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Old 11-03-2014, 09:36 PM   #1
birmingham
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Default Quantifying Battery condition

I have a 36V ezgo, with questionable battery wires, i have already ordered replacements will be here 2 days from now. The battery pack is 4 years old, cart performance is not very good, 1.5 miles round trip to the publix with some hills and she is about drained. any way to quantify where the batteries are at? very little flat ground around where i live.
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Old 11-03-2014, 09:37 PM   #2
birmingham
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Default Re: Quantifying Battery condition

its about a 2005
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Old 11-04-2014, 01:18 AM   #3
cgtech
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Default Re: Quantifying Battery condition

Your batteries are 4 years old, and underperforming, which means they are nearly paperweight status. If you wanted to calculate their capacity without expensive equipment, there are a few methods. Here is one i feel should be pretty accurate (barring a bone-head math error).

If you are considering this testing on your own, you need to be familiar with wiring batteries & lights in series. Decent at math (or using a calculator to do percentages). Ohms law to calculate loads. And some slang terms & what they mean. Like "volts" or "v". "amps" or "a". "amp hour" or "ah". "state of charge" or "soc" as a percentage (not to be confused with the terminology used in ohms law, these V, A, ah, are "slang" abbreviations used loosely around here). IF THIS SOUNDS CONFUSING, DO NOT CONTINUE. If you continue, do so at your own risk.

You would need 3 things. 1- a working voltmeter, 2- a way to time your test, 3- a device to apply a load of a known value (like a headlight bulb, or 3 if you wanted to do the whole pack at the same time, as i will describe). This test also assumes you know what your original battery capacity was (ie: 225ah @ a 20hour rate, the rating of a trojan T-105 which will be used in this example). Some understanding of ohms law wouldnt hurt either (3x 55watt headlamp bulbs in series = 165watts @ 36v totals 4.583 amps @ 36v). This test will drain 40% of a 225ah battery pack, and could overdischarge a pack with less than half of its original capacity. You could substitute 3 identical 12v bulbs that are less than 55w to do this test on a weaker pack with some math to calculate amp load over 20 hours. IF THIS SOUNDS TOO CONFUSING, have your batts tested by a shop. First, fully charge your batteries, wait 12 hours. Second, check & record your individual batteries voltages, compare to SoC tables on this site, this will be important to get any kind of accuracy out of your "redneck discharge test". If you were to hook your three 55w (4.583 amp) 12v bulbs in series to your battery pack + & - terminals for 20 hours, you would drain 91.66 amp-hours from your pack, or 40.7% of the capacity of a 100% capacity "great, seasoned battery". Knowing this, we need to know how much % capacity was actually drained, we will determine this by checking the individual battery voltages 1/2 hour after your test has ended, compare these "after-test voltages" to the SoC table you downloaded. If after the test, your batteries were only depleted 40% (still at 60% state of charge), your batteries are 100% rated capacity. If after the test, your batteries are depleted 60% (still at 40% state of charge), your batteries are at 75% rated capacity or 168ah. If after the test, your batteries are depleted 80% (20% state of charge remaining), your batts have 1/2 of their rated capacity or 112ah.

The purpose of this is to remove a known amount of energy from a battery, then compare how much %SoC should have been removed to how much %SoC was actually removed. These load values (X watt 12v bulb) will change the math to calculate results. The original Ah capacity will play a huge role in the calculations as well. You could over-discharge a failing battery, and make it worse (or even destroy it).

-If you have constructive questions or comments, feel free, i'll answer to the best of my ability.

-If you catch some "bone-head" math errors, please correct me, keep in mind this was typed on a phone, so dont bother me with grammer.

-If you want to call me a jerk or idiot for suggesting such lunacy (your idea is so much better, ohms law does not exist, ect, ect), go ahead also, i will not reward you with a war of words.

-if this post is deemed total garbage, have a moderator delete it
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Old 11-04-2014, 09:23 AM   #4
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Default Re: Quantifying Battery condition

CG, you do better on a phone than a lot of people do on a keyboard.

