Audio system capacities to solve voltage sag?

[Furyous]

I couldn't find anything about this on the web, but I am running 6 8v lead acid batteries in my golf cart based commuter. During acceleration, as expected, voltage drops and then rises and stabilizes when I get to top speed.

My question is whether I can effectively increase the range (and potentially the life of the batteries) by installing car audio capacitors across every 12v bank of batteries to reduce voltage sag? My guess is that it would improve performance as well.

[Cobb]

Yeah, you can buy some large off the shelf caps now. I have a 10 farad on my subs, but Ive seen up to 20 for less than 100 bucks.

[bobo333]

Capacitors drain their power very quickly, which is good for an audio system when you have a sharp bass hit that last for less than a second but i doubt it would do much for your car when you need the power during acceleration, unless you got a really really big cap or lots of them... in which case its probably cheaper/more effective to just get more/bigger batteries (which funny enough is also what i recommend for customers audio systems if they listen to music with more than just sharp bass hits (aka pretty much all music))

[mort]

Originally Posted by Furyous

I couldn't find anything about this on the web, but I am running 6 8v lead acid batteries in my golf cart based commuter. During acceleration, as expected, voltage drops and then rises and stabilizes when I get to top speed.

My question is whether I can effectively increase the range (and potentially the life of the batteries) by installing car audio capacitors across every 12v bank of batteries to reduce voltage sag? My guess is that it would improve performance as well.

Hi Furyous,
Audio caps for cars are rated at car battery voltage (16 volt is the nearest standard voltage), so there doesn't seem to be an efficient way to use them for a 48 volt system.
And...
Each farad can store about 100 joules at battery voltage (about 14.5 volts) 1 joule is 1 watt-second. So 1 F could store 100 watt-seconds. That's if you can convert the whole charge down to 0 volts. I guess you'd better assume you could use the top 30%, 14.5 volts down to 10 volts - you need 3 times as many capacitors.

The Stinger seems to be a combo capacitor & lithium battery which emulates 50 F.
If you attached 60 of these to pairs of batteries, 15 for each pair, you'd gain 750 farad at 48 volts. If you could use about a third of that (14.5 v down to 9.6 v) it would provide about 25,000 watt second. Or 20 amps at about 48 volts for 26 seconds. 20 amps might be noticeable and 26 seconds might be how long you want help accelerating. Or it could provide as an additional 100 amps for 5.2 seconds for climbing short steep driveways.
And that would only cost $11,700.
-mort

[Cobb]

I wouldnt worry. Just get bigger batteries or use less amps.

[Furyous]

So, even if I get 4 2.5 farad capacitors and wire them up to each 2 batteries, there will be no benefit, even with regards to battery life?

[Ryland]

They would help, but not for the reasons you'd think, the motor draws electricity from the battery in pulses, every movement of the motor changes what coils are connected via the brushes, lead acid batteries do not like these pulses and it starts to act like they are being discharged at the rate that is the peek of the pulse, so if you put a capacitor in there it evens out the peeks and valleys and the battery ends up with less stress, giving a longer range and longer over all battery life.

Bottom line tho is that if you are putting a high enough load on to see your 48v pack sag to under 40v, then that right there is shortening your range and battery life.

[Furyous]

Thanks! That hasn't been a problem. The pack was new in October and I rarely let the resting voltage drop below 50, nor have I seen voltage drop below 48 during acceleration.

So what you are saying is $100 worth of capacitors might be worth a few extra months if battery life? Why wouldn't everybody do this?

[Ryland]

I was told that not only would they extend the number of cycles you could cycle the batteries but that it should extend your range because it's going to be gentler on the whole pack.

I tend to see my resting voltage bounce back up to 50v higher depending on the state of charge, 52.6v is "fully charged at rest" if I remember correctly and I can drive 4 miles to work over some hills and still read around 52v after I turn the key off, but my voltage under load drops to 44v or even 42v sometimes and as the batteries discharge I see it get down close to 40v if I've driven a lot that day and am climbing a steep hill.

[Cobb]

Are these lead acid batteries? Have you tried the usual stuff like check the electrolytic level, add water as needed and maybe hook each battery itself on a charger over nite to "balance" it. Better yet, get a desulfur type of charger?