Battery Voltage vs. State Of Charge?

[William P. N. Smith]

Hi,

One of my golf carts has one of those Curtis meters that reads about
50% charge state when the golf cart controller thinks the battery is
fully discharged (and then goes into a slow, limp-home, battery
preservation mode). Which one is right, and what's the common
rule-of-thumb for voltage versus state-of-charge. Trojan T-145s in a
48V bank, if it matters. It's hard to get the Curtis meter out to
read it's exact configuration model number, but that's on my list.

I've been using 11.7 for discharged and 12.7 for fully charged for a
12V nominal battery bank in my weather station, which I probably
picked up from this newsgroup, is that pretty good?

[Yeah, I realize that voltage is pretty inaccurate without letting the
battery bank sit for 24 hours, but I don't have that kind of time to
take a cart out of service...]

Thanks in advance for any thoughts!

 

[Steve Spence]

Have you read http://www.uuhome.de/william.darden/carfaq9.htm ?

Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
http://www.rebelwolf.com/essn.html

 

[William P. N. Smith]

rule-of-thumb for voltage versus state-of-charge.

I've found my numbers. At one point:

Fluke DVM reads 49.0V
http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/Testing.aspx
says that's ~60% charged

The controller on the cart reads 48.8V, and claims that's 39% charged

The Curtis "battery meter" reads 8 segments out of 10. Curtis meters
read one of three discharge profiles:

N = 2.04VPC full, 1.73VPC empty
G = 1.97VPC full, 1.75VPC empty
W = 2.02VPC full, 1.85VPC empty

I can't make any of these correspond with the Trojan numbers above.

Specific Gravities read from 1.170 (50%) to 1.201 (65%)

My Brain Hurts! 8*)

[I'll take fully charged readings tomorrow morning...]

 

[William P. N. Smith]

Have you read http://www.uuhome.de/william.darden/carfaq9.htm ?

If you mean http://www.uuhome.de/william.darden/carfaq9.htm#charged

I've just read it, but it doesn't tell me anything new, except that
battery manufacturer's numbers trump everything else.

 

[Bruce in Alaska]

Hi,

One of my golf carts has one of those Curtis meters that reads about
50% charge state when the golf cart controller thinks the battery is
fully discharged (and then goes into a slow, limp-home, battery
preservation mode). Which one is right, and what's the common
rule-of-thumb for voltage versus state-of-charge. Trojan T-145s in a
48V bank, if it matters. It's hard to get the Curtis meter out to
read it's exact configuration model number, but that's on my list.

I've been using 11.7 for discharged and 12.7 for fully charged for a
12V nominal battery bank in my weather station, which I probably
picked up from this newsgroup, is that pretty good?

[Yeah, I realize that voltage is pretty inaccurate without letting the
battery bank sit for 24 hours, but I don't have that kind of time to
take a cart out of service...]

Thanks in advance for any thoughts!

If your figuring that 12.7 Vdc is a fully charged 12VDC Wet Cell then
you way undercharging the battery. Most folks figure 13.6Vdc for a
fully Floated charge, and 13.2Vdc for a fully Bulked charge. That's with
Temp Compensation considered. Check the Trojan Website, or ask the
Battery Technical guy where you buy your replacements.....


Bruce in alaska who uses 52.8 Vdc for our 48Vdc forklift chargers...

 

[William P. N. Smith]

If your figuring that 12.7 Vdc is a fully charged 12VDC Wet Cell then
you way undercharging the battery.

Hi Bruce. I wasn't talking about charging voltage, I was talking
about operating voltage. How do I tell, from just looking at the OCV,
how much of a charge I have left in my batteries?

 

[Steve Spence]

well, last I read it it gave info on full charge voltages and discharged
voltages.

Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
http://www.rebelwolf.com/essn.html

 

[[email protected]]

Hi Bruce. I wasn't talking about charging voltage, I was talking
about operating voltage. How do I tell, from just looking at the OCV,
how much of a charge I have left in my batteries?

Generally accepted opinion is cell voltage of today's alloyed lead
batteries is a very poor indication of state of charge. Particularly
open circuit voltage. IIRC pure lead batteries like some of the
Hawkers are more accurate.

