Why do lead acid batteries FAIL

My Aim here is to explain, in Layman’s terms and in a NON technical way :- “why a lead acid battery fails over time and how to calculate the available power”.

Some abbreviations I`ll use

  • LA –  means Lead Acid Battery
  • SOC –  Means State of Charge
  • A/h  – Means Amp Hours
  • Kw – Means Kilowatt Hours
  • V  – means Battery Volts

So let`s Start. As a starting point example let`s say you have 1 x 200 A/h 12V battery, now to make this 200A/h mean something in a easy way let`s convert that 200A/h storage to Kw Hrs of stored power.

(Note I am not taking any min or max battery limits into account in these explanations ,  you will need to use the supplied manufacturers battery specs in the real world)

The reason we do this, is because today most items power usage is measured in Watts or Kw  it`s very rarely measured in amps now.

So to start we need to work out the usable Kw of available power to do this. Calculate Kw Hrs of Power is done by calculating  12v x 200A/h  = 2.4Kw  of stored power of which at most you can only use (50%) or 1.2 Kw Hrs on a new battery. (if you use more than 50% on a continual basis you will badly damage the battery)

Why is this so?  Well to start with in an average LA battery there are plates of lead Phosphate, positive and negative ones. These are separated by an Insulator and there are 9 to 20 of these depending on the battery specs,  (and its important that the two active plates never touch each other). There is also a 10 to 30mm gap at the bottom of every cell in the battery. (see Fig 2 and 3)

Now as the battery ages and is cycled, bits of the active material become inert or fall into this gap, and once the space is full, the battery completely fails due to a short in that cell causing heat and high water usage. (see Fig 2 and 3)

In LA Batteries if you use more than 50% you damage the batteries, So how do you know when the battery is at 50%? Well no matter how old the battery is, it will be at 50% of its current available capacity when it gets to an idle voltage of around 12.1 volts,  a battery is full when the idle voltage is 12.7 Volts. (see Fig 1)

(NOTE:- with no draw and no charge and it has also been sitting for say 1 hr like this, “This is what I mean by idle voltage”.)

(I have also attached a picture at the bottom that illustrates this in more detail and at different voltages as well. (see Fig 1))

As I was saying that having one bank is 1.2 Kw in total USABLE Power means that you can use a1,000 watt appliance for 1.2 hrs.

If you have 2 banks that’s a total of 2.4 hrs for the same appliance at the same power usage.

If you have 3 banks that’s a total of 3.6 hrs for the same appliance at the same power usage.

If it’s a 2000 watt appliance then halve the above times. if it’s a 500 watt appliance  then double the above times.

Now what then happens to an LA battery when you start to cycle it?

In general an LA battery can be cycled 500 times  that’s 100% full down to zero,  but as stated above we must only use a max of 50% so if you take the LA down to 50% this means you can do that 1000 times,  if you only take them down by 25% then you can do that 2000 times and so on.

Battery SOC by Voltage (Lead acid only)
(Fig 1)
(Fig 2)

Now considering the past text, “you may say” my batteries have lasted 6 years but I have only been using 25% all the time. Well that’s great, but if you had half the batteries and used them to 50% they would have lasted 3 years,  you really have not gained anything by only using the 25% mark except carring twice as many batteries as needed and also the  hassle of removing and replacing them every  3 years.

Now this is the theoretical  maximums  we are talking about here.  We are also talking about a properly designed DEEP cycle LA battery. (NOT  A cheap car or truck battery these types of battery’s are only designed to have the first 10% only used of the battery on a regular basis) these batteries have VERY thin Plates and thus the active material fails when cycled quickly.

NOW  lets proceed to explain HOW they lose capacity over time. (NOT why HOW in layman’s terms)

Your existing LA batteries when they were NEW could say run that 1000 watt device for 1.2 hrs,  then they age because of usage, now that they are say 2 years old and done 500 cycles  to 50% on average they would have lost capacity,  I would guess and say 50% loss as you would have been cycled say once every other day or so.

The easy way to explained this is by converting the Kw Hrs or you can also use Amp Hrs as well here, it works the same way, all to become a  water tank of 1000 Ltr’s.

We digress

Things to consider as well,  when you take 100 amps out of a LA Battery are.

  1. If you take out 100amps  you have to put back 125 amps  to fully charge them (you put back 125 amps as there are losses  when charging the lead acid battery) LA are about 75% efficient, and to fill the last 20% this also takes 4 hrs of slow top up charge as well.
  2. Every time a LA cycles some Active Plate material falls off or becomes non active.  Now also at the bottom of the battery there is a 10to 30mm vacant space as this can also fill with active damaged plate material, (See Fig 2 and 3) The space slowly reduces, until the waste material touches the bottom of the plates and then it shortens that cell out, Bingo no more battery, they use water and also get very hot, or the battery goes flat quickly by self discharging.  

*** NOW how to Explain this ***

Lets say you have a 1000 ltr tank of water  (same as 1000 A/Hrs of battery ok)  now you draw out 500 ltr’s (50%)  and you then have to put back 600 ltr’s to fill it, but at the same time one ltr of sand was also added to the tank. (this is the example of lost capacity as active material has been lost as you use the battery) so now you have only 999 ltr’s , so you take another  500 A/h again  and you put back 600 and another one ltr of sand is added now you have 998 ltr’s of water not 1000,   this goes on and on and on and then after 500 cycles. You the average user still think you have 1000 ltr’s in the tank but really the capacity is now 500 ltr’s  as the tank now also has 500 ltrs of sand in it,  (the sand is a example of loss of Active usable plate area inside the battery bank) NOW consider  this you are still taking 500 amps all the time  but you are now using 100%  of the LA battery and it then fails quickly.

This is a rough idea as to what happens and how the LA battery drops in capacity over time and usage and why it fails quickly at the end.

NOW Consider Lithium Phosphate Batteries

  1. If you take out 100amps  you have to put back only 105 amps  (you have to put back 105 amps as there is still a losses  charging the  Lipep04  battery but only about 5%)  they are about 95% efficient
  2. In a Lithium Battery there is basically no loss to speak of

With Lithium however you also start with 1000ltr tank then  take 500  and you then put back 505 ltrs to fill it and there is 0.01 ltr of sand added  so basically there is no loss over time.

Lead acid for every 100 amps you take out you have to put 125 back in to fill it again plus you can only do 500 full cycles, max and the battery is finished.

Lithium LFP for every 100 amps you take out you have to put  105 amps back in plus you can do 2000 cycles down to 95% if need be  (we prefer 10 to 20%) and still have 80% of the original capacity left at the end of what the industry says is there useful life.

Another BIG advantage of LFP is that you can add all that the charges can put out right up to the last 5% this means much less time to charge, THUS less Generator time not only by the 20% loss saving but also because you can put in the FULL charge all the time.

(Fig 3)