Thoughts on using SLI/AGM batteries

[toddbailey]

I'm in the planning stages of converting a mtb to ev assist. I'm finding the Li battery packs quite expensive, LiFePO4 are not even close to my budget. Enter the old tech of SLA batteries, I need 36 volt at 12 amp/hr, I am thinking of 3x or 4x 15 to 20 amp/hr agm/sli deep cycle batteries in series. and a 3 or 4 simple 1 amp wall warts to recharge.

Any thoughts good or not?

thanks

fyi: looking at Voilamart E-Bike Conversion Kit 26" Rear Wheel 36V 500W or BAFANG Rear Wheel 500W 48V Hub Motor Electric

 

[amberwolf]

'm finding the Li battery packs quite expensive, LiFePO4 are not even close to my budget. Enter the old tech of SLA batteries,

A number of threads discussing the options
https://endless-sphere.com/forums/search.php?keywords=SLA+lithium&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

https://endless-sphere.com/forums/search.php?keywords=lead+lithium&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

There are lots of others under other search terms. this is a list of all the topics with SLA or Lead in the title, so has lots of irrelevant threads, but many relevant ones
https://endless-sphere.com/forums/search.php?keywords=SLA&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

https://endless-sphere.com/forums/search.php?keywords=lead&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

I need 36 volt at 12 amp/hr, I am thinking of 3x or 4x 15 to 20 amp/hr agm/sli deep cycle batteries in series. and a 3 or 4 simple 1 amp wall warts to recharge.

You can't use 4 in series to get 36v. You must use 3.

FWIW, I did exactly what you're talking about, because I couldn't afford anything better, and cooked the SLA pretty quickly, way back on the DayGlo Avenger's first operational motor system a decade ago. So here's some tips I learned the hard way:

If you use "wallwarts" then you must use ones that are current-limited so you don't burn them out or end up with a fire in the wallwart (meaning, intended as LED power supplies or as battery chargers), and they must be the right voltage for the full charge of SLA.

If they are not SLA-specific chargers you should stick with 13.6v units so when you leave htem on too long you don't destroy your batteries. If you use "12v" wallwarts you are not actually charging your batteries, as 12v is the reocmmended "empty" voltage (about 50% capacity). Don't use the heavy transformer type wallwarts; they're unregulated and will destroy your SLA by overcharging and boiling them (assuming they don't just burn themselves out or catch fire).

If you use mroe than about 50% of SLA capacity you will greatly reduce their lifespan, and will probably end up replacing them every 6 months to a year depending on how much you use the bike.

If you don't immediately recharge them as soon as you stop at the end of the ride, the SLA will be damaged (sulfated), more and more the longer they are left in a non-full state.

If you don't use them for a while, you need to maintain them by topping off their charge, and monitoring their voltage, or just like the above you will sulfate them and damage them as they self-discharge.

Since you can't (shouldn't) use more htan half the capacity of the lead, you have to use twice as much battery as you would with other chemistries to make up for that.

Since the way they measure capacity on lead is different from everything else, where lead is measured at a discharge rate (current) 1/20th or less than that you'd actually be using (and that which Lithium is measured at), you won't get anywhere near the actual capacity listed on the batteries, maybe half or perhaps a bit more (sometimes less). So you then have to also double the pack size for that reason, too.

That means you may need a pack up to four times the size you think you do to get the same range out of it without overtaxing the lead batteries and greatly shortening their lifespan. That means four times the size, and four times the already very heavy weight.

Those are all reasons why people use almost anything *other* than lead, as it is not actually cheaper except possibly on initial setup cost if you go really really cheap on everything...but it won't last anywhere near as long as a decent Lithium battery. So if you literally cant' afford anything else, and *have* to motorize it *now*, you could go for SLA while you save up for a good Lithium battery...but if you can afford to wait and save up now, that's cheaper in the long run.


Don't cheap out on the Lithium battery, either--there are LOTS of garbage ones out there. So look around the forums at the many threads about various batteries and projects like yours with needs like yours, and see what they used and what their results were (if they report them), and don't get the stuff that had problems. Only get the stuff reported as good.

