Battery

The battery is the piece that gives you power 24/7. There is an enormous range to choose from, but it all boils down to choosing a battery that suits your application. Some batteries are built to suit infrequent usage scenarios (e.g. UPS) while others are built for regular daily draining & recharging (cyclic usage in mobility applications) while still others are designed to tolerate occasional deep discharge and long periods of operation at partial states of charge (solar & deep cycle applications). Still others are designed to start motors (cranking applications). And newer Lithium models can offer fantastic cycle performance and weight reductions.

As the saying goes, “choose wisely”!

1) What autonomy do you need (how many days of overcast weather can it ride thru)?

This is like ‘how big is your fuel tank’? A smaller tank means you drive less km between refuel stops, but it also means the tank takes up less space and doesn’t weight the car down too much. It’s upfront cost can also be significantly lower!

A bigger battery means you can ride thru more cloudy days, but it also means you need to devote more space to the battery, consider it’s extra weight and have the budget for it.

A rule-of-thumb for a solar-only system (i.e. no generator) is a 5-day autonomy battery (i.e. 5 days of very overcast weather) using a battery suitable for periodic 80% maximum discharge (i.e. a ‘solar’ or ‘deep cycle’ battery). The more ‘critical’ the application, the more autonomy you should include (e.g. telecommunications and medical applications may need 10-day autonomy designs).

It is very common to skimp on the size of the battery, usually to save of cost or weight both. This can be done, but only when the client is aware that it will compromise the system’s reliability and result in reduced life expectancy of the battery. If it is necessary to cut back on the battery capacity you should consider mitigating the reduced reliability by increasing the solar size and incorporating remote monitoring and reporting for managing low battery capacity events.

2) How long do you want to battery to last?

There are many factors that impact on battery life, but the three main ones are it’s charging parameters, it’s usage cycle and it’s operating temperature.
- Setting the correct charging voltages can add years to the battery life. Always take the time to custom program your charging devices with the specific values recommended by the battery manufacturer.
- The heavier the daily use on the battery the harder it works and the shorter it’s life expectancy will be (especially with lead battery types). The easiest way to extend the battery life in a system is to increase it’s capacity (also by default increasing the autonomy).
- Temperature is the enemy of your battery. A rule-of-thumb for lead batteries is “every 10°C increase in average temperature will HALVE the battery life”. Do everything you possibly can to shield your battery enclosure from heat. Keep it out of direct sunlight. Ensure appropriate air flow, either passive vents or active fan-forced ventilation. In extreme conditions you may also consider air
conditioning, even though that will need more solar!

3) Do you need to minimise weight?

Many enclosure or pole mounted system need to reduce weight. Lithium batteries should be considered in these scenarios. You will usually find Lithium models are at least half the weight of an equivalent size/capacity lead model.

4) What will be the average temperature of the battery enclosure?

As mentioned in (2), temperature can kill a battery. If you can’t manage the enclosure temperature then you need to accept the fact that you’ll go through batteries at a very fast rate and include that in your expected maintenance schedule.

Ensure that your charging system includes temperature compensation measures so that the changes in enclosure temperature don’t result in unnecessary overcharging of the battery.

If you’re using Lithium batteries, ensure that your enclosure will not exceed the battery’s BMS operating range (usually ~50°C). Also ensure that the minimum temperature won’t be below the Lithium’s min threshold (usually 5-10°C).

5) Do you want a ‘smart’ battery?

Many Lithium battery ranges include models with Bluetooth communications built-in. These models can be connected to an App which will display important battery condition statistics such as State of Charge, Voltage, Temperature and any alerts or warnings.

Other Lithium models include active integrated communications links to the charging hardware. This can be connected to the internet and report to online portals for remote monitoring and management.

6) Factors to consider regarding chemistry & safety

Different batteries have different characteristics, both regarding their electrical specs, their weight, their temperature behaviour but also the risks associated with their chemistry.

Lead batteries will vent Hydrogen during the charging cycle. As Hydrogen is highly flammable these batteries may only be installed in appropriately vented enclosures. While Sealed Lead Acid (SLA) batteries may be less likely to vent Hydrogen, it can still occur so even sealed batteries need to be installed with air vents.

Flooded lead batteries contain liquid acid so need to be installed with containment and spill management peripherals as well as PPE for users. Lithium batteries do not vent any gasses during normal operation, but if punctured or forced into a thermal runaway state some chemistries can vent corrosive gases. Lithium-Nickel-Manganese-Cobalt (NMC) varieties tend to be more volatile but will usually be more compact. Lithium-Iron-Phosphate (LFP) varieties tend to be more stable and less prone to thermal runaway if punctured, but will be bulkier than an equivalent capacity NMC model.

All Lithium types are guarded by a BMS to monitor cell temperatures, voltages and current. Depending on the design of the battery, the BMS will usually shutdown the charging system and/or the battery itself before it can reach an operation