Right at the outset I am going to warn against the multitude of vendors screaming to replace your alarm battery with Lithium Ion Phosphate Batteries (LifePo4).
This is simply an effort at profit.
KEY TERMS
Load – This is the most critical concept for properly planning your solution. How much current (amps) is consumed by all the devices that you need to keep powered.
Depth Of Discharge (DOD): How much energy is cycled into and out of the battery during each cycle. It is expressed as a percentage of the total capacity of the battery.
Charge Rate: The speed at which a battery is fully charged or discharged measured in C – rates. e.g. Charging at a rate of 1C means your battery will go from 0-100% capacity in 1 Hour.
State of Charge (Soc) : The level of charge of a battery releative to it’s capacity measured in percentage of total capacity.
Battery Life Cycle: The number of charge and discharge cycles that a battery can perform before losing performance.
WHY THESE TERMS ARE IMPORTANT
Load: The current load of all devices will determine how long it will take for your battery to completely discharge without any input charge.
e.g. a 1Amp load will discharge a 7Amp Hour battery in 7 hours. Devices with a 1 Amp current draw will consume 4 Amps from your 7 Amp Hour Battery in 4 Hours.
Depth Of Discharge: To illustrate , lead acid batteries often have a specified life cycle, based on a 50% depth Of Discharge.
The rated life cycle is expected if the lead-acid battery does not discharge below 50% during bouts of power outage.
Where the battery is being discharged more than 50% of it’s capacity before the next charging cycle, it’s life cycle reduces and the battery performance degrades.
State Of Charge. When the Soc of a lead-acid battery is regularly lower than 50% of the battery capacity – it’s life cycle and performance will degrade.
The battery will perform significantly worse than what it is rated for.
Lithium Ion battery manufacturers use a 40/80 rule. That is that the battery should be charged when it reaches 40% Soc and discharged when it reaches 80% Soc. The same recommendations would apply to a LifePo4 battery.
LifePo4 battery life cycle will degrade if the battery is maintained at 100% Soc or below 40% Soc frequently.
Charge Rate: The major problem with effectiveness of batteries to sustain an alarm device, is the rate at which the device is able to charge the battery.
Most security devices have low charge rates. This implies that the battery will need a long period of time to recharge the current that was consumed.
e.g. If the devices charge rate is 0.5A and you consumed 4A during the period of outage, it will take 8 hours to replace the 4 amps that was consumed.
LifePo4 batteries usually have higher C charge rates, so they can charge quicker than lead acid batteries. Four times faster!
From the above it is seen that the battery health is dependant on Soc , and DOD.
Why does your alarm system fail during load-shedding.
Putting all of the above together.
The battery quality degrades due to;
- Inadequate battery capacity. The device load is such that the consumption during the outage exceeds the capacity of the battery, or regurlarly discharges the battery below it’s recommended DoD.
Security providers frequently do not provide a battery of adequate capacity to match the system load.
In other cases, consumers expand their systems with additional devices ,without giving consideration to the increased load effect on the battery.
In some Eskom direct power use cases, a single period of 4 hour outages may be experienced. The battery charger now needs to replenish double the consumed current, in the same period of time.
- The charger capacity of your device is not capable of recharging the consumed current, before the next outage.
Let’s look at some of the suggested remedies proposed by providers.
Let’s provide the Alarm with a higher capacity battery – say a 17 AH battery.
In the first scenario, your battery would be dead after 2 or 3 days of Stage 6 Load-Shedding. After adding a 17AH battery, it will now take about 5 to 6 days of Stage 6 Load shedding to get to the same state – a dead battery.
Now let’s replace the 7AH battery with a 7AH LifePo4 Battery.
All is good , and you now have a battery that should last 5 years, compared to replacing your lead acid battery almost weekly if Stage 6 is sustained.
But – the concern in this scenario is that of using a device charger that is not compatible with the charge/discharge cycle of LiFePO4 batteries. , this will.
- degrade the life cycle of the battery,
- and also maintain the battery at elevated temperatures for extended periods.
Additionally, Since these chargers are not compatible with LiFePO4 battery charge and discharge cycles you will experience more problems;
- The charger will treat a less than fully charged LiFePO4 battery as being fully charged when it reaches 13.8V (Common capacity for lead acid). You will never get the full battery charge capacity.
- The LiFePO4 BMS will cut off the battery at 11.7V . The battery will need to be kickstarted with an external DC source to start charging again. Lead acid chargers will only cut off the battery around 10V.
- You may start getting low battery errors on your system while the LiFePO4 battery is still more than 50% SOC.
- The battery may be damaged by frequent gate openings. These batteries where designed for gradual charge and discharge – not for the frequent charge cycles experienced with frequent driveway gate operaion.
The batteries do have Battery Management Systems that will prevent them from overcharging – but there are many other potential issues when using them with a lead acid charger.
*Some devices draw continuous power from the battery, and the only purpose of the mains power is to charge the battery. Even in these cases, the battery will be held at a high level of SoC for extended periods.
What are the recommended solutions.
Both solutions involve an external charger.
In the case of upgrading to a 17AH Lead Acid Battery – you can safely apply a charge rate of 1.7 A to the battery. This is a 3 times faster charge rate than the device is providing.
The cost of a 17AH Lead Acid Battery and charger, is magnitudes less than the cost of a Lithium Battery and compatible charger. The battery will work at 50% or above DoD for an expected 3 years.
This is where the Load Current becomes important. If the load is higher, and will frequently discharge the lead acid battery below 50% DoD there are two solutions;
A bigger capacity lead acid battery that is rated 4 times the Amp hour of your load.
e.g. 5Amp would need a 20AH battery and a 2 Amp Charger.
Or a LifePo4 battery that is 3 times the capacity of the load.
e.g. 5amp would need a 15AH battery, and a 2Amp Lithium Profile charger will suffice.
What is the benefit
The LifePo4 battery and profiled charger will have a significantly more expensive, initial outlay.
The expected cycle life of the LifePo4 battery will be around 5 years, and that of the lead acid battery around 3 years.
The deal breaker is the cost per watt hour
The cost of a Lead Acid battery and standard profile charger – broken down to cost per watt hour, is 1/3rd that of a LifePo4 Battery and LifePo4 profile charger over the rated life cycle of both batteries.
The lead acid solution gains further advantage after having changed each battery once, assuming that the charger continues to function through the life cycle of two battery deployments.
When you start to deploy batteries in solar solutions, often times the life cycle of these high capacity batteries can reach 10 years. In these cases the cost advantages may shift to LifePo4 – where the charger and inverters required to run the system are the same for both battery types.
This article has focused entirely on grid connected charging systems for security and gate systems – in which case an external lead-acid battery with charger is the recommended, and economical solution.
When offered a battery backup solution – pay attention to the charge rate of the device, many of them offer the same charge rate as your alarm device. If the charge rate is the same at 0.5A or 0.8A – it’s not going to be a viable solution – as demonstrated in this article.
The Sherlo backup systems are well rated to charge the recommended battery sizes, and are recommended.
To investigate available battery and charger solutions for Alarm Systems, CCTV and Gate Motors – follow these links to the online store.
If you prefer to have the solution installed, we will waive the shipping charge – a R395 Ex Vat Call out fee will be charged in the Durban area.
