Maintaining and un-used battery

[Ray13]

Hello. I’m in the process of building my first solar box that will be used for emergency purposes only. It will have an 18Ah (12V) AGM battery, a 40W (12V) solar panel, and a SunSaver-6L controller, to power several small DC LED lights. The whole system will be stored in the basement (unused) until I need it. Now for the part I need help in.

Will the battery go bad if it’s unused for months, and then I need it? Should I use it twice a month, once a month, or does it matter? I’m new to this, so any help is appreciated.

Thanks!

[Mike90250]

You will have to have a "float" charger connected to the battery. It will loose charge over time, and will need to be topped off monthly. The tough part, is to get a proper float charger for that small size of battery, that will meet the mfg's spec for float service. Too high of voltage will quickly ruin the battery, as will never charging it. You can't use a plain battery maintainer, it needs to have the exact right voltage for your battery for float service.

[SunEagle]

I know that a small UPS like the ones made by APC usually have small batteries in them. I wonder if using the charging system from one of those would be a good choice to keep a float charge on your 18ah.

[Mike90250]

UPS's usually have GEL batteries, and they have a different charge regime/voltage than AGM. Pretty close for short term (1 week) but likely far enough off, that there would be trouble in a year.
We are talking of the FLOAT voltage.

[SunEagle]

UPS's usually have GEL batteries, and they have a different charge regime/voltage than AGM. Pretty close for short term (1 week) but likely far enough off, that there would be trouble in a year.
We are talking of the FLOAT voltage.

I thought most of those smaller ah batteries were AGM type. Didn't know they were GEL. Thanks for the information.

[Sunking]

I thought most of those smaller ah batteries were AGM type. Didn't know they were GEL. Thanks for the information.

Cheap units from a box store may use Gel's, but larger professional UPS use AGM, some that are designed for long term outages will even use FLA.

[SunEagle]

Cheap units from a box store may use Gel's, but larger professional UPS use AGM, some that are designed for long term outages will even use FLA.

I believe you i just checked a couple of the APC batteries I have and couldn't find any reference to Gel or AGM on the battery. It did have a listing of RBC.

[Sunking]

What is the make and model?

[SunEagle]

What is the make and model?

The UPS is an APC 550. The battery only states it is a Genuine RBC sealed non-spillable lead acid battery and to go to APC to replace it. There is another label on the side stating it is a CS3 battery.

[Ray13]

You will have to have a "float" charger connected to the battery. It will loose charge over time, and will need to be topped off monthly. The tough part, is to get a proper float charger for that small size of battery, that will meet the mfg's spec for float service. Too high of voltage will quickly ruin the battery, as will never charging it. You can't use a plain battery maintainer, it needs to have the exact right voltage for your battery for float service.

On the battery itself, it has "Voltage Regulation in Standby Use is 13.6V-13.8V” http://www.apexbattery.com/universal...ery-d5745.html Is that the same as float charge? If so, I found a charger http://www.batterystuff.com/battery-products/g750.html which has a Float Voltage of 13.6V. Anyone one have opinions on this product? Thanks for all the help...lots to learn.

[Mike90250]

That looks like a great charger for your application. go for it.

Now if your battery is the right size for what you want to do with it......

Are you keeping the sunsaver and PV panel in a closed metal trash can (poor mans Faraday Cage)?

[Sunking]

On the battery itself, it has "Voltage Regulation in Standby Use is 13.6V-13.8V” http://www.apexbattery.com/universal...ery-d5745.html Is that the same as float charge? If so, I found a charger http://www.batterystuff.com/battery-products/g750.html which has a Float Voltage of 13.6V. Anyone one have opinions on this product? Thanks for all the help...lots to learn.

Here is the Specs for that battery.

It requires 13.6 to 13.8 volts depending on temperature. But there is a small catch as the charger must be able to supply at least a C/20 charge current, which means about at least 1 amp up to 5 amps max. So the charger is just on the cut line at .75 amps or a C/24. That means if you used the charger to recharge the battery you are looking at 24 hours to recharge vs 3 hours with a 5 amp charger. Amps and Hours are directly related.

[PNjunction]

If so, I found a charger http://www.batterystuff.com/battery-products/g750.html which has a Float Voltage of 13.6V. Anyone one have opinions on this product?

Sure - the NOCO's are nice units, although I would recommend stepping it up to at least the 1100 or 3500 model since they have agm-specific settings unlike the 750 which doesn't. If you deep-discharge your 18ah agm accidentally below 50% DOD, then I'd want the 3500 for sure. Note that these units don't have a real constant float - only "float monitoring", meaning that once fully charged, it will monitor the voltage and when self-discharge or external loads drop the voltage enough, it will initiate a charge and go back to monitoring.

While AGM's have a low-self discharge rate to begin with, just manually topping it off monthly (or letting the NOCO monitor it) will do fine rather than being on a constant float.

IF you do desire a constant float, then the canonical Battery Tender 1.25a "Plus" model will also do nicely as it will charge up to 14.6 volts bulk/absorb, and then constant float at 13.3v or so. There are a slew of other maintainer/tenders out there, but quite frankly, I think that is overkill for your application.

I'd go NOCO for your non-critical app even though it is primarily an automotive charger. I have the 3500 and the 7500 for some small non-critical agm-charging tasks. I like the convenience of being able to change the absorb voltage according to need - even for AGM's. My small Powersonic ups-style agm's like a higher absorb of 14.7. So do my bigger Odyssey's. However, my East-Penn Deka agm only wants to go up to 14.4v. In that case, even though it is an agm, I'd choose the "normal" mode. I also use the "normal" mode when the temperature is very hot, since the small Noco's don't appear to have ambient temp-comp like the BT+. Neither are as good as on-the-battery temp monitoring, but it's better than nothing.

[paulcheung]

Why would someone want to spend $35 or worse $75.00 to try to save $40?

[PNjunction]

Because if you don't do it right, you are now trying to save $80 or more in premature battery replacements.

Admittedly, small-scale projects where the consumer abuses the battery may not notice, care, or even reach the life-cycle problems during the life of the project. However, the thought of being precise and caring with a small battery carries over to possible future larger projects, where the financial outlay (say $4K of batteries and a $40 cheap charge controller) and subsequent failure of the project cuts into what could have been rent, food, or children's education money.

Dramatic? Yes. But how many proposals have we seen here where someone wants to go large is just diving headlong into expensive deficit-charge scenarios...