thoughts on the 50% DOD rule?

al128 Solar Expert Posts: 65 ✭✭✭✭
Hi there,

I read on var. sites (Lifeline, and others) that the *economic* optimum for a battery bank is where you have a 50%DOD cycle.

This seems that a bigger bank (lower DOD) will cost you more upfront (need more bat's) ... and a smaller bank (higher DOD) will cost by shortening the bank's life ...

any thoughts/experience on the 50% rule? ... it seems that people here go for ~30% DOD ....

just wondering


  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Re: thoughts on the 50% DOD rule?

    50% DOD is a good average maximum for long battery life--as you have researched...

    The other big issue is discharging below ~75% State Of Charge (SOC)... If you go below 75% SOC, the soft lead sulfates (that form when the battery is being discharged) will start to harden in hours--and hardened sulfates will remain and recharging will never convert them back... So the recommendation has been if you get below 75% SOC, you want to quickly recharge (within a day or less is ideal) to above the 75% level to stop the hardening process.

    Since we normally recommend 3 days of off-grid storage and never to go below 50%--you are really discharging the batter at 1/6 per day... or below ~66% after two days of no sun... You can run the other day too--but it is really a good idea to look at firing up the generator to get the battery bulk charged to 85% (aprox.), then let the solar finish off...

    The one exception are AGM batteries--from some vendor's documentation, you can cycle them down to 20% state of charge without worrying about sulfate hardening.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: thoughts on the 50% DOD rule?

    it depends on the battery, its type, and manufacturer. for common leadacid batteries the 50% rule does usually hold for them with some variances at times. with a 30% dod some like to be sure to not harm the batteries' lifespan that they have which are usually the cheaper leadacid batteries. costs may equal out in some cases because the higher cost batteries that have better dod characteristics are made up for in just obtaining more of the cheaper batteries.
    now as to concorde, they do have the ability to cycle deeper and can go almost as much as 70% dod or so before the cyclelife starts to decrease with acceptable cyclelife even as high as 80% dod. concordes are an exception so the rule stands generally at 50% dod for most other batteries especially when cyclelife characteristics are unknown for a battery. i err on the safe side and go with 60% dod for my sunxtender. .6x104ah=62.4ah usable.