12v 7ah batteries barely giving me a mile per charge

Hey so a few mnoths ago I bought 8 SLA batteries 12v 7ah each, and at first i had got about 3 to 5 miles per charge out of them and now I have them wired for 72v hoping my bike achieves 40 mph but the batteries for some reason are so weak that they barely will get me a mile on a charge and it wont go any faster than 32 mph. I am wondering why is this, the batteries were just bought like when fall began and they are already failing on me. Anyone got any recommendations for better batteries not too expensive i am looking at about maybe $150 budget to spend on ebike batteries that will; atleast give 5 or more miles per charge. Or is there a way I can fgix the batteries I currently have now?

Did you let the batteries freeze? Lead acid batteries do not like to be frozen.

How many amps is the bike trying to pull from a 7ah Sla?
Did you always recharge them immediately after use?

Going that fast and that far,, you just killed em. All is perfectly normal with your batteries, and the same thing will continue to happen if you do it again.

Lead can last 2 years easy, if you ride about 3 miles per charge, creeping along. But hit them with high rates of discharge, till they are empty,, honestly,, I'm kind of amazed they lasted as long as they did. My first e bike had lead, which I killed in 30 days, riding fast as they would go for 48v, till they reached 10v per brick. I was getting about 9 miles from 12ah of 48v lead, then 8, then 7, then 6. About that time my 36v 20 ah lithium battery arrived, which went 20+ miles full speed for almost 4 years.

Lead, those were the days. One thing we learned quickly is/was to cut Ah capacity in half. Even in perfect condition, once you started drawing motor loads you’d be lucky to see 3.5Ah from 7Ah SLA.

And then, as DM says when you hammer ‘em hard, deep and/or fail to recharge (even a small rate immediately after use) they most likely will never be quite the same again.

My suggestion, dump that crap and use batteries from this century.

And as lead acid starts aging out, it gets more and more important to get it right as the charger tops out. Lead acid chargers peak at about 14.5v, then drop back to 13ish v for floating. If you catch them right as they peak they're noticeably punchier.
Have you balanced them? If you don't hit them individually with a 12v charger pretty regularly they drift to different voltages, then the charger ends up cutting off early leaving a couple of the 6 down at low voltage, and a few get cooked from going up to 15ishv. You don't have to unwire them from the 72v setup. You can just clip on a 12v charger to each of them one at a time. The less ah ones usually drift apart faster too, compared to the bigger 12 or 14ah ones.

I know how it is trying to keep a pack alive on a budget...



But once you can possibly afford it, that much lead turns into this much lithium...




It will make you a happy ebiker

Others around here will advise you not to buy RC lipo. I say do it, but understand the risks and do extensive research. I recently purchased 20s of 10ah cells for less than $200 from hobbyking. Thats 740Wh of energy for damn cheap, however you have to consider that you need to purchase a balance charger and connecters, which'll set you back another $100 or so. By the time you're sick and tired of unplugging your pack to charge in parallel then you might have saved up enough money for a bulk charger. I'm on my second set of lipos, my old 15s 10ah pack still performs great after a year and a half of use (~2300 miles of use/ 90 full* cycles). I typically only charged to 4.14v per cell and only once ran it dead which should increase its longevity. RC lipo is like a crazy girlfriend, lightweight and fun to ride for like 3 years but she might burn your house down.

So my 7ah will be like 3.5ah capacity? And my bike I think pulls 40 amps to controller

They are likely beat into the ground. You can get them fully charged and do a capacity test, but likely they're all sulfated at least. +1 lipo and bulkcharge if you're going to be responsible with the little buggers. Specifically the multistars- 16ah should be good for 40a cont draws and you'll see huge range compared to the 7ah of lead.

cody196 said:

So my 7ah will be like 3.5ah capacity? And my bike I think pulls 40 amps to controller

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SLA? Yeah, and that’s on a good day. Only goes downhill from there.

I cut teeth on 2S2P HR9Ah SLA powering 24V/60A controller. 5Ah was the BEST capacity supplied when new and all but toast after about 3 months of daily use. $125 to replace 4qty every 3 months is/was quite an expense.

RC Lipo (not to be taken lightly) cost roughly the same $125 but I use it for 3 years or longer. And while being a 1/4 -1/3 the weight/size. Oh, they also deliver labeled Ah capacity and don't sag much.

They gotta be qualified and treated well but they’re a bargain in my experience.

will 4x 14.8v multistar 5200mah high capacity work for my bike? it will top out 67v wired in series too which should give me 35 mph
.