 

[markz]

I know there was one person who used a dc-dc converter, but the efficiency takes a nose dive. 12V 100Ah lead acid battery with a dc-dc converter, being lead acid you need to slice off I thought was 20 or 30% but it maybe 50%, so your left with 12V 50Ah with a really inefficient dc-dc converter to boost it up to 36V, then what amps will it use, do you get a 12V dcdc to 36V 1000W or 2000W, 30A or 55A. Are they even available, who knows. Whole bag of worms there.

You can do a search here on ES, the guy was using a 20V tool pack battery (Makita) to boost to 36V I believe. Again, totally inefficient but an option. I will see what I can find for a ES link.
https://endless-sphere.com/forums/viewtopic.php?f=2&t=90369&hilit=tool+pack+battery+dcdc#p1317117
Quote on summation at end of post, damn thats a fancy word I used there. summation

Can you just save up for lipo, wait until there is a sale on at Hobbyking, or dare I even mention this, buy a used battery. No, dont buy a used battery, you have no idea what the condition is, how it was taken care of. Dont buy used. I shouldnt have mentioned it.

Results:
So I finally got everything installed and the bike put together last night (though some of it is still temporary). So I took it for some shakedown cruising this morning. The short answer to my question is that yes, it will work for my intended purposes. The main caveat is that you may have to feather the throttle in demanding situations, and it really does help going up hills if you give a moderate amount of pedal assist.

The general booster behaviour is that it tries to always output whatever output voltage you set regardless of the input voltage. I set mine to 54V and the system's LCD reported 53.9V.

On accelleration, if you place too high of a demand via the throttle, the converter simply stops delivering significant amounts of current. It still powers the system LCD panel but the battery icon goes "empty" and the motor stops working. When the icon restores, you've got power to the wheel again.

The LCD doesn't show watts or amps, but it shows volts. Higher throttle demands cause the voltage to drop. It seems that when voltage drops below 50V or so, the risk of losing power to the wheel goes up. So monitoring the voltage is a good way to gauge the demands on the system and prevent any power cut-off.
So I rode the bike a total of 25 miles around Fountain Hills this morning. I recorded the last half of my testing ride using Sport Tracker Pro. Here's a link to that second ride. You can see that it does pretty much what I wanted. It "flattens" the hills a bit allowing me to maintain around 20mph going up many hills. I think the system will work better when I get a better 20mph gear. Right now, I'm over-spinning at 20mph in my tallest gear. The current gear feels comfortable at around 18mph. This jibes with my Sports Track Pro average moving speed of about 17mph. Spinning beyond 20mph is almost pointless. I need taller gearing.
http://www.sportstracklive.com/track/de ... st/2346622

 

[Balmorhea]

I'm in the planning stages of converting a mtb to ev assist. I'm finding the Li battery packs quite expensive, LiFePO4 are not even close to my budget. Enter the old tech of SLA batteries,

When comparing real usable capacity and cycle life, lithium batteries are cheaper anymore than lead-acid. The only way that lead bricks come out cheaper is if you don’t have far to go at one time, and you don’t need them to last long at all.

If you carry four 20Ah bricks with you (which have about 10Ah usable capacity), you will not enjoy your bike. And you’ll beat your back wheel to death, which will cost a lot more to replace than the difference between a lead pack and a decent lithium pack.

Back when lithium packs were more expensive and unproven, I used to carry a 50-something pound pack of 36V x 17Ah AGM batteries on my bike. I wouldn’t do it again even if they were free. I’d take a chance on the sketchiest, most suspect lithium pack on eBay before I’d use even the most highly regarded lead.

 

[MadRhino]

When you don’t have the budget to buy a proper battery for your bike, the solution is not to carry heavy sh*t lead acid batteries. It is in recycling. So many on ES had built good ebikes with recycled cells in the past, that all the info for making a good battery from recycled cells is here for you to learn, and save money with work and knowledge instead of buying crap.

 

[john61ct]

Yes, it's not like ICE cars, where you can get away with driving a $500 beater for a while

Lead batteries are just so inferior

same with most scrapped "recycled" LI cells

better to just delay getting into the game until you've earned & saved enough to buy decent packs.

