Battery Management:
Avoid deep discharge There’s a dangerous trend in renewable energy system design. Some unscrupulous salespeople are sizing systems for 80% depth of discharge (DOD) — often to rig buying criteria toward higher-cost-per-kWh batteries. Even if a particular battery technology can discharge deeper in the lab, you can’t aord to risk it. That’s because every battery needs reserve power in case of increased electrical demand or reduced energy production (for instance, during cloudy, foggy or windless days). Besides, while safe DOD levels vary, all lead-acid batteries die early when they discharge too deeply. A good solution is to size batteries for no more than 50% depth of discharge.
A few rules:
To stay below 50% DOD, size your system for two-times the true amp-hours needed, regardless of battery chemistry. If batteries are discharging below 50%, install extra batteries. Specify three to six days’ stored energy (batteries and/or backup generator) when going o-grid. Never exceed 80% DOD (50% DOD is optimal for enhanced power reserves and longevity).