I guess it's obvious the voltage will work since you've been running 72 nominal. You might want to confirm the low voltage cut on the controller though.

In many people's experience, you want to derate lithium batteries to about 1/3 their discharge ratings. So 10c x 5.2ah = 52a continuous. x 1/3 = 17a real world continuous rating, if you want to come anywhere close to the 5ah of capacity and keep them healthy.

So if you want to keep 40a draws I'd at the very least use 12ah multistars. 16ah would be better and still prob be less bulk and cost of that lead.

You'll want to read up on lipos if you're inexperienced. Abuse them like lead and it's possible to end up with more than dead batteries!!! Imo a good setup would include oversizing, and proper monitoring/maintanence with cell checkers or medics, and bulk charging with a meanwell style power supply. The most important part is the monitoring/maintanence though - some people even use a bms.

I do 16S cell count voltage and I like it. I’ve been using the Turnigy 20C 4S 5Ah hardcase. MultiStars don’t have quite the C rate and I prefer the form factor and added protection of the hardcase. Another advantage is that the hardcase come with 4mm bullet connectors which makes it much easier to series connect into one pack. I have yet to figure out how to series XT60 connectors without fabricating extra dongles?

Here's a nice little 16S pack made from 4S hardcase:

JST extension cables can be quite handy for breaking out the cell channels.

cody196 said:

will 4x 14.8v multistar 5200mah high capacity work for my bike? it will top out 67v wired in series too which should give me 35 mph
.

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If you've got a 72V controller, LVC will be 61-63V, so you'd want 5 14.8V packs in series for 74V nominal. 4 in series would work well for a 60V controller, and 3 in series would work well for a 48V controller. Now 5.2ah would only be rated for 52A continuous, and 104A burst. The most I'd want to pull from it would be ~30A. And unless you go wild on the throttle or climb some really steep hills, your 40A controller will only pull ~ 30A.

my controller is a 48v to 72v controller with LVc for each

If you buy the cheaper 20c packs from HK,, get 10 ah of 72v. ( 4 , 5 cell packs is 20s, so get 8 of them) You'll have great range, and will not beat the packs so hard. 40 amps is still a big controller for 5 ah of the cheap 20c packs. In reality, they don't like use at much more than 5c, 25 amps for 5 ah size.

Better packs can do better, but 5 ah is still pretty tiny. If you must, buy 5 ah now, but add 5 ah more later.

That little pack I posted normally doesn’t see more than 25A from my controllers. Occasionally though, I’ve used it as a backpack range extender for my 2kW ride and draw peaks of 35A or little more. It gets a little warm but it hasn’t suffered anything troubling.

But I say this a LOT about RC Lipo: DC IR (internal resistance) testing is your ticket to understanding genuine C rates.

Many RC manufacturers/vendors can and do put practically anything on a label. Don’t trust it. Even the most honest vendor may have old stock and defective stock unaware of it having degraded?

What I’ve learned (I also fly RC Aircraft, btw) is to look for the most common sizes/types of hobby packs. Product that frequently and consistently goes out of stock - the point is to buy the freshest batch possible. Unless it’s unavoidable, steer away from weird sizes, shapes, cell counts, etc. as they may often sit on shelves for longer periods of time compared to more popular sizes/configurations.

I mostly buy 20C Turnigy hobby packs and when received and not puffed or soft, immediately perform 3 qualification tests.

1) Capacity - Minimum 1C load between 4.2V down to 3V per cell, will all cells deliver close to what’s claimed on the label using those voltage parameters? If I don’t measure at least 4.85Ah from 5Ah labelled packs, the vendor will hear about it accompanied with my testing data and a request to return.

2) DC IR - See video… What I look for here are cells between 3-10mOhm/cell. I’m ecstatic whenever I receive a new batch of 20C labelled hobby packs which measure 1-3mOhm/cell. I usually buy more on the spot to take advantage of an exceptionally good batch. 8-10mOhm, meh? Not too excited there but they're usually workable for my demands. 10-15mOhm/cell is that "grey area" and I or may not bother with a return? Just depends... But 15-20mOhm/cell, I’m certainly gonna attempt to return those. Main point is that it doesn't matter what C Rate is printed on a label. What matters IMO is how much practical DC IR they exhibit.

https://www.youtube.com/watch?v=wP0MhOcPmQY
[youtube]wP0MhOcPmQY[/youtube]

3) Leakage - Simply full SOC charge connected to nothing, how much do the cells self-discharge over a period of a few days? if it only drops from say around 4.18V down to 4.15 and then it stops declining, I’m not too worried. But if it self-discharges more than that and then especially, continues to drop over several more days, that’s a “leaker” cell and should be returned to the vendor ASAP.

