SLA Deep Cycle, CAP wired in for even power?

Adding a large capacitance might actually help a little, especially if you do a lot of accel/decel driving. High currents in/out of SLA batteries are inefficient.

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

These are the capacitors which are used in some EV designs to buffer high currents to/from the batteries. They are cutting edge and still too expensive for most home projects. If you can find a good (surplus?) source for large electrolytic "beer can" capacitors, you should be able to do some experiments at a MUCH lower cost.

I used to have a several 115,000 microFarad (that's 0.115 Farads each) electrolytics. I think they were only 30V, but they would have worked fine on a 24V system. I found them surplus for $5 each. All tested good for leakage and capacity. Too bad I let them go.

Similar caps are sold for car audio use, but the brand markup on these seems steep.

http://www.electronixwarehouse.com/car/accessories/capacitors.htm

Suggest you check ebay or other electronic surplus store first.

I found a large 1.6 F audio amp capacitor on amazon for about $25. I've heard that that they over-rate the capacitors for advertising purposes...but how can they do that? Isn't 1.6 F equal to 1.6 F? Where's the wiggle room?

Car audio specs used to be totally overblown because there was little or no regulation. I haven't seen the blatant BS ratings for many years.

Regardless, I don't see how they can fudge this spec. Until I see evidence to the contrary, I believe their capacitance rating. Considering the voltage and physical size, the following seems realistic for a 1.7 F cap...

"Capacitance ± 5%, 20-24 Volt Surge, 105˚ C Dia, 2.99-Inch x h. 8.66 Lowest E.S.R. (≤0.0016 Ohm)"

If you do find evidence these guy are "cheating", please post a link.

This is the cap I purchased:

Pyramid Cap

What do you think, will it be enough to buffer the current and help even out the draw from the batteries?

Also...I've read some of the stuff on batteryuniversity.com and it seems that an SLA battery is considered fully charged at about 2.15 v per cell, is that true? My batteries used to sit at 13.7 v each (they are a nominal 12v) and now only read about 13.2 or so when fully charged... Is it common for SLAs to lose that much voltage from when they are new until a month or two in?

Jumped the gun!? Suggest you call to cancel order.

Read reviews, this item appears to have some meter problems.
IMPORTANT: Can't find spec, but 99% certain this cap will have maximum SURGE rating of 20-24V. Designed to run on single battery system, 12V-16V typical. RUNNING this cap AT 24V or above is asking for a catastrophic failure. LOW voltage caps must be connected in series to handle higher voltages. This can be tricky AND two 1.6F caps in series has an equivalent capacitance of only 1.6/2=0.8F.

Q=charge {Coulombs}
i=current {Amps=Coulombs/sec}
t=time {sec}
V=voltage {Volts}
C=capacitance {Farads}

Simplified(!)...

Q=C*V
i*t=C*V
i*t/V=C

AUDIO transient 20{Amps} for 0.1{sec} allowing for a 1{Volt} drop on large capacitor...
20{Amps}*0.10{sec}/1{Volt}=2{Farad}
This can help an audio amp, that doesn't already have large internal capacitors, handle larger musical transients.

EBike transient 20{Amps} for 5{sec} allowing for a 1{Volt} drop on large capacitor...
20{Amps}*5{sec}/1{Volt}=100{Farad}
I think you would need more like 10-100 {Farads} rated to 30{V} to make a safe and significant difference.

************
Temperature compensation is important for all batteries. We used to float charge our SLA batteries at 13.8V AT 20 deg C per the manufacturers spec. They lasted for years as BACKUP BATTERIES in our high end security systems. Not sure why your batteries would be reading low at full charge.

I hadn't thought of the 12 vs 24 volt setup. I won't use this cap on the ebike then...but 100 F would be difficult to obtain and wouldn't really be worth the weight in my opinion...so I guess I will just leave it as is. I'm going to keep the cap though...I want to use it to build a CD spot welder. All I have to do is get a hold of a good SCR.