Battery capacity question

[somemathguy]

Hi, I just came by some excellent batteries...a guy who owns an electric car was thinking of making the conversion from lead-acid batts to AGM for storage and safety. It's a 72 watt system with 12 golf-cart batts in series right now.

Anyways he obtained 12 Sprinter AGM 12V batts, but after installing them realized that their weight-to-energy ratio is significantly lower than standard lead-acid. So he wants to get rid of them cheap.

I checked them out on the company website and there is no amp-hour rating for these batts. They do, however, indicate the Power (in WPC - Watts per cell) as 306W in 15min discharge to 1.67V per cell at 20 degrees C.

I read online that AGM cells have a voltage of 2.02+- when fully charged. Using the formula Watts = Amps x Volts I determined

306W / 2.02V = 151Ah per 2V cell, or 151Ah at 12V for the entire batt. The batt is 10" x 7" x 9" so this seems reasonable.

Do these calcs seem correct? Does anyone know how to convert WPC to Ah?

Kevin

[whatsnext]

Somemathguy, A 15 min rating sounds like a starting battery to me. I have not looked at their site yet but are you sure the guy bought deep cycles?

[whatsnext]

Well, I looked at their site and they seem to have much higher ratings than the 285watt/cell C&D batteries I just bought as salvage. Mine weigh 57 pounds/each and I was told they are about 75Ah/each. I have 12 so I thought that 900Ah @12VDC was pretty good. Especially at $5.85/each for 10 month old bats. I work in the forklift buisness so I have plenty of people to gain real world data from but these VRLA AGM bats still are new to me so just disregard my earlier post. I've tried a couple of tests where I bring my stack down to 1.7V/cell and it takes forever with my 400 watt inverter and I just give up. They do take a long time to recharge though so I know the energy is there to start with. My batteries are supposed to have a live span of something like 1000 cycles which I can't imagine ever happening in their intended use.  I really need to graph two or three at a time to get an idea of their capacity. Let me know what you find out.

John.........

[ghurd]

AGMs tend to be Bigger for the same ah.  10x7x9" seems pretty small for an AGM with 151ah.

Also 306w/2.02v=151a, but that is for a 1/4 of an hour.  151/4=38AH.  38ah seems a little closer for the given dimentions.  Thats why he didn't want them on the car!

They have a lower float voltage.  Be carefull, use a controller or charger just for them or they will cook.

They are a 'pricy' battery good for RE.  I never saw a AGM suitable for starting.

G-

[somemathguy]

Yes AGM are expensive but these batts came at a good price...free! I agree that AGM do not perform well as starting batts because they cannot deliver a huge amount of amps as quickly as a lead-acid...this guy found that out when trying to accelerate uphill in the electric car...the extra weight, reduced Ah, and inability to deliver a large charge quickly meant that he could barely climb hills and had to limp home

[ghurd]

I'll bet they are more like 35ah.

Great batteries for you, bad for him.

They retail about $100 ea, last I saw.

Watch for over voltage charging.

G-

[whatsnext]

Somemathguy, I'd bet that they are closer to 80Ah. They sound like very similar models to my C&D 12-270FF. Go to cdtechno.com because I think you will find that your float charge is a bit higher than you think it is now.

John........

[ghurd]

I have some here about 7x7x7", 35ah, 13.6~13.8V float, 14.4V cyclic.  Yours are bigger.

G-

[somemathguy]

Ok I found a large Dynastar deep-cycle battery with the following info on it: 84Ah, 370WPC to 1.75Volts.

So that means my batts are about

(300WPC/370WPC)*84Ah = 68Ah

...maybe a bit more like 75Ah because the ratio above doesn't discharge the cell as deeply (My batts are rated at discharge to 1.67 Volts).