The introduction of micro-hybrid vehicles placed a new challenging service application on lead batteries identified as partial-state-of-charge operation (PSoC). Manufacturers and suppliers responded by investing manpower, time and capital. Their efforts yielded gains in charge acceptance resulting in improved PSoC life. One development was the introduction of speciality carbons characterised in part by their greater surface area. They are also used at higher dosages, usually greater than half a percent.
However, the downside was a weaker cold crank, shorter high-temperature life, and increased water loss. These attributes were influenced by the organic expander, typically a lignosulfonate. It turns out lignosulfonates such as Vanisperse A (a sustainable organic lead battery expander) and Vanisperse DCA (an organic lead battery expander derived from Norway Spruce lignin) adsorb onto the carbon’s surface. Vanisperse is a very effective carbon dispersant. High surface area carbons, coupled with the high dosages, adsorbed so much of the Vanisperse that there was not enough remaining to produce the desired organic expander functions.
Another issue is there are numerous available speciality high surface area carbons. Selecting the carbon and the appropriate dosage was a daunting task for battery engineers; hence, Borregaard initiated an investigation to assess the carbon-expander interactions on capacity, charge acceptance, cold-cranking and PSoC life. The objective was to provide practical information that would be useful in assisting engineers when selecting the carbon and Vanisperse additives once the critical service duties are defined.
Investigating carbons
Together with Jinkeli Power Sources, our laboratories investigated the interaction between Vanisperse and eight carbons, including the influence on key battery performance criteria with four carbons, in detail.
Vanisperse dissolves into the water and adsorbs onto the carbon’s surface. Depending on the carbon, this can remove a significant portion of the Vanisperse. The remaining soluble Vanisperse is the portion that serves as the organic expander; this portion is referred to as the “Effective Dosage” and can be estimated.
We measured the adsorption and found a strong relationship between the amount adsorbed and carbon surface area. We found a linear relationship with only one outlier carbon. The data indicates high surface area carbons can adsorb a significant amount of Vanisperse, and at high loadings can reduce the effective dosage well below the designed dosage.
The data also indicated the relative amounts adsorbed depend on the type of Vanisperse and the carbon variety. This suggests a carbon’s surface features and the Vanisperse composition may also play a role in the adsorption relationship.
We first investigated the interaction between the Vanisperse A and Vanisperse DCA with a traditional carbon having a relatively low surface area of 75 m2/g. Consistent with prior investigations, the charge current declined as the organic expander content increased. We also found striking differences between the two products, with Vanisperse DCA generating much stronger charge currents.
Vanisperse DCA’s best current was 0.58A/Ah compared to Vanisperse A’s 0.45A/Ah. This is a sizeable 29% improvement. We also noted the beneficial contribution from the carbon: charge currents improved about 15% with increasing carbon content.
Improving charge acceptance
Improving charge acceptance is a key performance criterion for micro-hybrid service. Based on results like those above, we conclude that Vanisperse DCA is the product of choice to improve charge acceptance for micro-hybrid batteries.
The C2 capacity test simulates the Society of Automotive Engineers (SAE) Reserve Capacity. This discharge rate probably better represents conditions experienced during an engine idle-off event. Of the two expanders, Vanisperse A provided the greatest capacity. Contrasting dosage trends were unexpected. The Vanisperse A combination had its greatest C2 capacity at a lower concentration and decreased as the dosage increased. Meanwhile, the Vanisperse DCA model suggests the negative electrode gains capacity with a higher Vanisperse DCA dose, and its optimum appears to be outside the studied dosage range. This means negative active mass (NAM) utilisation efficiency improves with increasing Vanisperse DCA content.
The Vanisperses yielded a similar maximum SAE cold crank, at about 78 seconds. However, their dosage responses were again opposed; Vanisperse A combination was most effective at lower dosages, while Vanisperse DCA combination improves with increasing Vanisperse DCA dosage.
