just confirming my math here... The T-105 is rated 225ah at the 20 hour rate. So 225ah/20h = 11.25a would be my target current for the capacity "test" right? and it should take around 10 hours to reach 50% DOD right?
I think I'll try it out today. The trailer has been plugged in for the last few days, it's at 100%. I'll just keep flipping on lights till I get to ~11a and keep an eye on the voltage and ah counter for the day.
1998 Ford E350 extended van, V-10, 3.73
1998 Fleetwood Terry 22lw, 24' long, 5000 pounds
Equalizer, Prodigy, Link-10, CPE 2000w generator
camperdave wrote: just confirming my math here... The T-105 is rated 225ah at the 20 hour rate. So 225ah/20h = 11.25a would be my target current for the capacity "test" right? and it should take around 10 hours to reach 50% DOD right?
I think I'll try it out today. The trailer has been plugged in for the last few days, it's at 100%. I'll just keep flipping on lights till I get to ~11a and keep an eye on the voltage and ah counter for the day.
The math is right, except that you'd like the 20 hour capacity rate for the capacity it has now, not the capacity it had when you bought it. Of course, you don't know the correct capacity now.
camperdave wrote: just confirming my math here... The T-105 is rated 225ah at the 20 hour rate. So 225ah/20h = 11.25a would be my target current for the capacity "test" right? and it should take around 10 hours to reach 50% DOD right?
Before starting take the SG to make sure they are really full. Might need an"overcharge" to get them full.
During the draw-down, your voltage will drop to about 11.6 by the time they are near 50%. To confirm and reduce the hit and miss stuff with voltage bounce back, take the SG.
It should be around 1.175 at 50% with loaded voltage near 11.6 and then you stop and let the voltage come back for an hour. If the voltage bounce back gets very slow at about 12.1 after an hour you can call it 50% close enough for the test.
With no SG measurement, you go to say 11.6 and do the one hour bounce back and see where it is. Then you might have to go back to discharge some more and over-do it. It can get time-consuming fooling around. The SG marker really saves you a lot of trouble.
The lights at 11.25 amps will dim as voltage drops during the test and amps will reduce as they dim. To keep a steady draw you have to turn on another light every so often or whatever, to maintian a steady load. If your load is an inverter powering 120v lights, then it goes the other way, where the inverter draw increases as voltage drops.
It is hard to keep a steady draw the whole time, but something close will be ok for saying if the batteries are near their ratings
2003 Chev 2500HD Gas, 2003 Komfort 26FS 5er
See Profile for Equipment List
As most people here know, the rate at which you draw down the battery affects its capacity by the Peukert formula. The battery is drawn down at 1 amp until the voltage reaches 10.5 V. That's the Peukert Capacity Cp, and that number can be used in the formula with the Peukert Exponent k to figure out how long (t in hours) you can draw I amps out of the battery before reaching 10.5 volts.
The formula is:
Cp=I^k * t
The energy stored in the battery can't just disappear. The P formula combines (at least) two effects - the actual energy lost due to resistive heating is one. That energy is really lost. The higher the current, the more energy is lost as heat.
The other effect is that there is a loss of voltage at the surface of the plates "surface discharge" and there are diffusion effects that cause the battery to reach 10.5 volts (battery determined to be empty) at current "I" even though if you let it recover for a while, the diffusion of the battery acid will result in some increase of voltage to above 10.5.
If you want to know the Peukert exponent k, you need to do multiple discharge tests at different rates, then solve for k.
Of course, most battery monitors don't use Peukert, and those that do have to fudge it to predict the future and guess how fast you will withdraw the remaining energy - since the remaining capacity depends on rate of withdrawal.
alright, I'm now 5 hours in. I didn't check specific gravity when I started, but did go buy a meter this morning so I can check it after I'm done (a little plastic meter, $5). I am just assuming that I was close enough to 100% charged since I've been on a maintenance charge for a few days now.
I did take a few SG readings after about 2 hours, they tested good, but one cell was weird in that it tested off the top of the chart. The other 5 cells were right where they ought to be.
So far after 5 hours, according to the Link-10 I have removed 55ah (75% SOC) and show 11.82v (it was flipping between 11.80 and 11.85 so I just split the difference) while drawing 10.9a. I was drawing 11.5a when I started so it's dropping a little, but not quite enough to warrant an added light yet.
I'm not going for total accuracy here anyway, just a ballpark. Fwiw, it was ~50f overnight, about 65f when I started, and is currently 85f. It will probably climb a little hotter into the 90's today.
I know a little about the Peukert effect, but was thinking it would be pretty negligible at ~10a. My actual draw in camping usage is typically more like 3-5a at most, so I figure this "test" will be a little more worst case than my actual usage.
Looking good. The second quarter may not be linear though if capacity is reduced and you overshoot 50% of the actual capacity
That 11.6v is a WAG based on what I get. It can vary depending on the wiring etc in your set-up.
IMO use the AH count to get to assumed 50% of rating, stop and check the voltage after an hour and SG (that style hydrometer is not as accurate as the glass tube with the bobber) Then if those indicate you are at say 40% you know your capacity is reduced.
You can't do successive tests without returning to true full with an overcharge. You get progressive capacity loss otherwise which means each test is with less capacity so you can't compare them. So it is a one shot test until the next time you can do an overcharge (such as an "equalize")
Running 500 watt working lights off of an inverter that powers down at 10.5 volts and timing how long it keeps the lights burning is the most practical way that I have found.
We do the same thing here at work with the UPS. We cut shore power and run off of battery power unit the UPS drops the load and time the minutes to fail time.
They give that extra data but I do not like getting battery acid every where. I really hate pulling those readings but new glasses make make it easier.
Now that I have flipped the polarity of the eight battery back to the OEM polarity and tests shows we have greatly recovered capacity I may pull some SG readings on the batteries that test out the best just for the heck of it.
* This post was
edited 07/31/12 09:54pm by Gale Hawkins *
camperdave wrote: I don't want to find out in the middle of a winter storm in December that I only have 80 ah available instead of the 110 that I think I have...
110 is half. If you get stuck with 160 capacity and continue to use 110 to get through that winter storm a few times those batteries will not even blink. No need to baby them if you are fine replacing them as soon as they do not perform. Nothing to lose at this point for cycling them all the way to 100% dod if needed.
Alright... here's my results. fwiw, I based my % numbers on the info on Trojan's website
So after removing 111ah over a bit more than 10 hours, I disconnected the batteries and let them sit overnight. Fwiw, the final voltage under a 10.8a load at 111ah removed was 11.52v
This morning the cells were all at around 50f, and the voltage had recovered to 11.96v. A few hours later, for a total of 15 hours after the load was removed, the batteries were now at 70f, and read 11.97v. I figured that was a good enough amount of time for voltage recovery and did specific gravity checks for each of 6 cells. below I'll list the cell, the SG reading, and the approximate state of charge % based on the Trojan website linked to above.
Total voltage over both batteries is 11.97v which is right around 40%. Measuring each battery individually gives me 6.03v (cells 1-3) which is ~50% and 5.93v (cells 3-6) which is ~35%.
So... I think I'm basically in OK shape, clearly one battery is stronger than the other but on the whole it looks like my overall capacity is down around 20% from specs after 7 years. Not bad! Should be able to get them through one more winter no problem.
Offline
The other 5 cells were right where they ought to be.
