Advantages of lithium batteries over lead acid (LA) battery

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1. Weight

Lithium batteries typically weigh 50% or less than comparable LA batteries. 

2. Capacity

You want maximum life from your battery bank. For longest life (greatest number of charge/discharge cycles) a LA battery should not be regularly discharged past 50% depth-of-discharge (DOD). Lithium batteries can typically use the full capacity of the battery, therefore you need 1/2 the amp hour rating for the same utilization. 

3. Number of charge/discharge cycles

Depending how the batteries are treated, Lithium batteries typically have a cycle life up to 10x more than LA batteries. Many Lithium batteries can go 6,000 cycles at 80% DOD in the manufacturers' spec sheet.

4. Flat discharge voltage curve

Lithium batteries have a very flat discharge curve at a higher voltage than LA batteries, maintaining a terminal voltage of over 13V. Lithium batteries exhibit this flat charge/discharge curve with a fast and steep rise (charging knee) at the upper end and a similar fast and sharp drop (discharge knee) at the lower end. 

5. Charge efficiency

Charge efficiency is defined by looking at how much energy must be returned to the battery after energy has been removed from the battery, to return the battery to the original SOC (State of Charge).

Lithium batteries are very close to being 100% efficient, meaning if you remove 100Ah from a fully charged Lithium battery, putting 100Ah back will restore it back to a fully charged state.

In contrast, LA battery efficiencies vary from 70% (flooded) to 90% (AGM/Gel), which will allow an extra 10-30% energy to be returned to the battery. As LA batteries age and sulfate this efficiency will drop resulting in more energy being needed to charge the battery.

6. Charge rate

LA batteries will accept their maximum charging current when they are deeply discharged. As they charge, the charging current is maximized until 80% State-Of-Charge (SOC) and then tails off exponentially, so the final 20% charge takes much longer. LA batteries can accept a charge current of about 25% of the battery/bank size. Lithium batteries charge much faster, typically at what the charge source can put out. They will continue to suck the maximum current from the available charge source until they are virtually at 100% charged. 

7. Physical orientation, fumes

Flooded LA batteries must be installed with the water ports facing upwards to prevent the electrolyte from running out of the battery. They also release hydrogen gas when charging in the absorb phase limiting where they can be mounted in a vessel. Sealed Lead Acid (SLA) (Gel or AGM) can be mounted on their sides, however in the event of overcharging conditions the valves will release the battery over pressure gasses and should still not be mounted in living spaces. LA batteries can experience thermal runaway whereby they start heating up, lowering their internal resistance, drawing more charge current, heating up further - a vicious cycle that can end with the battery exploding. Lithium batteries have no constraints as to their physical mounting orientation and do not release gasses. They may swell under abnormal charging conditions but should not burst, explode or catch fire.

8. Storage

LA batteries should be stored at 100% SOC. They will self-discharge at a rate of 4-6% per month. Left in a partially discharged state they will start to sulfate (a buildup of hard lead sulphate crystals) and this process cannot be reversed during charging resulting in a gradual loss of battery capacity. In long periods of storage, they need periodic charging. Lithium batteries do not like to be kept at 100% SOC. Before storage they should be dropped to around 70% SOC. Lithiums too will self-discharge, but only at 2-3% and do not suffer from sulfation. As long as they do not get completely discharged, they will be good.

9. Cost

When investigating lithium batteries for an upgrade, the first reaction is sticker shock. Yes, lithium batteries are expensive. The good news is they have come down dramatically from just a few years ago.