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Discussion Starter · #1 ·
This is for a 72 Volt pack of flooded cell batteries. It now takes long time to charge or the SOC meter never reports full charge. The problem developed after the vehicle was run at higher speeds and battery loading than normal. I fear there are shorted cells that are permanently limiting the pack voltage. Any techies know what is going on? Thanks.
 

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Check for loose connections, etc first. You probably did this already.

check each battery voltage individually after the pack has rested for several hours. Or even better, check specific gravity on each cell. If any of them are way out of whack from the others, Then you might try using a 12V (or 6V, depending on your pack makeup) car battery charger to individually top off each battery. If the 12/6v charger output is isolated you won't need to take any cabling apart to do this. charge each battery individually until it starts gassing in all its cells and then move on to the next one.

If even after manually balancing everything you have continuing problems, then yes you may have a damaged battery.

Fairly recently I had my first "low voltage experience" after trying to drive a bit too far on a bit too cold of a day on the freeways around here. I made it home but my 126v nominal pack voltage was off the bottom of the scale (under 80v) at a 30mph load by the time I got back. My pack bounced back just fine as far as I can tell, so I suspect as long as your batteries are balanced and not physically damaged they should too.

Good luck
 

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Discussion Starter · #3 ·
Madderscience, thanks for the troubleshooting suggestions. I will check the connections and balance the batteries if necessary. I forgot to mention the SOC meter sometimes jumps from 70 to 100% after the car is driven a bit. This might indicate connection problems. Other possibilities I thought of... 1) faulty info from the Curtis controller to the gauge 2) sediment that moves around the bottom of cells and shorts the cells. Are these possible? Any other causes? Just brainstorming, in case the connections check out OK. Thanks again for the tips.
 

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without knowing what algorithm your SOC meter uses I can't say whether jumping around is a problem or not. if it is analog, it is a specially calibrated voltmeter, nothing more. If it is digital, it could be the same, or it could be going so far as to be counting KwH and be taking into account rate of discharge, temperature and battery type in computing SOC, depending on how fancy it is.

Sediment shorting out cells is unlikely to be an issue that comes and goes, and is unlikely unless your batteries are old (or damaged) or perhaps a cheapo brand.

I am not aware of any SOC meters that talk directly to the controller but that does not mean that they don't exist.

Knowing model number of controller, SOC meter, and manufacturer/model of battery etc might allow for more than just general suggestions from the group.

Good Luck.
 

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I had a similar problem recently where I simply couldn't seem to get my pack up to a full charge. I don't have a SOC meter, but my voltage levels were a bit lower than I was used to and just couldn't seem to get them up.

After checking connections (all were fine) and checking the voltages of all the individual batteries (all were within .01v) I checked the fluid levels in the batteries themselves. I ended up adding almost 4L of water to the batteries that day! I hadn't checked the levels in the batteries since I'd installed them in the car and I'd driven them pretty hard over about 500 miles. Once the water levels were increased, I'm not getting a proper charge.

This may or may not be your issue but it's worth a check.
 

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Discussion Starter · #6 ·
Thanks ZeroGasoline and all posters. I filled the water yesterday so that might make the difference. Individual voltages were nearly identical so there do not appear to be bad batteries. Connections checked OK.

In this case the SOC meter is driven by the Curtis controller over a serial bus. If the problem comes back, I may see if Curtis has anything to say about the situation. The odd thing is once the car was driven just a few feet, the SOC went to 100%! I am wondering about surface charge, but I don't know very much about the phenomenon.
 

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I am wondering about surface charge, but I don't know very much about the phenomenon.
I'm wondering about this as well. I know this is a bit off topic, but how long would you expect surface charge to last? I took my batteries off of the charger yesterday, and two hours later I got a pack voltage reading of 79.9 volts (13.31v each). From what I keep hearing, my max voltage should be more like 78 volts (13v each). I figured that surface charge would dissipate after maybe 30 or 45 minutes, but it seems to be lasting much longer.

Can anyone clear this up?
 

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The surface charge can last a long while. I usually wait over 6 hours before taking any readings on my batteries for my weekly charting. If you have other devices on your batteries (the devices could be desulfators, equalizers, etc), the time would be less because they are slowly eating away at the surface charge. But the surface charge can last a long while.
 

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Discussion Starter · #9 ·
I'm wondering about this as well. I know this is a bit off topic, but how long would you expect surface charge to last? I took my batteries off of the charger yesterday, and two hours later I got a pack voltage reading of 79.9 volts (13.31v each). From what I keep hearing, my max voltage should be more like 78 volts (13v each). I figured that surface charge would dissipate after maybe 30 or 45 minutes, but it seems to be lasting much longer.

Can anyone clear this up?
Here is the best description of surface charge I know of...

http://www.smartgauge.co.uk/surf_chg.html

This effect is actually the opposite of what I was seeing. ZG, in your case, surface charge caused the batteries to exhibit a higher voltage initially and then drop. In my case, the SOC meter reported a lower condition and then jumped up. I think this is relevant because I can probably eliminate surface charge as the culprit. I am still checking to see if adding water solved the problem.
 

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I'd suggest reviewing your battery charging regime. You should be taking your pack up to at least 2.43V per cell at ~every charge~, and holding it there for an hour or more. Once every couple of weeks, up the charge voltage to 2.5V per cell for a few hours to equalize the cells. Then you'll begin to see water usage, which is a good thing and your batteries will have a better chance to live out their full life expectancy.

Also, don't forget that temperature makes a significant difference in cell voltage, both during discharging and charging. You need to adjust the voltage up in the cold months to get a full charge.

Water usage is a normal -and a necessary- part of good battery maintenance. Use distilled water only.
 

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