EV Engineering News

Practical aspects of off-grid EV charging

The holy grail for many EV owners is to obtain all of the energy needed for recharging from renewable energy sources, and while that’s difficult to justify on a purely economic basis if grid power is available, for those living off-grid it will be all but necessary to rely on renewable energy sources, as in this case running a fossil-fuel powered generator is the economically non-viable option (except on an occasional basis, as is its intended purpose).

An off-grid energy system basically consists of just four key components: 1) a battery to store energy; 2) one or more renewable energy sources (e.g. solar panels, wind turbines, hydroelectric turbines); 3) an appropriate DC-input charger for said source; and 4) a DC-to-AC inverter to power the house, EV charger, well pump, etc. from the battery. Since no renewable energy resource is available 100% of the time, you will probably need to incorporate a backup generator to supply an AC-input charger, but the good news is that this generator can be a lot smaller than if it were sized to power the house directly, because the inverter and storage battery will handle the peak power demands.

To properly size an off-grid energy system you need to know two key parameters: the peak power demanded by all of the loads that are likely to run concurrently; and the average daily energy consumption. The peak power demand sets the minimum size of the inverter, obviously, but it also influences the size of the battery (bigger batteries can supply more current, all else being equal), the size of the wiring, fuses/breakers, etc. The average energy consumption dictates pretty much everything else, from the number of solar panels or the swept area of the wind turbine to the storage battery capacity, and the power rating vs runtime of the backup generator, its attendant AC charger, etc.

A good way to get accurate values for these two parameters is with a whole-house energy monitor that uses current clamps on each incoming phase leg in the main breaker panel. If that isn’t an option (perhaps because the off-grid location isn’t yet up and running) then you can estimate the power demand with rules of thumb (e.g. assume that major appliances draw 80% of their breakers’ amperage ratings), appliance data labels, and even anecdotal evidence (e.g. the neighbor’s well draws 9 A at 240 VAC). As for estimating the average daily energy consumption, a typical value for us spoiled Americans is going to be in the range of 10-25 kWh per day for the first person in the house, and around another 2-5 kWh per day for each additional person.

Given that the size of the storage battery scales directly with this figure, it pays to determine it as precisely as possible rather than just relying on a thumb rule in an article you read in a magazine…ahem. Lastly, factor in the power rating of the EV charger, and if said EV will be charged when the renewable energy source isn’t producing—say, at night with a photovoltaic system—then you’ll also have to add the typical daily consumption of its battery capacity to that of the storage battery rating (in kWh, not Ah, since the two batteries will not be the same voltage unless the EV is a golf cart).

The storage battery is what makes an off-grid energy system possible, so it is arguably the most important component, but it is also one with a fair amount of flexibility. The three key parameters to consider here—assuming a nominal 48 V rating—are the energy capacity in kWh, the current rating vs time (that is, how much current is allowed over timescales of seconds, minutes and hours and/or continuously, often given in C rather than amps) and the cycle life vs Depth of Discharge.

When it comes to the size of the storage battery, bigger is better, budget (and space) permitting, but a practical minimum is enough energy capacity to run everything as per usual for at least a full 24 hours, so that if the renewable source isn’t producing you don’t have to immediately fire up the generator (or, worse, go rushing off to get fuel for said generator…at night…in the rain). For those wanting more concrete numbers, I feel that 200 Ah (again, at 48 V nominal) is the bare minimum for a single person (speaking from experience here), while 400 Ah would allow for an almost carefree lifestyle (assuming there is sufficient renewable energy available to feed said battery).

The most common choices of battery type are lithium iron phosphate (aka LFP or LiFePO4), all of the other lithium-ion chemistries (including recycled batteries from junked EVs), and, finally, lead-acid. To reach the de facto standard of 48 V nominal (the actual voltage can go up to near 58 V, depending on battery chemistry) you can either wire four 12 V batteries in series, or the appropriate number of bare cells (typically 16 for LFP). There are also ready-made “whole-house” batteries available, such as Tesla’s Powerwall, but these are often aimed at grid-tied solar systems to be installed by licensed contractors, rather than DIYers, and they carry a correspondingly hefty price tag.

LFP is arguably the best all-around choice for off-grid systems, as it has a very high cycle life (in the range of 2,000 cycles at 100% DoD to 10,000 cycles or more if operated over the range of 80% State of Charge down to 20%), and it can tolerate high peak and continuous current draws of up to 4 C and 1 C, respectively, with negligible impact on longevity or energy capacity. There are two main choices of LFP batteries: those intended as drop-in replacements for lead-acid batteries; and bespoke 48 V batteries—often in convenient 10-inch rackmount cabinets—that are specifically designed for off-grid use. The former come in a wide range of Ah ratings (from 7-10 Ah for a computer UPS to 200+ Ah for off-grid systems) and have an internal BMS that protects against overcharge, overdischarge and overcurrent. 

