Choosing A Solar System Battery

Choosing a solar system battery is a key step in building out a solar system. With the way the charge controllers work the system is designed to only work if all batteries are of the same class. You generally cannot mix-and-match gel batteries with LiFePO4 batteries or even other lead-acid batteries like AGM. Thus, once you choose your battery type you are going to be investing in that for the entire journey, or do a costly complete overhaul of the battery array.

When we started our Rally Creek off grid project we decided to go with gel batteries. Several factors were considered, price per amp hour was one consideration, but also the size and weight of the battery. We settled on 100Ah Gel batteries that we could easily move around. Our thought was weight wasn’t the primary concern, so why not save some money on the battery and get more amp-hours per dollar spent?

Turns out that may have been a big mistake.

Bottom Line: Choose a LiFePO4 battery.

Depth Of Discharge – A Key Component Of Usability

One of the key things we learned after diving into the inverter and charge controller books is that any lead-acid battery (sealed lead acid / SLA , gel, or absorbent glass matt/AGM) should only be discharged to 50% of the capacity. After testing in the real world that is somewhat true, however the charge controller and inverter we used does work until the battery reaches 64% depth of discharge. Our guess, that works well with brand spanking new gel batteries, but as these things age it will likely creep up.

LiFEPO batteries, on the other hand, are designed to be discharged to AT LEAST 80% of their capacity. Many people claim they can even be fully depleted without permanent harm to the battery, as long as it doesn’t happen often. Time to dive in deeper before we go “all in” on the gel battery array.

Thankfully we are only 4 gel batteries and $600 in on the investment.

Choosing A Solar System Battery By The Numbers

100ah Gel Battery 12V

We’ve been buying the Weize 100Ah 12V Gel battery for around $150 per unit — but it is now $170/battery on Amazon.

Labelled Capacity: 100 ah = $1.70 / ah
Maximum Usable Capacity: 64 ah = $2.66 / ah
Recommended Usable Capacity: 50 ah = $3.40 / ah

Weight: 60lbs
Size: 12.99 x 6.73 x 8.34 inches
Warranty: 2Y

Temp Range: -4F to 122F (charge and discharge)

100ah LifePO4 Battery

For comparison, let’s look at the Weize TPLI-12100AHd LifePO4 100ah 12v selling for $230.

Labelled Capacity: 100ah = $2.30 / ah
Maximum Usable Capacity: 100 ah = $2.30 / ah
Recommended Usable Capacity: 80 ah = $2.875 / ah

Sidebar: these batteries operate at slightly higher voltage 12.8v vs. 12v, so the proper comparison it to calculate the watt-hours of storage vs. amp-hours for a side-by-side cost-comparison. See the table below for more info.

Weight: 26.4 lbs (less than half that of a gel battery)
Size: 13 x 6.77 x 8.46 in
Warranty: 10Y

Temp Range: -4 F to 140F for discharge, but 32F to 113F for charging
Cycles: 2,000 if 100% discharged – 8,000 if 50% discharged

The Weize batteries shut off for protection below 32 F.

LiFEPO4 More Usable Battery Per Dollar

Based on the simple comparison above, the LiFEPO4 battery is a far better choice. You end up buying less batteries to power your system because you can use the entire battery (at the cost of a shorter 2,000 cycle life span) or 80% of the battery to get closer to the full 10 year covered warranty lifespan.

Granted the battery is nearly 50% more expensive, but you more than 62% additional “usable battery” if you stay within recommendations and 100% more usable battery if you run it to the supported but less life span 100% depletion on a regular basis. The interesting thing is that even with a shorter lifespan with full depletion, it will still outlast the 3-4 years of a Gel battery.

The expected lifespan also needs to be calculated. Assuming you push a gel battery, you might get 5 usable years. A LiFePO4, 10 years.

Temperature Considerations

If you are going to have the batteries in a colder environment, you will need to either choose self-heating LiFePO4 batteries in cold climates or you will be relegated to using gel batteries. The reason is that LiFePO4 batteries cannot charge safely below freezing (32F / 0C) or above 113F. Gel can charge (slowly) at -4F up to 122F.

Using the battery they are nearly identical in cold weather. In hot weather, however, the LiFePO4 can handle an additional 18F, up to 140F during discharge.

Total Storage Considerations

Another thing to consider, that may play a role in decision making is the total amount of energy you need to store in your batteries. This is where lead-based batteries can have an advantage.