We also note carbon has an adverse relationship with cold-crank (CC) discharge time. As carbon dose increased, CC discharge decreased just as reported by prior investigators.
Modelling exercises using the above performance models with emphasis on improved charge acceptance found the Vanisperse A combination was unable to provide satisfactory solutions. However, the Vanisperse DCA-carbon combination generated a wide range of workable solutions. For this reason, Vanisperse DCA is the preferred organic expander for PSoC duty.
Next, a study between Vanisperse DCA dosage and four varieties of carbon was completed. Carbon surface area ranged from 75m2/g to 1400m2/g. We found carbon type, as well as the carbon dose and Vanisperse DCA dose, have a strong effect on charge acceptance. The maximum charge acceptance occurs at a high carbon dose, >0.35%, and a lower Vanisperse DCA dose, <0.3%. The greatest charge acceptance was observed with the highest surface area carbon (1,400m2/g).
We conclude Vanisperse, carbon and their interaction can have a profound effect on key performance indicators.
Conflicting optimal doses
Life tests using the 17.5% depth-of-discharge (DoD) method found a strong correlation between improved charge acceptance and longer PSoC life. This indicates the improved charge acceptance translated into improved active mass conversion and to longer life as opposed to higher rates of parasitic reactions. Carbon type, as well as carbon dose and Vanisperse DCA dose, had a strong beneficial effect with maximum life occurring in the region of highest carbon dose and lowest Vanisperse DCA dose. The longest life was observed with the highest surface area carbon (1400m2/g)
The four carbons produced comparable maximum C2 capacities ranging from 8-8.2Ah. The maximum varied, depending on the Vanisperse DCA-carbon combination. Peak C2 capacity seems to be outside the dosage range for two carbons, 75m2/g and 225m2/g. This suggests the potential for even better capacity. Increasing the highest surface area carbon (1,400m2/g) dosage had an adverse influence on capacity.
With respect to cold crank, the low surface area carbons produced a longer discharge time. In the region of strongest charge acceptance and longest life, the two lower surface area carbons produced better cold crank. Increasing the carbon dose reduced cold crank. Increasing the Vanisperse DCA dose has a favourable effect on the cold crank. So, there is a situation of conflicted optimal dosages. Combinations with the best charge acceptance and longest life also had the weakest cold crank and C2 capacity.
To reduce the complexity, we compared carbon performance in the region of strongest charge acceptance and longest life (0.3% Van DCA, 0.5% carbon). This region was chosen since our goal is to provide a useful tool to assist in selecting the carbon and Vanisperse for PSoC service. In this way, the trends become clear. Higher surface area carbons are better at improving charge acceptance and PSoC life, but they tend to decrease cold crank and C2 capacity relative to the lower surface area carbons.
Full optimisation graphs of charge current, cold crank and C2 capacity for the four carbons are presented in Fig 2.
We performed optimisation exercises searching for solutions for three likely service duties:
• traditional SLI duty
• micro-hybrid PSoC duty
• best DCA/longest PSoC life
For a traditional SLI duty, where the highest priorities are cold crank and C2 capacity, a Vanisperse A/low surface area carbon combination was preferred.
For micro-hybrid duty, where improved charge acceptance and longer PSoC life is a higher priority, but good cold-crank is still necessary, Vanisperse DCA combined with mid surface area carbons met the requirements.
For service duty demanding the greatest charge acceptance and longest PSoC life, where the cold crank is a lesser priority, then Vanisperse DCA paired with a high surface carbon met the requirements (Table 1).
Finally, Vanisperse, carbon and their interaction can have profound effects on key performance indicators. Advanced carbons and Vanisperse DCA can improve charge performance, and this benefits PSoC life. Utilising response surface design of experiment (DOE) methods can map the complex additive behaviour to assist in optimising charge acceptance, PSoC life, cold crank and capacity.
This article was taken from a presentation given by McNally at the virtual 19th Asian Battery Conference.