These typically lack the ability to communicate with other batteries in a series string (or other devices), nor do they come with any means of monitoring total charge in and out of the battery. The former shortcoming can be addressed with an external active balancer (aka charge equalizer) that automatically shuttles charge from the highest-voltage battery in the series string to the lowest until all are at the same voltage. The latter shortcoming is easily handled by an external battery monitor/coulomb counter (ideally with the ability to report such data to a remote display or smartphone via Ethernet, Bluetooth or WiFi). Note, however, that charge equalizers typically have modest current ratings in the range of 1 A to 10 A, so they can’t correct a grossly out-of-balance pack, nor one with a battery that has significantly less Ah than the rest.

The higher price of the rackmount 48 V batteries will be offset somewhat by not needing an external balancer, of course, as well as being easier to wire up. They will also likely be able to communicate with other batteries (wired in parallel) and/or devices (e.g. to notify the inverter to shut down at the minimum DoD, rather than a minimum voltage) and include some form of monitoring and/or coulomb counting, along with a local display of such information. That said, an external battery monitor will still be more versatile and convenient, especially if the batteries are located in a different building.

The more adventurous (and technically capable) can drop the $/kWh metric even lower by repurposing old EV traction batteries for off-grid storage use, but this is not for the faint of heart (or short of time). Unless the EV was a golf cart, you’ll have to reconfigure the cells to deliver 48 V nominal, so don’t count on using the original BMS (which probably won’t work, anyway, without a lot of code-hacking) and you’ll still be taking the risk that the battery was abused so much that it has no real usable capacity left.

The last option is venerable old lead-acid, though perhaps it’s best to relegate it to the proverbial dustbin of history. Sure, the prohibitive weight of lead-acid batteries is less of an issue in off-grid systems than it is in EVs (until you need to build the racking to hold 10 kWh or more of capacity with them, anyway) and they are notorious for not liking to be discharged below 50% of their rated capacity, though even then you shouldn’t count on more than a few hundred cycles of life. Furthermore, continuous current draw needs to be limited to a fraction of a C (typically C/4) to avoid loss of capacity from the infamous Peukert effect. This basically means that a lead-acid storage battery will have to be 2x to 5x larger in energy capacity relative to LFP, which effectively eliminates all the cost differential between these two chemistries.

The next key component in an off-grid system is an inverter, which converts the nominal 48 VDC from the storage battery into the 120/240 VAC required by the house, EV charger, etc. Eliminate the cheaper “modified sine wave” type from consideration, as its output is really just a 50/60 Hz square wave of less than 50% duty cycle that a lot of devices won’t like (standard dimmers and induction motors being two notable examples). The other type creates a pure sine wave by sinusoidally modulating the duty cycle of a much higher-frequency square wave, then filtering out the high-frequency components with an LC network. If this sine wave is available directly, it’s called a “high-frequency” type, and if it feeds an internal 50/60 Hz transformer it’s called—somewhat misleadingly—a “low-frequency” type.

The LF type is supposed to be better at starting heavy loads like AC compressors by virtue of the energy stored in its transformer’s inductance, but better HF inverters will cope with such loads by going into current limiting mode, rather than outright faulting off (this should be explained in the user manual [you did Read The Fine Manual, didn’t you?]), so this might be a distinction without a difference. Furthermore, the LF type draws more no-load power (1-2% of rated power is typical) to maintain the magnetizing flux in that LF transformer, and this can add up to a lot of energy over the course of a 24-hour period.

The last major choice to consider is whether to go with a standalone inverter with separate DC- and AC-input chargers, or a unit that combines one or both charging functions (the latter is often called a “hybrid solar inverter” or AIO, for All In One). Note, however, that you do not want a “grid-tie” inverter in an off-grid system, as such will not function without the presence of AC from the mains. The AIO type can be a little less expensive, because it combines all three devices into one box, but its main advantage is the ability to prioritize whether energy for the AC loads comes from the solar panels (or, very rarely, a wind turbine), the AC mains or generator, or the battery. This is an excellent solution for partial off-grid operation—e.g. running off solar during the day and the grid at night—but if you’re going purely off-grid, there is more flexibility in using separate devices, as the intelligent prioritizing won’t be nearly as useful.

And speaking of solar and wind (and hydro, for those lucky enough to have it), we’ll cover that—and the appropriate DC-input chargers for each—next time in Part Two. 

This article first appeared in Issue 67: January-March 2024 – Subscribe now.


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