Last fall, I wrote about installing a PC power supply to power the remaining 12V DC loads in my RV, but that’s getting replaced with a DC-DC converter. The power supply did the job it was supposed to, without fail. But there are a few drawbacks with it:
The power supply converts AC power from the inverter to 12V DC. That means the inverter is on, and there’s an extra conversion that wastes energy.
Under load, the 12V output from the power supply is lower than I’d like. At the source, it drops to about 11V, and my rig’s 12V wiring is mediocre at best. Vent fans, water pumps, etc. all run a little slow.
The power supply doesn’t respond well to sudden load increases. This is most evident with the stereo system.
Last time I talked about this project, I left you hanging with the lead acid batteries from the old house power system gone, and the generator connected to the chassis batteries. But I didn’t have anything hooked up to power the remaining 12V house loads–things like the furnace, water heater, water pump, slide out motor, and a few lights.
There are lots of ways to approach this, but first it’s worth knowing how much 12V power I really need:
Water heater: just control logic here, no actual heating. It isn’t enough to worry about.
Furnace: about 8 amps running at 12V.
Water Pump: 4 amps just before reaching the high-pressure cut-out (in other words, when it’s under the heaviest load)
Slide out motor: 30 amps while in motion, more at the beginning when it’s sort of stuck in place.
Lights: since they’re all LED, not more than 10 amps.
Awnings: Just control logic here, and from what I can tell the only reason they use 12V at all is to power the on/off switch. I may be able to eliminate the need for 12V at some point in the future.
A couple of power outlets (10A max each), an antenna amplifier, and a small sound system.
Other RVs may have other considerations:
Leveling jacks: I have them, but they’re driven by the chassis/starting batteries.
2-way (LP, 110VAC) refrigerator: The control board runs on 12V on these. Very little power needed, but the refrigerator doesn’t run without it. If you have a 3-way (LP, 110VAC, 12VDC) refrigerator, you’re using 12V power not just to control the other two, but also to provide cooling–which is a significant demand.
Generator: Most RVs are configured to start the generator from the house batteries. I talked about why I don’t like that setup here.
When you look at the list of things that need 12V DC power to run, almost all of it is stuff that only needs to run for short periods of time. So while it might work, I really don’t want a big DC power supply running all of the time.
I’ll admit I didn’t have it all settled when I pulled the old batteries, but I had enough of a plan to get by. The slide-out was the biggest thing to deal with, and while I could in theory have wired it to run off of the chassis batteries like the generator and jacks, power for its motor and all of the lights in the slide out were fed in together–I didn’t want to try getting another set of wires into the slide out.
So the short term solution was pretty crude, but it’s still working just fine. I picked up a cheap 12V lithium jump starter, that’s able to be charged while the main leads are energized. It’s also not one of the ones with the smarts to limit current until the car’s battery is charged to a certain point, so it’ll work with no other battery in the system.
To test it out, I clipped the leads and connected them directly to the slide-out’s controller:
This configuration has no trouble running the slide out through its full travel multiple times. It’s micro USB charging port is more than enough to charge it and run the few 12V LED fixtures in the slide out.
For the rest of the 12V stuff, I’m using a 300W adjustable power supply. I’m not quite happy with it, as it’s not nearly as efficient as I’d like, so there’s not much point in dwelling on it at this point. It works fine, but I’ll post back when I have something better in its place.
With an extra set of leads, I now no longer carry the bulky SLA battery pack as insurance against a dead toad battery. It just isn’t necessary any more–the lithium jump starter does a better job.
A little while back, I posted a short writeup on a plug-in ammeter I’ve used to measure current on circuits with standard ATO blade-style fuses. When running through trying to find out which circuits have loads on them, it’s still as easy as it gets, especially when you don’t have good access to the wiring.
Before I go any further, let me point out that I have a degree in electrical engineering. If you’ve done any reading at all on here, you know that I mess with electrical stuff more than any sane person should. My “needs” in the tool department are more extensive than most, but I’m going to talk today about a tool that’s affordable and useful enough that every RVer should carry one.
This amounts to a bit of a side project, but provides a platform for really testing what the Volt lithium ion batteries are capable of. You can tell by the image above that I’m talking about putting lithium ion batteries, from a Chevy Volt, into a golf cart.
Why not just replace the lead acid batteries?
There are several reasons. The first is replacement cost–while this is a proof-of-concept, it can be done for about the same hardware cost as putting lead acid batteries in. These carts typically have 6 6 or 8-volt batteries (depending on whether it’s a 36V or 48V cart). Typical cost to replace the batteries is between $600 and $800. The Volt batteries, while used, can be had for about $100 per usable kWh. If we look at the lead acid batteries’ energy capacity to 50% depth of discharge, the standard configuration would be 3kWh of usable energy, or $200 per usable kWh. A single 2kWh Volt module is already 48V, so we could run on just one, at a cost of $250. But the goal is to make the electric carts go farther without charging on a golf cart, so we’ll install more than that.
