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Solar PV: Battery Types

By Ian Mander, 2 November 2019, updated 7 January 2020, 26-30 March 2021, 21 June 2021, 12 October 2024

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 lives of all these chemistries decrease when hot, and they all have reduced cycles when deeply discharged, but for both these problems some chemistries suffer much worse than others.

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 available any more. Use Li-ion or LiFePO₄ instead (both listed below).

Sealed lead acid (SLA)

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

Sealed valve-regulated wet cell and gel cell.
Absorbent glass mat.
Lead crystal.
Lead carbon.

I've also included some notes on SLA charging and the voltages that various states of charge correspond to.

Sealed valve-regulated wet cell and gel cell

This 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 sealed types cannot be charged to their full potential, in order to avoid excess hydrogen production.

Absorbent glass mat (AGM)

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
Very commonly available, and in a range of standard sizes.
Relatively inexpensive purchase price for a given capacity.	Short cycle life compared to other options, increasing the cost per cycle. 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 not fully recharged after use, or 500 or more cycles if treated quite gently and always fully recharged.
	Regular discharging of more than around 50% capacity will greatly affect cycle life. For reliable and extended long term use even lower depth of discharge is recommended, such as no more than 30%. Basically the shallower the discharge, the less damage occurs and the more cycles the battery will last.
	Higher self discharge than Li-ion or LiFePO₄. This 100 Ah battery (when new) discharges 3% of the total capacity per month.
	If an SLA is left not fully charged, sulphation of the lead plates will reduce capacity and increase internal resistance. This happens faster the lower the state of charge.
	High maintenance – a "topping" charge is recommended every two weeks or so to reduce sulphation. If the battery is old, it will likely need a topping charge more often.
	Relatively low maximum recharge rate, just 0.3C. This means that an SLA will not be able to take advantage of large solar panels to quickly capture available energy on a partly cloudy day.
	Ideal charging can be tricky, as is calculating the state of charge from the voltage. See SLA charging below.
	Low charging efficiency, about 80%. (This means that 20% of the energy fed into it cannot be retrieved, and that a solar panel would need to be 25% bigger to get the same "100%" amount of retrievable energy stored.)
High current output, so can be used to start a car (if new and bigger than about 18 Ah). This 100 Ah battery can output 1500 A for 5 seconds.	Voltage sag on heavy loads, especially if the battery is old. This can trigger low voltage responses (eg, buzzer or cut-off) in some devices before very much work has been done with it (eg, inverter).
	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. Lead acid batteries are typically rated by their C10 capacity, which is the capacity given by discharging at whatever rate will give a 10 hour runtime. The typical discharge current thus needs to be taken into consideration for calculating battery size, especially when aiming to use just 30% to 50% of the capacity. In other words, for this 100 Ah battery, the C10 capacity means discharging the battery for 10 hours at 10 A. The C20 capacity is higher, 20 hours at 5.15 A, giving 103 Ah. The C5 capacity is lower, 5 hours at 16 A, giving 80 Ah. The C1 capacity is lower still, 1 hour at 63 A, giving 63 Ah. Be wary of sellers primarily using the C100 capacity to avoid any significant Peukert effect, especially if there's much improvement over the C10 capacity. Also be aware that web pages sometimes have different values to the datasheet.
	Does not hold a high enough voltage to charge a DJI drone battery unless the SLA is connected to a solar panel (ie, being charged itself). This is annoying and reflects badly on DJI.
	Elevated temperature significantly reduces cycle life. Every 8 °C over 25 °C cuts the battery life in half. (This could double the cost of lifetime ownership in a hot climate.)
	Really heavy. Like, Really 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

