Can 7 aH SLA drive a 500W motor?

[scotinminnesota]

My original 12 aH SLA batteries burst, so I need to get new batteries. I'd love to go lithium, but that's a tough sell at this time.

I've got a 36 V 500 W ebike-kit motor and controller. It works great.

I'm considering going to 7aH SLA batteries, mostly due to cost (ebay: $16 x 3, free shipping). The weight is significantly less also.

Can a 7aH SLA provide the current for a 500W motor? 500 W/~42 V = ~12 amps

My daily commute is 5 miles one way. I can recharge at work if I have to. I think I'll be ok with the capacity. I pedal some on the way to work, depending on how hot it is outside. On the way home I'm usually pedaling hard and the motor helps with the hills.

My back up plan if I find I don't have enough capacity is to add another 7 aH battery and bring it up to 48V. If I can maintain the same speed at a higher voltage the current should be less. I'm assuming the power will be the same at 20 mph if I'm running 36V or 48V.

Any major holes in my thinking.

 

[Drunkskunk]

a 7Ah SLA battery probably won't live very long. You'll be pulling 12 amps out of it constantly, and 20 amps under acceleration, up hills, and into headwinds. Thats far more than the battery is likely to be rated for.
The pukert effect will be even worse than with the 12 Amp batteries, since you're C rate will be proportionaly higher. you may only get 40 to 50% capacity out of them.

 

[deardancer3]

Buy 6 and go 3s 2p. Your single system will be very short lived. and pretty short range.

Or at least go directly to the 48v system. dont forget the charger issues.

You might check nicad prices also.

d

 

[Russell]

If you want to stick with SLA's but shed some weight then go with 9 or 10Ah which are the same size as 7Ah batteries but will provide that little bit extra range. My original 36V/9Ah pack was under 18 lbs and could deliver a maximum of 5.4Ah or 60% of the 20Hr rating when used conservatively with a 250W motor and 15A controller.

Your original 3-12Ah SLA's may have delivered as much as 60% of rated capacity or 7.2Ah x 36V = 259Wh.

If you go with 3-7Ah SLA's they will probably deliver no more than 50% of rated capacity giving you just 3.5Ah x 36V = 126Wh, which means half the range. Since even 5 miles will drain them significantly they will not last very long.

For the same size and weight as the 7Ah batteries you could use 3-10Ah batteries and they might deliver 55% or 5.5 x 36V = 198Wh, still less than the 12Ah batteries of course but significantly more than the 7Ah. IMO this is the best compromise for your commute.

If you want extra speed go with 4-10Ah batteries since they will fit in the same space as your original 3-12Ah's though the weight will then be close to your original set-up as will be the overall Watt-hour capacity.

Finally remember as with anything you get what you pay for. Cheap generic SLA's are not usually designed for the high rate and deep discharges associated with an ebike and will last only a short time under such use and premium EV type SLA's will cost you. In the end there's no substitute for Lithium of any kind.

-R

 

[dogman dan]

No real solution to the prolem that won't take money.

If you must take that route, 4 of em makes sense, same watts to go a given speed but the amps will drop a bit, and Maybe. Maybe you will make it all the way for 25 cycles or so, till the batteries start dying. Riding really slow would help too, like 12 mph. You will have to keep the amps as low as possible to keep from murdering them pretty fast. 6 of em would work ok, two 36v packs paralelled, if you can afford three more really soon.

Time to start making buddies at the Home Depot, or anywere else batteries are recycled, and score some free cells.

Occasionally there is a broken lifepo4 pack selling cheap in the for sale section, but make sure it's REALLY cheap.

 

[TylerDurden]

36V 8Ah NiCad, 5C Max Rate..Triangular Layout

Five times the life of Pb.
Half the weight/size of Pb.
Faster charging than Pb.
Half the price of Li.

http://ebikes.ca/store/store_batteries.php

 

[dogman dan]

Yep, a good choice if you have the money, but can't afford lifepo4. Three new 12 ah sla's is cheaper though. And he doesn't have that much.

 

[Lyen]

Hi Scotinminnesota!

I was actually using 3 x 12v 7Ah SLA myself back in 2007 to communte locally in San Francisco. The realistice average I was able to achieved was between 3.5 to 4 miles only due to stop and go traffic and hilly area. Your 7Ah SLA should have no problem providing the current for the 500W motor you have when it is new. The thing to watch for is to top them off as soon as you returned home to prolong their life.

Lyen

 

[Russell]

I see why you want to buy the 7Ah batteries because they are cheap, far cheaper than I could find 10Ah SLA's;

http://cgi.ebay.com/12V-7AH-12-Volt...emQQptZLH_DefaultDomain_0?hash=item56329c8a01

If these are the ones you are looking at then they are the standard batteries in the PBQ line. Here's the data sheet;

http://www.pbq-batteries.com/datasheets/12V/pbq 7-12.pdf

They provide a nice little "discharge current Vs time" chart so you can see the expected run time at various currents but since you've been running SLA's already then you know what range you are getting and can compute what you'd get with the 7Ah SLA's (roughly half). You can extend that range with 4 in series IF you keep your speed down though that's easier said than done. You do have to factor in buying a 48V charger though you could make do with your old 36V charger and a 12V charger if you have one though that's inconvenient, especially at work.

