Lead-carbon batteries have been developed from traditional lead-acid batteries to provide better deep cycling ability and reduce sulphation of the negative plates when used in partial states of charge.  The first lead-carbon batteries were developed by the CSIRO (Govt. scientific research facility) in Australia in the mid-2000’s and commercialised around 10 years ago, first known as the “Ultra battery” and subsequently developed further by several lead battery manufacturers.  Narada have been producing the REXC series of lead-carbon batteries since 2013.  See the Technical section on this website for a detailed explanation of lead carbon technology.

The negative plate of a lead-carbon battery contains carbon. Different techniques are used by different lead-carbon battery manufactures to create this mix of lead and carbon on the negative plate. Some wrap the negative plate in a carbon film, others mix the lead and carbon in a matriculates, and others use proprietary nano-carbon particles.

The use of highly conductive carbon on the negative plate gives the lead-carbon battery the characteristics of a “super capacitor” that can more rapidly absorb charge current, and more rapidly deliver current under load than traditional lead-acid batteries.  This means they can be recharged faster and can deliver higher peak load currents without damage compared to other lead-acid battery types.

There are a few key factors that will determine how long your battery bank will last. These are

• how deeply you regularly discharge the batteries (“depth of discharge” or DoD)
• whether you have a stable temperature environment
• if you can get them fully recharged regularly

If you discharge the batteries on average to 50% DoD once a day, have them inside in a stable temperature that ranges from 15 to 25 °C, and have enough solar panels to recharge them completely when you get a full sunny day, then you can expect 10 to 12 years life.  If you discharge less than this you can get a lifetime of 15 years or more. On our Technical page there is a chart of cycle life vs depth of discharge which gives you an idea of how many times you can cycle the batteries for a given DoD. Lead carbon batteries have cycle counts for a given DoD that are 3 or 5 times that of typical flooded lead-acid batters or GEL/AGM batteries.

High temperatures are a problem for all battery types. Ambient temperatures over 30 °C will cause corrosion of the positive battery plate internally and can lead to battery failure within a few years.  If your battery bank is kept in an outside shed ensure the building is not subject to direct hot summer sun that could heat it up internally.

Although lead-carbon batteries can handle being used in a partial state of charge for extended periods, they do need to be fully charged from time to time to keep their full operating capacity.  Over winter it is very easy to end up with your battery bank in a permanent state of 50% discharge or worse.  You need to fully charge the batteries at least once per month. This can be achieved with a generator, or more economically these days by having more solar panels than you think you might need, so that in winter you can generate enough current to fully charge batteries on a sunny day.  You may end up with excess solar capacity in summer, but most modern controllers can simply ignore this excess capacity without causing harm – but do check your own controllers specifications.

In most cases the answer is yes.  As long as your solar charger settings can be changed to the slightly lower settings needed for lead-carbon batteries then they can easily replace your old battery bank.  You will have the bonus of more usable energy in the new battery bank because you can discharge the lead-carbon batteries more deeply without damage compared to traditional lead-acid batteries.  Most older lead-acid battery manufactures recommend that their batteries be discharged by no more than 30% to achieve a useful life of 5 years. The same size lead-carbon battery bank can be discharged to 50% and achieve a useful life of 10 years plus.

Narada lead-carbon batteries are sealed valve-regulated lead-acid (VRLA) type batteries.  This means the electrolyte that moves energy between the positive and negative battery plates does not need “topping up” with water as with flood battery types. It is held between the plates in a thick gel paste and the battery itself is sealed apart from a safety valve which only releases gas if the battery overheats or is seriously overcharged. Narada lead-carbons can be mounted in any orientation – on their side or upside down, and have the added advantage that they can be transported easily and safely as they are not classed as dangerous goods.

In normal daily use no gas is given off from the Narada REXC series batteries.  They do not give off flammable hydrogen gas like flooded lead-acid batteries unless they are severely overcharged, overheated or damaged.  They can be safely used inside a building, and this also helps keep them at the optimum operating temperature of 15 to 25 degrees C. There is a New Zealand standard – AS/NZS 5139 – that gives details on how to install a battery bank inside a building.  This includes not installing in a habitable room such as a living room or bedroom.  Installation in a garage, utility room or laundry is fine as long as the guidelines are followed. See the Technical section on Installation for more details.

Different battery manufacturers rate their batteries in different ways, but usually the capacity is given in amp-hours (Ah) over a certain time of discharge.  The C10 Ah rating means the capacity when the battery is fully discharged over a period of 10 hours.  The C100 Ah rating means the battery is discharged fully over 100 hours. Due to the nature of the chemical reactions in a battery, a slower discharge will allow the battery to give more energy than a fast discharge. So C100 or C120 ratings are always higher than C10 or C20 ratings. A C100 rating is generally more applicable to solar setups since batteries for off-grid use are not usually discharged in 10 or 20 hours, but more likely over 2 to 4 days if the weather is cloudy. So the C100 or C120 rating is generally a good indication of battery capacity.  But remember that most batteries are not designed to be 100% discharged, so the useful capacity is less than the total capacity.  Narada lead-carbon batteries are designed to be discharged more deeply than other lead-acid types, and a warranted for 5 years at a 65% depth of discharge.  See here for more information on depth of discharge (DoD).

