Forklift Batteries: Publshed Capacity vs ActuAl

Anawa

This thread is a follow-up on a post made in another discussion in the forum concerning forklift battery maintenance. It was stated that the published 20hour rating (on their website) for GB Industrial batteries is inaccurate. Since forklift batteries are "rated" according to their duty cycle (6 hours), the 20hour rating is derived by using a multiplier to the 6hour capacity. A multiplier of 1.35 was posted to convert the 6ah capacity to the 20hr capacity. I acknowledge that I may have mis-understood the post and I'm asking the forum for clarification and/or correction.

I currently use 2 GB forklift batteries in series for 48 volts. I made my purchase last year based upon the published 20 hour capacity of 804ah. If the multiplier of 1.35 applies to these batteries, then I have only 689ah which is a significant disparity in what I thought I was getting. The model number of my batteries: 12-85-13. 

Follow-up by the forum appreciated. 

Paul
in Georgia

BB.

Hi Paul,

I edited your post to talk about 20 "Hour" capacity rating system (standard)--That is the current draw over 20 hours from 100% to 0% state of charge.

The Amp*Hour capacity of a battery, is the resulting number (i.e., draw 20 amps * 20 hours = 400 Amp*Hour).

For lead acid batteries, the higher Amp rate you discharge the battery, the less "apparent" capacity it has (internal resistance, ability of chemicals to react quickly enough, etc.).

GB apparently had an issue with their published 20 hour rating values being incorrect (I guess this is the original thread you were talking about?):

https://forum.solar-electric.com/discussion/350391/determine-ah-capacity

Apparently, if one takes their 6hr rating and muliply by ~1.35x, then you will have a more accurate 20 Hr capacity number (don't know if this is still an issue or not--or what their final resolution was). Note that 1.35x only (probably) applies to this model/brand of GB batteries--It will probably be somewhat different numbers for other brands/models of batteries.

I do not know the present state of the GB issue--Perhaps somebody else here does.

You may have to call your battery retailer and/or GB directly to figure out what is happening now. And if there was a spec sheet error--Then it is possible that they would "make good" on the reduced capacity value.

-Bill

Anawa

Bill, thanks for the edits.

But now, I'm thoroughly confused. Not directly by your response, Bill, but a comparison of the multipliers.

This link at the GB site:  http://gbindustrialbattery.com/Battery_Options/AH_Ratings.html  reflects a multiplier of (1.57+) which is significantly higher than the one 1.35 that PhotoWit used in the post, but it reconciles with the 20 hr published capacity on their website. Perhaps PW will chime-in and let us know if the 1.35 is obsolete, or if GB is using something different, or as usual, I misunderstood. 

I'm not sure what all this means to me if I find out that the battery I bought does not have the capacity I thought becuase I purchased a 12-85-13 and that was what was delivered. At a minimum, I will need to re-set the Mate3 settings if the battery is not 804ah. 

Paul
in Georgia

BB.

There is no one "magic coefficient" for 6 hr to 20 hr rate conversion factor.

What happen with GB... Perhaps somebody did a test once on one model of battery and then used that conversion factor over time and product in a spreadsheet to marketing (or somebody in marketing "did the math" on one product and used it across the product line--trying to be helpful).

Note that the Peukurt Factor (a experimentally derived equation+value from 100+ years ago) is a exponent (i.e. e^x). So, a "small" change in P.F., can have a larger "real capacity effect":

http://www.solar-electric.com/deep-cycle-battery-faq.html

Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some manufacturers and vendors have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets:

Battery Type	100 hour rate	20 hour rate	8 hour rate
Trojan T-105	250 AH	225 AH	n/a
US Battery 2200	n/a	225 AH	181 AH
Concorde PVX-6220	255 AH	221 AH	183 AH
Surrette S-460 (L-16)	429 AH	344 AH	282 AH
Trojan L-16	400 AH	360 AH	n/a
Surrette CS-25-PS	974 AH	779 AH	639 AH

Some Peukert Exponent values (not complete, just for info). We don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20

Note that simple flooded cell have larger PF value. AGM, Li Ion and other batteries have PF closer to 1.0 (1.0=no capacity change based on discharge current level).

https://en.wikipedia.org/wiki/Peukert's_law

For a one-ampere discharge rate, Peukert's law is often stated as

C p = I k t , {\displaystyle C_{p}=I^{k}t,}

where:

C p {\displaystyle C_{p}} is the capacity at a one-ampere discharge rate, which must be expressed in ampere-hours,

I {\displaystyle I} is the actual discharge current (i.e. current drawn from a load) in amperes,

t {\displaystyle t} is the actual time to discharge the battery, which must be expressed in hours.

k {\displaystyle k} is the Peukert constant (dimensionless),

The capacity at a one-ampere discharge rate is not usually given for practical cells.[citation needed] As such, it can be useful to reformulate the law to a known capacity and discharge rate:

t = H ( C I H ) k {\displaystyle t=H\left({\frac {C}{IH}}\right)^{k}}

where:

H {\displaystyle H} is the rated discharge time (in hours),

C {\displaystyle C} is the rated capacity at that discharge rate (in ampere-hours),

I {\displaystyle I} is the actual discharge current (in amperes),

k {\displaystyle k} is the Peukert constant (dimensionless),

t {\displaystyle t} is the actual time to discharge the battery (in hours).

Go to the Smart Gauge website--They have A LOT of white papers about Peukert factor, spread sheets, and such if you want more information (there are, at least, 2 pages in their Technical section):

http://smartgauge.co.uk/technical1.html

Of course, none of this helps with GB's issue (other than understanding the issues)... Talking to your supplier/GB tech support is probably the place to start--Hopefully, somebody else here can give an update on the issue.

-Bill

cgserrant

I had the same issue going off GB calclation from 6 hour rating to 20 hour rating as my battery SG was not matching. Now my battery is not GB but Enerysys General forklift battery. After contacting enerysys  5 times a tech finally said to me the conversation is 1.245 of the 6 hour rating. After changing in my system voltages and SG seem to be inline. Hope this helps. Also enersys are also the makers of solarone batteries have the same tech as these http://www.enersys.com/HUP_Batteries.aspx? They too have a line of solar battery http://www.enersys.com/PowerSafe_RE_Batteries.aspx?langType=1033

Anawa

Thanks cg for the comment. I've had a lot of problems reconciling my SG with the "rated" ah for the batteries. This may very well be the reason. Looks like l may need to re-set my system and see what happens. 

Paul
in Georgia

cgserrant

I charged my batteries at 31 volts temp compensated till SG was 1.265 to 1.275 then reset the flexnet dc with the adjusted 20 hour rate from the calculated 6 hour rate using 1.245 multiplier. All seems good now but I am also upset that I have less amp hours than I thought  but at the same time I  now that I will not kill my battery /Investment by cycling too low. On the good side I have taken these batteries under load to 40% and the still held a load of 1000 watts but then my gene kicked in and they took 170 amps from gen and 70 from the solar to bulkup. greedy puppys. Big lead plates can take lots of power. With forklift batteries I find the best guide is solar one chart http://www.hupsolar.com/pdf/HUP-Installation-Manual-Users-Manual.pdf. Is a good starting point as every system different.

Cheers
Chris

cgserrant

Also how old are the batteries?

Anawa

I put them into service in early March 2015. The Solar One manual for forklifts is very good, thanks. I'll post an update on my situation soon.

Paul
in Georgia