Battery Assessment: Page 4 of 5


Inside this Article

Filthy battery bank.
Take stock of your battery bank’s health with this step-by-step assessment guide.
Using a battery room for storage
Using a battery room for storage can lead to poor accessibility to the batteries for maintenance, and can be a safety hazard.
Class C fire extinguisher
Besides eye protection, acid-proof gloves, baking soda, and distilled water, a Class C fire extinguisher and an eyewash station are important to have near the batteries.
An eyewash station
Besides eye protection, acid-proof gloves, baking soda, and distilled water, a Class C fire extinguisher and an eyewash station are important to have near the batteries.
Battery terminal corrosion
Battery terminal corrosion can lead to poor system performance and/or reduced battery life.
Improperly installed temperature sensor
Only properly installed temperature sensors can adjust the charge regime correctly. This one should have been installed two-thirds of the way up on the side of the battery, and between two batteries.
Testing the battery voltage
Testing the voltage of each battery is a quick way to identify a weak or failing cell.
Checking electrolyte level
The electrolyte level in this battery is very low, exposing the plates to air, which causes permanent damage.
Testing specific gravity of the electrolyte
The specific gravity of the electrolyte can be tested with a hydrometer.
Testing specific gravity of the electrolyte
The specific gravity of the electrolyte can also be tested with a refractometer.
Checking cell temperatures
Cell temperatures that are too high can impact the number of available cycles.
Battery Temperature Sensor
The BTS communicates the battery cell temperature to the inverter/charger and the charge controller to adjust the charging setpoints.
Filthy battery bank.
Using a battery room for storage
Class C fire extinguisher
An eyewash station
Battery terminal corrosion
Improperly installed temperature sensor
Testing the battery voltage
Checking electrolyte level
Testing specific gravity of the electrolyte
Testing specific gravity of the electrolyte
Checking cell temperatures
Battery Temperature Sensor

When working with batteries and their electrolyte, be sure to put on goggles and chemical-rated gloves, but first remove all jewelry and/or watches, which could accidentally come into contact with battery terminals and cause arcing. A good flashlight is usually needed to see the level of the electrolyte inside of the battery—but be sure its housing is not metal, either. 

The electrolyte on a properly filled battery should be approximately 1/4 inch below the fill tube. If the battery is overfilled, the electrolyte can flow out of the vent caps when the battery is charged, as bubbles of hydrogen and oxygen that are produced cause the electrolyte’s level to increase.

Sometimes, battery cables or other obstructions can make maintenance of one particular cell, battery, or even an entire battery string, difficult. Pay attention to these types of problems and correct them, as seemingly small improvements can have a dramatic effect on the frequency of future maintenance and battery longevity.

Specific Gravity

While a voltage measurement provides a quick side-by-side comparison (identifying potentially weak batteries or cells), the overall battery voltage fluctuates significantly depending on if the battery is being charged, discharged, or if it has been at rest. For flooded batteries, specific gravity (SG) readings show the actual concentration of sulfuric acid in the electrolyte and thus are a much better indicator of the battery’s state of charge (SOC) for each cell and the entire battery. SG can only be measured on nonsealed, flooded-type batteries.

Measure and record each cell’s SG with a hydrometer or refractometer, using all the same protective equipment as you’ve used for checking the electrolyte levels (see “Methods” in this issue). To correlate an SG value to the SOC level, check the SG graph or table from the battery manufacturer’s specifications. For example, a typical SG reading for a battery at 100% SOC is 1.265. If the difference in SG between any two cells is greater than 0.025, the battery bank needs to be equalized to ensure that all cells receive a full charge.

Cell Temperature

Battery capacity and cycle life are typically provided for a battery at a temperature of 77°F (25°C). If the battery is colder or warmer, the specification and performance will vary. At lower temperatures, the battery’s available capacity (measured in amp-hours) will be decreased. At higher temperatures, the battery’s life (number of cycles) will be diminished.

The “Typical Life Cycles vs. Average Depth of Discharge” graph shows the number of charge/discharge cycles versus the depth of discharge (DOD) for a typical flooded lead-acid (FLA) battery at 77°F and 95°F. Most FLA batteries will be able to supply only half as many cycles when the battery temperature is increased by 10°C (18°F). For example, at 25°C (77°F), a battery may be rated at 2,000 cycles at a 20% DOD. But when operated at the higher temperature of 35°C (95°F), it will only provide 1,000 cycles under the same cycling conditions.

Factors like lack of ventilation, heat or sunlight exposure, and high charging voltages and currents can also increase a battery’s internal temperature. Operation at consistently high internal temperatures can significantly reduce a battery’s life.

Comments (8)

Brian Ellul's picture


Good article!

Regarding the load testing, can you give an indication of what should be the voltage under load for a 2v Cell. I would like to test a set of brand new (however left unattended) forklift batteries before purchasing them however I'm finding it difficult to arrive at a definate conclusion as to their actual state/capacity.

