Sealed Deep-cycle Batteries vs. Open Vented Batteries – Off Grid Solar

Date: October 24, 2012

The decision between sealed deep-cycle battery, often called a maintenance free battery and open-vented batteries when considering an Off Grid solar system is often made by a financial one. Typically battery capacities are measured in Amp-hours, that is the amount of time a battery can deliver a Amp or Amps. This decision can be made also when comparing the same sized batteries against another one.

Typically, batteries are also measured at certain rates it can deliver those capacities, i.e the automotive industry usually measure battery rates at rates which suit cranking a starter motor, similarly deep cycle batteries are measured at a rate which suits a ‘slow discharge’. This assists consumers in making an informed decision of exactly the capacity of the battery they may in interested in.

When this is applied to the advantages and disadvantages of sealed deep-cycle batteries and open-vented batteries it often depends on the type of application intended for it use, financial constraints and similarly the amount of use the batteries are subjected to.

Sealed deep cycle batteries are robust, hence the cost is often associated with the fact that they are able to take more extreme use in terms that they will not suffer the same way which a sealed battery will. They are sealed which means that they do not require nearly as much periodical checking like their open-vented counterparts.

Because they are sealed, means that they also do not ventilate hydrogen gas as open-vented deep-cycle batteries do, which is often why they state that they are maintenance free – although they do still require that the terminal connections are kept clean, the bolts are tight and they still require a regular charge to prevent self-discharge.

Open-vented batteries are still very good, are still robust, however the most critical parts regarding open-vented batteries is that they do require attention regarding how they are discharged (they will not last as long if they are regularly deeply discharged) and require that the electrolyte levels are checked regularly, as this is often the failure of these types of batteries.

Obviously, when it comes to making a decision on whether a sealed battery in regards to an Off Grid solar would be better than an open-vented battery we will also have to look at how long either one will last. The longevity depends on the amount of ‘cycles’ (that is the amount of times that the battery can be discharged & recharged) depending on the level of discharge or commonly known as “Depth of discharge”.

All manufacturers must state the amount ‘of cycles’ a battery can perform based on the ‘Depth of discharge’ – this information is measured as a percentage and are liable for any false claims. Solar charge controllers in an Off Grid solar system are able to record and store this information in Amp-hours in and Amp-hours out and shows the Depth of discharge as a percentage for each day.

The design decision of the size of the batteries is based on the users requirements and typically is designed to discharge 20% daily on average.

A Depth of discharge of 20% per day is designed to ensure that the batteries will last their maximum life-cycle, typically 10-12 years for open-vented and 18-20 years for sealed batteries.  The user must bare in mind that open-vented batteries will require attention, once a month is best, however once every 3 months would be critically acceptable.

Typically, sealed deep-cycle batteries are more expensive than open-vented types. That is, the upfront cost is more for sealed deep-cycle than for open-vented. However over the the life-cycle of the battery especially if it is used for regular and or occasional deep depth of discharge, then sealed deep-cycle batteries will last longer, hence saving over the life of the battery.

By approaching the method of amount of cycles that are expected from each type of battery, it can be worked out what the cost is per year over the life of the batteries. If the user knows what their electrical demand will be daily, which is usually done at the design & quote stage and that demand does not differ too dramatically from day to day, season to season then it may just be a matter of financial constraints.

To work this out it is matter of knowing the amount of cycles expected from the battery/s and dividing this by amount of days in the year i.e. a sealed deep cycle battery manufacturer claims 6500 cycles at 20% Depth of discharge – 6500/365 = 17.8 years –  the cost of the battery is e.g $1110/17.8 years = $62.35/year.

An open-vented battery manufacturer claims 4200 cycles at 20% Depth of discharge – 4200/365 = 11.5 years = the cost of the battery/s is e.g $720/11.5 years = $62.60/year. Therefore, the cost per year is actually close to each other, although the upfront cost of sealed batteries will be considerably more hence they will last 6.3 years longer and the advantage of the sealed batteries is that they do not require the electrolyte levels are checked regularly.

Similarly, for the benefit of having a sealed battery but reducing the size and increasing the daily Depth of discharge works out approximately the same in regards to cost per year, again the upfront cost will be considerably more for sealed deep-cycle – as these batteries are generally 2 Volt cells and for a 48 Volt system, 24 are required, or for a 24 Volt system, 12 are required.

A sealed deep-cycle battery manufacturer claims 3500 cycles at 50% Depth of discharge (as the same sealed battery above), 3500/365 = 9.58 years. The cost of the battery is $1110/9.58 years = $115/year. An open-vented battery manufacturer claims 2500 cycles at 50% Depth of discharge (the same open-vented battery as above), 2500/365 = 6.85 years. The cost of the battery is $720/6.85 years = $105/year.

As per the examples above, there is no great advantage in cost on a yearly basis, the main advantage as mentioned previously is that sealed batteries do last longer, do require a lot less maintenance and although that the initial upfront cost is more when deciding on Off Grid solar, they will provide less risk in reliability and vulnerability  than open-vented types.

By contrast, there is no real advantage in purchasing a lesser capacity in any of these battery types, or in other words, discharging the battery a greater Depth if discharge  - infact it becomes more expensive, as replacing a set of batteries becomes quite expensive, especially as the Depth of discharge increases towards near full discharge.

In conclusion, both sealed and open-vented batteries offer advantages. The cost per annum is relatively comparable, both are reliable, offer a good life-cycle in terms of their cost, but the decision when all this is weighed up is what the user can ultimately afford.