Off-Grid Battery Bank

The off-grid battery bank – the heart of the utility free home

Due to the fact that our domestic power consumption is peaky and most of the sources of renewable power are intermittent and uncontrollable, power must be stored to balance these conflicting concepts. Virtually the only way to store power is in the form of an off-grid battery bank.

There are several different types of battery storage available, but because this is probably the most expensive component in the utility free home, it’s very important that the right choice is made.

The traditional and almost universal method of storing power is in a Lead acid battery. Even with the few recent developments in lead acid battery technology (and please read the rest of this text before spending extra money on them), this power storage technology has remained fundamentally the same for the past 150 years.

There is however a fundamental paradox with using Lead Acid battery technology for an off-grid battery bank:- renewable energy sources are inherently unpredictable, but to keep a lead acid battery healthy, it needs to be fully charged. How then do you decide how much charge to put into a lead acid battery today when you don’t know how much renewable charge you’re going to get tomorrow? If you fully charge your lead acid batteries today (via the grid or your generator) there will be no spare capacity tomorrow if it turns out to be a sunny and windy day. If you don’t put enough charge into your lead acid battery today and tomorrow has little sun and little wind, then you’re going to be short of power and your batteries will suffer.
There are complicated control systems that can reduce the effects of this paradox, but the problem still remains:- you need your off-grid battery bank to be discharged so that they have capacity to be charged by your renewable sources, but this is going to damage/age lead acid batteries.
What the renewable energy, utility free home needs is a battery technology that is equally happy at zero charge as it is at 100% charge.

Introducing the Lithium Iron Phosphate Off-Grid Battery Bank

LiFePo4 off-grid battery bank

24v 150Ah LiFePo4 battery

Lithium Iron Phosphate batteries are fundamentally the same as the lithium Ion batteries within all our mobile phones, laptops and most battery powered gadgets we use today, but Lithium Iron Phosphate has some very significant advantages.

LiFePo4 off-grid battery bank technology

Power and stability within the chemistry

Here’s a list of all the advantages of LiFePo4 batteries:

  • Safe technology — will not catch fire or explode with overcharge
  • Over 2000 discharge cycles life compared to typically around 300 for lead acid
  • Double the usable capacity of similar amp hour lead acid batteries
  • Virtually flat discharge curve means maximum power available until fully discharged (no “voltage sag” as with lead acid batteries)
  • High discharge rate capability, 10C continuous, 20C pulse discharge
  • Has a far simpler re-charge algorithm. LiFePo4 batteries are perfectly happy with a single stage constant voltage charge. This means your charger will be more reliable, cheaper and less complicated.
  • Unlike lead acid batteries, can be left in a partially discharged state for extended periods without causing permanent damage
  • Extremely low self discharge rate (unlike lead acid which will go flat quite quickly if left sitting for long periods)
  • Does not suffer from “thermal runaway”
  • Can be used safely in high ambient temperatures of up to 60C without any degradation in performance
  • Maintenance free for the life of the battery
  • Can be operated in any orientation
  • Does not contain any toxic heavy metals such as lead, cadmium, nor any corrosive acids or alkalis thus making LiFePO4 batteries the most environmentally friendly battery chemistry available
  • LiFePO4 cells are of solid construction — there are no fragile/brittle plates made of lead which can be prone to failure over time as a result of vibration
  • Can be safely rapidly recharged — when fully discharged can be brought to a state of over 90% fully charged in 15 minutes
off-grid battery bank comparison

Energy Density Comparison

Sizing your Off-Grid Battery Bank

Even calculating the size of your off-grid battery bank is easier with Lithium Iron Phosphate Batteries. The steps for lead-acid batteries is as follows:-

  1. Decide on the output voltage of your off0grid battery bank system
  2. Decide on the kWhr capacity required. The average UK home consumes 12kWhrs every day.
  3. Divide that figure by the voltage to get the required Ah capacity (e.g. at 24v, this would be 500Ah for the average UK home).
  4. Decide how far you expect to discharge the batteries [N.B. the greater the discharge percentage, the sooner they will need replacing].
  5. If, for example you are willing to discharge down to 70%, the calculation is the required Ah (e.g. 500Ah) divided by 1-0.7 = 1667Ah.
  6. Select a model of battery (with the correct voltage, or multiples of a battery model that when wired in series will equal the system voltage) based on price/size that when wired in parallel will provide the calculated Ah capacity.

Luckily, there are battery bank size calculators on the internet.

The steps for LiFePo4 batteries is:-

  1. Decide on the output voltage of your off0grid battery bank system
  2. Decide on the kWhr capacity required. The average UK home consumes 12kWhrs every day.
  3. Divide that figure by the voltage to get the required Ah capacity (e.g. at 24v, this would be 500Ah for the average UK home).
  4. Select a model of battery (with the correct voltage, or multiples of a battery model that when wired in series will equal the system voltage) based on price/size that when wired in parallel will provide the calculated Ah capacity.

What’s the bottom line

Lithium Iron Phosphate batteries are unquestionably more expensive than Lead-Acid batteries, but they are still better value for the following reasons:-

  1. With a little care whilst designing your off-grid power system, the expensive inverter-charger will no longer be required. You will be absorbing more of the generation capacity of your renewable sources and if your backup generator is DC (see our DC Battery Charger page) instead of AC, the inverter (which can cost as much as your batteries) is not required.
  2. Lead-Acid charge/discharge cycle is only 80% efficient (and it degrades with age). This means that 20% of your generator fuel and 20% of your renewable energy is lost. The Lithium Iron Phosphate charge/discharge cycle is 97% efficient.
  3. The capacity you need is the capacity you buy. You don’t need 70% more capacity just because the battery technology can’t cope with being discharged.
  4. Unless you spend a lot of money on lead-acid batteries, you are not going to be able to absorb all the renewable power that is generated because you can’t leave lead-acid batteries discharged for long waiting for renewable charge. Lithium Iron Phosphate batteries can be left indefinitely at a very low level of charge, ensuring that there is adequate capacity to absorb renewable power.
  5. With a lifetime number of charge/discharge cycles compared with 300 for some lead-acid batteries, the lifespan of the Lithium Iron Phosphate battery is six times longer than lead-acid batteries.

Lithium Iron Phosphate Batteries available from UFL

off-grid battery bank model choice

off-grid battery bank cells

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