In the previous FAQ we discussed Ah capacity ratings over different depth of discharge times.  Batteries in solar systems are unique in how they are used because their use and recharging are subject to the vagaries of the weather and to differing patterns of demand for power.  The lifetime of a battery depends on temperature (temperatures higher than 25 °C shorten life), depth of discharge (deeper discharge shortens life), the charging profile and the time between charges (time spent partially charged shortens life).  There is a standard called IEC 61427 that defines a service life for a deep cycle battery for solar use that attempts to simulate typical solar battery use. It defines the service life as the time at which the battery no longer has at least 80% of its original capacity and the testing is done at elevated temperatures (40 °C) and at a range of state of discharge and cycle times.  When comparing battery capacity and lifetime check that the capacity is done to the IEC 61427 standard if you want to compare them.

Batteries’ capacity is normally calculated at 25 °C. At higher temperatures the capacity of a battery actually increases, but the higher temperature speeds up chemical reactions and leads to corrosion on the battery plates that cannot be reversed and therefore the battery life is shortened. For every 10 °C increase in temperature the speed of the chemical reaction roughly doubles.  A battery operated at 40 °C will likely have less than half the life of one operated at 25 °C for the same usage pattern.  So it is very important to control battery temperature to ensure long life.  Even short periods of the batteries getting hot in summer will shorten their life. Mounting your Narada lead-carbon battery bank in a garage or utility shed that protects the batteries from direct sunlight, or excessive winter cold is ideal.  The batteries do not gas like flooded lead-acid batteries so are safe to have inside.  If kept in a shed or lean-to outside the house or building always put the batteries on the shaded (south) side of the building to protect them from direct sun, and insulate them if possible if they are subject to winter temperatures below 10 deg C.

You never get anywhere near the full rated power of your panels over the entire day even in full sunlight.  A rule of thumb to calculate power output in New Zealand is approximately as follows:

Summer:  5 times the panel rating in kWh
Autumn/spring: 3.5 times the panel rating in kWh
Winter: 2.5 times the panel rating in kWh

For example if you have 5000 watts of solar panels (5kW) then you could produce a maximum of 25kW of energy per day in summer, but only 12.5kWh in winter.

This is based on optimal panel mounting – i.e. north facing and tilted at the optimum angle (around 35 to 40 degrees tilt for NZ depending on how far north/south you are).  If your panels are on a flat roof then you will get considerably less power output per day – as little as half in winter compared to the above figures.

One way to measure battery efficiency is to measure how much energy has to go back into a battery during charging to make up for the energy used during discharge.  Figures given by Trojan, a major battery manufacturer of all battery types, say flooded lead-acids need 107 to 120% as much energy to recharge as they produce during discharge. GEL/AGM type batteries (which include Narada lead-carbon) are somewhat more efficient with 105 to 109%.  Lithium ion are 105 to 115%.  There is quite a range in these efficiencies because the efficiency changes depending on the state of charge.  A battery at a 50% state of charge has a higher efficiency than one that is nearly fully charged.  So over 95% of the energy coming from your solar panels will get into your batteries when they are half charged, but as they get to 90% full the efficiency will drop to nearer 80% and even less as the last bit of energy is squeezed into the batteries.  Because Narada lead-carbon batteries can operate at partial states of charge without sulphating to the extent that flooded or other AGM batteries do, they can generally be operated with higher conversion efficiencies than other battery types, resulting in less wasted energy.  Remember that there are other inefficiencies in your system also – such as the inverter. The inverter will waste between 5 and 15% of the energy it takes from your batteries at low voltage (48, 24 or 12V DC) to convert mains voltage (240V AC).  So when you combine battery efficiency and inverter efficiency you can easily be losing 20% of your solar power compared to usable mains power at the 240V socket.

A large solar battery bank has a lot of stored energy and this can result in huge short circuit currents in the DC low voltage cables if wiring mistakes are made.  A 48V bank of 1000Ah batteries will produce a short circuit current of over 10,000 amps which is enough to instantly melt cables and tools or blow up batteries if they are not protected by appropriate fuses or breakers.  Even a single 2V battery cell has a large short circuit current so take simple precautions when wiring batteries together in series and bolting cables to terminals.  Always use spanners or wrenches with insulated handles so you don’t accidentally touch the other terminal when you are tightening a bolt on the terminal. If you have a metal spanner, wrap the handle in insulation tape to prevent shorts.  Always ensure there is a circuit breaker or heavy duty fuse in the battery cables between the batteries and inverter/charger. These should be good quality items, not cheap TradeMe types which have been known to melt and catch fire at currents way less than their ratings. See this image of a cheap $30 ANL fuse and holder that melted at a constant 70 amps despite its 150 amp rating.