Manufacturers have always recommended that batteries in a battery pack should be charged to full levels, particularly for storage. However a certain charge state that works for one type of battery doesn't always work for another. I recently received an electric car newsletter discussing this very topic and summarizing that a lithium battery that is partially charged, not fully charged, is best for longevity when kept in storage. Is this true? And if so, for all lithium battery types? Conversely, the article stated that regular topping-up is recommended.
Such thoughts are far from the average driver but typical daily use of an electric vehicle will require a daily charging habit. However, is daily charging bad if it's not needed? While a peak charge is not harmful if the level drops to a stable voltage within a short period of time, persistent charging could affect the battery chemistry (also note that a lithium phosphate battery should not be discharged below a recommended level of 33%).
Before we discuss charging further, let's consider driver habits. Heavy current demand on batteries from aggressive acceleration and up-hill climbs will tap more heavily into available power. While a 33% remaining level is recommended for lithium phosphates, power demands can poke through this 'floor' from time to time, even with 50% charge remaining. A demanding pedal push causes a downward spike. Sure it would be momentary but consider the troughs created by such demands. Excessive drops repeatedly dipping below that safe level could affect the stable chemistry and shorten battery life. I'm not sure how the lithium substrate and cadence that enforces the lattice bonding is affected. No one has bash tested these batteries to any great degree outside the manufacturer's laboratory (perhaps due to cost) and one would assume the manufacturer performed some of their own tolerance testing (not that I can locate decent test results). Therefore being conservative with driving habits is always a sound recommendation. Persistent excessive demand will indeed shorten pack life and using the lower gears will help preserve power demand when necessary. If your electric vehicle is a direct drive type, selecting an economy mode will protect the battery pack.
Back to charging, we all know that overcharging can ruin a lithium battery. When my batteries reach their top charge of 4.2 volts each, after shunt they drop back to 3.4 volts or so over a given period (I should measure this time, I believe it's around 40 minutes or so). If one constantly charges a pack after short drives one could argue that batteries reach their top voltages more often which would result in a shorter life, even for top 'safe' voltages, particularly if the current is high (32 amps from some commercial charge station providers). A lower current charge means less time where a battery sits at its peak voltage. It is not recommended for lithium batteries to be kept at their topmost voltages for long periods. As some studies have concluded, excessive electrolyte at this level can result in faster drain.
What battery pack do you have? Most 'do it yourself' electric car builders use SLA (Sealed Lead Acid) batteries simply because they are a low-cost option, however their life cycle is approximately half that of lithium chemistries. With lithium batteries, phosphates are more stable than the cobalt types being produced for some commercial vehicles. While cobalt chemistry has a greater energy density (good for performance) life span is expected to be less than the phosphate batteries (and manganese and polymers for that matter). Also, without management, overcharging can be a fire hazard, described as 'thermal runaway' (whereas phosphate chemistry just vents and melts in such a situation).
Most lead acid electric vehicles don't have battery management; such batteries are typically charged to full but management can help. A friend's pack regularly sulphates; some of the batteries become overcharged while others haven't reached their full charge state. This only serves to reduce plate area and charge capacity so a BMS (Battery Management System) on a lead acid battery car would be just as useful, even if the lead acids are more robust than lithium batteries.
Ultimately the jury is still out on the typical percentage idle charge. Some say 45% for lithium batteries when left alone. Others disagree and there are too many variables to consider; environment, temperature and so on. If you go away on vacation and the car is sitting around, some insist that lithium batteries last longer on a partial charge but as I just mentioned, no one can agree on a charge state. To accommodate the variables, I tend to believe an idle state of 85% charge is sufficient. Somehow the lithium substrate ages when left lying around fully charged, particularly in warmer temperatures. I am not sure how true this is for phosphate chemistries albeit reasonably documented for cobalt batteries.
Driving around short distances and charging regularly is fine; the batteries should last longer from frequent partial charges than deep charges anyway. When I do charge, I prefer to do so during the daytime/afternoon where possible if I haven't driven very far. Note your energy usage in this regard; off-peak or shoulder periods may be a better time for charging. I could leave charging another day but that just adds to the eventual charging time when I need to do so, and doesn't allow capacity if I have to make a long drive somewhere. Ultimately lifestyle will govern how often you charge your pack. Given driving habits and when to charge, I prefer to charge every day.
While off-peak is cheaper I don't like the idea of my car charging all night when it doesn't need too, constantly shunting on/off to retain the top voltages, having achieved the balancing some hours earlier (and the my charger's shunt action is loud). Despite initial investment, solar panels on the roof of my house help to lower the cost and provide virtually free energy for my vehicle now. With energy prices going up, my EV is still cheap to run and I get a good night's sleep.
Happy electric motoring,
Carmel M Morris
Carmel MD Morris is a published author, technical writer and engineer. She converted a 1983 Mitsubishi Starion coupe to an all-electric vehicle and now drives happily around town with no gas worries. Using conversion guides, anyone can convert a gas car to electric, www.convertyourgasguzzler.com.