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Old 12-30-2013, 02:29 AM   #1
kojo
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Default Battery Load Test

With my 36v battery pack fully charged, with a DVM hooked up, what should my voltage drop down to within the first 40 yards. How much more voltage drop would the DVM show after 4 to 5 miles while still driving. Thanks for any replies.
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Old 12-30-2013, 07:04 AM   #2
JohnnieB
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Default Re: Battery Load Test

Regardless of SoC or AH capacity, the more amps drawn, the lower the voltage drops.
The lower the motor's RPM, the more amps it can draw, so the first few feet when accelerating from a stand-still and when the cart is slowed to a crawl going up a steep hill, will be the highest amp draw.

How much the voltage actually drops depends on what percentage of the stored capacity the amp draw is, plus some other factors.

Stored capacity depends on the batteries AH rating, SoC and Temperature.
Batteries loose about 10% of their storage capacity for each 15°F below 80°F.

I have 245AH batteries and a 500A controller so when my pack is fully charge, I can put slightly over a 2C load on it at 80°F and a 3.4C load on it at 20°F. The load factor increases as the SoC decreases, so if they are at 50% Soc, I could hit them with a 6.8C load, which would very likely irreversibly damage the batteries.

Physical damage to the plates starts occurring at 1.5VPC (Volts per Cell), which is 27V for a 36V battery pack, so never, ever, load your batteries down that far. (The danger of doing this is greater on the trip back from wherever you've been since the SoC is lower)

When battery manufacturers do non-destructive testing of their products, the limit the loading to 1.75VPC, so that should be a fairly safe number to limit your loading to while driving your cart. That will be 31.5V for a 36V pack.

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While driving, the pack voltage is going to fluctuate considerably, depending on the driving conditions.
Voltage decreases rapidly when amps are drawn, but recovers at a slower rate.

To estimate SoC after driving cart, let the batteries rest for at least five minutes, preferably 15 minutes before reading the voltage.
The battery voltage will climb for over an hour after the cart stops, but most of the change occurs in the first 15 minutes.
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Old 12-30-2013, 12:46 PM   #3
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Smile Re: Battery Load Test

Thanks JohnnieB. for the info. I was hoping you would share some of your knowledge with me. Last night when me along with 3 others went out it was all highway, and it was the 1st time I've had the DVM hooked up while driving, the DVM read around 34.8 to 35.2 for about the 1st 30 yards then slowly got up to 35.8. After about 5 miles I looked and it was 34.6, all going around 10-12 mph and a couple of times 18 mph for about 150 yards. When we got back home I let her sit for about 2 hrs and checked pack voltage and it was 37.8v. Does these #'s look pretty good to you Johnnie. Thanks again for any advice and info you can give me.
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Old 12-31-2013, 05:07 AM   #4
JohnnieB
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Default Re: Battery Load Test

You were carrying a fairly heavy load, the air temperature was cooler than 80°F and the Under-Load voltages weren't dropping below 31.5V, so they were good.

Watching the pack voltage in real time will drive you a little crazy at first because it bounces all over the place, but you eventually get a feel for what is happening. For the most part, all it is telling you is how hard the motor is working and about the only thing to watch for is how low the voltage drops down to and if it stays above 1.75VPC (31.5V for a 36V pack) you are doing okay.

As the batteries age, the voltage will drop lower on the same hills when the batteries are at the same SoC. You'll also see the voltage drop lower in cooler weather.

------
The distance traveled wasn't mentioned, but the pack ended up at 88.5% SoC, so you can probably travel about 4.5 times that distance carrying the same load at the same temperature without discharging the pack below 50% SoC.

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For the voltage to drop quickly and recover slowly, is normal.
The acidity of the electrolyte touching the plates drops quickly as the sulphate ions migrate into the plates and it takes time for more acidic electrolyte further away from the plates to mix with the less acidic electrolyte and reduce the overall acidity throughout the cell.
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Old 01-01-2014, 12:32 AM   #5
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Smile Re: Battery Load Test

Thanks for the help and info. JohnnieB., myself and others can always count on you for advice. To everybody, HAPPY NEW YEAR.
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