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Old 24-07-2019, 13:11   #1
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Realistic expectations from lead acid

I bought a pair of 12V 100A deep cycle batteries about 8 months ago and just recently had a chance to test them. I am no expert on the subject and would like to get some advice from those with real world numbers so I know if mine are busted or if this is what I should expect.

I had the pair of batteries fully charged at the store where they sell them. Total adverised capacity is 200A. Batteries read 12.77VDC. I hooked them up in parallel and connected a 4A 12V DC fridge to them. Batteries read 12.44VDC under load. The fridge cuts off if the voltage goes below 12V. The fridge ran for 7 hours and then gave a low voltage error, meaning it cut off when batteries went below 12V. I measured 12.22VDC.

If we assume the compressor ran for the entire 7 hours, that gives us 7 x 4A = 28A total consumption. Batteries are rated at 200A. I know there are inefficiencies in lead acid batteries and manufacturers always exaggerate their ratings but, are my results normal? Should I accept these batteries or ask for another pair?
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Old 24-07-2019, 14:47   #2
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Re: Realistic expectations from lead acid

All kinds of issues, first they weren’t fully charged most likely. Then they almost certainly are not real deep cycle as 12V deep cycle batteries aren’t common, but 12V starter batteries with deep cycle labels abound.
Then batteries are rated usually for a 20 hour discharge to 10.5V, although for daily use it’s best to not go below 12V.

I think other than not being deep cycle, your biggest issue was they weren’t fully charged, usually a 12V battery that is fully charged is 13V.
That is a generic statement and can differ slightly by manufacturer, but I don’t think 12.77 was close to fully charged
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Old 24-07-2019, 15:12   #3
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Re: Realistic expectations from lead acid

Yes to all the above, likely very sub-par batts to start with, but setting that aside,

If the store knew what it was doing they would've told you come back next day, 6+ hours charging is required to get truly Full, and most chargers quit too early by default, even top shelf ones need adjusting the profile to match the battery mfg specs.

Next you can't just let them sit without damaging them, even if they **were** Full when you brought them home.

Need to at least top up every few weeks.

To get detailed care advice, link to the batteries, your charger, and get an ammeter and DMM.
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Old 24-07-2019, 15:24   #4
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Re: Realistic expectations from lead acid

These batts read 12.77 new? I would have thought more like 12.9. My refer may get down to 4.5a, but not till it has run for a while. It starts out at 6.8a.
8 month old batteries will be pro rated so don't expect new free batteries.
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Old 24-07-2019, 16:19   #5
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Re: Realistic expectations from lead acid

FLA batteries sitting for 8 months will self discharge at least 5% per month, so they would have been at 66% SOC at best when you tested them.


And sitting that long at partial SOC will have damaged them already.
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Old 24-07-2019, 21:01   #6
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Re: Realistic expectations from lead acid

The batteries were on the boat and in use, I have solar panels and a working charge controller to keep them charged. The store has proper charging equipment and charged the batteries overnight. I am willing to assume that the batteries were well charged before my “test” began which the store allowed me to conduct at their facilities.

I am just surprised that I only got 28A out of something that was rated at 200A. Is that what real world numbers look like?
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Old 25-07-2019, 10:08   #7
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Re: Realistic expectations from lead acid

Without knowing exactly which brand or type (flooded versus AGM) batteries you have, your numbers do not sound very unusual.

I am only familiar with Lifeline and Odyssey batteries. I have two of each brand. All four are 6 years old and load test at above 90% capacity after many cycles. They have never been discharged below 50% state of charge (SOC) defined as an open circuit voltage (OCV) of 12.1 volts after 8 hours of resting (no charge/no discharge/no load).

Both brands claim to be deep cycle batteries. They were all between 12.7 and 12.8 volts OCV when purchased prior to charging. I charged them immediately using the three-stage factory recommended charge cycle.

Lead acid batteries require about 20 "forming" cycles between 50 and 100% SOC to reach 100% capacity. After the forming cycles, your 2 parallel 100 A/H batteries should deliver 10 amps for 10 hours (at 25 degrees C) before discharging to 50% SOC. Discharging them below 50% SOC - and not recharging them promptly to 100% SOC - will result in accelerated sulfation and reduced life.

On the topic of sulfation, you may find this article helpful: https://www.practical-sailor.com/issues/37_53/features/Fighting-Sulfation-in-AGMs_11691-1.html

Just to be scientific, here is my load-testing method: I discharge the test battery into a 2.4 ohm 300 watt resistor (including wiring resistance - that's a 5 amp load). The resistor is switched off when the terminal voltage reaches 12.1 volts using a voltage sensor-switch. When the load is removed from the battery, its resting terminal voltage will "float up" and when it rises above 12.1 volts, the load is switched back on. This process continues, switching the load on and off as 50% SOC is approached, until the OCV finally settles at 12.1 volts. The whole process takes about two days. Connected to the load is a clock that records the total time the load was connected. For a 100 AH battery, I expect a total discharge time of 10 hours: 10 hours X 5 amps = 50 amp/hours.

