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Old 01-06-2016, 08:50 PM   #1
ducowti
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Default Voltmeter vs Battery Status Meter - redundant?

I've installed a voltmeter on my TXT PDS, but haven't run the battery down to see how the displayed voltage drop translates to battery status. Can anyone offer some practical relation, or is it worthwhile adding a battery status meter (fuel gauge style)?

Thx.
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Old 01-06-2016, 09:24 PM   #2
teecro
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Look at post 3 here http://www.buggiesgonewild.com/elect...batteries.html from that chart one can tell battery status quite well with a voltage meter....
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Old 01-06-2016, 11:11 PM   #3
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Heres my thoughts, and they are worth what you paid to hear them which is nothing so here goes nothing.

I prefer a volt meter.

There are a lot of different battery/range/capacity/fuel gauge type devices. They do kind of work for a basically stock golf cart. They attempt to give a linear display of usable range. For a golf cart on a golf course they work ok. Some are complicated some are simple. Some of the ones that come with battery management systems measure power out verses ambient temperature, battery temperature and discharge rate. They can all be wrong under the right circumstances.

A volt meter is not as simple to use or understand, but if you learn to use it it will tell you a lot more than a battery gauge. At rest a volt meter will give a very accurate state of charge. Under a load a volt meter can tell you a lot about the condition of your batteries.

The main issue using a volt meter is the recovery time of lead acid batteries. It can take minutes of hours for batteries to level out. Fresh off of a complete charge they will read high, after a steep hill with a big load they will read low. Once they recover from charging or a heavy load you will get their true condition.

To sum it up, drive your cart with both, see what works for you.
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Old 01-07-2016, 09:19 AM   #4
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Default Re: Voltmeter vs Battery Status Meter - redundant?

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Originally Posted by Revolution View Post
Heres my thoughts, and they are worth what you paid to hear them which is nothing so here goes nothing.

I prefer a volt meter.

There are a lot of different battery/range/capacity/fuel gauge type devices. They do kind of work for a basically stock golf cart. They attempt to give a linear display of usable range. For a golf cart on a golf course they work ok. Some are complicated some are simple. Some of the ones that come with battery management systems measure power out verses ambient temperature, battery temperature and discharge rate. They can all be wrong under the right circumstances.

A volt meter is not as simple to use or understand, but if you learn to use it it will tell you a lot more than a battery gauge. At rest a volt meter will give a very accurate state of charge. Under a load a volt meter can tell you a lot about the condition of your batteries.

The main issue using a volt meter is the recovery time of lead acid batteries. It can take minutes of hours for batteries to level out. Fresh off of a complete charge they will read high, after a steep hill with a big load they will read low. Once they recover from charging or a heavy load you will get their true condition.

To sum it up, drive your cart with both, see what works for you.
Thanks, this is very helpful. After reviewing the chart referenced in teecro's post above I realize the voltmeter's information is more complex but probably more practical.

To segue a bit, re your notes about load and charge, would that explain why, after it's 4th lap of lesser loads, my cart 'died' during it's haul of 15 5yo kids and 6 adults?! I was doing hay rides at a holiday party towing a 5 x 8 Lowe's utility trailer with 4 adults/2 kids in the cart, and 2 babysitters/13 kids in the trailer. With lesser loads the cart did great over 4 or 5 runs, but on the 4th or 5th run with 15kids and 6 adults she got half way before crapping out. Let it sit an hour and then she was fine (just driver).
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Old 01-07-2016, 11:27 AM   #5
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Default Re: Voltmeter vs Battery Status Meter - redundant?

edit.
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Old 01-07-2016, 12:14 PM   #6
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Quote:
Originally Posted by ducowti View Post
Thanks, this is very helpful. After reviewing the chart referenced in teecro's post above I realize the voltmeter's information is more complex but probably more practical.

To segue a bit, re your notes about load and charge, would that explain why, after it's 4th lap of lesser loads, my cart 'died' during it's haul of 15 5yo kids and 6 adults?! I was doing hay rides at a holiday party towing a 5 x 8 Lowe's utility trailer with 4 adults/2 kids in the cart, and 2 babysitters/13 kids in the trailer. With lesser loads the cart did great over 4 or 5 runs, but on the 4th or 5th run with 15kids and 6 adults she got half way before crapping out. Let it sit an hour and then she was fine (just driver).
The voltmeter's information is more accurate, but its usefulness and practicality depends on whether the person receiving the information understands what they are seeing.

The voltmeter provides battery voltage in real time, but with a single exception, the battery's voltage is constantly changing, sometimes quickly, other times slowly. The single exception is when a fully charged battery is on float charge. Other than that, the battery is self-discharging, or actively being discharged, or recovering from being actively discharged, or being charged, or the surface charge due to charging is depleting, so the battery voltage is changing.

A typical 36V charger shuts off when the battery pack voltage is about 45V, and over the next 12 to 24 hours the charging induced surface charge depletes and the voltage drops to roughly 38.2V. After that, the battery continues to self discharge, but at about 1% of the storage capacity per day, so the voltage drop is very slow.

It is called a surface charge because it exists only on the outer surface of the plates and the electrolyte in intimate contact with the surface of the plates. The amount of electrical energy stored in the surface charge is such a small portion of the usable energy stored in the battery, it can be disregarded for most range, run-time and SoC (State of Charge) estimates.