Something I find funny about the eBay ad above is the "2 year warranty!" I wonder what that covers because they certainly won't last you 2 years...or even through (or to???) the summer months :wink:

-R

 

[scotinminnesota]

Thanks for all the great advice. I'm still a little unsure of what I'm going to do.

It looks like the 3 x 7aH isn't going to last long even if I recharge at work. After a while, it might not be enough to get me to work. If I can't run at full speed, I'm not getting the time saving of commuting with an electric bike. It was kind of a pipe dream.

The 4 x 7aH would give me a taste of 48V to see if this is the voltage I want my next long term pack. The cost with a 48V charger would be ~$85. I would probably need to recharge the 20 lb pack at work. I tried that with my 12 aH (~28 lbs) and it was a pain.

Replacing the original 3 x 12aH is about ~$75 dollars. I know that would work. I was hoping to drop some weight, but a different technology is the way to do that.

Two strings of 7aH would only be $84 and would have plenty of capacity, but it's a little heavy for my bike.

I might check out the recycling center and see if I could pick up some UPS batteries. Also, I'll check with Home Depot to see if I can get a few recycled battery packs and build a pack.

Thanks for the help,
Scot

 

[Lyen]

Thanks for all the great advice. I'm still a little unsure of what I'm going to do.

It looks like the 3 x 7aH isn't going to last long even if I recharge at work. After a while, it might not be enough to get me to work. If I can't run at full speed, I'm not getting the time saving of commuting with an electric bike. It was kind of a pipe dream.

The 4 x 7aH would give me a taste of 48V to see if this is the voltage I want my next long term pack. The cost with a 48V charger would be ~$85. I would probably need to recharge the 20 lb pack at work. I tried that with my 12 aH (~28 lbs) and it was a pain.

Replacing the original 3 x 12aH is about ~$75 dollars. I know that would work. I was hoping to drop some weight, but a different technology is the way to do that.

Two strings of 7aH would only be $84 and would have plenty of capacity, but it's a little heavy for my bike.

I might check out the recycling center and see if I could pick up some UPS batteries. Also, I'll check with Home Depot to see if I can get a few recycled battery packs and build a pack.

Thanks for the help,
Scot

Scot, if you REALLY want to save money & weight. Get yourself a few hundred 18650 lithium batteries from used computer battery and bundle them together. The Tesla electric car uses the 18650 cell to power it too. If you are lucky, you may find used cells for the same price as the new SLA at 1/4 the weight.

Your Truly,
Lyen

 

[scotinminnesota]

Thanks Lyen, I think that's where I want to get to. I'm going to talk to our IT group at our company. We've standardized on one or two types of laptops, it shouldn't be too hard to start acquiring matching cells from laptop batteries that are still good. I've still got a lot to learn about building a lithium battery pack. I like the idea of using recycled batteries for my ebike.

If I can't get enough lithium cells or the expertise to build a pack in the next month, I think I will just buy the 36 V 8 aH NiCad battery pack. That should last me at least two to three years. I'm hoping to put on 600 - 1000 miles each year. It's more money, but then I won't have to make this decision again for a while. I was being a little too cheap, I'm really trying to keep my ebike expenses in the range where it's paying for itself vs the car costs.

Scot

 

[kartopla]

Hey buddy, I'm also looking for a cheap power solution, and discovered that for about 100 bucks I can get a 6.6 ah lipo 10s 3p from buying 10 Turnigy 2200 mah 3s 20c at 10 bucks each. Throw in a 24 dollar charger and there you have it. You won't get lighter or smaller than that. Just parallel 3 2200's and series 3 plus take the 10'th pack and open it up and make it a 3p from a 3s. No more heavy SLA. The bike will lose a ton of weight.

 

[Ykick]

I guess you pedal a lot if 7Ah SLA can get you 5 miles? My experience with fresh 9Ah HR, BB brand batteries is that they barely deliver 5Ah useable capacity. For my no-pedal 48V rig that's somewhere between 5-8 miles depending on wind, grades, weight and terrain factors.

Li-poly is an attractive lightweight approach but you must take precuations and even then, I'm not sure I would charge li-poly at work? If something went wrong you could wind-up in very sticky liability mess with your employer.

And remember, there's a lot of difference between High Rate SLA and standard SLA. Many standard SLA can't deliver high rate current without considerable voltage sag and/or they'll wear down and need to be replaced very frequently. Even HR doesn't last that long if you consistently pull max Ah out of them.

Have you considered building a 12S 1P Headway Lifepo4 pack? Around $250 for the cells but if cared for they'll last much longer than SLA with better performance. Much safer than Li-poly and the weight savings over SLA will feel too good to be true.

 

[SHARKBITEATTACK]

I wouldn't count on it. I tried using a set of 7ah batteries on a 24v 450 watt scooter and it accelerated like a car that was missing a couple pistons. They worked ok when I put them in parallel and overvolted the motor to 36v though. 8)

 

[vanilla ice]

The 7's should work fine but they wont give good range. Or life span. The more you run in series the less load each battery will have to deal with at a given mph. So 48v is better than 36v. If you only need 5 miles and you pedal, you should be ok with four 7's. How hilly is your run?