Battery voltage drops while charging?

[PNjunction]

.. I got my Battery Tender in the mail today. That's the stronger 5A charger I was talking about earlier. So I plugged it in and since it uses the same quick connect cable type as the BatteryMinder Plus I've been using I just popped the rings that were already on the battery into the cable and the charge light didn't come on...

Make sure that you connect the battery and THEN apply ac power to the charger. That got me as well with my battery tenders at first.

[samcrut]

Make sure that you connect the battery and THEN apply ac power to the charger. That got me as well with my battery tenders at first.

Yeah. I've noticed that. I've had to "reboot" the charger if I moved it from one bank to another one. Seems odd but it was pretty cheap so not complaining too much.

A little update...
The batteries are charging almost up to 15v now. I got my hydrometer in and most of the cells are in the red. Some are in the blue. Red is low, blue is fine, green is good. No green yet. I'll keep it on the Battery Tender another day to top it off as much as possible and then put it back on the BatteryMinder Jr to get back to desulfation.

[samcrut]

I think these batteries are pretty much revived. I haven't left them off the charger long enough to get a cold read on them but they're holding at about 13v now and the hydrometer shows several cells up in the green (good) zone and the rest are in the blue (fair) zone. Nothing in the red at all. I'm not sure which did more good, the Battery Tender or the BatteryMinder Plus, but now that they're showing a good charge, I'll leave them on the BatteryMinder Plus to see if it can't desulfate them further to get some of those blue cells up into the green.

I'll get a load tester once some checks come in. No way I'm going to sit there and listen to my air compressor blaring away for who knows how long to find out if the amp hours are there.

Very happy with the results.

[Sunking]

I think these batteries are pretty much revived. I haven't left them off the charger long enough to get a cold read on them but they're holding at about 13v now

A fully charged battery should measure 13.6 to 13.8 volts on float charge.

and the hydrometer shows several cells up in the green (good) zone and the rest are in the blue (fair) zone. Nothing in the red at all.

You hydrometer means nothing, it is not capable of measuring the specific gravity. The type you have is about as useful as the old IDIOT lights in cars like the HOT light. To be useful it has to be capable of temperature compensation and actually read the SPG like 1.2250.

Very happy with the results.

Wait until you do an actual load and capacity test. I bet the battery voltage crashes within minutes with a C/20 load.

[samcrut]

Got my load tester in today. I hooked it to the batteries. It was reading 14v. I turned it on for 10 seconds and the voltage dropped to a little over 12v while the load was on. When I turned it off, it popped up to 13v.

Now I don't know what a C/20 is, but this load tester has a heat coil in it and that SOB was putting out some serious heat. According to the instructions, I have a good battery bank. It's a Schumacher BT-100 100 amp Battery Load Tester.

As for my hydrometer being worthless, I just didn't bother to write down all the numbers when measuring. It does say that it's temperature compensated, but I doubt that much matters at room temperature. Here's an image of the scale.
hyd2.jpg

So... They started at 5V three weeks ago and now they're charging to 15v and leveling off at 14v. All of the cells are better than 1.2 sg. They'll crank 100A and stay over 12v. Any other tests?

[inetdog]

Got my load tester in today. I hooked it to the batteries. It was reading 14v. I turned it on for 10 seconds and the voltage dropped to a little over 12v while the load was on. When I turned it off, it popped up to 13v.

Now I don't know what a C/20 is, but this load tester has a heat coil in it and that SOB was putting out some serious heat. According to the instructions, I have a good battery bank. It's a Schumacher BT-100 100 amp Battery Load Tester.

As for my hydrometer being worthless, I just didn't bother to write down all the numbers when measuring. It does say that it's temperature compensated, but I doubt that much matters at room temperature. Here's an image of the scale.
[ATTACH=CONFIG]2201[/ATTACH]

So... They started at 5V three weeks ago and now they're charging to 15v and leveling off at 14v. All of the cells are better than 1.2 sg. They'll crank 100A and stay over 12v. Any other tests?

The only remaining test would be to do a full discharge, to a voltage set point corresponding to say 50% SOC, and actually see how many amp hours are available. The load tester measures internal resistance of the battery and the short term power delivery capability, but unless you leave a C/20 load (for a 100AH battery, C/20 = 100AH divided by 20H which is 5A) on for a long enough time (say 10 hours), you do not know how much storage the batteries have regained. Your Battery Tester puts a 100A load on, which for a 100AH battery would be C/1. And I doubt that it is designed to apply that load for very long without overheating. If it can be set to draw a lower current, you may be able to use it as your test load. Or you could use an inverter driving heaters or light bulbs.
I am hoping for you on this remaining test, but am not wildly optimistic.

[Sunking]

The only remaining test would be to do a full discharge, to a voltage set point corresponding to say 50% SOC,

That does not work because SOC does not mean anything but you are on the right track. It is called a load capacity test. To perform it you need to use a known load, and use the manufacture load data sheets.

