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Jubal 04-07-2015 11:52 AM

Optimum charging voltages for 6V GC2 batteries
I received this info from the tech service at Interstate batteries.

Charging recommendations for 6-volt deep cycle batteries. GC2-XHD, GC2-HD, GC2-RD
Thank you for your inquiry here at the home office of Interstate Batteries, Inc. See the attached information on the charging recommendation for your batteries. Let me know if you need any additional information.

Jeff Barron
Engineering & Technical Service Specialist
Interstate Batteries, Inc.
10200 Plano Rd. Suite #400
Dallas, Tx. 75238
Ph. 469-221-4655
Fax 469-374-6117

Nominal Battery Voltage

Charging Current

Bulk Charge Voltage (2.41 VPC)

Absorption Voltage (2.41 VPC)

Absorption Time in Hours

Float Voltage (2.23 VPC)

Equalization Voltage (2.6 VPC)

Equalization Time in Hours

12 Volt (6 cells) battery voltage

C / 10 charging current

14.46 bulk charge

14.46 +/- .05 absorption charge

2 to 4 absorption time in hours

13.4 float charge

15.6 equalization charge

2 equalization time in hours

As you can read the absorption charge is 14.46 volts. The absorption charge begins when the battery has reached the maximum voltage of the charger and the amperage then starts to decrease.
CAUTION Charging at 15.4 or 15.6 will cause a lot of off gassing and electrolyte depletion. Be present to monitor the batteries during the high charge rate cycle!
I am having trouble transferring the WORD document docx file.

fudge_brownie 04-07-2015 01:34 PM

2 Attachment(s)

These numbers surprise me. There are three things that I question.

First, this information is contrary to the link that you posted from their web site.

This is the link, on the top below is a screen capture.

There is a second place on the Interstate website were they provide the same information. Here is the link, bottom is the screen capture.

Remember they may be talking about a single six volt battery, so the charge rates need to be doubled if you have the dual six volt battery option. Both these above links are stating the proper charge voltage for dual six volt batteries is 15.3 at the absorption stage.

Second, I have had this conversation with their phone technical support (not customer service) and confirmed with them that the correct charging rates are what you read on the website table.

Third, the description in the reply that your received from Interstate states "12 Volt (6 cells) battery voltage ". I think they are quoting you from an incorrect table. They should be citing numbers for a 6 volt (3 cells) battery. Someone appears to have slipped up and pasted the wrong battery information.

Every indication I have received from Interstate is that the correct charge voltages for their GC2 series of batteries is:

Bulk 7.23 volts in a dual 6 volt arrangement 14.46
Absorption 7.65 volts in a dual 6 volt arrangement 15.3
Float 6.7 volts in a dual 6 volt arrangement 13.4
Equalization 7.8 volts in a dual 6 volt arrangement 15.6

The support engineer went on to state that the reason for the high voltages is the particular chemistry they are using in this model. He did not elaborate what this consisted off, but it was very clear that these were the charge rates. This particular chemistry has been in use by Interstate for some time. The earlier U2200 series that was used by Escape in the past has a similar chemistry, it was tweaked with the newer GC2 series.

I would certainly agree there is the potential for off gassing at these high rates, but off gassing can occur any time during a charge operation, will there be more at a higher voltage I am not qualified to answer. Your past advice is also a very good recommendation, if you are charging any battery it is important to maintain the water levels. I have seen marketed products that you can add to your battery to improve performance, I would regard adding anything but distilled water to your battery akin to buying tonic from a snake oil salesman.

Jubal 04-07-2015 03:18 PM

I am grateful that our batteries are in the optional front storage box.

sallemann 01-25-2016 02:31 PM

For those of you using a solar panel with the dual Interstate 6V batteries: did you change your controller to set the absorption voltage from 14.4 to 14.8 (or anything else)? I've read a couple articles that suggest this should be done (for example,

Before doing this, I wanted to see what others on this forum have done.

FYI: We have the Samlex SCC-30AB solar charge controller.

Thanks in advance for any feedback.

padlin 01-25-2016 03:37 PM

Don't have the Samlex but I'm using 15.3vdc. If I had the option of 14.4 or 14.8 I'd go with the higher. 15.3vdc is Interstates spec for them although most solar controllers or converters do not go that high.

It's a somewhat argued about spec., as you can see from the above posts.

sclifrickson 01-26-2016 02:00 AM

I'm equalizing our dual Interstate 6V's at 15.6V with our Blue Sky 3000i solar controller. Seems to be working well. That reminds me, I need to check fluid levels.

fudge_brownie 01-26-2016 08:59 AM

Any time I charge my batteries I am using the 15.3V setting, this is the recommended rate from Interstate. The difficulty is obtaining this rate. Two barriers are: most solar controllers (and battery chargers) do not exceed a 14.8 charge rate and the amount of solar panel combined with angle of the sun means there may not be enough hours of daylight.

I am also using a Blue Sky controller, in my case the Sun Charger 30.

sclifrickson 01-26-2016 09:06 AM


Originally Posted by fudge_brownie (Post 125471)
I am using the 15.3V setting, this is the recommended rate from Interstate.

Per Interstate, equalization for the GC2's is 2.6V per cell, so that should be 15.6V, no?

fudge_brownie 01-26-2016 09:29 AM

The absorption voltage is the important charge rate, it is the rate used most often and is the charge rate for the last 20% of fill. It often takes the longest and will determine the final charge. Equalization is a maintenance charge and done at the most every 30 days, in many cases it is not done at all, particularly if the battery level does not drop below 50% of charge.

The absorption rate for dual GC2 six volt batteries is recommend by Interstate to be 15.3. You might be interested in this post regarding: Is Your Battery Really Charged?

sclifrickson 01-26-2016 09:37 AM

Ok, got it. That's how I have my Blue Sky set up as well. The capability for the higher, recommended 15.6 equalization charge was the reason I went with the Blue Sky, and it seems to be a limiting factor with most chargers out there, so I guess I fixate on it a bit.

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