You don’t need RC Charger to perform meaningful testing but it makes life much simpler along with some good connectors (not automotive crap) and a few other things like cell checkers, RC Power meter or better yet, CA (cycle analyst).

Test loads can be improvised using any old AC heater elements, toaster ovens, clothes irons, space heaters, practically anything with 1000W AC heater or more can be scavenged for load resistance. Parallel ‘em up until you reach a power draw that works best for your pack configuration.

Well,, I'm just saying pull 40 amps from 5 ah of cheaper 20c packs enough, they suffer. You see more sag immediately vs 30-40c stuff. Carry 10 ah,, and you half the size of the amp spike each cell sees, and they perform pretty good on 40 amps controllers.

Not saying an occasional fairly short 40 amps pull from them will kill them. But Lot will. I don't know how you ride,, city riding in general, much less harsh than highly technical off road riding. City is often lots of starts,, but followed by lots of coast to the next light. Off road,, or on a race track, It can be more like WOT amps spike, followed by a hard braking, then another WOT. 20 of those per mile, you need a bigger pack than 5 ah. The ride is all amp spikes.

I tend to write shit based on the worst ride I can give em. Because many do ride like I do. Tell me different,, and I can adjust my advice to your ride.

dogman dan said:

Well,, I'm just saying pull 40 amps from 5 ah of cheaper 20c packs enough, they suffer. You see more sag immediately vs 30-40c stuff. Carry 10 ah,, and you half the size of the amp spike each cell sees, and they perform pretty good on 40 amps controllers.

Not saying an occasional fairly short 40 amps pull from them will kill them. But Lot will. I don't know how you ride,, city riding in general, much less harsh than highly technical off road riding. City is often lots of starts,, but followed by lots of coast to the next light. Off road,, or on a race track, It can be more like WOT amps spike, followed by a hard braking, then another WOT. 20 of those per mile, you need a bigger pack than 5 ah. The ride is all amp spikes.

I tend to write shit based on the worst ride I can give em. Because many do ride like I do. Tell me different,, and I can adjust my advice to your ride.

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I ride on the road with traffic of course, and don't have too many stops in the town i live in, look up Genesee, Michigan on google maps and you can see what the town looks like, those type of roads.

And you haul ass? trying to keep up with traffic? I'm assuming you do this,, or why else would you be running 72v.

If you are blasting out from the starts, pulling the full 40 amps or whatever for the first 30 feet,, you may as well take a screwdriver and short the terminals of your batteries. then,, if you ride like that more than maybe 5 miles per charge,, you may as well just take a hammer to the batts and pound holes in them.

Or,,, maybe you don't ride like that,, I can't say.

dogman dan said:

And you haul ass? trying to keep up with traffic? I'm assuming you do this,, or why else would you be running 72v.

If you are blasting out from the starts, pulling the full 40 amps or whatever for the first 30 feet,, you may as well take a screwdriver and short the terminals of your batteries. then,, if you ride like that more than maybe 5 miles per charge,, you may as well just take a hammer to the batts and pound holes in them.

Or,,, maybe you don't ride like that,, I can't say.

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lol, when i start from a stop i dont smash the throttle in i gradually get going and pedal too to help not strain it so much. however there has been a time or two where i forgot to charge the batteries over night.

would refilling batteries with like epsom salts help restore its life since they arent too old.

cody196 said:

however there has been a time or two where i forgot to charge the batteries over night.

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Unfortunatley that is very hard on them, especially if they are run down a lot at that point.

As for fixing them....for the most part, I've not found SLA to be recoverable once killed, but hey, if they're already dead, you can't make them any worse, right?

If you look around old stuff from a few years back, you'll find various threads discussing de-sulfation and other fixes of SLA, that might help you start your experiments.

cody196 said:

would refilling batteries with like epsom salts help restore its life since they arent too old.

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That sounds similar to the old idea of dropping an aspirin in each cell. The concept was that the acetylsalicylic acid would raise the PH of the liquid but that was when Lead Acid batteries had actual lead plates. I do not think that epsom salts (Magnesium sulfate = MgSO₄) will provide much relief to you tired, sore cells but feel free to try it.

Do you know much about rc lipo batteries ? You may want to save you money for a longer lasting safer battery solution. More plug and play and not a battery charging hobby with a BBQ or fireplace to charge.