80% Depth of Discharge is what Cell Voltage for 48v sys with AGM Batteries

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rekhill
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80% Depth of Discharge is what Cell Voltage for 48v sys with AGM Batteries

Hi, reikiman (David)

I am using 4 BB EB50-12 Batteries for a motorcycle, KLR650, conversion I am
working on. The BB data sheet says:

50Ah @ 20 hour rate F.V.(1.75V/cell)

I am a little confused because I have read (Table 7.2-1) in THE ZERO-CARBON
CAR, by William H. Kemp (2007), on page 281 that 100% depth of discharge is
equal to 1.75 volts/cell per the table. Mr. Kemp is using(ed) Optima AGM.

I tried to get the sales guy in Cal with BB to discuss Depth of Discharge
before I bought the batteries, currently setting on the shop floor, and
could not get a straight answer, understandable but frustrating. I know
voltage is fluid and they, BB, don't want to get "caught" with their pants
down on that issue (warranty). But, I do need to understand where 80% is so I don't
create problems with battery life and the response from BB did not satisfy me.

My plan is to install a WESTBERG item no: 2DC10-76(COMBOMETER #2..Volts, 0-50VDC
..Current, 0-250ADC with a 250A=50mV Shunt. I would monitor system voltage and
decide when to call it a day at 80% depth of discharge more or less based upon
?.?? volts per cell time 24 cells.

Please help.

Thanks
Bill Rose
Fayetteville, TX 78940

reikiman
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

Lead acid batteries are rated at the 20 hour discharge rate. This means the discharge is at C/20 amps.

The spec sheet is here: http://www.bb-battery.com/productpages/EB/EB50-12.pdf

C/20 means to take the rated capacity, 50 amp-hours, divide by 20, and that's the discharge rate. 50/20 = 2.5 amps is the rated discharge rate. The battery is spec'd to deliver 50 amp-hours over a 20 hour period at 2.5 amps discharge.

Experience with lead acid batteries show that if you discharge the battery at a higher rate, you extract fewer amp-hours in total.

Experience with electric motorcycles shows you'll want a discharge rate of 100 amps or more, depending on how fast you want to go. The battery pack you're discussing is pretty small, 48 volts 50 amp hours. That's the size pack normally delivered in a 30 miles/hr electric scooter. I used to own one of those (an EVT 4000) and it was quite fun, max 30 miles/hr, and the best range I got was 20 miles. The KLR650 is a much bigger beast and deserves a better battery pack.

For this size bike 72 volts is much better, and would be even better with a few more volts.

In any case .. the target discharge rate of 100 amps (or more) using 50 amp-hour batteries means a 2C discharge (double the rating) making this quite a bit higher, and you'll end up extracting quite a bit less total amp-hours. It's likely the usable capacity will be 50% - meaning you'll only get 25 amp-hours out of the 50 amp-hour battery.

Another note is that it's not a good idea to deeply discharge lead-acid batteries. Even the deep discharge batteries like the B&B.

The battery will tell you when you've pushed it too far. The voltage falls off a cliff and your speed drops to zero. If that happens, the battery is exhausted and you've pushed it way too far.

- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
- Electrified Electra To

rekhill
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

Thanks, for the quick response. I agree, my 48v sys with 50 amp-hour batteries is not going to give me much performance, i.e., range and/or speed. I am still uncertain of the meaning of 1.75v in the referenced BB spec sheet. Does it mean that after 20hrs at the C/20 discharge rate of 2.5 amps, I will arrive at 1.75v per cell with a 100% depth of Discharge?

If that is correct, then will the 50% usable capacity your suggesting equal a 50% depth of discharge and a cell voltage of 2.030v? With my 48v system, when the sys voltage gets to about 48.72 do I arrive at about 50% depth of discharge, assuming a cell voltage of 2.100v (50.4v) for 0% Depth of Discharge?

Thanks,
Bill

Bill

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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

I deal with the whole battery, not the per-cell voltage, so am not entirely sure how many cells are in one of these batteries. The whole battery voltage is nominally 12v but when fully charged is 13.2 volts or more (not remembering the specific number). During discharge it is common to see voltage at 11.5 or fewer volts per battery. If the voltage strays below 11 volts that's a deeply discharged battery, especially if that's the resting voltage.

What I mean is - there is a voltage sag during discharge .. and there is the voltage at rest, with no load on the battery. During discharge voltage could sag towards 11 volts, then snap back to 12.8 volts or whatever the current state of charge is.

State of charge for lead acid is roughly related to the voltage at rest, when there's no load on the battery. This works because lead acid batteries have an actual discharge curve, where the voltage actually drops over the period of the discharge. This isn't true for lithium batteries, where the resting voltage remains pretty constant over the discharge until it's depleted.

- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
- Electrified Electra To

LeftieBiker
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

I think this is what you are looking for: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Voltages It looks to me like 80% DoD at rest is about 49.68 volts?

antiscab
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

I am still uncertain of the meaning of 1.75v in the referenced BB spec sheet

Hi Bill,

1.75vpc = 10.5V for a 12 monoblock

Thats important, as it means as a worst case scenario, you may actually have 5 x cells at 2V, and one at 0.25V

that approach doesn't work when you have more than one 12v monoblock

A Lee hart batt bridge can be used to determine if you are about to reverse a cell

Your best bet is to not use voltage to determine SOC (its *very* inaccurate)

Use an Ah counter like a cycle analyst instead

You will likely not get more than half the rated capacity out of a lead acid battery due to the high discharge rate needed.
So for your batteries, 25Ah @ 46V is about all you will get (1.15kWh)

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

rekhill
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

Thanks, LeftieBiker
I've bookmarked the link...it is answering a lot of questions, even with just a quick look.

Bill

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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

Hi, Matt
Thanks for the suggestion. I'll add a cycle analyst. I know I am not going to get much range out of this set up, but I only need to go 6 miles for a round trip to Fayetteville, Tx from out in the country where I live. 25ah x 48v = 1200wh / 150wh/mile = 8 miles? If I keep the speed at 30mph more or less, I probably can do much better than my wild guess?

Bill

antiscab
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

25ah x 48v = 1200wh / 150wh/mile = 8 miles? If I keep the speed at 30mph more or less, I probably can do much better than my wild guess?

My Vectrix uses 125Wh/mile when I do 50mph or more continuously or 70wh/mile at 35mph (on the odd occasion I go that speed)

at 30mph, you should do better

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

rekhill
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Re: 80% Depth of Discharge is what Cell Voltage for 48v sys ...

Hi, Matt

I went to the Vectrix website. You've got a really "cool" scoot! How are you measuring wh/mile? Does it have an on-board computer? Are you able to measure amps, too?

Bill

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