Unless you really get into the game and learn how to spot bargains and test the cells, huge investment in time there.

 

[markz]

When you don’t have the budget to buy a proper battery for your bike, the solution is

....to save money in order to buy the proper battery for the application. There are many options, DIY battery pack (whether tab welding or soldering), Home Depot batteries, Hobbyking lipo's, prebuilt packs but just dont go with some random pack builder find a half decent builder from the search function of Endless Sphere.

 

[toddbailey]

I gave up on the idea of using sli or agm batteries. Too much weigh and too short of a lifespan.

Instead I came across LiFePO4 batteries on Amazon, I settled on 4 Miady brand rated at 12 volt and 16 amp/hr.
the pack weighs in at just under 16 lbs, sli/agm would have tipped the scales at about 50.

For charging I have a ~12 volt and a ~54 volt charger built for this chemistry. My only concern is equalization while each 12 volt battery has it's own internal bms, the 4 is serial do not. I'm thinking I need some sort of equalizer/balancer but not finding any on the market for a 54 volt pack. for not I'll just charge with the charger then measure voltages for as battery and if I see any imbalances occuring I can change the low battery using the 12v charger

 

[dogman dan]

Run your battery conservatively, and your 12v sections will need very little balancing. What I mean is don't ride top speed all day, and stop to charge when the battery is only about 3/4 empty, and it will keep its balance amazingly.

just to be on the safe side, do the 12v charge about once a week, and it should be fine. Balance any time you run it 100% empty though.

 

[LeftieBiker]

I have a 20AH (probably now about 17AH) 24V Ping pack and charger I could sell you, as the bike they are on isn't selling. It's about 5 years old, but doesn't have a huge number of cycles on it, and it has a BMS and is in a padded cloth cooler. You could use a boost converter to get 36 volts and about 11AH. Make me an offer.

 

[amberwolf]

For charging I have a ~12 volt and a ~54 volt charger built for this chemistry. My only concern is equalization while each 12 volt battery has it's own internal bms, the 4 is serial do not. I'm thinking I need some sort of equalizer/balancer but not finding any on the market for a 54 volt pack. for not I'll just charge with the charger then measure voltages for as battery and if I see any imbalances occuring I can change the low battery using the 12v charger

If the batteries ahve their own internal balancers, then (assuming their FETs/etc can handle the full series voltage of all the batteries in series (they might not!)) it doesn't matter that you're "bulk charging" them with the 54v charger (as long as it really goes up to the correct voltage for all four in series, which may be higher than that), or using the 12v.

As soon as the least-empty battery fills up, it will disconnect *all* the batteries from the 54v charger, until it drains down any high cells inside itself, then will reconnect them. If it's already balanced inside, then it wil simply leave them all disconnected, and the rest of them will never finish filling up. If they're already all close to each other, that wont' matter much, but as they drift further apart, it will begin to.


If you were to get some form of "balancer" for the type of pack you now have, you would not want a device/bms/etc., you would want four 12v chargers. (well, really 13.6-14.4v, depending on what it says on each of your 12v batteries for actual full charge voltage to engage balancing) They need to be "isolated" cahrgers, meaning the AC input side is completely unconnected to the DC output side. Then you put plugs on each of the 12v batteries so you can hook up all four chargers at once, one to each battery, and plug them all into the same AC outlet set, at the same time. This completely replaces the "54v" charger.

 

[gogo]

If you never discharge past half capacity and recharge immediately after any use (not 'a few hours later'), lead can work. A good example would be a lawn mower that gets plugged in anytime its not being used.

 

[LeftieBiker]

If you never discharge past half capacity and recharge immediately after any use (not 'a few hours later'), lead can work. A good example would be a lawn mower that gets plugged in anytime its not being used.

If the battery is deep cycle then you can discharge down to about 25% - again, as long as you recharge immediately after stopping. If these batteries aren't actively discharging they should be actively charging, or sitting at 100% - never sitting partially discharged. I rode for quite a few years on lead batteries, both in bicycles and in electric scooters (the Vespa type ones, not the skateboards). I got ten years out of the AGM batteries in my Oxygen Lepton, and then sold it with about 60% capacity remaining.