This is the fifth DIY solar post in a series on solar power systems, now onto the deep cycle battery for use as our solar batteries.  While this application was done for an RV solar panel system, the concepts are intended to apply for any do it yourself solar power system.  We hope you enjoy this entry (The Solar Series: Choosing Solar Batteries) and links to the rest of the DIY solar panel system series can be found at the bottom.

Choosing the type of solar batteries to fill the required capacity dictated by our solar power system sizing is one of the simpler steps as there are far fewer choices than with selecting panels.  The industry standard for solar batteries is the Trojan T-105 as they’re durable and cost effective, so you could skip this whole post and just pick up those.  However, if you’re trying to make the most informed decision or have specialized needs, let me go through some background on different battery technologies and manufacturers.

Battery Applications – Deep Cycle Battery VS Starter Battery

Although this series of posts is intended to apply beyond just RV solar panel systems, that does provide us a good base to compare different battery types.  Everyone knows a standard automobile has a starter battery under the hood.  Its main purpose is to provide a very high current in a short burst to turn over the starter motor to get the engine going.  Its ability to do so is rated in Cold Cranking Amps (CCA) which will be higher for an 18-wheeler than a Honda Civic.  While a starter battery can hold a moderate amount of overall charge, that’s not its purpose.

This is where deep cycle solar batteries come in.  They are specialized for storing energy in applications that dissipate charge much more slowly than a starter battery and their capacity is rated in amp hours.  In my RV solar power system, when I have all the electronics going it will drain the solar batteries at 20 amps, where the capacity per battery is 110 amp hours (I have four).  A large starter battery can provide up to 200 amps for a few seconds but maybe only has a 60 amp hour capacity.

One point I have to make is there are batteries called “dual purpose” on the market which claim to provide the functionality of deep cycle and starter batteries in one handy package.  Naturally all this means is it’s bad at doing both of them.  I originally had one in my RV which I removed and replaced with a starter and put the solar batteries in a separate compartment in the back.  Avoid dual purpose batteries at all costs.

Battery Types – Flooded, Gel Cell & AGM

Flooded lead acid batteries are by far the most prevalent type of starter or deep cycle battery.  They contain conductive lead plates suspended in a battery acid liquid solution of electrolytes.  Starter batteries have a greater number of thinner plates for maximum surface area and maximum current, whereas deep cycle batteries have a lesser number of thicker plates to store more charge.

Flooded lead acid is the oldest battery technology so is the cheapest, and if treated correctly more than adequately durable.  Because the electrolyte solution is just water with dissolved battery acid ions, it can evaporate over time, or convert to hydrogen and oxygen gas if the batteries are overcharged, requiring an occasional top-up with distilled or de-ionized water.  One variation to offset this is sealed lead acid batteries where this gas cannot easily escape.

Gel Cell batteries function exactly the same as flooded, except the electrolyte is replaced with a gel.  This means the batteries don’t need to be held upright, are less sensitive to vibration and the electrolyte will not evaporate.  However, they are more expensive.

AGM stands for absorbed glass mat and is the newest and most expensive technology.  Here the electrolyte is held next to the plates by a fibreglass mesh.  They are more resistant to extreme temperatures, hold more charge per unit of weight and don’t have the electrolyte evaporate or need to be upright, partially justifying their cost.

Deep Cycle Battery Manufacturers

Trojan – I already mentioned Trojan because they’re the brand I chose and are very popular for RV and off-grid solar power systems.  Their classic T-105 is a 6 volt flooded lead acid designed for golf carts, which just so happens to work perfectly for RV solar batteries as well.  Keep in mind you’ll need to buy them in pairs to achieve the required 12 volts.  They sell several other voltage and capacity combinations, so if there’s a distributor in your area, investigate after researching on their website.

Surette/Rolls – Another heavyweight in the renewable energy and transportation deep cycle and solar battery industry, Surette specializes in more heavy duty applications and is slightly harder to find.  However, their products are well reviewed, they have more variety especially in larger battery banks, so they’re definitely worth looking into (website here).

