Depth of discharge vs longevity

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Hi folks,
I am new to this forum, but not new to EV's. I have had an EV on the road for over two years now and I wanted to pass on some calculation I have done on FLA batteries. I often see discussions that FLA batteries will last much longer if they are not discharged very much. However, my calculations suggest that overall the longevity is not reduced very much in terms of total miles driven on a battery pack.

The US batteries web site has information on the number cycles vs. DOD for their batteries. They show that at 20% DOD their batteries are good for 3300 cycles and at 50% DOD they are good for 1150 cycles. That may initially seem as a big difference, but overall the amount of energy cycled through the batteries is not that different (only 13% less energy cycled at 50% DOD - this means only 13% fewer miles overall).

Here are the calculations I have made (for simplicity let us just do the calculation for one 6volt golf cart battery of 230 Amp hrs).

At 20% DOD the battery will have put out 276 Watts
At 50% DOD the battery will have put out 690 watts

276 Watts for 3300 discharges = 910 Kilowatts over the life of the battery
690 Watts for 1150 discharges = 794 KW

So the actual difference between the two discharge rates over the life ofo the battery is only 13%.
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MN Driver

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The difference between pulling 20% of a lead-acid's battery capacity and 50% of the batteries capacity might not be that big of a deal. ...but get the numbers for 80% DOD or what sort of damage that sulphation does as it accumulates if you don't charge ASAP after the end of a discharge.

Most people doing conversions are aiming for 50% DOD but lots of people through either improper or optimistic calculations end up pulling their cars down to a lower DOD affecting the battery life. 80% DOD has been known kill a battery in a couple hundred cycles based on peoples conversions here, the deeper the amperage load you take them to while down deeper the worst things get. Remember these things are rated at a 20hr rate and those DOD ratings are likely light loading with little or possibly even no pause once discharged before being charged. Too optimistic to be considered serious IMHO.

"690 Watts for 1150 discharges = 794 KW"
The units you are shooting for are Watthours, not watts. Watts is momentary and measured as volts * amps while the current is being drawn. Watthours is a measurement of the actual cumulative energy used.

...battery manufacturers are also known to stretch the truth or leave details out. In this case your calculations, under conditions likely nowhere near the rates or conditions an EV is in aren't even showing 80% DOD.

I hope this helps.

Sunking

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Here are the calculations I have made (for simplicity let us just do the calculation for one 6volt golf cart battery of 230 Amp hrs).

At 20% DOD the battery will have put out 276 Watts
At 50% DOD the battery will have put out 690 watts

276 Watts for 3300 discharges = 910 Kilowatts over the life of the battery
690 Watts for 1150 discharges = 794 KW

So the actual difference between the two discharge rates over the life ofo the battery is only 13%.
OK these numbers do make any sense. Watts is a given amount of heat power at a given moment in time. So for say 200 watts on a 12 volt 200 AH battery only means at a given moment in time the battery supplies 17 amps. There is no time element associated with it to conclude a DOD.

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Soundboats

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Sorry, MN driver, I did mean to say KW-hrs. You are correct. I was sloppy in my units.

I also agree that folks need to watch their DOD. The curves provided by US batteries are show a break point at around 70% DOD. Taking your batteries below that significantly decreases the battery life.

I would like to share my experience since I am now on my second battery pack.

I converted a Nissan pickjup to an EV with the following stats:
Volts 120
Motor - NetGain Impulse 9"
I drive about 100 miles a week and usually recharged after 7-15 miles. Only once did I have to stop on the side of the road and wait 10 minutes before I could drive the last 1/2 mile home.
The first battery pack was Trojan 105's. It lasted 8200 miles. Since I run about 450-500 watt-hrs/mile (includes lights and heater), the batteries lasted for about 4 Megawatt-hrs of use and about 800 or so charge cycles during the 2 years the batteries lasted.

The Toecutter

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Sorry, MN driver, I did mean to say KW-hrs. You are correct. I was sloppy in my units.

I also agree that folks need to watch their DOD. The curves provided by US batteries are show a break point at around 70% DOD. Taking your batteries below that significantly decreases the battery life.

I would like to share my experience since I am now on my second battery pack.

