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Old 10-08-2017, 08:09 AM   #1
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How to tell if batteries are shot

Hello,
We recently installed a VictronConnect BMV-700 to try and figure out what is up with our batteries. We have some ~1 year old interstate srm-24s.

Anyway, this morning the battery monitor shows a voltage, nearly at rest of only 12.05 volts, but we've only drawn 28 hours off of them.

Looking at state of charge tables, we should not get to 12.05 volts until nearly 50% (70ah?) has been drawn off.

Does this mean the batteries are pretty much shot?
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Old 10-08-2017, 08:38 AM   #2
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Could be: low water in batteries, a problem with the converter, a wiring problem, or a battery problem. I would start by pulling the batteries and take them to an auto parts store and have them do a load test.
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Old 10-08-2017, 08:45 AM   #3
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Factory Battery amp hr rating is nothing more then a close estimate as to what the battery is going to have. Batteries lose storage capacity as they are used, and harsh use i.e. running dead or extremely low causes this faster. 12volt interstate grip 24 batteries are not true deep cycle battery. They are rated CCA (cold cranking amps). They are designed for faster release of their stored power.

Check the water level in the batteries and make sure that is good.

What kind of charger are you using?

Is ALL the Power being used going through the shunt?

Is the Victron set up with the correct amp hours available base on the batteries you have and not just what Interstates inflated numbers?

Simple answer is the batteries are not performing as you desire or meeting your needs so gotta change something. My bet is the batteries.
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Old 10-08-2017, 10:14 AM   #4
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Thanks,
The water is full, and we recently upgraded the charger to a PD 4655 and installed solar with a Victron charger. The bank capacity is set to 140ah, but that’s sorta a separate issue, as I’m just looking at voltage vs amp-hours drawn, and tying to tell if this indicates worn-out batteries.

The cabling should be fine, I just redid it too. I can tell you for sure that all negative power is going through the shunt. (Attached)

The battery date stamps are about a year old, but we bought the trailer in June, and I don’t know what was done to the batteries before that.
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Old 10-08-2017, 10:18 AM   #5
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For $10, pick up a battery hydrometer with temperature compensation and check each cell after fully charging your batteries. This is a simple and often over-looked technique to verify the integrity of your cells.
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Old 10-08-2017, 10:23 AM   #6
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Also, as far as a load test: we can run the toaster off the inverter, which pulls ~ 80 amps off the batteries and they drop to 11volts, then bounce back.

So, they are not totally dead, but it seems the capacity is ~1/3 what it should be, if I’m reading things right. But I don’t know if I’m reading things right.
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Old 10-08-2017, 10:25 AM   #7
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Fully charged they are 12.8v plus, what do I check for with the hydrometer? I thought those just gave state of charge.
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Old 10-08-2017, 10:51 AM   #8
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If you know your batteries are fully charged, but your hydrometer readings indicate a specific gravity substantially lower than the expected 1.265 in each cell, then you can be assured that one or more of the cells is damaged.
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Old 10-08-2017, 01:08 PM   #9
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From the image this was with some current being drawn from the batteries at the time. The battery voltage vs charge state is not really accurate then. Needs to have batteries with no load and given time for the voltage vs charge state to be reasonably accurate.
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Old 10-08-2017, 06:26 PM   #10
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Alano, thanks, I understand better now. Didn’t know it worked that way.

HiJoe- I figured a 0.5a draw was low enough — have you seen that cause a significant impact to the resting voltage when the draw is that low?
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Old 10-08-2017, 06:51 PM   #11
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Hi

What temperature are the batteries? The "magic voltages" move by +/- 0.6V or so going to pretty cold or pretty warm temperatures.

How long were the batteries charged? Something like 24 hours is a good idea to get to the "full capacity" number. You get to 80% pretty fast. The last 20% takes a *long* time.

Have you checked the wiring on the BMV-700. Everything must be wired through the shunt. No bypass allowed !!!

There are two shunts for the 700 series. You can be off 5:1 if the shunt does not match the settings on the unit ....

Lots of issues ...

Bob
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Old 10-08-2017, 06:52 PM   #12
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At that instant no, but if it wasn't resting open circuit for at least an hour before measuring, previous loads could cause the voltage to indicate lower.
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Old 10-09-2017, 08:38 AM   #13
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Originally Posted by cerickson View Post
Alano, thanks, I understand better now. Didn’t know it worked that way.

HiJoe- I figured a 0.5a draw was low enough — have you seen that cause a significant impact to the resting voltage when the draw is that low?
Hi

As a practical point - you can't get rid of the parasitic 0.5A sort of load on an AS. If you have 200AH of batteries, anything under about 2A gets into the "does not matter" range.

You have three things that impact a reading:

Resistance of the wires and the battery it's self will cause a voltage drop as current increases. Remove the load and the drop goes away. It's fast.

The chemistry of the battery reacts to charging and discharging. It's action is more like a capacitor. You run the battery up to 13.7V for a while and it will "hang there" even after the charger stops doing it's thing. Do a really heavy discharge and you get the reverse effect. (it goes low and slowly drifts back to a higher voltage).