East Penn – Another well known battery manufacturer, they manufacturer more conventional consumer batteries, something like an AC/Delco.  Their EPGC15G is similar to the T-105, and they also have a variety of Gel and AGM batteries such as the 8A27 I was also recommended (website here).

Other – I listed three big players, and like I said there isn’t a huge variety in batteries because they aren’t overly complicated so manufacturers succeed by making a lot of them well with small profit margins.  If you want to try another company, make sure you research their products and ensure you get good product data (explained next section).  While sourcing batteries locally can be cheaper due to their weight, when they are shipped drained there are some good deals online for the T-105s.  Lastly, I 100% recommend against used batteries.  I saw some great deals on Craigslist in my area, but you have no idea how they were (mis)treated before you got them and they could die sooner than you hoped, ruining the value proposition.

Solar Battery Parameters

As I mentioned, the above established manufacturers will be able to tell you a lot about each of the solar batteries they make, which you can use to make them last as long as possible, which is really the only concern once installed.

Voltage Set Points – When my solar batteries are 100% charged, have been allowed to rest for several hours and have no current being drawn from them, they settle around 12.6 Volts, which is fairly standard.  While you would hope you could just apply a voltage of 12.6 to them during the day to return them to full charge, unfortunately it doesn’t work this way…at all.  I don’t want to get into the nitty gritty, and will touch on it more in the Charge Controller post, but a voltage somewhat above this 12.6 is required during the bulk charge, then once it reaches full charge it needs to be held there for a specific period of time, then to prevent overcharging it needs to be lowered to a specific float voltage.  These all vary per battery and without knowing the exact numbers, you won’t be able to achieve peak charging efficiency or battery life.

Capacity Degredation VS Cycles – There isn’t really a formal term for this variable but you’ll usually find it as a graph.  All it indicates is that over time, a battery’s capacity will decrease each time it is discharged and recharged.  What counts as a ‘cycle’ can vary, usually 50% discharge is the number because below that isn’t good for the batteries.  As the three manufacturers listed above are all reliable and competitors, you’ll find they’ll be fairly good and in line with each other that sometime after 500 cycles there will be a rapid decline in how much a battery can hold.  This also illustrates the importance of not overdrawing your battery bank.

Selecting Batteries

Now that I’ve outlined all the considerations around different solar battery manufacturers and types, and you’ve figured out which are available in your area, I wanted to finish with a quick and easy comparison method between two or more different batteries.  It’s just two simple numbers derived from a spreadsheet.  The first is dollars per amp hour ($/AH) and measures cost effectiveness, the second is amp hours per pound (AH/lb) and measures weight effectiveness, which is only important for mobile applications and even then not a huge factor unless comparing flooded to AGM.  Simply fill in the blanks yourself following this example.

Mfgr	Surette	East Penn	Trojan
Model	S-460	EPGC15G	T-105
Cost ($)	310	189	186
Capacity (AH)	350	215	225
Weight (lb)	117	63	62
$/AH	0.89	0.88	0.83
AH/lb	2.99	3.41	3.63

As I said right off the bat, I picked the cheapest and most readily available Trojans and they’ve worked great ever since.

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So that’s it on the selection of solar batteries.  Over the coming weeks I will be creating posts for each aspect of the creation of off-grid solar power systems with the hopes that you can follow along and use the the info for your own DIY solar panel system.  Of course it gets a little technically involved in spots, so feel free to ask for clarification in the comments or drop me an e-mail.

Topics

RV Solar Panels – Goals, Rationale

How Do Solar Panels Work – Power, AC & DC

Solar Power System Sizing – How much power do you need?

Best Solar Panels – Selecting the best solar panels for you

Solar Batteries – Selecting the best solar batteries for you

Solar Charge Controller – Choosing between MPPT and PWM

Installation (coming soon…)

Maintenance (coming soon…)

The Solar Lifestyle (coming soon…)

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