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Solar Power

Lithium Ion batteries vs Lead Acid

There are a few tools to compare solar batteries and what they do, their cost effectiveness and efficiency in the real word. There is also a lot of deceptive statistics and a few bogus warranties around.

The most common measurement is DOD (Depth of Discharge). Don’t confuse this with SOC (State of Charge) because it is its opposite. DOD is how much you can use, SOC is how much you have left. Traditional lead acid batteries can be run safely down to 50% DOD, while Lithium Ion can go to 80%. DOD is measured in Kilowatt hours (Volts x Amps/hr) - basically it means the number of kW you can use before it conks out.

So to achieve a similar result in terms of DOD, Lead Acid needs to be 30% larger than Lithium in kW. Another rule of thumb is double the amount of Amp hours.

As far as price goes, for the same amount of Amp hours, Lithium is about 3.5 times more expensive.

For weight, generally not relevant for home use, but highly relevant for electric vehicles, lead acid are about 4 x heavier for the same kW.

Most Ah ratings are calculated at the 20 hour rate. For lead acid, the higher  the load, the more volts required and the lower the level of useable energy. The higher the load then more you need to increase Ahr capacity. But for Lithium this isn’t the case - a load 10 times greater will still have the same volts. So Lithium is much better for higher loads, and the longer this load is on, the more exaggerated the difference between the two chemistries.

Charge rates are important - lead acid can only charge normally up to about 60% SOC before it starts hitting resistance - it gets harder and harder to charge it up to 100%. Higher rate charging does not make it charge any faster. However with Lithium the charge rate is fairly stable up until around 98%. Therefore in both charge and discharge, Lithium is ahead.

For cycle life, there are many different measurements of cycles at various levels of DOD, but if you.consider full charge/ full discharge, then lead acid will give you 400-1500 cycles while Lithium gives 2000-4000. So for longevity, Lithium gives you between 2.5 and 5 times more cycles depending on how the battery is used. Other factors to consider are the time and cost involved with replacement as well as the less efficient use of power to charge lead acid up in the first place and whatever costs that might incur.

There are some other issues around safety - lead acid gives off hydrogen, an explosive gas as well as liquid sulphuric acid that can splash into your eyes and slosh all over the place (but not if you have sealed lead acid or AGM type). Lithium has safeguards built into the battery, but if there was a general fire near the battery there is a possibility it could heat up and explode.  On the balance on safety they are probably about equal.

Lead acid can be taken away and recycled. Lithium batteries can be removed and replaced easier due to their light weight, but at this stage there are not general recycling facilities, at least not near Christchurch, New Zealand.

It might come down to this - first see if you are going to have heavy loads like heating, washing machines, microwaves etc. then see if you prefer to have something really modern with a longer life for a higher price or something cheaper now but maybe more expensive over the longer term.

February 18th 2017