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12 Volt Planet

How long will my battery last?

20 January 2022  |  Paul

A topic that often comes up during our conversations with customers is 'What is the lifetime of my battery?'. This can be a tricky figure to put a solid number on, however we are able to make an estimation as long as we account for differences in battery specification, type and usage.

1. What are the main factors that determine battery lifetime?

There are two essential considerations when looking at battery lifespan, and these are the no. of Rated Cycles and the Depth of Discharge (DoD) usage of the battery.

No. of rated cycles

A 'cycle' is a discharge & recharge of the battery. Most manufactures will specify the rated cycle count at various Depths of Discharge (see below) and this provides you with a rough guide to the number of cycles you would get from a battery before it reached the end of its life, based upon your usage of the battery (i.e. how deeply you discharge it per cycle). Note that lithium batteries tend to have a much greater number of rated cycles compared with lead-acid batteries.

Depth of Discharge

Depth of discharge (aka DoD) is a term referring to how far a battery is depleted as a % of its fully charged state. Generally speaking, the shallower the DoD, i.e. the smaller the amount of energy removed, during each cycle the greater the number of cycles that you'll be able to get out of the battery before it reaches the end of its life. 

So you can see that the no. of cycles and DoD are linked and, based on your expected DoD usage, you can estimate how long your battery will last. Alternatively, you can use the manufacturer's datasheet to determine the best DoD for your battery to maximise its lifetime (this will have limitations as very low DoDs will probably not be practical to adhere to).

2. A note on 'Usable Energy Capacity'

Amp-hours (Ah) is a measure of the energy storage capacity of a battery and can often be found indicated on the specification label or elsewhere on the battery (i.e. 77Ah, 110Ah, 320Ah). Theoretically, it means that a 100Ah battery, for example, could supply the full 100Ah before being completely discharged, however, for lead-acid batteries this comes at a cost because they should be discharged to no more 50% DoD or they will become damaged. Conversely, a lithium battery can be safely discharged to anywhere between 80-100% DoD (depending on the battery) without suffering damage. So, the usable energy capacity (usable Ah) in the battery will be dependent on the Ah rating, safe level of DoD and the battery technology.

For example:

For a 100Ah rated lead-acid battery that should only be discharged to around 50%, the actual usable capacity is around 50Ah (50A supplied for 1 hour, 25A supplied for 2 hours, 1A supplied for 50 hrs etc. etc.)

But for a 100Ah rated lithium battery that can be discharged to, say, 90% DoD, the actual usable capacity is around 90Ah (90A supplied for 1 hour, 45A supplied for 2 hours, 10A supplied for 9 hrs etc. etc.).

This is probably the single most important thing to keep in mind in day-to-day usage of your battery. Exceeding the safe DoD of a lead-acid battery will seriously reduce its lifetime.

3. Comparing lifetimes of different battery technologies

To help understand how lifetimes of various battery technology compares, we have looked at our TN Power AGM Battery - 100Ah & TN Power Lithium (LiFePO4) - 100Ah batteries to calculate the number of usable Ah they would offer over their lifetime, based on a typical DoD and no. of rated cycles provided at that DoD.

  • TN Power AGM Battery - 100Ah:  approx. 1100 cycles at 50% DoD and 50Ah useable capacity per cycle = 1100 x 50 = 55,000Ah usable lifetime capacity
  • TN Power Lithium (LiFePO4) - 100Ah : approx. 2000 cycles at 100% DOD and 100Ah useable capacity per cycle = 2000 x 100 = 200,000Ah usable lifetime capacity

As you can see, the greater number of cycles achieved by the lithium battery makes the total useable lifetime capacity compare very favourably with the AGM (lead-acid) battery, offsetting the higher additional purchase cost. Assuming the same energy usage in your system with both battery types, you would need to replace your AGM battery several times to replicate the lifespan of the lithium battery. So, although lithium batteries are much more expensive to purchase, their lifespan is considerably longer than a lead-acid battery.

Furthermore, lithium batteries are optimally kept in a partially charged state for greatest longevity, so if your lithium battery DoD is only around 50%, the number of rated cycles increases, e.g. approx. 7000 cycles at 50% DoD = 350,000 Ah usable lifetime capacity for our 100Ah lithium battery.

4. How long will my battery last before I have to re-charge it?

Knowing the safe DoD for your battery and the usable Ah capacity is important to allow you to work out how many hours of the run time your battery has before you'll need to recharge it. We will again compare lead-acid & lithium batteries for this which will be our TN Power AGM Battery - 100Ah & TN Power Lithium (LiFePO4) - 100Ah batteries as used before.

As we don't want to cause damage to our AGM we will be keeping to a 50% DoD so we know we now have 50Ah of usable energy. Our lithium battery, however, can be run down 100% so we have a full 100Ah to use with that battery. 

To work out the run time we have with these batteries we will need to know how many Amps each appliance we are using will draw (if an Amp figure is not available but you have a wattage, simply divide this by the voltage to work out the current draw in Amps) & for how many hours we will use them. Below we have selected some products to help with this exercise. To work out the number of Ah used per 24 hr period we use the following equation:

(Number of units being used) x (Current draw in Amps) x (Hours used per 24 hr period) = Total Ah required per 24 hr period

  • 4 x LED Interior Light @ 0.2A each for 6 hours = 4 x 0.2A x 6hrs = 4.8Ah per 24 hrs
  • 1 x USB Power Socket @ 3.1A for 3 hours = 1 x 3.1A x 3hrs = 9.3Ah per 24 hrs
  • 1 x 12 Volt Fridge @  1.73A for 9 hours = 1 x 1.73A x 9hrs = 15.57Ah per 24 hrs

      Total = 29.67Ah per 24 hr period

We now know that we are looking to use 29.67Ah per 24 hrs or 1.24Ah per hour (29.67A/24). We can now take our usable energy per battery and divide this by our energy requirements per hour to get an estimate on how many hours our battery may last. 

  • TN Power AGM Battery - 100Ah : 50Ah (@50% DoD) / 1.24A per hour = 40hrs running time
  • TN Power Lithium (LiFePO4) - 100Ah: 100Ah (@100% DoD) / 1.24A per hour = 80hrs running time

So it can be seen that the AGM battery would last 40 hrs before reaching 50% DoD and needing to be re-charged, whereas the lithium battery would last 80 hrs before reaching 100% DoD and needing to be re-charged.

These figures are based on the batteries not having any charging sources attached during this period (for more information on this we recommend reading our leisure battery guide).

We hope you have found this article helpful however if you have any further questions please do not hesitate to contact our support team.