battery voltage dropping fast

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hillbilly
hillbilly Solar Expert Posts: 334 ✭✭
Well my saga with batteries continues. A new one for me this time, and I'm wondering what more knowledgeable folks make of this. So due to some ailing batteries, I've cut my bank in half (thinking that the "good" batteries might last a bit longer if not paralleled with the two weakest batteries). I wanted to get a rough idea of just what the capacity might be of the better half of the battery bank once I had rewired everything, so I had been discharging at about 12amps or so (which would be a bit less than the C/20 rate of the batteries). Keeping an eye on the voltage and the SG levels, things seem fairly normal until the batteries get down to about 24.4 volts or so, at which point the voltage starts to drop pretty fast although the SG levels are still indicating a much higher SOC.

Some numbers: I had estimated that the batteries might have about 250ah capacity and thus used this figure for my trimetric settings. When about 70+ AH are withdrawn is when the voltage starts dropping much faster than the SG. The voltage under load starts to go down to less than 23V within another 10-15AH withdrawn, meanwhile the SG readings are still up at 1.230-1.240 and fairly consistent from cell to cell. The SG levels in the past have for the most part been pretty consistent with my under load voltage readings and my resting voltage readings (with a few problem cells which are not in the current bank). Not quite sure which numbers to trust more at this point, but it does make me concerned that perhaps the "good" batteries are not quite as good as I had hoped. Is there any chance that they are behaving this way due to the fact that they are being cycled a bit deeper than usual, since it seems to happen when the battery capacity drops below about "75%" SOC which is about as deep as they would normally be cycled.

Any ideas?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: battery voltage dropping fast

    Not anything necessarily bad... Roughly 24.0 volt resting voltage at 50%--so I would expect below 12 volts under load.

    Have you logged the voltage of every cell/battery and see if there is a cell/bank that is the major source of the voltage depression?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
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    Re: battery voltage dropping fast

    Bill, I haven't done that yet but will likely do that if I see the same thing again. I should mention that the resting voltage wasn't 24V, that was the loaded voltage ("mostly resting" voltage was 24.6 at that time). Once I put the load on the voltage starts to drop quickly, but only once the battery bank has been withdrawn by at least 70ah, before that the voltage drops quite slow and steady.

    This morning specifically: woke up, checked voltage and AH from full on the trimetric (very slight load of .5amps, voltage 24.6, AH from full -75) turn on some loads to start pulling about 12amps and the voltage dropped to 24V (right about where I would expect it to be). Then in a very short time frame the voltage drops all the way down to 22.8V after drawing only another 10AH from the battery.
    The evening prior to this, with up to 15-20amp loads running, the lowest the voltage reached was 23.9V; in other words the voltage drop was following a very "normal" pattern (1.4V drop over 70ah withdrawn, vs 1.2V drop when another 10ah is withdrawn) in the evening, but then dropped much faster this morning.

    That kind of voltage reading under a C/20 load would normally indicate about 40%SOC, whereas the SG reading of 1.230-1.240 would indicate a SOC of more like 75%... seems a very large discrepancy.
  • blackswan555
    blackswan555 Solar Expert Posts: 246 ✭✭
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    Re: battery voltage dropping fast

    When and how was your last EQ ?
    Tim
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
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    Re: battery voltage dropping fast

    EQ was quite recent, as I had been fighting to resurect the whole battery bank for the last month or so. The SG levels were all at or above 1.260-1.270. I'm watching the voltage drop right now, but starting from a less discharged battery bank (I wanted to rule out any issues with the colder A.M. temps).
    So far, starting with 18ah drawn out the rested voltage was 25.3V dropping quickly to 25V under a 12amp load.

    Checked the voltage again a bit later with 23ah drawn down, voltage read 24.7V.

    Now with 30ah drawn down the voltage is 24.6V. I'm waiting to see what happens over the next hour or two...???
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
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    Re: battery voltage dropping fast

    I have a tad bit more detail now, but still am a bit unsure quite what to make of it all.

    Some specific stats from the 4 surrette s-460s I have currently connected (350ah rated):

    This morning started a steady discharge of about 11.5-12.5amps, the battery bank had about 18ah withdrawn last night, and the starting voltage under load was 25V. Kept checking periodically and noted the AH reading each time the voltage dropped a bit:

    -22.5ah = 24.7V
    -30ah = 24.6V (took individual voltage readings and SG readings; 6.17,6.17,6.17, 6.15, SG levels 1.256-1.264)
    -36ah = 24.5v
    -42ah = 24.4v
    -51ah = 24.2V
    -58.5ah = 24V
    -63ah = 23.8V
    -67ah = 23.6V
    -69ah = 23.4V
    -71.5ah = 23.2V
    -73ah = 23V (took individual voltage and SG readings again: 5.96, 5.96, 5.97, 5.12)

    The one battery with the low voltage had SG levels of 1.243, 1.230, and 1.230, which still seems to me to indicate a fair bit of capacity, while the voltage level indicates otherwise. My gut feeling says that the true capacity is somewhere between these two opposing readings, but I'm not really sure which one to trust or worry about? It does appear that the one battery is not as healthy as the others (this battery does have one cell that has a lot of sediment that shows up when under higher voltage charge, so it could just be that I've lost a lot of capacity due to plate shedding).

    I put all the numbers down here to illustrate how much faster the voltage started dropping, what was interesting is that the sharp voltage drop seemed to be happening much sooner this time than the other day. This could be partly then due to the colder am temps, but hard to say for sure?

    Any further ideas or thoughts are most appreciated...
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: battery voltage dropping fast

    now maybe it may have been mentioned elsewhere, but what is missing here is how often these readings were made and just exactly what was the item drawing those currents.
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
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    Re: battery voltage dropping fast

    You mentioned that the voltage drop seems to follow a "normal" curve in the evening, but drops off more quickly in the morning. Are your batteries located where they get very cold overnight? During the day they will be heated somewhat from being charged, and then from being used, but then overnight with light use (so no internal heating) and colder outside temps I would expect a drop in capacity.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
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    Re: battery voltage dropping fast

    @Niel- these readings were taken when I saw the voltage dropping but was over a fairly constant load of roughly 12amps. I wasn't paying a ton of attention to time, but looking at the discharge those number occurred over about a 4.5hr window. The loads were mixed lights (cfl) and the laptop and stereo, but stayed constant throughout the day. There was almost no sun yesterday so I had the PV breakers off for the test.

    @techntrek, yes the batteries are in an unheated (but moderately insulated) box outside. I was thinking that the same thing on temps, which is why I made sure to start yesterday mornings discharging from a much more full battery. I wasn't sure if the temps could make the voltage drop faster, or if it was just that the voltage was dropping faster one the batteries started getting lower in capacity; from all I can tell so far it's both. I should add that the electrolyte temps don't seem to change all that much right now, the main thing that really changes those temps is longer higher voltage charging warming them up. Otherwise they seem to hover pretty much right around 40-50F.

    It seems that the one battery is just holding it's voltage ok for a little while and then starts to fall much quicker than all others, so my guess is that it has lost a lot of it's capacity. That all seems normal, the thing I don't quite get is why is the SG still reading as high as it is; which would you all trust more on this the SG reading or the voltage reading?