Batteries & Battery Management Systems (BMS)

In very broad terms there are two basic types of batteries in use with solar applications, lead-acid and lithium. I'm not going to try and cover them all, just concentrate on what the main differences mean in practical terms for solar applications. 

The two common types of lead-acid are AGM and Gel.  Both types are maintenance-free but need to be carefully treated as far as their use patterns are concerned. Heavy discharging regularly reduces their life cycle, so to prolong their life, there should be a limit to the amount of energy drawn from them overnight, typically down to 40% of full charge (SOC) or so. Just as important is that they are not left in a low SOC condition for any length of time. They need to have a full charge applied periodically.   

The latest iteration of lead-acids is the lead­ carbon or Pb-C battery.  These batteries tick a lot of boxes! The initial premium paid up front for these over and above AGMs and Gels is overtaken by their longevity and usability.  

Unlike lithium packs, lead-acid batteries don't have any inbuilt battery management system (BMS). This is important if you're using a hybrid system, where the lack of battery data feeding into the inverter will seriously affect the system operation and life of the batteries. There are some cheap low-voltage hybrids on the market advertised as comparable with lead-acids but I've learned the hard way to avoid this idea completely. On the other hand some manufacturers offer factory BMS kits to go with more expensive low voltage hybrids, as is the case with inverter/charger installations. 

Where lead-acids come into their own is with an inverter/charger installation. This arrangement has long been what a solar installation is and there are some compelling reasons for its continuing use. Lead-acids can be fully discharged if necessary so that their full capacity is on hand in an emergency. Large battery banks can be configured at a relatively low cost. Individual batteries may be slotted into a bank with a failed unit if necessary to cover an emergency. The issue of their never being fully charged is easily overcome because there will be a BMS system with an inverter/charger installation, and there is definitely also a strong mains charger included. 

AGM and Gel batteries are manufactured in New Zealand and are readily available. The initial purchase price is considerably less than that for lithiums. They are completely safe, not having the thermal runaway characteristics of lithiums. 

Lithium batteries have become widely available as a result of global heating and the uptake of electric cars. In conjunction with this is the proliferation of hybrid manufacturers whose products are best suited to lithium. Lithium battery packs contain an in-built Battery Management System. 

There have been inherent problems with the risk of thermal runaway and fire although battery temperature (among other parameters) is continuously monitored by the Battery Management System, and the system shuts down if things get too hot. Each lithium pack comes with its own inbuilt BMS, so you know the inverter is going to be capable of monitoring the SOC accurately. They are quite expensive, although, like the Pb-Cs, this would be somewhat offset by their usability and long service life. 

The charge/discharge cycles of lithium and lead acid need to be considered differently so that in regular use, it's common to discharge lithium more heavily overnight than is the case for lead acid. They can be discharged down to 10% SOC, so you could install a smaller capacity pack than a larger set of lead-acids at the same price. However, you have to bear in mind the total capacity available in times of grid failures. 

Lithium batteries charge up more efficiently than lead-acids, and you can notice this when the sun is setting and you're trying to get the last drop of energy out of the array. Additionally, they can be fast-charged to 100% capacity, unlike lead-acids, which need a much slower absorption charging phase to top up the last 20% of storage. In practice, this means that for a lot of days, you're more likely to have a fully charged bank heading into the evening with lithium than you might with lead-acid, which may not be taking full advantage of what the array has to offer later in the afternoon.

The function of the battery management system is to safeguard the batteries, preventing overcharging and over-discharging.  It consists of a battery shunt, a temperature sensor a box with the associated computing and electronics constantly monitoring voltage, current and temperature to ensure the system is operating within set parameters.  As already mentioned, this is an integral part of a lithium battery pack but not so for lead-acid batteries. A system with lead-acid batteries without a Battery Management System is unable to perform this vital task.