This 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 normally (but not always!) more expensive than 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 and 6000 cycles at 20% depth of discharge – fairly similar to LiFePO₄. 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.	Deep discharges still reduce battery cycle life, just not as much as normal SLA and AGM varieties.
Lead crystal is less expensive than LiFePO₄ and in some cases is the same as SLA/AGM. For example, in March 2021 the cheapest 18 Ah lead battery of any type at PB Tech is lead crystal $109, the same price as the cheapest SLA/AGM of that size from TradeMe including shipping. The next cheapest at PB Tech is AGM $140 (compared with 22 Ah lead crystal $142).	Depending on size (and maybe quality of SLA it's being compared to), up to roughly 1.5 times as expensive as normal sealed lead acid. (The price has dropped from about 2 times as expensive as SLA since I first listed this variety.)
Much more resistant to sulphation, so can be discharged to 0 V and "fully recover". Test results here showing capacity 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). This makes lead crystal quite attractive for situations where an SLA will be abused by being over-discharged.	No balance charging or protection circuitry inside like LiFePO₄ has – but it can clearly cope without over-discharge protection.
Even with full discharges to 0 V it's still rated for 628 cycles, which likely works out to just cents per cycle.	To get that many cycles requires optimum charging, which is not quite the same as other SLA battery types.
Can be left two years without top-up charge.	Must be charged 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. For larger lead crystal batteries this could be quite difficult.
High current output, so can be used to start a car.	Still does not hold a high enough voltage to charge a DJI drone battery unless the lead crystal battery is connected to a solar panel and is being charged.
Better temperature range than normal SLA.	Heavy (just like any other SLA). For example, a 100 Ah "EV" model is 34 kg, the average weight of a ten and a half year old boy.
Available in a wide range of sizes from PB Tech, mostly as click and collect. 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.	There may be a mistake in one of the datasheets, because 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 in good condition is 40 Ah. (However, 7.1 A is the most I've actually measured, which works out to a 23.7 Ah lead crystal battery, very close to what I've got.) A 100 Ah lead crystal battery would need to be charged at 30 A – at least 360 W of actual solar panel output.

Lead carbon

Also sometimes called lead foam, this is a newish version of lead acid which is a variation on AGM that uses carbon for the negative electrode, which is said to increase cycle life on deep discharge by reducing sulphation. Depending on brand, I've seen claims from 1100 cycles @ 40% depth of discharge (DOD) – which is not great – to 3500 cycles @ 50% DOD for "supercapacitor" lead carbon batteries – which is similar cycle life to lead crystal and LiFePO₄. Prices also vary, from about the same as SLA or gel cell up to just as expensive as LiFePO₄.

Lead carbon batteries still have many of the disadvantages of SLA, such as being very heavy and needing to be kept cool or they'll lose a significant amount of cycle life. I don't have any test data for relative performance of lead carbon vs lead crystal, so a lot of the available information will simply be sales hype, such as exaggerating the capacity by using the C100 discharge capacity, not the nomal C10 or C20 capacity. The Peukert effect (see the short description above) means using a slow discharge gives an apparent 20% extra capacity, which is a concern is intended use is at higher discharge currents.

Lead carbon has more limited availability and available sizes than lead crystal batteries in New Zealand. Update October 2024: PB Tech now has several lead carbon batteries in its deep cycle battery range. The performance of lead carbon batteries appear to relate directly to how much "carbon" technology has been incorporated, with the best performing ones costing about the same as LiFePO₄. Without any clear cost advantage for the longest lasting lead carbon batteries over LiFePO₄ or lead crystal there's not much going for this battery type.

SLA charging

Giving a lead acid battery an ideal charge is quite tricky. Float charge (trickle charge when full) is 13.4 to 13.8 V. Full charge can be up to 14.7 V at 25 °C, less if it's warmer than that. This higher "topping" voltage reduces sulphation. The battery must not be kept at its topping voltage for more than 48 hours, and should be reduced to its float voltage. This is especially important for sealed systems.

State of charge table:

State of Charge	Sealed or Flooded Lead Acid battery voltage	Gel battery voltage	AGM battery voltage
100%	12.70+	12.85+	12.80+
75%	12.40	12.65	12.60
50%	12.20	12.35	12.30
25%	12.00	12.00	12.00
0%	11.80	11.80	11.80

Alternate charge table (there are many different ones). Resting voltage levels – not under load, not charging, and preferrably in that state for an hour or two before measuring:

100% = 12.73 V	50% = 12.10 V
90% = 12.62 V	40% = 11.96 V
80% = 12.50 V	30% = 11.81 V
70% = 12.37 V	20% = 11.66 V
60% = 12.24 V	10% = 11.51 V

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% 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 for a given amount of energy storage. A 40 Ah Li-ion battery is $330 (without protection), the same price as a 25 Ah LiFePO₄ battery (with protection).	Quite expensive purchase price relative to SLA. A 40 Ah Li-ion battery costs about the same as a 100 Ah SLA battery.
Can deliver very high currents. If each 5 Ah cell used in a Li-ion battery is rated at 20C then a 30 Ah Li-ion battery can in theory deliver 600 A continuous.	May explode if roughly treated, or shorted, or charged too quickly, or over charged, or over discharged. Explosion may result in release of toxic smoke and sometimes flames.
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 for big batteries; "some assembly required" since it would have to be made up from smaller batteries and would need a protection circuit. Care must be taken while charging because of the explosion risk.

