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Celebrating the independent kiwi spirit of invention.


Solar PV: Battery Types

By Ian Mander, 2 November 2019, updated 7 January 2020

 

Background

When I was at Jaycar buying the solar panel I was also offered a deal on a 100 Ah SLA battery and battery box. I declined.

SLA batteries are useful, but they are not necessarily the best battery for what I want. I was also unsure if I wanted something with that large a capacity, and it's a big heavy battery – over 28 kg!

What are the battery options for a solar PV system? All the options listed below need to be kept cool. The cycle life of all these chemistries suffer when hot.

 

Nickel metal hydride (NiMH)

I'll get this one out of the way. For the solar storage capacity needed it's not realistic to use NiMH. Large capacity NiMH cells are simply not made any more, deferring to Li-ion and LiFePO4 (both listed below).

 

Sealed lead acid (SLA)

There are three main types of SLA, also known as valve-regulated lead acid (VRLA).

The sealed valve-regulated wet cell is a flooded lead acid with valves to allow the release of hydrogen gas. The gel cell (or Gelcell, a brand name) is similar but uses silica dust to turn the electrolyte into a putty, or gel. Both these types cannot be charged to their full potential, in order to avoid excess hydrogen production.

Absorbent glass mat (AGM) is a variation on VRLA to make them more physically rugged. They have the same charging regime as flooded lead acid, which means they are much more tolerant of over charging. Whether they are better at deep cycling depends on how the plates are made – as for any other lead acid type, they can be made for deep cycle or starting. If it has a rating for CCA (cold cranking amps) then it's a starting battery and inappropriate for solar storage.

SLA Pros SLA Cons
Relatively inexpensive purchase price for a given capacity – eg, 18Ah SLA, 18Ah AGM (both sold out). Regular discharging of more than around 50% capacity will greatly affect cycle life.
 

Short cycle life compared to other options.

Cycle life: "Depending on the depth of discharge and operating temperature, the sealed lead-acid provides 200 to 300 discharge/charge cycles."

It may be less than 100 cycles if treated roughly and deeply discharged often, or 500 or more cycles if treated quite gently.

  If an SLA is left discharged, sulphation of the lead plates will reduce capacity and increase internal resistance.
  High maintenance – a "topping" charge is recommended every two weeks or so to reduce sulphation.
High current output, so can be used to start a car.

Does not hold a high enough voltage to charge a DJI drone battery unless it's connected to a solar panel.

  Elevated temperature reduces cycle life. Every 8 °C over 25 °C cuts the battery life in half.
 

Strong Peukert effect, where the energy delivered reduces as the current increases. Smaller effect for AGM, moderate for gel, largest effect for flooded lead acid.

 

Heavy.

A 100 Ah AGM SLA is 28.4 kg (note that there is nothing visible on the battery or in the datasheet to confirm it is AGM as claimed).

These disadvantages – particularly the short cycle life which reduces even more for deep discharges – make SLA very unattractive for either solar PV storage or portable power pack (multi-function jump starter) replacement.

Lead crystal is a variation of AGM. Its plates are made from a lead calcium selenium alloy, and has a silicon dioxide (SiO2) electrolyte which goes crystalline at a certain point in its charge/discharge cycle. It is roughly twice as expensive as other types of SLA but is claimed to have a much greater cycle life.

Lead Crystal Pros Lead Crystal Cons

Lead crystal claims 1600 cycles at 80% depth of discharge – similar to LiFePO4 – and 6000 cycles at 20% depth of discharge.

From the specs the sweet spot for total Wh during its life is 40% to 60% depth of discharge, with 20% to 80% not too bad.

Roughly twice as expensive as normal sealed lead acid.

Lead crystal is a little less expensive than LiFePO4. No balance charging or protection circuitry inside like LiFePO4 has.
Much more resistant to sulphation, so can be discharged to 0 V and "fully recover". Test results here showing life to 50% was not much affected after 6 months two-daily cycling to zero volts (except a period of six weeks where it was left at zero volts). Deep discharges still reduce battery cycle life.
Even with full discharges to 0 V it's still rated for 628 cycles, which likely works out to just cents per cycle. That many cycles requires optimum charging.
High current output, so can be used to start a car. Does not hold a high enough voltage to charge a DJI drone battery unless it's connected to a solar panel.
Better temperature range than normal SLA. Heavy (just like any other SLA). For example, 100 Ah "EV" model is 34 kg, the average weight of a ten and a half year old boy.
Can be left two years without top-up charge. Must be changed at 0.3C to push moisture out of plates back into electrolyte. If not charged with enough current, performance will eventually deteriorate and will need waking up.
22 Ah battery available in roughly the same size as 18 Ah and I was able to fit one in my jump starter/portable power pack without modification.

This may simply be a mistake in the datasheet. The product photo in each of the two datasheets show they are a different height.