The concept is valid and without breaking out a calculator or building a spreadsheet, the math looks okay to me.

I've been kicking around ideas on how to discharge batteries in ways that are relatively safe, easy and inexpensive for a couple different reasons and I always come back to light bulbs or heating elements since they are more or less constant current devices.

One reason being ballpark measurements of AH capacity, the others relate to charging.

I've got a 36V set of T-105 batteries that have been on float charge for over two years and want to find out what their AH capacity is before I start doing some charger games with them, so I order three 12V/55W bulbs.

The amp flow through a 12V/55W bulb is less than half of the amp flow used in the manufacturer's 20Hr capacity testing, so going over 20 hours ought to be safe enough.

The calculated AH capacity will be a little high due to the lower discharge rate and the Peukert Effect, but shouldn't be off by much, around 5%.

Also, the batteries ought to be allowed to rest a few minutes after the load is removed, so the voltage will recover. But I doubt is the recovery will be very much since the discharge rate is so low.

Now I gotta figure out how to mount the bulbs so the don't get broken, or start a fire, or blind me while I'm trying to do other things in the garage while the batteries are discharging.
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Old 11-04-2014, 09:38 AM   #5
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Default Re: Quantifying Battery condition

JohnnieB, hope you are ok with me borrowing this State Of Charge chart to accompany my post.
Attached Images
File Type: jpg JohnnieB's SoC Chart.JPG.jpg (66.2 KB, 0 views)
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Old 11-04-2014, 10:24 AM   #6
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Default Re: Quantifying Battery condition

Quote:
Originally Posted by cgtech View Post
JohnnieB, hope you are ok with me borrowing this State Of Charge chart to accompany my post.
Absolutely okay.

A lot of the stuff I post is "borrowed", so use whatever you can use, whenever you can use it.
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Old 11-04-2014, 11:16 AM   #7
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Default Re: Quantifying Battery condition

Quote:
Originally Posted by JohnnieB View Post
Absolutely okay.

A lot of the stuff I post is "borrowed", so use whatever you can use, whenever you can use it.
That's why this forum is the greatest. When a member need info someone will post the answer, because it's on this forum for all members, And I Thank all of the members that help others fix their own cart, and don't have to take it to a shop if you don't want to.
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Old 11-04-2014, 03:41 PM   #8
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Default Re: Quantifying Battery condition

Great no frills, nuts and bolts information for us electrical challenged folks. One question though. What changes to the load would be needed to test one suspect battery.
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Old 11-04-2014, 04:02 PM   #9
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Default Re: Quantifying Battery condition

The one suspect battery will show itself at the end of the test of the whole pack, it will have the lowest SoC. Or just test it individually, with one bulb. If you use a 12v bulb to test one 6v battery, cut the bulbs watt rating (and therefore amp draw) in half. You will have to charge that individual battery back up by itself, dont just plug your cart charger in to 5 full & 1 empty battery then expect it to charge up.
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Old 11-04-2014, 05:50 PM   #10
birmingham
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Default Re: Quantifying Battery condition

This is a great answer, My question upon reading it again may not have gotten at what i really wanted it to.

My cart runs strong for the first mile after being charged, I have one of the colored bar charge indicators, it will be down one bar out of 8 or 10 i think, I go in do my shopping (Publix) then load back up my home performance is quite poor and my hills performance is a slow walk, the bar charger at this point will only be down 2 to 3 bars (i am assuming that there is a small microchip in there and that the bar charger is just converting a SOC chart into colored bars that are easy for my wife to understand.) If this is the case why am I seeing such a large drop in performance when the bar charger is displaying decent charge? possible that there is something wrong with my controller or motor? the only thing that i can feel getting hot is the motor, some heat in the wires between the controller and motor. Warm.

mostly stock cart, more aggressive rear tires for hunting, stock height stock size tires. no back seat. no headlights no taillights. I am excited about modifying this thing.

Another side question, If my pack is toast is it ok to keep my current charger, go to 8 6volt batteries, charge the last 2 with an automotive charger, and because they will always be charged separately i could run my 12 volt accessories off of them? Bad idea?
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