 

[William P. N. Smith]

Generally accepted opinion is cell voltage of today's alloyed lead
batteries is a very poor indication of state of charge. Particularly
open circuit voltage.

Oh, I understand that, but what other means do I have of telling how
much further my golf cart will go (for instance) than measuring
voltage?

[Yeah, the Club Cars watch amp-hours and light a separate indicator,
but all of them have the Curtis meters on them nowadays...]

 

[William P. N. Smith]

The curtis is adjustable, and you can make it read anything you want.

Ah, I must have a different one than you do. All the golf carts I've
ever seen use
http://curtisinstruments.com/index.cfm?fuseaction=cProducts.DownloadPDF&file=revC_newdes906.pdf

Which are fixed voltage.

 

[[email protected]]

Ah, I must have a different one than you do. All the golf carts I've
ever seen use
http://curtisinstruments.com/index.cfm?fuseaction=cProducts.DownloadPDF&file=revC_newdes906.pdf

Which are fixed voltage.

The one I'm familliar with is the much more complex 933 series. Mine
is a 933-E3648 it is a 34-48 volt auto-range unit, and by connecting
it across only part of the battery pack, can apparently be used to
monitor higher voltage packs.It also performs an automatic "lockout"
when voltage reaches a programmed low. The meter resets itself to full
when a properly charged battery is connected - or a partially charged
battery if the voltage meets the minimum reset voltage programmed.

 

[wmbjk]

Generally accepted opinion is cell voltage of today's alloyed lead
batteries is a very poor indication of state of charge. Particularly
open circuit voltage.

Oh, I understand that, but what other means do I have of telling how
much further my golf cart will go (for instance) than measuring
voltage?

[Yeah, the Club Cars watch amp-hours and light a separate indicator,
but all of them have the Curtis meters on them nowadays...]

You might check out the owner's manual for the Link 10 (Emeter) here
http://www.xantrex.com/web/id/72/docserve.asp Even with all the
customization available with that device, it's still not perfect. Are
the Curtis models on the Club Cars similarly programmable?

Wayne

 

[William P. N. Smith]

You might check out the owner's manual for the Link 10 (Emeter) here
http://www.xantrex.com/web/id/72/docserve.asp Even with all the
customization available with that device, it's still not perfect. Are
the Curtis models on the Club Cars similarly programmable?

Nope, they look at voltage only, though they have some proprietary
algorythm for figuring state of charge. On my Club Cars they seem to
work pretty well, but the 4WD buggy with the T-145s is _way_ off. I
need to go back to the designers and see what they think might be
wrong, but it'd be nice to be able to know what's 'right' before I
do...

They are not programmable, except that you can buy them in the three
versions denoted in a previous missive. I dunno which version the 4WD
version uses...

I've looked at the Emeter, and it looks interesting, but it feels like
it's just be another source of error.

 

[Bruce in Alaska]

Hi Bruce. I wasn't talking about charging voltage, I was talking
about operating voltage. How do I tell, from just looking at the OCV,
how much of a charge I have left in my batteries?

You can't, you can only guesstimate and thats with a +/- 20% guess.
Too many variables between battery systems, but you might be aboe to
make a line graph for a single battery system imperically....


Bruce in alaska

 

[wmbjk]

Nope, they look at voltage only, though they have some proprietary
algorythm for figuring state of charge.

Then something like the Emeter would be a huge improvement. Although
at ~$200, it may be overkill for a golf cart. I can tell you that on
*our* home power setup, having to assess battery state of charge by
voltage would be, dare I say it... suspect design.

On my Club Cars they seem to
work pretty well, but the 4WD buggy with the T-145s is _way_ off. I
need to go back to the designers and see what they think might be
wrong, but it'd be nice to be able to know what's 'right' before I
do...

They are not programmable, except that you can buy them in the three
versions denoted in a previous missive. I dunno which version the 4WD
version uses...

I've looked at the Emeter, and it looks interesting, but it feels like
it's just be another source of error.

When you read about the logic is uses, it's easy to see why there's
going to be some variations in getting it to consistently recognize
that a full charge has been completed, and therefore to reset. Most of
the variables in programming for that don't exist in the golf cart
application I wouldn't think. I bet that if you tried an Emeter (or
any of the similar competitors), you'd never go back to monitoring by
voltage alone.

Wayne