Choosing Solar Batteries

What battery technology do I use?

This technical note applies primarily to the 12V sealed lead acid batteries used in Urban Solar photovoltaic (PV) LED lighting systems. It provides an introduction to the basics of battery selection, testing battery state of charge, battery performance and key factors affecting their service lifetime.

Battery selection

Sealed lead-acid batteries are used in most solar lighting and backup power systems. There are several reasons for this, but essentially they are chosen because there is no other technology (NiCad, NiFe, etc) that competes on an overall cost vs. performance vs. reliability analysis.

Advantages of lead-acid AGM (Absorbed Glass Matt) batteries include the wide operating temperature range, low cost, deep cycling with no memory effect, and simple charging methods. The only drawback is their large size and weight.


Battery lifetime and the number of discharge cycles available, depend on depth of discharge, rate of discharge, temperature, and charging voltage. Depth of discharge and temperature are the most significant factors in determining the system lifetime.


Battery capacity (how many amp-hours it can hold) decreases as temperatures lower and increases as temperatures rise. The standard rating for batteries is at room temperature (77°F).  At a freezing 32°F temperature, capacity is reduced by ~ 20%, while at an extremely cold -22°F temperature, battery capacity drops to ~50%. Capacity is increased by ~12% at 122°F.

Even though battery capacity at high temperatures is higher, the battery lifetime is shortened.  As a general rule, for every 15°F increase in operating temperature above 77°F, battery life is cut in half.  Conversely, while battery capacity is reduced by 50% at -22°F, battery lifetime increases by about 60%.  The important factor in system design and battery size selection is the average yearly temperature.

Selection of a quality battery with operating temperature specifications that exceed your environment’s average temperatures is a key metric that will result in a positive solar experience and help end the Solar Myths.

Daily cycling

Typically, the lighting system is used every night, 365 days per year. In a solar lighting system, the total number of cycles defines the lifetime of the battery.

Battery storage and testing

All batteries will lose their charge when sitting on the shelf, so it is important to understand how to store batteries and keep them charged up prior to deployment. It is recommended that stored batteries are never allowed to self-discharge below 65% state of charge (SOC) before recharging.

For lead-acid batteries used in Urban Solar products, the shelf life is defined as the time required for the battery to degrade from 100% to 65% state of charge (SOC). Once a stored battery reaches 65% SOC it should be recharged, and batteries should be deployed with a minimum of 80% SOC, especially if deployed during winter.

Note, these voltages are for batteries that have been at rest for two hours or more. Measuring the battery voltage during charging or under load will not be an indication of the true SOC of the battery. The battery needs to be ‘at rest’ for a couple of hours after charging before measuring voltage.

There are more accurate methods of determining the true health of a battery by measuring specific gravity and performing a load test, but these are not usually practical or readily available. For batteries that are less than a year old and have been stored in reasonable temperatures (<86°F), the voltage measurement will give a good indication of battery SOC and under these conditions the batteries will be in good condition.

The self-discharge rate is approximately 3% per month when the storage temperature is maintained at 68°F. The self-discharge rate will vary with storage temperature and the remaining capacity. The following table is a general guide as to when stored batteries will need to be recharged:

Storage Temperature Shelf Life/Recharge Interval
32-68°F (0-20°C) 12 months
70-86°F (21-30°C) 9 months
86-104°F (31-40°C) 5 months
106-122°F (41-50°C) 2 months

If batteries have been neglected, by being stored at high temperatures and not maintaining their charge, then it is recommended that new batteries be purchased for deployment in the lighting system.

How to charge batteries

Urban Solar recommends that batteries be properly stored at room temperature or lower and not be stored for more than four to five months prior to deployment. Under these conditions, the batteries will be ready for deployment and charging the batteries individually will not be necessary.

If charging is required, there are a couple of options:

  1. Purchase an approved battery charger and individually charge all batteries prior to deployment.
  2. Allow the solar panels to recharge the batteries.
  3. Purchase new batteries.

A safe battery charger with overcharge protection will likely cost about $200, and will only charge one or perhaps two to three batteries at a time. Given that it can take six to eight hours, depending on the charger and SOC of the batteries, this could take considerable time.

Allowing the solar panels to recharge the batteries is a good choice if deploying systems in the summer, but will take considerably longer in winter. If this method is chosen, the energy control module (ECM) will not turn on the LEDs until the batteries are charged to approximately 12.3V or 65% SOC.  This could take just one or two days in summer, but a week or more in the winter depending on SOC and weather conditions.

The best policy is to store batteries in a cool dry place get them into service as soon as possible.

Solar charging

The charging voltage, and total amount of battery recharging available from the panels can vary significantly over the year, so typical systems are designed for the worst case scenario – i.e. winter. Battery charging voltage also changes with temperature, which makes the need for having a temperature compensating charge controller to help preserve battery life. The charge controller uses two and three-step charging methods, which initially deliver maximum current to the batteries until they reach around 80% charge and then switch to a constant voltage or trickle charge to protect the batteries from overcharging.

It is vital that the system be designed to have, at worst, a neutral energy balance during the most demanding time of year. Typically this will be in the middle of winter when the energy available from solar charging (Ein) is at a minimum while the system lighting on-time demand (Eout) is highest. To achieve this balance, solar lighting systems need some kind of smart energy control management to optimize lighting performance with a fixed solar array size and battery bank capacity. Urban Solar uses an Energy Control Module (ECM), which, in addition to controlling the charging and monitoring the health of the batteries, can be programmed for an optimal operating profile based on customer requirements for lighting intensity and duration, system location, and overall limitations on system size. With smart energy control management, and a properly designed system, the batteries should last five years before replacement becomes necessary.

(Reference: Enersys)


Batteries are the fuel tank for your solar powered LED lighting system. They are the first component in a solar system that will reach end of life.

In a properly designed system, with correct temperature rated batteries and a smart solar controller, users can expect to receive three to five years of reliable battery performance. Understanding battery technology and design, specific to a stand-alone solar system, will result in a reliable and positive solar experience.