Reason #2: Capital Cost and Replacement Frequency
Actually, the second reason is cost as well. But from a different standpoint. Typical lead acid batteries used on a golf course are only good for about 2 years. If you assumed year-round golf weather and no rainy days, that would be a little over 700 discharge cycles. The Volt battery should be capable of well more than that, and with proper management should be able to last 10 years or more. In most cases, that will mean that the batteries will get moved to a new cart when the rest of the cart has reached the end of its life.
In addition to faster charging, we also have the ability to increase the amount of energy on-board, making charging stops less frequent. Typically, on the course where the testing is going to happen, a cart is done for the day after two rounds of 18 holes. How much energy is that? We sent a cart out with a regular golfer with a monitor attached. After 9 holes, the cart had used about 0.65kWh, or 2.6kWh after 36 holes. That’s in keeping with the 3kWh expected of a new set of batteries. It also means that with 3.6kWh of lithium capacity, it’ll be an improvement over the status quo.
But 3.6kWh doesn’t quite make 3 rounds of 18–it would only be about halfway through the back 9 on the third round. We could install another module (bringing capacity to 5.4kWh and just over 4 18-hole rounds, for about $250 in batteries. We could also install rooftop solar for about the same price, and extend the range of the 2-module configuration. In two rounds, those panels should be able to pick up 1kWh pretty reliably most days, making for a solid 3 rounds. Anything more, and they’re reducing charge time and cost.
Reason #3: Charging Cost
It might come as no surprise that the third reason is cost also–from a charging standpoint. The lithium batteries should be more efficient in charging, and can do so much faster. Faster charging means fewer charging stations are needed, and a cart might even be able to return for another round after a mid-day charge. Getting an extra round out of a cart without an overnight charge means a course needs fewer carts, which of course saves money.
Now, with fewer carts, other things may wear out sooner (in time). Carts get banged up on the course–whether it’s running rough in search of a lost ball, bumping into things, wear and tear from spikes on the floor mat or tees on the upholstery, they don’t last forever. At least some of the mechanical wear is reduced though, as the cart is lighter than before. Each 2kWh section is about 42lbs–so we’ll have 84 lbs of battery taking the place of almost 400. That means that the cart ready to roll has shed 1/3 of its weight, not counting passengers and cargo, which should help the life of at least some of those components.
How easy is it to do?
Simply replacing the source of power is really easy. Take out all of the boat anchors, and connect the power leads to the 42lb module, and the cart will run. In a Club Car DS with a separately excited motor and electronic controller, here’s the result:
Ok, so I was kind of cheating–with the electronic controller there’s a pre-defined ramp-up in power when you floor it. I was holding the brake during the ramp period so that I got off the line quicker. In an older Club Car cart, with a series motor and no electronics, it’s simply how fast you step on it that matters.
Getting a quick launch is actually a lot easier that way, but also becomes something that we don’t want out tearing up the course. Not everyone is going to practice a quick launch with a brake hold; it’s far more likely that someone will just stomp on the pedal. More on that later.
But it looks a little lonely without company. To protect the pack from overcharging or over-discharging, we need a controller and new charging system. That’s a little more sophisticated, as it’s something that doesn’t exist off-the-shelf. But since we’re going to the trouble, there are a few other things that can be done at the same time:
Charge/discharge control–turn the charger on and off at the right times and disable the cart from moving if the battery is depleted.
Solar charging–control power from 240W of solar mounted on the roof going into the batteries. Simply turn off controller at night to eliminate need for blocking diodes.
Temperature monitoring–detect problems, and prevent use in temperature extremes.
Data logging–this is a big one. We hope to learn a lot about how these batteries really work. With a fleet of carts, this should prove invaluable.
Simplified cell balance monitoring–we’re not going to automate balancing, but we will keep an eye on it.
Remote shutdown–should course staff see someone starting to do something they shouldn’t, the cart can be turned off remotely until someone goes out to address the problem in person.
Geofencing–carts aren’t allowed on the greens or on tee boxes. Geofencing means that the cart can be disabled or sound an alarm if someone tries driving in one of these areas.
Anti-theft–should someone try to steal a cart, it will lose its connection to the course’s wireless network. It’ll sound a Hollywood-style bomb countdown, then self-destruct. No, not really (or maybe it will–don’t steal golf carts!) Seriously, it’ll be set to stop working if it can’t phone home for a couple of days–long enough that intermittent connection problems don’t interrupt play.
That all probably sounds complicated, but there’s a lot of overlap with the house battery system that’s been installed since February. The whole system should be ready for testing on the golf course within a week. Here’s one more video, showing the cart tooling around with the solar panels working.
I’ll follow up on more of the controls, wiring, and the rest before long.
So it has been quite a while since I’ve posted anything on the lithium battery project. I had a week of dry camping sneaking up on me, and made a kind of mad dash to get the RV out of winter hibernation (still in use, but parked for a couple of months), get the batteries installed and anchored down for travel, and everything hooked up and running.
If you’ve followed this project from the beginning, you know that one of the reasons I wanted to go to a 48V DC system was for generating AC power. At 48V, generally you get more capacity (in terms of power) for your money; because of it’s popularity in off-grid installations, there’s arguably more choice, particularly as you move to higher capacities.