170 mm is slightly taller than standard 18 Ah SLA batteries.

These advantages make it suitable for portable power pack replacement, but the heavy weight, lack of cell balancing, and the need to charge it quickly (at least sometimes) all make it less desirable for solar PV storage while camping, or in an RV, but could still be suitable for a home setup.

With the short circuit current of my 200 W solar panel rated at about 12 A, the greatest capacity lead crystal battery I'd be able to charge fast enough to keep it in good condition is 40 Ah. A 100 Ah lead crystal battery would need to be charged at 30 A.

Also see Lead acid battery characteristics on the solar charge controller page.

 

Lithium ion (Li-ion) and lithium ion polymer

There are several different chemistries of Li-ion battery. They each have their own benefits, and generally trade energy density (think capacity) off against power density (in practice, how much current they can deliver).

Lithium ion polymer (sometimes abbreviated as Li-po) is a variation which can provide much higher current. This is the sort of battery used in drones and cellphones because it can be made in any shape, not just cylindrical, making it very convenient. This makes Li-po the go-to battery for a huge range of applications these days.

Li-ion batteries last longest when operated between 30 and 80 percent charge. They should be stored at about 50% charge.

Each Li-ion cell is about 3.7 V, so the closest replacement for a "12 volt " SLA is a three cell Li-ion, at 11.1 V. It must be fitted with battery protection for over current (in or out), over charge, over discharge, and temperature.

Li-ion Pros Li-ion Cons

Cheaper than LiFePO4.

A 30 Ah Li-ion battery is $253 (without protection), about the same as an 18 Ah LiFePO4 battery.

Quite expensive purchase price relative to SLA.

A 40 Ah Li-ion battery is $330 (without protection), about the same as a 100 Ah SLA battery.

Can deliver very high currents.

A 30 Ah Li-ion battery can in theory deliver 600 A continuous. (Each 5 Ah battery pack used in it is rated at 20C.)

May explode if roughly treated, or shorted, or charged too quickly, or over charged, or over discharged.

Better cycle life than SLA for the same depth of discharge.

This chemistry is the least affected by depth of discharge, meaning a larger-than-needed battery can be avoided.

No better than half the cycle life of LiFePO4.

 

Elevated temperature hastens permanent capacity loss.

Very light weight.

A 40 Ah Li-ion battery only weighs 3.5 kg (protection circuit, and protective box not included).

Li-ion is the hobbiest option. "Some assembly required" since it would have to be made up from smaller batteries and would need a protection circuit.

 

Lithium iron phosphate (LiFePO4)

LiFePO4 is a safer variation of Li-ion with a lower voltage and energy density. Its lower voltage means four LiFePO4 cells in series at a total 12.8 V working voltage are an ideal replacement for a six cell "12 volt" SLA battery.

LiFePO4 Pros LiFePO4 Cons

Safe and physically rugged. Will not explode like Li-ion can (and sometimes does).

Expensive purchase price – three times that of SLA, or about 1.5 times that of Li-ion.

Long cycle life – extremely long if shallow depth of discharge is used.

Compared to Li-ion, cycle life is twice as long at 100% depth of discharge, three times as long at 40%, 4.5x as long at 20%, 2.5x as long at 10%.

Cycle life more affected by depth of discharge than Li-ion.
Less than half the weight of SLA. Not even close to the light weight of Li-ion pouch cells (ie, without a hard protective shell).
For a given capacity, standard charge rate is twice that of SLA.  
Increased tolerance to modest overcharging compared to other lithium battery chemistries. Higher self discharge than other lithium battery chemistries which can lead to balancing problems in later life. This is avoided by balance charging.
Replacement batteries can be bought with protection and balancing circuits built in, in the same physical sizes as SLA batteries. For example, this 18 Ah LiFePO4 battery.   No clear (or affordable) options with built-in protection between 25 Ah and 100 Ah (other than buying multiple 18 or 25 Ah batteries, but those sizes are the most expensive per Ah).
Some LiFePO4 batteries come with Bluetooth and you can use a cellphone app to check its voltage, state of charge and temperature. These ones are crazy expensive.
Holds a high enough voltage to be able to recharge DJI drone batteries. The DJI car charger will not function under 13.0-13.5 V. Cannot be used to jump start cars, which means they are not really suited for replacing the battery in a portable jump start power pack.
  A small memory effect has been detected. This will not ever be a problem in a solar PV system but is a potential problem (no pun intended) for electric cars because the discharge voltage is very flat, and so, as this article explains: when the state of charge is determined from the voltage a large error can be caused by a small deviation in the voltage.

The advantages of LiFePO4 make it a clear winner for a new solar PV system, but not for a portable power pack unless giving up jump start capability. The initial purchase price is a definite financial commitment. Is the greater cost up front worthwhile for the extra cycles that LiFePO4 offers when they are slowly but steadily and stepwise dropping in price?

 



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