Inspired by a previous post, I wondered how many battery-electric vehicles (cars that run on just batteries, not hybrids with plugs) there are in Minnesota. This information is not readily available from the DMV. But combining this data and this data from DOE yields an answer as of 2014: 644.
When I started tinkering with off-grid solar one of the first questions I asked myself was “how long is it going to take to charge this battery?” Or, similarly, “how much power will I produce in a day”? Initially, the answer seemed easy: I’ve got a 100-watt panel, Minnesota gets about 4 hours of peak sun on average per day, so I’ll get 400 watt hours per day! A 960 watt hour battery should be charged in two and a half days!
Wrong. Way off. Maybe more like 250 watt hours per day, and more like four days to fully charged.
I realized this quickly when attempting to charge a fully depleted battery. In reality, it took into the fourth day for the charge controller to switch to battery maintenance mode (showing it was completely full).
Under perfect operating conditions and when grid-tied, you may actually get close to that nameplate 100 watts. However, when your system is connected to a battery the voltage drops. Many charge controllers (except for the really good expensive ones) will match the voltage of the panel to that of the battery to facilitate charging, which is almost always a lower voltage than the panels potential peak. The rest of that potential is wasted. So while my panel will produce 5.29 amps at 18.9 volts under optimum conditions (5.29 amps x 18.9 volts = 100 watts), when connected to my battery, it will probably only produce 5.29 amps at between 11 and 13 amps (5.29 amps x 12 volts = 63 watts).
Panels are built this way on purpose to make sure power can continue to flow to the battery even during overcast conditions when voltage may drop a little (gotta make sure that water flows downhill!). Grid-tied panels don’t have this problem, since the grid can usually accommodate your voltage, and in a grid-tied system you’ll probably opt for one of the fancier MPPT controllers (see link above).
So, in summary, I’m probably getting 65 – 70% of my nameplate wattage (by design), and a realistic estimate for charging a fully-depleted 80 amp-hour battery from my 100 watt panel is 3.5 – 4 days.
Another takeaway for this amateur: it’s about amps, not watts. The websites where you shop for panels always have the watts in large font, but the small print tells you the optimum operating current, or amps. This number times hours of sun gives a much better estimate of the output (5.29 amps x 4 hours = 21 amp hours per day) for a battery-tied system.
The battery was totally dead this morning (11.4 volts) after about 16 hours powering the freezer. The charge controller indicated the panel was charging the battery for about 4-5 of those hours, but it was extremely cloudy. I turned off the inverter and reconnected the freezer to grid power (it’s nice to have a backup to the backup!) After charging all day today, the battery is up to about 60%. I hope I didn’t do any permanent damage to the battery.
My estimate of the charge time was pretty close, but my estimate of running time for the freezer was pretty far off. I don’t think I calculated any loses from the inverter, which the internet tells me can be 15% or more. It was also a hot day, hotter than when I measured my freezers usage initially, which could have had an impact. Next time I’ll definitely be measuring the total watt hours used (I forgot to hook up the meter) so I can try to estimate what was lost to the inverter.
One more day of charging before I do any more experimenting.