My last post dealt with designing your product to get long battery life. The title implied it was about primary batteries but in reality most of what I wrote would apply to get the best efficiency from any power source. This week I’m going to continue in a similar vein but with considerations for using rechargeable batteries.
The first thing to keep in mind regarding rechargeable batteries is you have to separate charge duration from actual battery service life. Many people think of charge duration as “battery life”, particularly non-technical consumers but they are considerably different things. Charge duration is the amount of run time a product gets from a single charge cycle. The primary things that impact charge duration are the same things I discussed last week – sleep mode and active mode current draw and duty cycle and the various environmental conditions that can reduce battery output.
With rechargeable batteries, the actual service life is typically measured in recharge cycles rather than in service hours. Charge duration is still measured in the number of hours from fully charged to a cutoff voltage (usually battery chemistry specific) but there are a number of factors that will change the voltage/time discharge curve for individual batteries over time or simply reduce the number of charge cycles a battery can endure. As it turns out, most of these factors are design related so you do have some control over them.
Instead of repeating it all here, the article at the link below gives an excellent overview of what you need to pay attention to in order to get the longest service life from your batteries. The article mainly discusses lithium-ion batteries but most of it applies to any rechargeable battery chemistry, it is also somewhat dated but still very applicable since the general concepts apply to almost any battery chemistry with only slight differences in the details.
The main take-away from the article should be that the service life of a rechargeable battery is very much dependent on how the battery is treated. Charging in particular is very stressful on any battery and most anything you do to charge a battery faster will also decrease its service life, often disproportionately so. Trying to extend battery life by lowering the cutoff voltage is very unwise since the discharge curve for most batteries is so steep around the specified cutoff voltage (particularly lithium based batteries). The modest improvements you might get by cheating a tenths of a volt on the cutoff voltage or saving a few minutes when charging can easily be overshadowed by the reduction in service life this treatment inflicts on a battery.