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Manoc396
29 September 2010, 1047
Hi,
I was on EM and getting my bike running right around the time the site crashed, so I'm just now getting back to posting some progress. My 2006 Lifan conversion (http://www.evalbum.com/2388) is running really well overall but I'm concerned about the batteries. I already searched and checked the "How To:" section.

Batteries: six, 12-volt 35Ah, AGM.

Chargers: two, 36-volt, 4A chargers which are supposed to be 3-stage chargers (bulk, absorb, float).
http://www.batterystuff.com/battery-chargers/36-volt/JAC0891-102.html
The charger has an LED on it which is amber while actually charging but should turn green when it is float-charging.
In case it helps, I've posted some of my charge logging values at http://myelectriclifan.blogspot.com/

Question 1:
What voltage should I really expect for a full charge?
At the moment, I expect each battery to be fully charged at 12.9 volts each, making the pack 77.4.
I've used the PakTrakr to watch the batteries, and when it says 83 or 84 volts while charging, I know the pack will settle around 77 volts when I unplug the chargers. However, the chargers don't turn green and I don't know how far to push it. There were two large sparks at battery terminals as we connected charging leads months ago, so in my mind, there's a chance one or both chargers are damaged. All the batteries seem to take, hold, and release a charge just fine, but I've never had the guts to let the chargers run long enough to turn Green. Before the site crashed, I could have sworn somebody mentioning the pack having more like 80 or 85 volts at a full charge, but that doesn't jive with any numbers I've found in research.

Question 2:
How come some batteries charge way "ahead" of others during charging?
I consider the pack relatively balanced because there's only .2 volts difference between the highest and lowest 12v battery. (If that's not as good as I think, please let me know.) Still, as the PakTrakr shows most batteries reaching 13.1 or 13.3 volts while charging, some batteries (the highest 1 or 2 of the pack) are shown to be at voltages like 14.8 and even 15.3 which really worries me, because it seems outside the appropriate charge voltage for AGMs. Could this be the two, 36-volt charger configuration? Is this dangerous/unhealthy for the batteries? Should they be more balanced?

Question 3:
How discharged is too discharged, and is it resting charge or riding sag that should be considered?
I've researched the lowest voltage to which an AGM deep cycle can normally be taken repeatedly in order to preserve its life as much as possible, but I don't know if voltage sag while riding is equivalent to being that deeply discharged. I can start a ride with the pack resting at 75 volts, go about 10 miles, park it and the pack will rest an hour later at like 72 volts. However, in transit, I may have sagged the batteries below 63 volts, so is 63 volts too low (and life-shortening) or just a consequence of how/where I rode? My research left me with the impression that I should just try not to discharge them such that they rest below 70. And of course, not to leave them discharged for long.

By the way, I had the idea for a series of organized pages, like a "top 10 things to do" and "top 10 thing NOT to do" for the various types or each various component. Like one of them would be battery sulfation maybe or, as Frodus likes to harp on (j/k)... not using a 12 volt switch for 72 volts! If it contained both the top 10 items and facts/experience to support them, I think it'd really help noobs like me and maybe save people some money and frustration too.

Thanks for your help,
-Colby

P.s. Recent video: http://www.youtube.com/watch?v=mxkecR8QJho

frodus
29 September 2010, 1215
Before I start, here's a VERY good website devoted to batteries:
http://www.batteryuniversity.com/




Question 1:
What voltage should I really expect for a full charge?
At the moment, I expect each battery to be fully charged at 12.9 volts each, making the pack 77.4.
I've used the PakTrakr to watch the batteries, and when it says 83 or 84 volts while charging, I know the pack will settle around 77 volts when I unplug the chargers. However, the chargers don't turn green and I don't know how far to push it. There were two large sparks at battery terminals as we connected charging leads months ago, so in my mind, there's a chance one or both chargers are damaged. All the batteries seem to take, hold, and release a charge just fine, but I've never had the guts to let the chargers run long enough to turn Green. Before the site crashed, I could have sworn somebody mentioning the pack having more like 80 or 85 volts at a full charge, but that doesn't jive with any numbers I've found in research.

That sound about right. Most 12V individual chargers charge AGM to somewhere between 13.8 and 14.5V in my experience.
Look here:
http://www.batteryuniversity.com/partone-13.htm
Each 12V battery has 6-cells, so 14.4V is about 2.4V a cell. 6 of these for a 72V system would be 86.6V fresh off the charger. This is not an issue unless other batteries (see question 2 answer) are charged quicker than others.


Question 2:
How come some batteries charge way "ahead" of others during charging?
I consider the pack relatively balanced because there's only .2 volts difference between the highest and lowest 12v battery. (If that's not as good as I think, please let me know.) Still, as the PakTrakr shows most batteries reaching 13.1 or 13.3 volts while charging, some batteries (the highest 1 or 2 of the pack) are shown to be at voltages like 14.8 and even 15.3 which really worries me, because it seems outside the appropriate charge voltage for AGMs. Could this be the two, 36-volt charger configuration? Is this dangerous/unhealthy for the batteries? Should they be more balanced?

They should be balanced at the top of the charge, Not at rest. Rest may or may not give you an idea of how the battery SOC is. Do this, charge each battery individually with a 12V smart charger until it is completely finished. If you don't start out with an equalized pack (at full charge), you'll see your pack become unbalanced. Thats one of the main problems with series charging the pack, the pack could become unbalanced, because all batteries may not discharge the same due to slight differences in manufacturing. Try not to go above 15V a battery. The fact that you split into two "packs" at 36V each is a good idea. Better than one at 72V. You really need to post the model/make of the battery you're using. All 35Ah batteries are NOT created equal.




Question 3:
How discharged is too discharged, and is it resting charge or riding sag that should be considered?
I've researched the lowest voltage to which an AGM deep cycle can normally be taken repeatedly in order to preserve its life as much as possible, but I don't know if voltage sag while riding is equivalent to being that deeply discharged. I can start a ride with the pack resting at 75 volts, go about 10 miles, park it and the pack will rest an hour later at like 72 volts. However, in transit, I may have sagged the batteries below 63 volts, so is 63 volts too low (and life-shortening) or just a consequence of how/where I rode? My research left me with the impression that I should just try not to discharge them such that they rest below 70. And of course, not to leave them discharged for long.

10.5V (1.75V/cell) each 12 battery is the 80% DOD. That would be 63V. Don't take it below this. If you can set low voltage cutoff in the controller, set it to 60V.




By the way, I had the idea for a series of organized pages, like a "top 10 things to do" and "top 10 thing NOT to do" for the various types or each various component. Like one of them would be battery sulfation maybe or, as Frodus likes to harp on (j/k)... not using a 12 volt switch for 72 volts! If it contained both the top 10 items and facts/experience to support them, I think it'd really help noobs like me and maybe save people some money and frustration too.

Did you see Ed's post? It seems we both agree on several things :)
http://www.elmoto.net/showthread.php?350-12V-switchs-on-72V-applications-(part-2)&highlight=switch

It may be a good idea to do something like this:
http://www.diyelectriccar.com/forums/showthread.php/want-build-ev-do-starti-6441.html