[see
Forum Thread this is taken from for more]
Is it safe to discharge Lead Acid batteries to 20%?
It's not a matter of
personal or
battery safety; it's about battery
economics and
longevity. A lead acid battery won't explode even if you drain it dry. All that draining it to a low depth of discharge does is to
dramatically shorten it's possible life span. The reason you typically see 45 to 60% depth of discharge (DoD) for lead is that is usually where the best economics lie for the average off-grid usage. Use the manufacturer's datasheet to understand life-cycle vs. DoD.
What's important is to understand your application. How many charge cycles you want out of a battery will enable you to understand the best DoD for you and allow you to compare different batteries at different costs in an apples-to-apples way.
Let's work an example. Say you want to cycle a battery everyday for 10 years; you need 3,650 cycles. Battery A costs $68.50 and has 500 cycles at 65% DoD. Battery B costs $1,000 and has 10,000 cycles at 90% DoD. Which is right for you?
Assuming the full DoD everyday, you need about two As for one B. Since 10,000 > 3650 you'd only ever need 1 so buying battery B will costs $1000. For Battery A, to get 3,650 cycles at the DoD you'd need 2x3650/500x$68.50 = $1000. So, in this case the price is the same.
As an exercise, which is better if Battery B gets 1000 cycles at 45% DoD? Which is better if you only need it for 50 cycles (e.g., emergency use over 10 years)?
Other factors:
- The time value of money. One Battery A would last over a year, the money you didn't spend on Battery B can be making you money. Or, if you can't afford battery B, buying a Battery A now would hold you for a year so you could save up for it - and in that time Battery B's price might come down.
- In addition to cycles being greatly affected by DoD, the actual power from lead chemistry depends on the current draw rate and ambient temperature, see Battery Power .
- Weight, lead acid batteries are heavier. Not much savings if you hurt your back or crack your floor.
- Battery maintenance/venting, FLAs in particular need a lot of care.
- Low Temperature operation
- Extra work in replacing more batteries (remember, they're heavy).
- You generally get cash back for recycling lead batteries, you might have to pay to dispose of lithium.