Lithium iron phosphate (LiFePO₄)

LiFePO₄ is a much safer variation of Li-ion with a lower voltage and energy density. Its lower voltage than Li-ion 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 – two to three times that of SLA, or about 1.5 times that of Li-ion (although this is dropping).
Long cycle life – extremely long if shallow depth of discharge is used. Compared to Li-ion, cycle life is 2x as long at 100% depth of discharge, 3x 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. Some sellers (and apparently manufacturer datasheets) claim cycle lives which are unsupported by independent test data, such as 3,000 cycles of 100% depth of discharge down to 80% of original capacity. It is questionable if even the manufacturers have tested this sort of claim, because even with accelerated testing it would take at least 10 months to complete 3,000 cycles.
Very consistent voltage throughout discharge.	This can make it hard to calculate state of charge from the voltage.
Less than half the weight of SLA. This 314 Ah LiFePO₄ battery is just 25 kg. More than 4 kWh!	Not even close to the light weight of Li-ion pouch cells (which don't have a hard protective shell).
Maximum charge rate is often 1C – much faster than SLA – and short term burst charging up to 2C. This means larger solar panels can be used to capture available solar energy more quickly. (Solar panels are inexpensive compared to batteries.)
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.
Almost all commercially available LiFePO₄ batteries come with a built-in BMS providing protection and balance charging.
Can provide higher discharge currents than SLA.	Most BMSes limits the maximum discharge current, sometimes to about 2C, usually less, because a high current BMS is expensive.
Batteries are available in the same standard physical sizes as SLA batteries, making SLA replacement easy. For example, a 6.4 volt 4.5 Ah LiFePO₄ battery.
Larger capacities are available in the standard SLA sizes – for example, a normal capacity 12.8 volt 18 Ah LiFePO₄ battery is the same size as this 30 Ah LiFePO₄ battery.	More capacity in the same size doesn't mean the LiFePO₄ battery can cope with all of the same tasks, such as vehicle starting.
LiFePO₄ batteries are available in a variety of voltages (6.4 V, 12.8 V, 25.6 V, 51.2 V), and a good range of capacities: 12 volt LiFePO₄ batteries can be found in 7 Ah, 10 Ah, 12 Ah, 18 Ah, 20 Ah, 25 Ah, 30 Ah, 50 Ah, 100 Ah, 150 Ah, 200 Ah, 230 Ah, 280 Ah, and 314 Ah capacities (and that's just from Jaycar and SETL Energy).
Some LiFePO₄ batteries and BMSes come with Bluetooth so you can use a cellphone app to check its voltage, state of charge and temperature.	Some of the premade Bluetooth-equipped batteries 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.)	Should not be used to jump start cars unless the LiFePO₄ battery is quite large, which means they are not really suited for replacing the battery in a typical portable jump start power pack.
No nickel or cobalt.	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.

Conclusion

The advantages of LiFePO₄ make it a clear winner for a new solar PV system, whether mobile or fixed (such as in a home where weight is not so important). However, the initial purchase price is a definite financial commitment, so it needs to be asked: 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? YES! They last longer, are less fussy about charging and temperature, provide a better voltage under load, don't lose a huge amount of capacity under heavy load, are much lighter, and so on. There are so many benefits the choice is clear.

The only reason I went for a lead crystal battery in my portable power pack was I didn't want to give up jump start capability. The improved life cycle characteristics of lead crystal provided a reasonable trade-off.

There is probably no reason to now buy a normal SLA or AGM battery. (Car starter batteries are not sealed lead acid.) For small batteries such as a portable jumpstarter/power pack where specifically wanting a lead battery, for the same or similar price, a lead crystal battery will last much longer than SLA or AGM, perform much better, and be less hassle, especially with deep discharges. For larger batteries where weight isn't an issue, and there's some particular reason to not buy lithium (there are not many reasons that hold up to inspection), it appears that lead crystal is a winner over lead carbon.

For large solar storage batteries where weight is even slightly an issue, LiFePO₄ is the clear chemistry of choice.