The Rolls 4500- and 5000-series have been updated to use carbon-enhanced negative electrodes, Rolls calls it “Advanced NAM Carbon Blend”. Carbon-blend and partial carbon negative electrodes help greatly against (permanent) sulphating of the battery, and to get many more cycles out of it. In practical terms this overcomes the biggest weakness of the lead-acid chemistry; lead-carbon batteries can sit at partial-charge for long periods of time without the need for a full 100% charge, and all that without ‘hard’ sulphating (the type that results in permanent capacity loss). This shows in an astounding 4300 cycles at 50% Depth-Of-Discharge, using IEC 61427-1, which simulates real-life off-grid working conditions that batteries face, including sitting at partial charge for some time.

Batteries with a partially carbon electrode were first developed in the early 2000’s by CSIRO, the Australian government research institute. They called it the “UltraBattery” because it was designed as a hybrid supercapacitor and lead-acid battery. They found that another effect of the carbon electrode was batteries no longer sulphated, resulting in a cycle-life of tens of thousands of cycles! The UltraBattery could sit at partial charge forever, without detrimental effect. A separate carbon electrode is expensive to make, and it was found that simply adding carbon to the mixture that forms the negative plate had a similar effect. In fact, adding carbon to the negative plate does not meaningfully change the production cost of the battery. The lead-carbon battery was born: A regular lead-acid battery with carbon added to the negative plate. While not quite as good as a separate carbon electrode, Lead-carbon batteries turned out to be much less sensitive to sitting at partial charge for long periods (a full charge cycle is still recommended every now and then), and they have dramatically improved cycle-life vs. a regular lead-acid battery.