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.
I’m going to keep working through the details one subject at a time, but I thought I’d report out on how they’ve done so far. First of all, compared to the lead acid batteries, big loads are no sweat. Previously, running the microwave for more than just a few minutes was enough to have the inverter hitting its low voltage limits. As those batteries got older, even a couple of minutes was too much. Even with cables only about 3 feet long, deep cycle lead acid batteries just can’t supply the kind of power needed for that with a relatively small (~3kWh) battery bank. Now, I’m baking several batches of cookies using the convection oven without issue.
With the refrigerator now running off the lithium battery bank, it was interesting to see just how much it taxed the inverter on start-up. With a 12-volt battery bank, especially with lead acid batteries, the conventional approach is to seriously oversize the inverter to handle the draw of the compressor starting. With this inverter, the startup load is big enough that the fan momentarily kicks on–but otherwise, it’s no big deal.
The inverter has done quite well overall. I did manage to make it complain about overloading, but not enough that it shut down–in other words, I tested the 20-second surge specification. It just so happened that I managed to print a 10-page document with my color laser printer while the stand-alone ice maker, refrigerator, and convection oven (more cookies!) were running. It beeped to let me know that it was above the 2kW continuous rating, but never interrupted power.
I have also noticed the autotransformer going to work when I’ve been passing shore power through. When on 30-amp service at an older park, it’s boosting the voltage on the inverter sub-panel when big loads (like the air conditioner) are starting. You can hear everything on shore power slow while it’s on–such as the fan on an air purifier–and measure the voltage dropping to ~105V. Yet move that air purifier over to an outlet fed by the inverter, and that doesn’t happen–even when it’s just passing through shore power (i.e. not inverting).
As far as the battery monitor/charging controller I put together, it has been working fine. I do think I’m going to end up with cell-level monitoring though–there’s just a little variation in cell voltages at this point that I’d like to be able to keep an eye on without having to access the pack and measure by hand. Ultimately I want the system to be as hands-off as possible, even if it means a little more work up front.
While I’ve not been tracking power in and out so far (it’s on the to-do list to start logging that), I can say that it’s more than meeting my expectations. Running the refrigerator, cooking using the microwave, watching TV, keeping the laptop and external monitors going, handling lights, and numerous other small loads. A week dry camping, making normal use of everything inside, needed no charging at all. I expect once the solar panels are up on the roof I’ll be good to go for quite a while.
As I catch up on documenting all of the assembly work and compile data I’ve collected so far, hopefully the project will start to come into focus for those of you following along.