The temperature of the battery impacts it's voltage. Heavy charge or discharge can heat the battery up. The hotter the battery gets, the lower its voltages will be. With a quick discharge, the inside of the battery can get pretty warm without the case showing much of an immediate rise. Sticking a temperature probe down inside the battery .... not recommended

You are chasing three things when you try to "settle" the battery. It can get confusing ....

Bob
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Old 10-09-2017, 09:10 AM   #14
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Two Group 24s - 85*2 Ah = 170 Ah, so you are conservative with your 140 Ah setup.

.15 (used) *170 = 25.5 Ah

Pretty close considering there is current draw. But voltage measurement at 12.06 indicates 40-50% charge as you said. Let them charge until you think they are full, then disconnect the batteries from everything for several hours and measure voltage. If not 12.6 or better, they are not fully charged or they are needing replacement.

I would think another problem is the monitor telling you there is 85% left at 12.06 volts. Did you follow the synchronization process in the manual?

Also make sure that all current drawn goes through the battery monitor shunt. It is normally placed between the trailer common bus and the negative battery post. There should be nothing connected to the negative posts but the jumper between the batteries and the shunt.



Al
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Old 10-09-2017, 10:15 AM   #15
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Hi

One other gotcha: Some of the monitoring panels are less than perfect. I would *assume* that the Victron does a good job reporting voltage. Just in case, check the batteries at the posts with a multimeter.

Bob
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Old 10-09-2017, 10:58 AM   #16
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Quote:
Originally Posted by Lumatic View Post
Could be: low water in batteries, a problem with the converter, a wiring problem, or a battery problem. I would start by pulling the batteries and take them to an auto parts store and have them do a load test.
I concur.
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Old 10-09-2017, 10:59 AM   #17
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Quote:
Originally Posted by Al and Missy View Post
Two Group 24s - 85*2 Ah = 170 Ah, so you are conservative with your 140 Ah setup.

.15 (used) *170 = 25.5 Ah

Pretty close considering there is current draw. But voltage measurement at 12.06 indicates 40-50% charge as you said. Let them charge until you think they are full, then disconnect the batteries from everything for several hours and measure voltage. If not 12.6 or better, they are not fully charged or they are needing replacement.

I would think another problem is the monitor telling you there is 85% left at 12.06 volts. Did you follow the synchronization process in the manual?

Also make sure that all current drawn goes through the battery monitor shunt. It is normally placed between the trailer common bus and the negative battery post. There should be nothing connected to the negative posts but the jumper between the batteries and the shunt.

Al
The percentage here I think can be misleading, as it is calculating that based on what's gone across the shunt, based upon what I said the bank capacity was. So, it is sort of meaningless if the batteries are worn and the true capacity is less.

I'm positive that all current is going through the shunt -- I just re-cabled it myself with 2/0 cables.

The voltage vs. current drawn is the part I'm trying to figure out -- and fully charged they are at 12.8+ volts.

The synchronization process, as far as I can tell, is just to let them get to a full charge (which they've done via. the solar many times now). Am I missing something there?

uncle_bob:
It has been between 40 and 65 degrees here, would those kinds of swings cause that much voltage swing? The low-voltage/high-current-drawn scenarios have been measured at ~ 40 degrees.
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Old 10-09-2017, 11:38 AM   #18
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load test batteries

How do you know when your batteries are shot?
The best is a "load test". There are a number of instruments available to the DIY'fer to perform this test. Not to mention, hauling them to your friendly local auto parts store or even a "Battery Warehouse".
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Old 10-10-2017, 08:29 AM   #19
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Originally Posted by cerickson View Post

uncle_bob:
It has been between 40 and 65 degrees here, would those kinds of swings cause that much voltage swing? The low-voltage/high-current-drawn scenarios have been measured at ~ 40 degrees.
Hi

Fully charged at 40F would be about 13.2V rather than 12.8V. That *assumes* the batteries got down to 40F. If the temperature is moving around, the battery box and batteries may "lag" a bit. The charger output should go from 13.7 to 14.1 if it was temperature compensated and tracking the battery voltage. The "stop using" voltage would be about 12.4V.

The other gotcha is that battery capacity drops as temperature goes down. That plus a charger that does not "understand" the battery temperature (and does not fully charge as a result) gets you into even more trouble when it's cold out.

All this moving voltage stuff is why you buy a BMV-700 in the first place for capacity. The second part of it really needs to be a charger with a temperature probe. An ideal setup would feed temperature into the BMV....

Bob
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Old 10-13-2017, 04:00 PM   #20
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Don't know about anyone else but I never get correct state of charge reading from my Victron BMV-702.

After being home from camping where I had full hookups. Trailer has set for 2 days.
Reads 12.32 Volts and 100% state of charge. State of charge is never correct.

Shows consumed 0 ah temp 50F Have set to 160AH on stock interstate batteries. Had them tested a few months ago.

All my loads are run through the Victron shunt.

Any ideas what is wrong with my system. Also why 12.22 after two days with nothing on but the co2 deector?

Thanks Dave
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