Low-cost digital switching brought pulsed techniques to battery charging in the 1990s, no matter what the cell chemistry. Now a Latvian-based company is bringing a variant of that technology to lead-acid battery formation. Gerry Woolf reports.
More than a decade ago, the pulsed charge was the latest craze for lead-acid battery technology – a means of breaking down the sulfation that inevitably formed during the cycling of lead-acid batteries. It led to longer battery life and is now widely adopted throughout the battery industry, whether you operate forklift trucks or if you charge a humble cell phone using a lithium-ion cell.
Charging a battery is a three-part process – the charge has to be got into the cells, it has to be optimised and it has to terminate.
No matter what the chemistry, the process taking place during charging results in a chemical change within the cell electrodes and the inevitable evolution of heat and gases. It is quite possible to push more electrical energy into a cell than it can accept.
There are at least two key processes involved in this chemical conversion. One is the “charge transfer”, which is the actual chemical reaction taking place at the interface of the electrode with the electrolyte and this proceeds relatively quickly. The other is the “mass transport” or “diffusion” process in which the materials transformed in the charge transfer process are moved on from the electrode surface, making way for further materials to reach the electrode to take part in the transformation process. This is a relatively slow process which continues until all the materials have been transformed.
Both of these processes are also temperature-dependent.
The battery charging process thus has at least two characteristic time constants associated with achieving complete conversion of the active chemicals which depend on both the chemicals employed and on the cell construction. The time constant associated with the charge transfer could be one minute or less, whereas the mass transport time constant can be as high as several hours or more in a large high capacity cell. This is one of the reasons why cells can deliver or accept very high pulse currents, but much lower continuous currents. (Another major factor is the heat dissipation involved). These phenomena are non-linear and apply to the discharging process as well as to charging. So there is thus a limit to the charge acceptance rate of the cell. Continuing to pump energy into the cell faster than the chemicals can react to the charge can cause local overcharge conditions including polarization, overheating as well as unwanted chemical reactions, near the electrodes thus damaging the cell. The beauty of fast charging is that it forces up the rate of chemical reaction in the cell (as does fast discharging) and it may be necessary to allow “rest periods” during the charging process for the chemical actions to propagate throughout the bulk of the chemical mass in the cell and to stabilize at progressive levels of charge. And that “rest period” is what is provided for in pulsed charging regimes.
Now if you can do this for battery charging, why not battery formation?
That’s exactly the approach that AEL, a small electronics manufacturer based in Ogre, Latvia has undertaken, on the request of a well known Russian battery manufacturer. The object of the exercise is no different from that of pulsed charging – it should be possible to save both time and energy in producing batteries.
It’s well appreciated that lead battery formation is best achieved with either a circulated and cooled electrolyte (for industrial batteries) or for automotive batteries, formation is best achieved by placing forming batteries in water baths. It’s thus apparent that continuous formation is an inefficient process which liberates heat. Pulse formation reduces the heat generated in the process.
Tests that have been carried out by AEL indicate that it is possible to reduce formation times between one third and one half (six hours as opposed to 12 hours, for a 60Ah starter battery), presenting a significant reduction of temperature and gassing during the formation process. Furthermore, batteries formed using the pulsed formation process also show a notable increase in initial battery capacity – up to 10% more than that of the reference batteries formed with standard formation methods currently used by manufacturers.
AEL’s Formac system can produce high current charging pulses from CC to 100Hz and for durations as short as 3 ms. The process by which the pulsing algorithm is optimized for any battery is entirely down to the manufacturer says Kaspars Garkevics, who is marketing the product, “and is clearly related to the formula of the active mass and plate mixes in each battery design.”
So is it a battery designer’s plaything or a valuable cost-cutting tool? You’ll have to take a flight to Riga to find out!
