Deep Cycle Batteries and their capacity
Lead acid batteries vary in capacity according to how quickly they are discharged. You often see a specification like C20, C100 etc. This relates to their capacity in amp hours at the 20 and 100 hour discharge rates.
Australian standards specify that stationary batteries (eg deep cycle solar batteries) should show the 120 hour rate (C120). Traction batteries (for electric fork-lifts, golf buggies etc) are usually specified at the 100 hour rate (C100). Traction batteries are sometimes called 'starved electrolyte' since they do not have a large reservoir of acid. The difference in say their C20 and C100 hour rates is not as large as you would typically find in a stationary battery with a large reservoir of electrolyte.
Parallel Connection of Deep Cycle Batteries
1. BATTERIES IN PARALLEL: Smaller capacity batteries are sometimes cheaper (per Amphour) than larger batteries. I believe this is because the smaller batteries are more ‘mass produced’. This cheaper cost sometimes leads customers to place several in parallel to achieve the desired higher capacity rather than purchasing larger capacity batteries.
Ray Prowse, who used to inspect failed systems for the Solar Energy Industries Association of Australia, commented that in every case of system failure, there were batteries in parallel. While this may not be the core reason for the failure, it is an indication of poor practice. Noel from Solar Tasmania subsequently commented, ‘Our experience with parallel batteries, even when wired correctly, is don’t try … even when they are identical type, age and capacity …’.
The biggest problem with parallel batteries is that they will tend to charge/discharge unevenly, which can lead to sulphation and stratification of the electrolyte.
If you must place batteries in parallel, you should limit this to two parallel strings. Great care needs to be taken to ensure that there are equal current routes through both strings. The positive lead should come off one string and the negative through the other. The cable size must be identical, as should the crimped terminations.
Australian Standard 4509.2 stipulates that ‘… the number of parallel strings be minimized …’. The standards also stipulate that each parallel string must be fused and earthed (or fused on both positive and negative if the system is floating). So to conclude, we would suggest that you try and avoid connecting batteries in parallel, and to seek the advice of an experienced and qualified solar installer in this regard. |