Why is this? Well, to start, most of the internal components (everything from wires to traces on circuit boards, transformers, etc), have to be sized based on current (remember that the power, P, dissipated across a wire is equal to the resistance times the square of current through it). When we quadruple the voltage, we quarter the current.
That means a given component–like a wire–at 4 times the voltage carries 4 times the power with a quarter of the relative losses. It also means that we can buy inverters with much larger capacities, which just aren’t practical at 12V.
Since we’re wanting to be able to run everything through the inverter, we’re going to need a big one. Two air conditioners, water heater, microwave, washer and dryer, dishwasher, refrigerator, TVs, ice maker…you get the idea. If you start looking at pure sine wave inverters, you’ll notice a few things:
There isn’t much price difference between an inverter and one that also includes a charger.
Broadly speaking, there are 2 classes: Chinese-made, cheap-looking stuff, mostly found on eBay and similar sites, and more expensive stuff from brands most are familiar with (e.g. Magnum, OutBack).
Once you start getting to 6kW and larger, split-phase power is pretty common.
There’s a big range in surge ratings, and the intervals over which they can be sustained.
I was uncomfortable with most of the cheap stuff. When you notice errors, even as simple as spelling things right, and you know there’s essentially no warranty or support, it’s a big risk. Not to mention possible consequences if something goes wrong.
I also wasn’t interested in a lot of the bells and whistles on the higher-end stuff–I’m building my own energy management and automation system, and that kind of stuff would be under-used. In the process, I ended up stumbling upon AIMS Power, a company based in Reno, Nevada, that’s been around for about 15 years. They offer a 10kW split-phase model that would do everything I was after, and was pretty reasonably priced at about $2350 delivered.
That was still a lot of money to spend, though, on a project still in its early stages, and with a company where I really didn’t have any independent reviews or first-hand knowledge of their products. But like with my own business, they make their stuff available on Amazon. Though I won’t speak to his engineering credentials, Terry Bradshaw has talked about their stuff.
As I poured over spec sheets, there was one thing that gave me some hesitation about the big inverter–it’s standby power consumption. At 200W, it would put a real dent in how long I could go in boondocking mode. So I decided to do two things: test their product out with a smaller version, and tweak my system design to include 2 inverters–one for all of the big loads, like air conditioning, and a smaller one that would handle the always-on stuff, like certain outlets, computers, and the refrigerator.
The model that I’d use for that task is the AIMS PICOGLF20W48V120VR. I could get it delivered in 2 days with Amazon Prime, which also meant I had some assurance it wouldn’t be a total dud and could return it without hassle if there was a problem. And instead of a $2000 roll of the dice, this was only $628.
Let’s start with the basics on this model. First of all, it’s outputting 120V AC power at 60Hz. Be careful as there’s a lot of off-brand stuff that’s set up for “world” markets, at 240V and 50Hz, which won’t do you much good here. A dead giveaway that you need to pay really close attention is a universal receptacle, that takes a variety of international plug types.
It’s capacity is 2kW, which by itself is enough to run a single large appliance like a microwave, and it’s a 48V nominal inverter. It’ll limit what we can use in terms of capacity, as it’ll only operate down to 40V (the Volt battery pack’s usable range goes lower), but at this stage it’ll also serve as a secondary battery protection mechanism–it’ll shut down well before we need to worry about the battery.
It also has built-in charging, though it’s not specifically for our battery pack and would over-charge if left on its own. But it is a 15A max charger, with a variable current limit (remember, 4X the voltage means this is equivalent to an 60A charger at 12V).
Getting a little more into the smaller details, it has a pretty reasonable (for our use anyways) power consumption at 60W max. Under load, it boasts a 6kW 20-second surge rating–which means there’s plenty of time to shut something off should we get the microwave and two hair dryers going at the same time. It also allows selection of “grid” or “battery” priority–this allows some configuration of how the transfer switching works. All of the circuit boards are conformal-coated, making it resistant to dust and moisture that can cause damage. I don’t particularly care for the style of terminal strip they use–it’s not easy to reliably torque the flat head screws as the whole strip buckles and twists. But several other big-name manufacturers use the same ones. Perhaps most importantly, I can download a user manual on their website, and it’s well written in English.
While I’ve only been using it for testing so far, I’m pleased with the purchase. It’s doing what I need it to do, seems well made, and price and delivery were just right.
I mentioned in the previous post that a new battery monitoring system was going to be built to allow use of run-of-the-mill charging equipment, and that it’s cost would be less than $100. It’s also going to be internet connected–we’ll be able to log state of charge, voltage, and a number of other performance metrics to a Google spreadsheet if we want (and that’s easier than it might sound). How do we do this?
Here’s where things start to get pretty interesting–and where money really starts leaving never to be seen again. I’ve talked briefly about the battery pack itself, and still plan to write more about that decision process, but it’s in-hand and looks well suited to do the job.
That’s the easy part. Now we’re into figuring out how to charge the battery, how to protect the battery, and how to use energy from it. Before we go too far, let’s look at a couple of things about the battery pack.
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.