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Blake Dickinson: Chargers ain’t chargers

Wednesday, 26 November 2014 ( #695 )
Blake Dickinson joined AeroVironment Inc in 1995 and is the Technology and Applications Engineering Director for Efficient Energy Systems. He has a BS and MS in Mechanical Engineering from Texas A&M University and completed two years of postgraduate research and class work at UC Davis focusing on advanced batteries and fuel cells for electric vehicles. He has over 20 years of experience in testing and development of all types of energy systems and power electronics.
There are three industry-accepted definitions of charger types for charging motive power batteries: conventional, opportunity and fast. The main differences are related to charge rate, intelligence and safety features. Conventional chargers are designed to support single-shift operations and to charge only one battery voltage and capacity type. Furthermore, since they do not measure temperature, they should not be used for opportunity charging. They are designed for one cycle a day (eight-hour discharging, eight hours charging and eight hours cooling).
Opportunity chargers, as the name suggests, are designed to charge a battery during periods of downtime for the truck, such as operator breaks. At a minimum, to operate safely and provide long battery life, opportunity chargers should have the following key upgrades over conventional chargers: 1.) Battery temperature measurement 2.) Anti-arc disconnect 3.) Real-time clock 4.) An adjustable charge algorithm 5.) Multi-voltage capability The battery temperature measurement and adjustable charge algorithm are needed to prevent battery overheating, excessive gassing, thermal runaway and overcharging. The anti-arc disconnect is required to prevent damage to the connectors and chargers and improve operator safety. The real-time clock is used to schedule EQs and daily full charge requirements. Fast chargers are opportunity chargers that have additional intelligence to accommodate the higher charge rates required for heavy-duty applications. The additional features compared to opportunity chargers include: 1.) Battery monitor with real-time voltage and temperature measurements 2.) Closed-loop EQ control (battery monitor controlled) 3.) Added safety 4.) Additional data storage The battery monitor is required to provide real-time battery voltage and electrolyte temperature measurements for closed-loop control. The closed-loop control provides maximum charge return without overcharging the battery. The battery monitor will also allow for closed loop control of the EQ process. This allows any battery to go to any charger and get a complete EQ while preventing overcharging that can occur with a charger controlled EQ process. Furthermore, a fast charger with a battery monitor will include additional safety such as water level detection and under/overcharge protection. Finally, fast chargers generally have more data storage that can be used to verify operator compliance, battery performance and charger faults over a longer period of time. How to Select the Proper Charger Type When choosing the appropriate charger type, it is important to understand the specific application requirements such as: - Usage/duty cycle - Available charge time per day - Available days/times for EQ - Dedicated or universal charging requirements - Ambient temperature conditions - Battery type (flooded, high gravity, waterless or sealed) - Accessory requirements (auto watering, stack lights and vent control) The usage and available charge time of the application is the first criterion to use in selecting a charger type. If the battery is used more than once a day (1.0 EBU), you should not use a conventional charger. If the application is multi-shift and requires less than 1.5 EBUs per day, then this is probably a good application for an opportunity charger. Above 1.5 EBUs, you should consider using an intelligent fast charger due to its higher charge rate, closed-loop control algorithm, thermal management control and data management capabilities. Besides usage levels, the next criterion in determining the charger type to purchase is the universality requirements of the application. If several types of vehicles with several types of batteries will be sharing chargers, then a multi-voltage charger is required. If there is only one unique battery capacity for each battery voltage level, such as a 36V, 1,000 Ah battery and a 48V, 850 Ah battery, you can utilise an opportunity charger. But, if the application requires supporting multiple battery capacities in the same voltage, such as a 36V, 1,000 Ah and a 36V, 850 Ah, then you will need to use a fast charger with the battery monitor technology. If you will be dealing with multiple battery types (flooded, high gravity, waterless and/or sealed), then it is critical to either dedicate each vehicle to a specific charger or use a battery monitor to provide battery specific information to the intelligent fast charger. This will allow one charger to be able to handle any battery type that is connected. If the EQ time is limited or broken up over several days and/or the trucks will use multiple chargers, then it is best to use a fast charger with a battery monitor-controlled EQ. If you are looking to auto water your batteries, the good news is this is a feature that is generally available with all charger types. But, only the intelligent fast charger with a battery monitor can guarantee it only happens once a week, preventing overwatering of the battery. Furthermore, intelligent fast chargers allow the auto watering feature to be used with multiple batteries supported by a single charger or, conversely, trucks that utilise several chargers during the week. Finally, only intelligent fast chargers have stack light and ventilation control capabilities.