Capacitors and SLAs

[cannonball2]

Yeah I agree on the money better spent on batteries thought. But I just cant afford or rationalize maybe the high expenditure on a first time project. Im still largely a fossil fuel guy. Hate to have a mega$$ battery pack gathering dust because I couldn't appreciate being moved by electrons. Im going with 15ah SLAs. They are compact and weigh 29lbs(3), and importantly cheap. I have read about all I could find on them and know how to baby them. Found a 4 stage pulse charger that aids in de-sulphation. Am still curious about the super caps as a protection for over amping the batts. From what I read current can be drawn through the caps with out discharging them as long as the battery supplies the amperage, once the batts. amps are exceeded the cap momentarily rises to the demand protecting the batts. I guess once discharged they would only recharge to what amperage the battery had left to supply, which is all you would have anyway. MY theory is anyway that a properly sized group of caps would have a threshold that was entered only when the poor SLAs had reached max amps and the caps would only supply the need draw, not totally discharge in a flash of amperage. Granted a prolonged overload would totally empty the caps, but possibly the SLAs would have escaped any harm. The upfront costs for the caps are high, but with care will cycle almost forever. In practice I will probably just ride the SLAs, but I find the concept interesting.

 

[Theon]

It's a great idea, just not cheap for the benefit.
The Caps should be able to hold off voltage drop for a couple of seconds.
But unfortunately you'd need a big bank of caps to give you say 30 seconds of full voltage acceleration. Then you'd want your controller to automatically throttle back the Amps as it senses the voltage starting to fall, otherwise your back to over drawing from your batteries.
I can see how on a race car it would help give that boost needed to get past someone.
But $$$.

 

[snellemin]

The caps would only help on burst power. Voltage drop will be less when instant power is required from the sla. Less voltage drop will give you the sensation of a bit more torque. Your controller will also run a lot cooler with just caps installed as close to the controller as possible. Has to do with a reduction in ripple current.

 

[cannonball2]

The caps would only help on burst power. Voltage drop will be less when instant power is required from the sla. Less voltage drop will give you the sensation of a bit more torque. Your controller will also run a lot cooler with just caps installed as close to the controller as possible. Has to do with a reduction in ripple current.

Thanks for your input! A few questions. Do the caps constantly recharge from the battery assuming they have given a power burst, and will they reach full capacity? I was thinking of each 12v SLA having it own bank, but you are suggesting a total caps bank upstream of the SLAs? It appears this could be done for a one time investment of less than $200 for the caps. Then battery replacement costs would be in the $100 range. Will this possibly extend the life of the SLAs given good care and improve ride ability, or is it just an interesting experiment?

 

[Theon]

The Caps will try and hold your voltage up, voltage will eventually drop under load.
When the larger load reduces, voltage should pick up again and recharge the caps as it climbs.
For the price vs spending on a bigger battery bank, I'd have to say interesting experiment.

 

[snellemin]

I'm with Theon. However I wouldn't spent that much money on big caps. I would rather get several smaller caps put in parallel as close as possible to the controller itself. It will smooth out the voltage a lot that the controller sees. So less stress on the controller itself. The roads are not as smooth as you might think. The load changes continuously. Picture a straight line for the voltage. Now picture that same line with a bunch of little peaks through it. That distorted line is what your controller sees. What you want is get rid of those peaks in the line and smooth them out as best as you can. That can be done with small extra caps just like in your RC.

But I would rather see you spend your money on a lithium battery pack when it's time to replace your SLA's. The weight savings alone will give you a better ride and a better power to weight ratio.

 

[Theon]

Voltage filter cap's sound like a very good idea for the controller.
So does the Lithium's.

 

[cannonball2]

Thanks for all the input and info guys! Any recommendations on cap size and voltage. The super caps are in the low 2v range. Will go lithium after the SLAs expire. Actually think I have enough 4s lipos in my rc stuff to make an experimental pack, wouldn't be but 5ah enough for a neighborhood ride, but sure would be light!

 

[snellemin]

You don't need super caps. Just low ESR's like 680uF. Four or six in parallel. Voltage of the caps should be higher than your battery voltage ofcourse.

 

[cannonball2]

One last question, I find either 35v or 50v. Which is best? Thanks so much for all the input!!

 

[cannonball2]

One last last question. Should the caps be wired to detach from the batteries. I understand caps have electrical leakage, will it draw down the batteries over extended periods?

 

[Theon]

I'd have the batteries isolated whilst not in use.
If your going to stick to 24 v 35 would be enough.
I'd just get the 50's.

 

[cannonball2]

Thanks! Will order the 50s as my system is 36v which I may expand to 48.

 

[16v4nrbrgr]

I've done caps and SLA's on my Razor dirtbike and they are worth it, it keeps the heavy current draw from acceleration up to speed from drawing down the batteries and gives you better performance for a longer period of time. When the batteries do eventually run down, it does give you less warning before the low voltage cutoff due to the performance not degrading gradually throughout the ride, since it keeps the max voltage from a standstill pretty constant until later when the bootstrap effect cannot be sustained by the batteries recharging the caps when the throttle is released or load is reduced at cruise or top speed.

 

[cannonball2]

I've done caps and SLA's on my Razor dirtbike and they are worth it, it keeps the heavy current draw from acceleration up to speed from drawing down the batteries and gives you better performance for a longer period of time. When the batteries do eventually run down, it does give you less warning before the low voltage cutoff due to the performance not degrading gradually throughout the ride, since it keeps the max voltage from a standstill pretty constant until later when the bootstrap effect cannot be sustained by the batteries recharging the caps when the throttle is released or load is reduced at cruise or top speed.

Thanks for the info! Cap size for a 48v 15ah system?

 

[Theon]

If your going 48v you'll want more than 50v caps.
60v at least.

 

[cannonball2]

Would a cap like this work? http://www.ebay.com/itm/1-TCI-60v-1...380?pt=LH_DefaultDomain_0&hash=item2eb64f4aa4

 

[pstan]

They do something similar with amplifiers in car audio systems, put a cap in line to supply a boost of power to the amplifier when needed. It may be a place to start looking for a capacitor that would work for you, or at least a place to get some more knowledge on what might work. Not sure if it helps, but just thought I'd pass on a different take on what you're doin.

 

[Mike B]

Would a cap like this work? http://www.ebay.com/itm/1-TCI-60v-10...item2eb64f4aa4

That capacitor will store 180 Joules of energy. Which is 180 watt-seconds.

A 48 volt, 20 AH battery will store 1 kilowatt-hour of energy.

That's 3.6 million watt-seconds.

Save your capacitor money and buy a better battery.

 

[16v4nrbrgr]

If you use caps you wanna use ones in the Farad range of capacitance, like those used on car audio systems, the highest capacitance and voltage ones you can find. I used two 24V, 6 Farad capacitors in series for a 48V 3 Farad capacitor bank when I ran 48V.