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Something that still has me baffled is how do you know the DOD of the battery?. For example I am running a 120V system down the road, how do I know when its 50% DOD, 80% DOD etc..

Mostly it’s a matter of statistics and wild estimates!

I think the way most DOD (depth of discharge) meters work is that they monitor the charging of the battery and when it gets over a certain “full” voltage for a period of time the meter resets itself to flag the batteries as “full.” Then, when current is drawn, the meter keeps track of the amp hours used and compares that to what the user programmed in as the pack capacity. Some other formulas, like peukarts, can help improve upon this estimate based on the rate the power is being drawn. But it’s still hard to take into account every factor, like the age of the batteries and things like really cold weather.

Comments 2
  1. — Jimmy Carter    Apr 14, 2007 14:21 PM    #

    I think that the person asking the question wanted to know what the voltage would be at 50% DOD and 80% DOD, in case they are using a volt meter….

  2. jerry Halstead    Apr 14, 2007 15:21 PM    #

    My understanding is that the voltage reading doesn’t give a reliable indicator of DOD%. Things to consider:

    * voltage sags under load, going up a steep hill the pack voltage could drop significantly

    * without a load the voltage of a lead acid battery will start to slowly rise (I’ve sat on the side of the road a few times waiting for a low battery pack to self-charge enough for the last block home…and it always worked)

    * chemistries and age: each battery type will have different characteristics, and capacities change as they get older

    With that said, after driving an EV for a while and watching the meters you can get a sense of how your pack is doing by watching the pack voltage under typical load.

    The battery university talks a bit about depth of discharge with different battery types.