How to Compare Solar Batteries

(This article has been reproduced from OffGridDIY)

Alternative energy, such as solar power often needs to be stored in batteries because the power is generated at times when we don’t need it.  The same is true to some extent for wind power as well as hydro.

The most cost effective, practical way of storing electrical energy at this time for home use is using lead acid batteries.  But unfortunately, while the basic chemistry remains the same, lead acid batteries are available in a very wide range of models and prices.  Some batteries can literally cost three or four times for the same basic rating.

How does one compare these seemingly similar batteries at such different prices?  Or for that matter, batteries with widely different prices and specifications?

Without writing a novel on the various different technologies employed in modern lead acid batteries, one can sum up the basic differences by simultaneously considering two technical parameters and comparing that to the price.

Energy Storage Capacity

The function of a battery is to store energy.  How much energy it can store is obviously an important factor to consider when comparing differently priced batteries.

All batteries will be rated in terms of an Ah (Amp hour) rating.  This is a somewhat less than useful measure, even though it is widely adopted, because you can’t directly compare all batteries using this figure alone.

You need to know what the battery nominal voltage is.  A 100Ah 2V battery does not store as much energy as a 100Ah 12V battery.  So, unfortunately you need to do a bit of maths in your head.  Its simple really – just multiply the Ah figure with the nominal volts to get an energy figure in Wh (Watt hours).  So a 100Ah 2V battery stores at about 200Wh, while a 100Ah 12V battery stores at about 1200Wh.

Design Life in Cycles

The other important thing to know about any battery is how many cycles it can endure at a given depth of discharge.   This information is not always readily available, but the better battery manufacturers sometimes publish a chart like the one below:

tdcycles

So from the above chart, if you routinely discharge the battery to 50%, then it should last about 400 cycles, while if you only discharge it to a depth of 30%, then it will last about 800 cycles.

This information will often explain why one battery costs a lot more than another, when they both have the same energy storage rating.

Obviously these charts are plotted using ideal conditions for each battery.  Battery temperature is one of the most important variables affecting their performance.  Most batteries need to be kept nice and cool (20C is often quoted).  Again, the more expensive batteries can sometimes perform well over a much wider range of temperatures.

Defining a Battery Cost/Performance Index

So if we combine these two crucial battery attributes, we are in a position to compare apples with apples (at least to a basic extent).

To do that, I suggest using this formula:

BCPI = Battery Cost Performance Index (you saw it first here)

BCPI = Battery price x 1000 / (Ah x Volts x Depth of Discharge % x Cycles at that DOD)

This will give a figure in Rand per kWh Cycles which you can compare directly between batteries.  It should show which battery is effectively the cheapest over the long run (provided you can believe the published specifications).  We would recommend that you choose the same depth of discharge (DOD) when comparing different batteries.  50% DOD is probably a good starting point to compare different batteries because that is a reasonably deep level of discharge, one should not routinely go deeper than that with most deep cycle batteries.

An example of using this metric would be as follows:

If we use the battery whose cycle chart is shown above, its price, at the time of writing this article, is R2700 excl.

Its a 150 Ah, 12V battery and from the chart above, you can see it should last about 400 cycles at 50% DOD.

So the BCPI = 2700 x 1000 /(150 x 12 x 0.5 x 400 ) = R7.70/kWh Cycles

If we compare that to the specs of the OmniPower 120Ah listed on OffGridDIY:

Price R2352.05 (at time of writing), 120Ah 12V – if we consult its data sheet it shows 1750 Cycles at 50% DOD.  So in this case the BCPI would be:

BCPI = 2352.05 x 1000 /(120 x 12 x 0.5 x 1750) = R1.87/kWh Cycles

 

We Can Now Compare Differently Priced, and Sized Batteries

Provided you use the same depth of discharge data (for example 50%) you can now compare different batteries.  A battery with a lower BCPI should be cheaper to own over the long run, because it costs less per “kWh Cycle”, or to state the same thing in a different way, it offers more “kWh cycles” per Rand.

So maybe using this (somewhat simplistic) metric, it might now become a bit easier to make sense of the vast array of battery options out there?

The problem comes with the lower specification, cheaper batteries which typically do not have reliable published depth of discharge vs cycle life data.  Without that information, we can’t really compare.  It seems that in those cases, its back to the old premise of “you pays your money and you takes your chances”.

We Can Also Use this to Estimate the Optimum DOD for a Given Battery

By calculating the BCPI at different depths of discharge for the same battery, you can use this index to try and estimate the most cost effective depth of discharge to choose for that particular battery.

omnipower_dodcurve For example, from the specs of the OmniPower 120Ah listed on OffGridDIY, we see that the estimated number of cycles at 30% DOD is about 2750, at 40% DOD 2200 and at 50% DOD 1750.

So if we compare BCPI figures for the same battery at those different depths of discharge then we get:

BCPI(@30% DOD) = 2352.05 x 1000 / (120 x 12 x 0.3 x 2750) = R1.98/kWh Cycles

BCPI(40% DOD) = 2352.05 x 1000  / (120 x 12 x 0.4 x 2200)  = R1.86/kWh Cycles

BCPI(50% DOD) = 2352.05  x  1000 / (120 x 12 x 0.5 x  1750)  = R1.87/kWh Cycles.

So from the above it would seem that the most cost effective depth of discharge for this battery would be about 40-50%.

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