Mid Frequency Inverters – A Game Changer for Lead Acid Battery Manufacturing

Everyone is talking about Li-ion batteries – the proverbial “belle of the ball” in the booming EV market. But did you know that most EV’s still use a traditional lead acid battery to power the car’s electronics at startup?  Gas powered cars with internal combustion engines still make up 90+% of the worldwide market; they aren’t going away anytime soon. And that means lead acid batteries aren’t either!

The assembly of reliable, high-performance lead-acid batteries for use in automotive, marine and industrial applications, however, poses a significant challenge. The basic application involves welding a series of lead castings or “tombstones” which make up the cores of the individual battery cells. These castings must be consistent and precisely controlled in order to assure the robust, long-life of the finished battery. Unfortunately, the intrinsic properties of the castings themselves vary, making it difficult to achieve consistency with traditional AC resistance welding which is susceptible to current spikes, line voltage variation, secondary resistance and other challenges. Even advanced AC weld controls, which feature secondary current output, can’t completely overcome the dynamic variability of the lead castings. As a result, production floor operators are constantly tweaking weld parameters to maintain even marginally acceptable consistency.

We’ve found that the best way to address this lead acid battery manufacturing challenge is to use a closed loop mid-frequency inverter that combines precision-controlled secondary power (V  x I) with monitoring and real time feedback to adapt for the differences in resistance in the lead castings, as well as other variations in the weld process (e.g. electrode wear, cabling, etc.). Why? Inverters automatically maintain constant power and consistent heating profiles, resulting in dramatically increased yields and decreased dependence on operator process tweaking.

CHANGE IN TOMBSTONE RESISTANCE VS. TIME

Let’s take a look at the tombstone manufacturing process. As molten lead moves from the smelting pot into the mold, its resistance properties begin to change – and can keep changing for up to eight days.  Welding is downstream from a number of other post-mold operations, so it’s not possible to accurately predict when each batch will be welded. And because that time varies, the exact resistance properties of each batch is a moving target that can’t be accommodated by a static weld control.

The tombstone’s rate of resistance change is so rapid that weld parameters that are producing good welds one minute may be producing marginal or unacceptable welds twenty minutes later – even for units within the same batch. Consequently, production floor operators often have to readjust weld process settings whenever the flow of production is interrupted for more than 15 minutes. With such a narrow process window, the resistance welding operation is never able to reach a level of process stability to support sustained high volume production demands.

FUNDAMENTALS OF CONSTANT POWER CONTROL WITH CLOSED- LOOP FEEDBACK

There are two primary kinds of welders: open-loop and closed-loop. The definition of an open loop control system is simple: no feedback. Open loop controls do not react to variations – like the dynamically changing nature of lead- acid battery tombstones – that affect either the primary or secondary of the power transformer. This renders them poorly suited for this kind of complex welding challenge.

Closed-loop controls with constant power feedback, on the other hand, dynamically monitor changes in secondary voltage as they relate to changes in secondary resistance. The secondary current can then be controlled to deliver consistent power from weld to weld. Ultimately, the use of a closed loop, constant power control is the only practical method for ensuring uniform heating profiles in a situation where the resistance of the materials being welded shows a significant variation over time, as seen in lead-acid battery manufacturing.

REAL-WORLD BOTTOM-LINE RESULTS

AMADA WELD TECH has been working with leading lead acid battery manufacturers for more than twenty years. Our focus has been characterizing the resistance welding processes and refining the effective use of closed loop inverters with real time feedback and constant power control. Among the most significant benefits has been the expansion of the effective process window using the same weld schedule from 15 minutes to as long as 72 hours without any operator adjustments. The use of secondary power with real-time closed- loop feedback significantly increases overall weld consistency and dramatically reduces bad welds.