That's a cheap-and-dirty tester that loads the battery at the factory-specified C/20 rate. It cost less than $150 to construct, runs unattended, and gives repeatable results.
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Old 25-07-2019, 10:19   #8
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Re: Realistic expectations from lead acid

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Originally Posted by hd002e View Post
The batteries were on the boat and in use, I have solar panels and a working charge controller to keep them charged. The store has proper charging equipment and charged the batteries overnight. I am willing to assume that the batteries were well charged before my “test” began which the store allowed me to conduct at their facilities.

I am just surprised that I only got 28A out of something that was rated at 200A. Is that what real world numbers look like?
First the corect unit of measure is Amp Hours (AH) not Amps (A). Yes it makes a difference.

Here's a soc vs. voltage chart I found online at https://www.energymatters.com.au/com...age-discharge/


State of Charge
Sealed or Flooded Gel AGM
Lead Acid
100% 12.70+ 12.85+ 12.80+
75% 12.40 12.65 12.60
50% 12.20 12.35 12.30
25% 12.00 12.00 12.00
0% 11.80 11.80 11.80

A flooded lead acid batteries SOC is best measured with specific gravity.
see; https://www.altestore.com/howto/batt...of-charge-a81/
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Old 25-07-2019, 10:40   #9
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Re: Realistic expectations from lead acid

Couple of points / questions.


I've never seen rested 12 volt batteries read 13 volts. Various manufacturers I checked usually say 12.8 or 12.7. This is for a battery that has had no loads or charging for at least an hour. Should be easy to check your manufacturer's specs online.


Cutting off the batteries at 12 volts when under load, is probably too high a setting. At you noticed, they quickly went up to 12.22 once the load was off. My guess is that they would have been a good bit higher if left sitting for an hour. If this was after they had "rested" for quite a while it would indicate they were around 60%.



My cheat sheet for charge levels show 12.7 100%, 12.5 80%, 12.1 50%. 50% is the number I've always heard as what you should use for max discharge for good cycle life.




I agree that most 12 volt batteries that say deep discharge really aren't. The "D" batteries (4D, 8D) are exceptions. If you want true deep discharge look at 6 volt Golf Cart batteries. They are designed for deep discharging. Also usually a lot cheaper at the box stores (under $100 each).



My 2 cents.
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Old 25-07-2019, 10:52   #10
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Re: Realistic expectations from lead acid

Quote:
Originally Posted by hd002e View Post
I bought a pair of 12V 100A deep cycle batteries about 8 months ago and just recently had a chance to test them. I am no expert on the subject and would like to get some advice from those with real world numbers so I know if mine are busted or if this is what I should expect.

I had the pair of batteries fully charged at the store where they sell them. Total adverised capacity is 200A. Batteries read 12.77VDC. I hooked them up in parallel and connected a 4A 12V DC fridge to them. Batteries read 12.44VDC under load. The fridge cuts off if the voltage goes below 12V. The fridge ran for 7 hours and then gave a low voltage error, meaning it cut off when batteries went below 12V. I measured 12.22VDC.

If we assume the compressor ran for the entire 7 hours, that gives us 7 x 4A = 28A total consumption. Batteries are rated at 200A. I know there are inefficiencies in lead acid batteries and manufacturers always exaggerate their ratings but, are my results normal? Should I accept these batteries or ask for another pair?
Batteries are mfg. dated. Did you buy them from a dealer that realizes that or one the uses first in last out and you were the last? Walmart would be an example of that. The stockers don't know to rotate the stock.
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Old 25-07-2019, 21:27   #11
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Re: Realistic expectations from lead acid

if they have been used for 8 months only on solar and no dock charging you may have already trashed them.

the 4a fridge load probably cycles on and off. so you maybe only got 14 ah if the frige runs 50% ....


a 200ah battery dropping from 12.77 to 12.4v with a 4a load is not in good shape.

from 12.77 to 12.22 (your 2 un loaded measurements ) is probably from 100% down to 60%. so you should have gotten 80ah. or about 20 hours of 4a constant fridge running untill it shut off.
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Old 26-07-2019, 06:20   #12
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Re: Realistic expectations from lead acid

Just curious - sealed or flooded and what charge profile are you using:


Bulk voltage?
Absorption?
Float?


Thanks.
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Old 26-07-2019, 06:31   #13
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Re: Realistic expectations from lead acid

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I am just surprised that I only got 28A out of something that was rated at 200A. Is that what real world numbers look like?
No it's not and you need to recharge till absorption acceptance current is less than 0.5%C (try for lower) and repeat your test. If the voltage since gets down to 12.2 at only 28AH you batteries need replacing.

My 440AH rated batteries will sometimes be -90AH in morning and still at 12.45V under refrigeration load and they are 2 years old.

BTW - your statement of having solar aad working controller is pretty meaningless as far as getting the batteries charged. You need to provide numbers of what voltage they get the batteries to, how long they are held there, and the acceptance amps at the end
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