Many electronic systems can benefit from using customised battery packs that can be installed quickly to reduce product assembly costs, are designed to fit snugly into the available space, and facilitate easy connection to both the systems and to chargers.

There are an infinite number of ways in which battery packs can be customised to produce the optimum capacity, size, shape and performance for each application. The very nature of products that require custom packs usually means that rechargeable batteries are preferred; it’s hardly ever worth producing a customised pack to throw away after a single discharge. In most cases, the batteries themselves will be standard products and it’s in the packaging and connections that the customisation is evident. An example is shown in Figure 1.

Figure 1: Most battery packs use standard cells with customised packaging and interconnect.

At one time, NiCd (nickel cadmium) batteries were commonplace but these have fallen out of favour due to the potential for chemical pollution at the end of the batteries’ life, and because of the ‘memory effect’ which meant that users had to completely discharge and recharge the batteries on a regular basis, otherwise their ability to hold a charge diminished rapidly. Li-Ion (lithium ion) is now the technology of choice and this, together with lithium polymer for more exotic battery shapes, such as the flat ones found in cameras and mobile phones, dominates the market. A very popular option is shrink-wrapped packs of standard AA or AAA cells, as shown in Figure 2.

Figure 2: packs of shrink-wrapped AA or AAA cells can be recharged to 80% of their original capacity up to 300 times.

Battery cells do vary in quality and performance but despite the plethora of brands on the high street, there are only a handful of battery manufacturers who produce in large quantities. It will come as no surprise that most small batteries are made in Asia. Providing your custom battery pack supplier uses cells that are certified by UL (Underwriters Laboratory) and buys only from ISO9001 approved factories you can have a high degree of confidence that the products will meet their claimed specifications. Cheaper batteries from sources that are not qualified in this way are a high-risk option in terms of performance, longevity and even fire hazard. After ensuring that the basic requirements are met, you should always ask for samples for evaluation in your application before committing to buying large quantities.

You will be able to recharge a good battery pack to 80% of its initial maximum level at least 300 times. Depending on the frequency of charge/discharge, this will give a good guide to its likely operating life.

Economic quantities

Battery pack assembly is labour-intensive and will normally be carried out in the Far East, although for small quantities of higher power packs local assembly may be more economic.

The economic quantity at which it is worth considering asking a supplier to produce smaller custom packs depends on the weight, quantity and mode of transport that will be used to import the batteries. Because packs vary widely, it’s difficult to give precise numbers but for a typical small battery pack weighing around 50 to 60 grams, airfreight for 1000 units can be economical. Sea freight is cheaper for larger quantities, say 5000 or more, but you need to allow about 4 weeks, including customs clearance, rather than just a few days for delivery.

An example application: the power to find your dog

Retrieva Systems manufactures a range of dog collars than enables owners to find their dogs using a mobile phone, Internet connection or a complementary hand-held receiver. The waterproof and tamperproof collars , shown in Figure 3, integrate a quad band modem, GPS chipset and a 433MHz RF transceiver. They can record up to 7000 locations so that owners can even see where their dogs have been when out of sight. The collars need reliable, compact and lightweight power sources.

Figure 3: Retrieva’s dog tracking collars are powered from a compact Li-Ion custom battery pack that was designed, developed and delivered within 2 weeks

Retrieva was using re-chargeable batteries from one of the major European suppliers but the product was about to become obsolete. The original Li-Ion battery pack was rated at 3.7V nominal voltage with a capacity of 1230mAh. Cyntech worked with leading Hong Kong-based manufacturer, Mike Energy, to develop a size-for-size replacement, complete with integral safety circuit. Evaluation samples of the new packs were delivered within two weeks and after successful trials they were adopted for use in all Retrieva products. Each battery pack has a flying lead and connector to match that of the original, so it is a drop-in replacement. An added benefit is that the new pack, which was significantly less expensive than the original, has 1280mAh capacity, 4% more than the original. With normal usage, the batteries need only to be recharged every 5 to 7 days. Retrieva’s managing director, Jon Bryan, commented, “Not only are the new battery packs good value but we enjoyed very good technical and commercial support during the changeover. It was a painless process from start to finish.”

Dave Mellor, Director, Cyntech Components Tel: 01908 821811
Email: dave.mellor@cyntech.co.uk