Having determined the energy requirements for Connect Monitor, the next step was to select the battery chemistry and construction. There are many, many different types of non-rechargeable battery available nowadays, each of which has their own pros and cons: it’s easy to get it wrong if you don’t carefully think through all the requirements. In our case, we wanted:
- High power density, as even though weight is not a problem, we didn’t want the device to be any bigger than it had to be.
- Excellent behaviour at low temperature: it’s cold inside a cooler, and colder inside a freezer. All batteries lose some capacity and voltage at low temperatures, but some types lose more than others.
- Very low self-discharge rate. If you are looking for an operating life in the 5+ years range, you need a battery with a shelf life much, much longer than that. Most battery types have a shelf life of only a few years.
- Inherent safety. High-performance batteries have a lot of energy stored inside, so they are all potentially hazardous when physically damaged (and should, therefore, be handled with respect). However, as various consumer products have shown, some battery types can misbehave under “everyday abuse”. Failure isn’t acceptable to us.
After evaluating many battery types and operating conditions – including giving our engineers the opportunity to have lots of fun trying to make them catch fire or blow up – we settled on a “Lithium Thionyl Chloride” (LiSOCl₂) type battery with a “bobbin-wound” construction style. LiSOCl₂ batteries feature:
- the highest power density of any commercial battery type, allowing us to pack all the power we need in a relatively compact A size battery.
- the longest shelf life of any battery – up to a phenomenal 25 years!
- the best low-temperature tolerance, being capable of operating as low as -60° Celsius (C) and with little voltage or capacity degradation between room temperature and 0°C.
- A nominal voltage of 3.6V rather than 3V as per other high-performance types, giving more headroom for voltage sag without electronic components shutting down.
- Inherent safety due to the bobbin wound construction. Unlike the alternative “spiral wound” type, bobbin wound LiSOCl₂ batteries are inherently safe without relying on an internal safety device, as they have very high internal resistance. In fact, it’s possible to directly short circuit a Connect Monitor’s battery without it getting hot enough to be hazardous.
The main disadvantage of this battery type is they are only suitable for very low current levels, and they prefer continuous load. They don’t like sitting around for a long time and then being asked to provide a big burst of power, so you wouldn’t use one in an emergency defibrillator, for example. But as very low current and continuous use is exactly what we want here, that’s not a problem for us.
The other disadvantage is that they’re not cheap. However, compared to the next best alternative, lithium manganese, the cost premium is not large, and the performance is better in every area that matters to us.
So that’s why Connect Monitor has a 4000 mAh, 3.6V, A size, LiSOCl₂, bobbin construction battery!