battery charging 201

[shrpshtr]

all,

now that I have a fancy DVM installed (thanks scottyB) I am seeing my battery SOC real-time. what I am noticing is that it consistently is charging my pack to 39.1-39.3v. I've verified that SOC @ 12 & 24 hrs post-charge with a pretty accurate fluke meter. I've also checked each battery to make sure that they are properly hydrated and that doesn't seem to be a problem. at rest, it appears to lose about .1v per day.

should I be concerned? i will usually plug the charger in once-a-week if not driving it and after each time out without exception of discharge amount, distance traveled, time running, etc.

thanks in advance.

shrp

[JohnnieB]

A pack voltage reading of 39.1V to 39.3V after 12 hours rest, is a bit on the high side.

What make/model batteries and how old are they?

What make/model charger are you using?

A self-discharge rate of 0.1V per day equates to loosing about 3% SoC per day, which is also might be a bit high (1% is more typical), but could be due to a parasitic drain or how the discharge rate was determined.

[shrpshtr]

The batteries are Trojan T-105's about 14 months old IIRC. The charger is the PowerWiseII charger that came with the cart. It's worth noting the voltage has held at 38.7 the last 24-36 hours or so now.

[JohnnieB]

Trojan put a date stamp on the negative terminal. Letter is the month and number is the year. Fourteen months ago would be Aug-15, so they would be stamped H5.

A PW-II is a good charger.

The At-Rest voltage is a bit high, but some batteries run a bit higher than others. Check the individual battery voltages. All six should be no more than 0.1V from the others.

[shrpshtr]

checked the batteries tonight and they are stamped E5. so, they are from may 2015. i haven't charged the pack in a week now. DVM shows (& confirmed by fluke) 38.5v. each battery is exactly 6.4v.

did i actually break them in optimally/properly to get the "added" voltage? Or maybe just get lucky and have a strong pack or something?

not complaining, just want to make sure i'm not over-charging or wearing the batteries out.

shrp

[mutant]

hi shrp-i have been experiencing the same thing-12 hr after charging my pack is showing 39.8 and are all 6.6 individually...they also have a similar daily discharge rate....batteries are carquest(east penn/deca) and my charger is a 28115 304 powerwise....
i was very through with topping off the batteries after every use while i was breaking them in-seeing this is my first electric cart and i am being very particular-this cart was abused before i got her and i am amending her history.

[JohnnieB]

Must be a Carolinas thing.

I'm not sure why both sets of batteries are reading higher than average, but I have to point out it is an average, so by definition, some batteries will read higher and some will read lower.

At some point in time the Trojan Battery company (and other battery manufacturers) ran tests on a number of their products. I don't know what the number is, but I suspect they used a large sample size of randomly selected batteries from the standard production run. Statistically, 68% of the Trojan batteries tested were within 1-SD (Standard Deviation) of 2.122VPC (Volts per Cell), 95% were within 2-SD and 99.7% were withing 3-SD, but I don't know what the voltage is for 1-SD, so I don't know how meaningful the deviation your batteries are away from average is.

The battery voltage is mostly determined by the concentration of sulfuric acid in the electrolyte and temperature. The nominal SG of the electrolyte in a Trojan buttery is 1.277, so the electrolyte is 27.7% sulfuric acid and 72.3% water by weight.

So at 2.211VPC (39.8V pack voltage), I suspect the SG of the electrolyte in mutant's batteries was 1.366 and at 2.183VPC (39.3V pack voltage), I suspect the electrolyte in shrpshtr's batteries was 1.338.

Since batteries don't manufacturing sulfuric acid, the question is how did it get there?

One possibility would be that higher SG than normal electrolyte was added to the batteries at the factory.

Another possibility would be that electrolyte (Battery acid) was added instead of distilled water to bring the fluid up to the proper level above the plates.

Another possibility would be stratified electrolyte. Being heavier than water, the sulfuric acid sinks towards the bottom, so after a battery sits for a while without being agitated, the SG of the electrolyte towards the bottom of the cell is higher than the electrolyte at the top of the cell. When you measure the voltage, you read the highest voltage on the entire plate.

Another possibility would be the batteries are very low on water. During gassing, only water is converted to gases. Since the same amount of sulfuric acid is now mixed with a lesser amount of water, the SG of the remaining electrolyte is higher.