The batteries are brand Fulmen 680AH - 20cm width x 12cm depth x 46cm tall, I've charged one cell, voltage reached 2.11v and SG=1.28.
I discharged this cell with a small load. Discharged for 85 hours at 3.3 amps (not easy to find loads at such a low voltage). At the end of the discharge test, the voltage came down to 2.01 and SG=1.19.
During discharge, the voltage read 1.95v. Is this OK since I'm finding it a bit low? How can I determine the battery capacity based on my tests?

Thanks a lot for your help.


Christopher Freitas's picture

Yes - that was my error. I had changed the text during editing to show the watts (which decrease by a factor of 4 when the voltage is halved) and then switched it to amps (which only halves when the voltage is halved) and forgot to switch the values. I apologize for this oversight and appreciate you bringing it up.


Richard Bosse's picture

I believe there is a mistake in your calculations under the heading "load testing." For the 12 Volt battery, R=E/I. R=12V/500A, R=0.024 Ohms. For the 6 Volt battery, I=6V/0.024 Ohms, I=250 Amps not 125 Amps as stated in the article. For the 2 Volt battery, I=2/0.024, I=83 Amps not 30 Amps as stated in the article.
Rich's picture

Hi Christopher,
Thanks for the quick reply! The setpoints are 29.6 v, absorb (2 hours), 26.4 float and 31.0, EQ. I believe those are pretty standard for this application but I could be wrong. The batteries were using a bit of water, about normal, I'd say, and I was aware that the Outback is set up for AGM out of the factory as to not inadvertently overcharge them if the default was FLA.
It's true that I could no doubt make the RTS work, but what would be the point? I'd really like to understand the chemistry more and know what I should expect from using the BTS.
Maybe the reduced SpG at elevated temperatures is what results in longer battery life, albeit at a reduced SOC. Or maybe the BTS really provides the most benefit at reduced temperatures where the setpoints would increase to drive the SpG. higher than w/o the BTS. The Trojan tech was not impressed with my particular results!

The house location experiences very little temperature fluctuation during the year.

Although I can't imagine what I could be doing wrong it would be interesting to have others verify (or refute) my findings!

Thanks for your time. This battery technology still seem like a trial and error implementation, even after all these years of experience with it.

PS: If this tread is getting a bit "over the top" for the 'Comments" section, let me know!!

Christopher Freitas's picture

Hello Marty - Interesting problem. You didn't mention what the setpoints are that you are using for the Trojan T105s. I've found that with PV systems that the settings may need to be different than what is recommended by the manufacturers since they are typically thinking of applications which are charging 24 hours a day continuously (such as the utility grid) and not a source which floats the battery for only 6 or so hours a day.

The bottom line is what you are observing - the specific gravity is better with the higher setpoint - by defeating the RTS you are essentially raising the setpoint up a couple of tenths. This could be done also by having the RTS connected and just adjusting the float (and absorb probably as well) a couple of tenths. This way the setting will be compensated for the change in temperatures better over the year.

BTW - the default settings in the OutBack products are intended for sealed type batteries - which are always lower setpoints then recommended for flooded type batteries.

You also didn't mention if your batteries are using any water - all flooded deep cycle batteries should use some - if they don't then they most likely are not being charged fully and may become sulfated.

Let me know what your setpoints are currently and how much water you've been adding.

Christopher's picture

After many posts and phone calls, I've never been able to find the answer to a BTS (Outback calls theirs RTS) problem I had. Never having used an RTS before, I properly placed and connected a new one via an Outback MX60 to a new bank of 4X2 Trojan T-105s. I noted that it was operating correctly and reducing the Absorb set point a few tenths to compensate for a slightly elevated ambient temperature in accordance with a table of values I had, temp vs change in voltage.
Everything was looking fine until I measured the temperature compensated specific gravity (at Float) which showed a consistent under charge at about 1.230 (SOC 80%). Nothing I could do (e.g., lengthen absorb time) would bring these new batteries up to 100% SOC or 1.27 SpG. An EQ worked but the RTS is not used during equalization.
Trojan couldn't explain it but was adamant that the SpG be brought up to 1.27, whatever it took. So, my new RTS is disconnected and the batteries happily float at 1.26+ after 3 years of use.
I'd like to find out how to implement the RTS correctly but have yet to discover what to expect from the RTS implementation.
Ideas? Next issue?

robert wimberly's picture

Good article and focuses on some of the most important checks. I would only add that when checking specific gravity you may be slightly mislead if you do not correct for the electrolytes temperature. The normal correction factor is 0.004 points of gravity for every 10 degrees above or below 80F. For example if you are outside in a cool area at 40F then a reading of 1.265 g/ml is really approximately 1.249 g/ml.

Justine Sanchez's picture

Hi Robert,
Good point! And timely as our next issue of Home Power includes an article specifically focused on checking specific gravity, where we do touch on the need to adjust for the electrolyte temperature. Thanks for posting!
Justine Sanchez
Home Power Magazine

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