Most lead or lithium batteries recommend no more than wiring 4 batteries in series — increasing the output voltage each time, 12 volts for 1 battery, 24v for 2 in series, 36v for 3, and 48v for 4.

Sidebar: Series wiring means connecting the positive of one battery to the negative of the next. First battery positive lead goes to the “source” and last battery negative lead to the source.

However, lithium batteries ALSO recommend wiring no more than 4 batteries in parallel AND if you are doing this, no more than 2 in each set of parallel connections. Some manufacturers do allow up to a 4×4 array and anything in between, one even claims they support a 6×6 array. This is highly dependent on the controllers in each battery, so be sure to check with the manufacturer first!

Siebar: Parallel wiring means connecting the positive of one battery to the positive of the next and connecting the negative of one battery to the negative of the next. The first battery connects the positive and negative to the source.

Gel batteries often have no limitation on how many you can wire in parallel, which theoretically means infinite power storage capacity (though total cable length of the connections and other factors, including the size of the earth, play some role in this).

That means for lithium the maximum storage is usually 4 parallel connections of 2 batteries in series or 8 batteries connected together. Total maximum storage: 8x each battery’s watt-hours is your peak. This means to maximize the array your battery bank nominal voltage will be at most TWICE the voltage of each battery (series wiring increases voltage). Thus starting with a 12.8v battery means would run at most a 25.6v solar system (referred to as a 24v solar system). It also means multiplying the “Usable Wh” in our table below by 8 for lithium batteries as the top limit you can store, so choose your starting point wisely.

Sidebar: If you start with a 12.8v 100Ah LiFePO4 battery, your maximum system storage would be 8,192 Wh (8 kWh) and be a 24v solar system.

For gel batteries you can wire up to 4 in series, which means the maximum voltage of the solar system would be 48v. You can then wire as many of these 48v series in parallel, creating a huge amount of storage.

Sidebar: for charge controllers to work efficiently and to get the maximum use of your batteries you should MUST use the same voltage rating on every battery in the battery bank, the amp-hour rating should be identical, and the batteries should all be of similar age.

Battery Comparisons For Choosing A Solar System Battery

Volts is nominal voltage.

Usable Wh is based on recommended maximum depletion of an in-use battery. 50% for any lead based battery (SLA, Gel, AGM) and 80% for a lithium based battery.

$/Wh/Year is the comparison in cost per usable watt-hour of storage for each year of the life of the battery. Highlighted entry is the initial purchase we made. Percentage is the cost reduction from the highlighted baseline.

Battery	Type	Max Array*	V	Ah	Wh	Usable Wh	Warranty	Cost	$ / Wh	$ / Wh / Year
12v 100Ah	Lead		12	100	1200	600
Weize LP12-100-W	AGM	*P2S : infinite					2y	$155	$0.2583	$0.1292
12v 100Ah	Lithium		12.8	100	1280	1024
Weize TPLI-12100AH	LiFePO4	4P: 4kWh 4S: 4kWh					10y	$230	$0.2246	$0.0225 (82.6%)
12v 200Ah			12.8	200	2560	2048
Expert Power LBXLFP12_200BT	LiFePO4						2Y	$490	$0.2393	$0.1197
NewtiPower 12V200A	LifePO4	4P2S ?? : 16kWh					10y	$400	$0.1953	$0.0195 (84.9%)
Renogy	LiFePO4	4P4S: 32kWh					5y	$770	$0.3760	$0.0752
Victron BAT512120610	LiFePO4	5P4S: 40kWh					5y	$1110	$0.5420	$0.1084
Weize TPLI-12200AH	LiFePO4	4P: 8kWh 4S: 8kWh					10y	$500	$0.2441	$0.0244
24v 100Ah			25.6	100	2560	2048
Redodo 24V100AH	LiFePO4	4P2S : 16kWh					5y	$520	$0.2539	$0.0508

Note (*) – the max array configuration for many manufacturers is undefined, misleading, or wrong. Reputable manufacturers often list 24V systems (2 in series) as the recommended maximum and no more than 4 in parallel, so we use that as our baseline. NewtiPower doesn’t specify and insinuates infinite connections or at least a *P2S system (infinite parallel, 2 series) but we are pretty sure they just snarfed that from a gel battery site. Weize puts 10P and 4S all over their US website, but does that mean either-or or a 10P4S system? But then the Weize manuals all say “4 series maximum” and “4 parallel maximum” all over the place and do not indicate if a 4P4S (16 battery array) is OK. Redodo is very clear, 4P2S maximum array.