The batteries in a 36V battery pack drop about 1.0V per 100A of current being drawn from them. The drop is nearly instantaneous, but the voltage recovery is relatively slow. It takes an hour or more to fully recover, but most of the recovery occurs in the first few minutes. Attached is a chart showing the voltage recovery of a 36V-225AH battery pack after a 10.2 mile run.

As you see, the voltage increased by 0.52V over an hour, which changed the SoC estimate from 68% to 82%. When figuring out when it is time to head back to the house and charge the batteries, using the voltage reading only a few minutes after the cart stops to estimate what is left in the batteries is okay since the error is on the safe side.

However, for doing range estimates based on % SOC used per mile over a measured distance, you will underestimate the cart's max range if you use a voltage reading take too soon. On the posted chart, using the voltage when the cart stopped, the estimated range is 16 miles, while using the voltage after an hours worth of recover is 28 miles. (When doing range testing, I wait 30 minutes, which is over 90% recovered and close enough for my needs)

Another thing that shows up while the batteries are actually under load is that the high current cables, connections and contacts have resistance in them and voltage is dropped across that resistance according to Ohm's Law. However, once the current stops flowing the voltage drop across the resistance disappears.

It takes a while to figure out what the voltmeter is telling you, but once you understand how a battery reacts to current being drawn from it, the fog lifts.

------------
That was quite a load for the cart.

The controller might have gone into over-temperature performance reduction rather than under-voltage performance reduction.

Are you running a stock PDS controller?
How tall are the tires?

With the right upgrades, you hay rides will do better next year.
Attached Images
File Type: jpg Voltage recovery after Load.jpg (93.1 KB, 0 views)
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Old 01-07-2016, 02:29 PM   #7
ducowti
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Quote:
Originally Posted by JohnnieB View Post
The voltmeter's information is more accurate, but its usefulness and practicality depends on whether the person receiving the information understands what they are seeing.
Thanks for the lengthy explanation, also very helpful. Understood on interpretation, and I'm a fairly intelligent fellow so should be fine for me. It's for when I loan it to friends or my brothers I'm more concerned


Quote:
Originally Posted by JohnnieB View Post
------------
That was quite a load for the cart.
I was impressed!


Quote:
Originally Posted by JohnnieB View Post
The controller might have gone into over-temperature performance reduction rather than under-voltage performance reduction.

Are you running a stock PDS controller?
How tall are the tires?

With the right upgrades, you hay rides will do better next year.
AFAIK everything, except the Freedom chip, is stock including the little bitty tars. I'm not with it to confirm, but they're the small ones.
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Old 01-07-2016, 03:19 PM   #8
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Default Re: Voltmeter vs Battery Status Meter - redundant?

JohnnieB explained this very well. The problem for me is the continuous cycling of the digital gage. This would drive me crazy and I would have to put it on a switch so I could turn it off when not needed. But it is real time and more accurate if that's what you want/need.

The three big golf cart manufactures use the Curtis model 906. It is very common. I have attached the manual for it. 3 pages but you just need the first page to get the gist of it's operation. I like it because it operates like a fuel gage in a car and is easily interpreted by casual users of my cart. I tell them when it gets to no more than half empty, bring the cart home. Worked great for the last 15 years.

I'm not trying to advocate for one of the other but either way get a good meter so you don't have to "fix" it frequently.

I attached a chart you can print out if case you go the digital route. Just pick the color and voltage you like, cut it out and tape to the tee holder or however you like. (I lifted it from someone but can't remember who so I can't credit them)


Edit: I may have a problem. Let me know it those attachments don't work
Attached Files
File Type: pdf SOC table tee holder in color.pdf (50.3 KB, 0 views)
File Type: pdf curtis gage manual.pdf (912.0 KB, 0 views)
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Old 01-07-2016, 05:47 PM   #9
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Quote:
Originally Posted by ducowti View Post
1. Thanks for the lengthy explanation, also very helpful. Understood on interpretation, and I'm a fairly intelligent fellow so should be fine for me. It's for when I loan it to friends or my brothers I'm more concerned

2. I was impressed!

3. AFAIK everything, except the Freedom chip, is stock including the little bitty tars. I'm not with it to confirm, but they're the small ones.
1. If I loaned my cart out much, or if my wife drove it much, I'd add a "Fuel" gauge in addition to the voltmeter.

Personally, I prefer an analog type meter for a battery "Fuel" gauge, but I'm running 42V and nobody makes analog (or bar-graph) meters for 42V.

2. As odd as it sounds, you might have been within specs for the cart.
Max recommended loading for cart is 800 lbs, so the 4 adults and 2 kids on the cart might have exceeded that, but the 2 adults and 13 kids on the trailer probably did not exceed the towing limit.

A cart with a 36V battery pack weighs about 1000 lbs and according the the operators manual, you can tow a maximum of three carts.

3. Stock tires are typically 18x8.50-8, which are about 18" tall, but can vary an inch or more in either direction. The numbers on the side wall are group size rather than actual size. (IE: The tires on my cart are stamped 18x8.50-8, but measure only 17" tall.)

------------
Taking the load limits into consideration as well as it being a completely stock PDS, I'm leaning further towards the controller overheating than battery voltage dropping too far.
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Old 01-13-2016, 09:12 PM   #10
ducowti
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Default Re: Voltmeter vs Battery Status Meter - redundant?

Having both will be helpful. Thanks again for your help gents.

(Ignore the ghetto bluetooth radio holder bracket)
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File Type: jpg cart 3.jpg (237.0 KB, 0 views)
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