An aging and seldom-used automotive battery that had to be kept charged, prompted this engineer to take a new look at an old design.
Many years ago, I was stranded at home due to a flat battery–literally stranded as I lived on a farm some 10 miles from the nearest town. The solution I came up with owes a lot to that most indispensable item “the home lab” and was constrained by the components on hand, the ubiquitous LM723 and 2N3055, a good selection of resistors, and some trim-pots. It proved easy to build on a piece of Veroboard, was reliable, and as I later discovered ‘a good idea,’ being simply just a current limited constant voltage supply, requiring little in the way of maintenance or documentation.
Thirty years on, and the battery of an aging and seldom used SUV that has to be kept charged, prompted a new look at an old design. I’m not a luddite, I spent much of my career programming PLCs in the mining and chemical industries, but I cut my teeth in an RF R&D lab, so at heart, I’m an analog guy and a consideration of requirements convinced me to use the older programming language called ‘solder’ and to implement the logic required with analog components. This also means that the circuit can be used to “upgrade” any older charger. I do love an analog solution!
Charging lead acid cells
A little research reveals that automotive lead acid batteries are different than deep cycle or stationary batteries. Automotive batteries are designed to maximize current capacity for cranking and do not respond well to deep discharge or float charging (also known as stage 3 charge cycle). The plate structure of starter batteries maximizes surface area and the electrolyte specific gravity (SG) is higher than in other batteries in order to deliver the high cranking current. Like stationary batteries, automotive batteries allowed to remain in a state of deep discharge experience permanent sulfation, where the small lead sulfate crystals produced during periods of discharge convert to a stable crystalline form and deposit on the negative plates. Float charging automotive batteries, on the other hand, can easily cause over-saturation leading to oxidation of the positive plates also shortening the battery’s life. Charging voltages and charging cycles are therefore quite critical and are different for automotive and deep cycle types; furthermore, charging voltage should be de-rated with ambient temperature at a rate of 3mV per degree C above 25ºC.
Figure 1 shows stage 1 and 2 charge cycles. Stage 1 and 2 can be accomplished by the circuit of Figure 2, which in current limit forces a relatively constant charging current for stage 1 and, as charging current decreases below current limit constant voltage mode for stage 2. A good rule of thumb here is that when the current no longer decreases, the battery is fully charged.
Figure 1 Stage 1 and 2 charge cycles
Figure 2 The original power supply unit (PSU) operates in constant current mode (CCM) until the load current drops below the current limit threshold. The adjustment sequence is: Adjust VR2 10k pot to set Vout = 14.1V under no-load condition.
Hard or permanent sulfation is a function of time as well as state of discharge so if vehicles are not in regular service, it’s advisable to have some means of monitoring battery voltage and restarting the charging process when the voltage drops to some point below full charge voltage. Consider discharge rate for the vehicle when setting the setpoint for initiating stage 1 charging.
Data on exact values of charge rates, current, voltage, and float voltage differ from source to source. The main take-away from most sources though, is that, in order to optimally charge the battery without reducing its life, don’t allow it to overheat, don’t allow hard sulfation to occur, don’t allow gassing, and don’t over saturate. This Design Idea attempts to accomplish this as simply as possible using no equipment other than a soldering iron, a screw driver, and a multimeter.
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Seagan Yi-O’Kelly has a background in plant automation and analog design.