I converted a Nissan pickjup to an EV with the following stats:
Volts 120
Motor - NetGain Impulse 9"
I drive about 100 miles a week and usually recharged after 7-15 miles. Only once did I have to stop on the side of the road and wait 10 minutes before I could drive the last 1/2 mile home.
The first battery pack was Trojan 105's. It lasted 8200 miles. Since I run about 450-500 watt-hrs/mile (includes lights and heater), the batteries lasted for about 4 Megawatt-hrs of use and about 800 or so charge cycles during the 2 years the batteries lasted.
What was your truck's max range to 100% discharge?

The 'sweet spot' for max range times cycles is around 30-40% depth of discharge. If you drive 15 miles a day, you want a pack sized for 50 miles range to full discharge.

Conversions with large packs, like Brian Matheny's S10 "Polar Bear", this truck having 120 miles range, have gotten 45,000 miles out of their pack(in the case of "Polar Bear", the pack wasn't even dead yet, had survived an accident, and the vehicle was sold). AC Propulsion routinely got over 40,000 miles out of packs of Optimas during the 1990s(their product is different now and they aren't as durable; the same is true for the Trojans as their design changed recently).

Green Machines

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A couple of questions?

- Is DOD measured as a % of the total volts of the battery (pack)?

- If so, is it measured from the 'top' of the charge ie: 13.5v for a 12v battery. Or is it from the stated voltage of the battery ie: 12v for a 12v battery?

- Is it measured under load (what the voltage dips to while in use) or at rest?

Mine is a lightweight car of 72v with a DC motor. Full charge is a little over 80v. My self imposed DOD of 25% sets the alarm off when the voltage under load drops to 60v. (that's 25% gone from the top of the charge). If I then give it a rest the pack will then show something like 74v.

Am I doing it right?

GerhardRP

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A couple of questions?

- Is DOD measured as a % of the total volts of the battery (pack)?

- If so, is it measured from the 'top' of the charge ie: 13.5v for a 12v battery. Or is it from the stated voltage of the battery ie: 12v for a 12v battery?

- Is it measured under load (what the voltage dips to while in use) or at rest?

Mine is a lightweight car of 72v with a DC motor. Full charge is a little over 80v. My self imposed DOD of 25% sets the alarm off when the voltage under load drops to 60v. (that's 25% gone from the top of the charge). If I then give it a rest the pack will then show something like 74v.

Am I doing it right?
There is a good table of voltage vs. state of charge at http://www.batteryfaq.org/ These voltages are for the cell after it has rested for a while.
If the voltages you note are at 100 F with floodies, then your rested 74V corresponds to 60% of the charge still available. Or 40% DOD.
Gerhard

Soundboats

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My original Trojans were rated at 220 amp-hrs so at 120 volts (nominal) I had the equivalent of about 24 KWatt-hrs of energy stored. Since I use about 450 - 500 watt-hrs per mile, my maximum is about 50 miles to full discharge. I must have bought the Trojans just after they changed. I was rather dissapointed in their performance.

I had bought Trojans 10 years ago for an electric 26' dory I built and they lasted over 5 years. This recent batch however was definitely not as good. I now have the US battery 2200 (6 volt 230 amp-hrs) in my EV and I hope they last longer.

Does anyone have any "real" information on the longevity of the Lithium Iron Phosphate batteries? Manufacturers claim they are good for 2000 or more charging cycles. At three times the price however they need to be that good or they are not cost effective in an EV.

Thanks everyone for chiming in on this discussion.

GerhardRP

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My original Trojans were rated at 220 amp-hrs so at 120 volts (nominal) I had the equivalent of about 24 KWatt-hrs of energy stored. Since I use about 450 - 500 watt-hrs per mile, my maximum is about 50 miles to full discharge.
If your 450-500 W-hr per mile is at 35 mph, then you are running a draw of 1C where the Peukert compensated capacity is about half the 20 hour rated capacity, or about 12 kW-hrs. Do you have a controller that gives battery current logs for a typical drive? I could estimate the actual Peukert limits are to your range (and test my program .)
Gerhard

Soundboats

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Hi,
This is an answer to Gerhardt's question.
No I have no monitor on my controller, but soon will.
As you guessed, I usually drive about 35 mph though sometimes I go as fast as 50 on one road in my neighborhood. In the summer it works out to be about 400 watt-hrs/mile and in the winter about 500 because I have to use my heater and lights.

My EV is now in my garage waiting for a new controller. I ordered a NetGain with the monitor and hope to have it delivered in a few weeks. My old controller was a Logisystems 700 amp that blew out exactly 3 days after the warranty expired. Luckily the folks at Logisystems fixed it at no cost to me, but it blew out a second time only 4 months after I re-installed it.

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