Battery Assessment: Page 2 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

The battery temperature sensor is critical to ensuring proper charging. The sensor needs to be well-attached to the side of the battery, about two-thirds of the way up from the bottom. It should be located toward the middle of the battery bank—not on the end or sides, where it would have more airflow or be influenced by other heat sources. Check the sensor wiring for damage, such as breaks or nicks in the insulation.

Battery Measurements

Once the visual inspection is complete, move on to taking measurements—the only way to really ascertain what’s truly happening inside of a battery. First, sketch out the battery bank layout and assign a label to each battery and each cell.

In the “Labeling Batteries for Consistent Data” illustration, four 6-volt batteries are wired in two series strings for 24 volts and the two strings are wired in parallel. Each battery is made up of three 2-volt cells with a cap on each cell, which allows individual electrolyte inspection and measurement. Permanently marking each cell of the battery with an identification number or letter is very useful for future reference and comparison.


Voltage is the easiest measurement to make, since it only requires a digital voltmeter to be connected to the terminals of each battery. Measure and record each battery’s voltage in a table similar to the example. Some batteries allow measurement of each individual 2-volt cell, while others with multiple cells in one case allow access only to groups of cells—such is the case with nearly all 6-volt and 12-volt batteries.

The battery should be kept in an open-circuit condition while making these measurements—this is accomplished by disconnecting all charging sources and all loads. Allow  the battery to rest for 30 minutes, and then make all of the measurements in as short of a time period as possible. To get accurate state-of-charge measurements, many battery manufacturers suggest waiting six or even 24 hours for the battery voltage to stabilize. However, this can be difficult at sites where the battery system is the only power source. When voltage is used to compare one battery to another, a shorter time period can be used.

The voltage variations between the individual batteries provide valuable information on the batteries’ overall condition. Low-voltage readings can reveal weak or failed batteries, and wide variations can indicate the need for an equalization charge, an intentional overcharging of the battery bank to bring the weakest cells up to their full charge. Equalization also mixes the electrolyte to eliminate stratification of the acid and helps remove any sulfation on the lead plates. This can help to balance capacity differences between cells and allow the battery bank to be charged to its maximum potential. This will be discussed more in the second part of this three-part series.


All batteries have a higher voltage when no loads are connected. Once a load is connected, the battery’s voltage decreases. The amount that the voltage drops is a good indicator of the battery’s health and can be used to compare each battery in a bank to find poorly performing batteries or cells.

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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