Batteries & Battery Management Systems (BMS)
In broad terms there are two basic types of batteries in use with solar applications, lead-acid and lithium. The inverters that they supply their DC energy to are classified as either low or high voltage, and so the battery bank you choose will be either a low or high voltage bank. For household applications lead-acid banks are generally 24 or 48 volt, these are low voltage banks. Lithioum battery banks are either low voltage 48v banks or high voltage 120-800volts.
The two system configurations that use batteries are hybrids and inverter/chargers. The latter are far more versitile in this respect and may usually be connected with either, whereas with a hybrid inverter some sort of lithium pack is required. The proliferation of hybrids on the market has gone hand-in-hand with the availability of lithium batteries through the arrival of electric vehicles.
Battery technology is not standing still, and the price of lithiums especially continues to fall as the number of types increases. There are currently two main lithium-based varieties suitable for home solar applicatians, rack mounted high voltage modules and low voltage Lithium/Iron/Phosphate (LFP) batteries in a form similar to lead-acids. The former have inbuilt battery management systems (BMS) to regulate their charging and temperature control functions while the latter category sometimes require this management to be added. The inherent risk of thermal runaway with lithium is managed with BMS, with battery temperature, state-of-charge (SOC ) and charging/discharge current continuously monitored.
The two most common types of lead-acid are AGM and Gel. Both are maintenance-free but need to be carefully treated as far as their use patterns are concerned. Regular heavy discharging reduces their life cycle count, 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. There are charts available for each battery model showing the expected number of cycles against the amount of discharge per cycle. 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. As of October 2025 I see that new manufacters are joining the Pb-C market and prices are coming down.
Unlike lithium packs, lead-acid batteries don't have any inbuilt BMS. However most inverter/charger manufacturers produce an in-house battery processing unit to manage charging regime and monitor the bank's SOC. This is a necessary component. Don't try to use lead-acids with a hybrid inverter, they are for inverter/chargers.
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. All of the lead-acid types are readily available. The initial purchase price is considerably less than that for lithiums. They do not have the thermal runaway characteristics of lithiums.
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 should 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 size of your lithium battery pack is measured in kilowatt/hours (kWh) capacity. They may be stacked together so that the upper limit is determined by the space available for solar panels. Lead-acids come with various voltages and amp/hour (Ah) ratings, and may be arranged in series, parallel or series/parallel to make up your required voltage and capacity.
