can affect some electronics.  I also want to be able to feed power to batteries and run house loads at same time—even if shore power is limited.

This is actually much simpler to do than it sounds.  All it takes is a charger separate from the inverter, and leaving the inverter’s shore power input disconnected.  In fact, that’s exactly the setup I’ve had for my existing inverter, as its transfer switch failed long ago.

As long as the charger’s maximum current can be set accordingly, it will charge the batteries at the maximum rate permitted by the shore power connection.  And so long as the average power draw is less than that, the batteries still get charged.

If not, the batteries will gradually discharge, and we’re back to needing to run the generator for an hour at some point.

Charging Sources

I want to be able to retain the full charging functionality I have now: charging the chassis batteries from the house system, and charging the house batteries from the engine alternator.  I’d also like to be able to start the generator from either battery source, but if that happens it’ll be much later in the process.

The chassis batteries can be charged using the 3-stage charger that currently supports the 12V bank.  It’s plugged in to a shore-power/generator-only outlet, which should be fine.  It’s not the most efficient approach, but it’ll be rare that the chassis batteries should need charged when boondocking.

Going the other direction is fairly simple as well, but it requires hardware that’s a little less common.  We’ll need a DC-DC boost converter.  These can be found for less than $50 with a 600W capacity.  If you look at a normal truck alternator (160-amp in my case), there really isn’t that much power available.  The alternator’s rating is a peak output, and here that’s only about 2000W.  By the time you consider the output at cruising speeds, and all of the other chassis loads, 600W will take most of what’s available.

Last but not Least

With what seems like a long wish-list of features, the goal is to keep is as simple to operate and maintain as possible.  The batteries themselves should be virtually maintenance-free, and if the rest of the system is kept simple, it should prove very reliable.

Reliability and simplicity also support the last objective: a safe system.  No fires, explosions, leaks, funny smells, or anything of the sort can be tolerated.  As a nuclear engineer, I appreciate the need for multiple and independent safety systems.  Things will go wrong, it’s important that the result isn’t a big deal.

Leave a Reply