The load can be as small as C/20, or as high as 1C. Actual test use high discharge rates to minimize the time involved. Typical is C/2. The charts are needed to adjust for the Peukert Law Factor because a 200 AH battery at the 20 hour rate (5 amps) is 72 AH at the 1 hour rate (72 amps). It is a timed test by applying the known load and keep it applied until you reach a battery terminal voltage of 1.75 volts per cell. So if you apply a 50 amp load and it take 2.0 hours to reach 1.75 vpc the battery capacity is 2 hours x 50 amps = 100 AH @ 2 hour rate. Then from th emanufacture data sheets you convert that to whatever discharge rate you want like 20 hours which will be around 200 AH @ the 20 hour rate.

[samcrut]

I guess I have some 12v lighting I can clamp on but we're talking about like 40w so that could take quite a while.

Well, I mean I HOPE it would take quite a while. I'll try that later and time it.

This load tester is 100A, all or nothing. It's not for extended burning. After about 20 seconds the element is glowing hotter than a stove top.

[Sunking]

I guess I have some 12v lighting I can clamp on but we're talking about like 40w so that could take quite a while.

Well, I mean I HOPE it would take quite a while. I'll try that later and time it.

This load tester is 100A, all or nothing. It's not for extended burning. After about 20 seconds the element is glowing hotter than a stove top.

What is the voltage and AH of the battery under test?

[samcrut]

Two 115AH, 600 CCA, 12v batteries in parallel, so potentially 230AH on the upper end.

[Sunking]

Two 115AH, 600 CCA, 12v batteries in parallel, so potentially 230AH on the upper end.

Well you can only test one battery at a time.

Speaking of time that is your challenge. How much time do you have and how much time it takes to do the test. Did I mention time is the key element?


OK you have a 115 AH battery. Not AH = Amp Hour which is time. Did I mention time is the key element?

OK enough nonsense. It is all really simple 5th grade math. The amount of energy a battery has is a function of the voltage and amp hours where Watt Hours = Voltage x Amps x Hours. Your battery voltage is fixed at 12 volts so we will not even take it into consideration because we already know the voltage. What we want to know is the Amp Hours. So you have a battery rated at 115 AH. That rate is specified most likely at the 20 hour discharge rate, or C/20 where C = the AH specification. So in theory your battery should be able to deliver 115 AH / 20 hours = 5.75 amps. All we did is factor out the time element HOURS which leave us with Amps. So if you put a 5.75 amp load on that battery it should be able to deliver 5.75 amps for 20 hours at which point the battery voltage should be 10.75 volts at 100% DOD with a load on it.

So the question is how much time do you want to spend? Back to the time issue again. You can speed up the test up to 10 hours by applying a 10 hour discharge rate of 115/10 = 11.5 amps, or even go to as low as a 2 hour test by applying a C/2 rate of 115/2 = 57.5 amps. That is your challenge is to find a load current, apply it to the battery, and measure the time it takes to reach 10.5 volts. Then all it takes is a quick calculation of amps x hour = amp hours. So if you apply a 10 amp load and it takes 2 hours to reach 10.5 volts the battery capacity is 10 amps x 2 hours = 20 amp hours at the 2 hour rate.

To calculate the load use 11 volts to find the resistance and power required. Let's say you want to test at the 8 hour rate. 115/8 = 14.3 amps. Resistance = V/A = 11/14.3 = .77 ohms and power = V x A = 11 volts x 14.3 amps 157 watts. So the challenge is now how to do that.

Well 2-75 watt 12 volt car head lamps wired in parallel will work, or 240 feet of # 14 AWG 7-strand copper wire. Or a 12 volt 150 watt heating coil rated at 100% duty cycle. Butt if it were me and I had a good ole fashion iron used to iron clothes as they are rated around 800 to 1200 watts and a 2000 watt inverter, I would just use that and discharge at about the 1C rate and getter done in about 1-hour. If I am correct about your batteries at the C1 rate the test will be over in just a few short minutes.

The rest is for you to figure out.

[samcrut]

Well this is going to take a while. I have a 20w MR16 bulb clipped to one of the batteries and 15 hours in, it's still showing 12.5v.
20w/12v=1.666A
1.666A*15 hours = 25AH so far. If they have 100AH left in them, it'll take 2.5 days to finish this test, but based on what I'm seeing so far, I'd say it's safe to say that the desulfation is working.

Oh yeah, and I used the hydrometer again before I started this load test and all cells were in the green. That's a specific gravity of 1250 or better across the board.

[samcrut]

69 hours into my battery test... The 20W halogen bulb has been shining non-stop. The battery is now showing 11.6v and still going. By my math that comes to 115AH, which is the battery's brand new rating. I'm guessing the bulb isn't putting out a full 20 watts, because I find it hard to believe that a battery from 2010 is performing BETTER than new. Sure, it's a very gentle test at this wattage, and I'm sure a heavier load would significantly decrease the AH rating, but I think it's safe to say that this battery has made a full recovery from it's 5V state last month.

[Sunking]

I'm guessing the bulb isn't putting out a full 20 watts, because I find it hard to believe that a battery from 2010 is performing BETTER than new. Sure, it's a very gentle test at this wattage, and I'm sure a heavier load would significantly decrease the AH rating,

It is called Peukert Law Batteries are rated at the 20 hour discharge rate so for a 115 AH battery you would need to discharge at a constant 5.75 amps or around a 70 watt load. At 20 watts you are discharging at the 100 hour discharge rate. So for a battery rated at 115 AH @ 20 hours will be up around 150 AH or more at the 100 hour rate.