36 volt 1% daily discharge values, question?

[R.Boyd]

Ok, I've heard it said on here on many occasions that battery pack voltage typically decreases daily by about 1%. I ran the numbers in Excel to see what this would look like on a typical 36 v system and this is what I've come up with. I figured this would be a good thread to explain this and clear up any misunderstandings. When I did the calculations I started with 38.6 volts which is my understanding of what US2200 batteries should be when fully charged. See screen shot of the spreadsheet. Whatever the voltage should be for each day after the 1% discharge is what I used to calculate the next day's voltage value. What doesn't make sense to me is how quickly the pack value drops to complete dead. I've heard folks say you need to charge every 30 days when not in use, but if that was follow the pack would sit completely dead for a couple of weeks before being recharged. I've heard people state that sulfation starts after 6 hrs and irreversible damage occurs after that. So the question is, is my math correct and am I correct in thinking the pack needs recharging after sitting for 7 days. My 36 volt cart has sat for 7 days and doesn't drop much at all and nowhere near the 50% mark. Please explain the math behind the 1% discharge rate. I know I'm probably not calculating this correctly, but would like to know how to.

[R.Boyd]

Oh, and yes, I realize that other variables affect the rate of discharge. The calculations were done with the assumption that the temperature and other environmental conditions were constant or at a steady state.

[Sergio]

Sorry but that is not correct, a Flooded Lead Acid battery will lose up to 1% of its stored capacity, not voltage every day.

A fully charged cell is about 2.15v and an empty cell is about 1.75v

[cgtech]

Originally Posted by Sergio

Sorry but that is not correct, a Flooded Lead Acid battery will lose up to 1% of its capacity, not voltage every day.

A fully charged cell is about 2.15v and an empty cell is about 1.75v

The chart is correct, the description is not quite. Lost capacity generally cannot be restored, and you would never get more than a year out of a set. I think you meant 1% of State of charge.

[Sergio]

The chart is not correct, and I did mean to say "stored capacity" or SOC.

His chart shows an SOC of 50% after 7 days since it was based on a 1% voltage drop per day.

[Mooncarter]

Would it be safe to say the math is correct, but it is being applied to a situation that does not exist?

[Mooncarter]

With a 1% loss of SOC per day I come up with a loss of 25.31% SOC after 30 days. Voltage of pack 36 or 48v or whatever not important.

Is this close? This is assuming the discharge rate slows slightly each day.

[R.Boyd]

Ok would this be correct?

[Sergio]

That looks better, pretty much the same as Mooncarter calculated as well (My Excel calculation was 74.7% remaining capacity after 30 days - if started at 100%).

That is also in the heat of summer as the self-discharge rate is considerably less at cold temperatures.

[R.Boyd]

Originally Posted by Mooncarter

With a 1% loss of SOC per day I come up with a loss of 25.31% SOC after 30 days. Voltage of pack 36 or 48v or whatever not important.

Is this close? This is assuming the discharge rate slows slightly each day.

If you go off of the last spreadsheet I made it would be a 26% loss after 30 days so you are dang close.