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May 13 , 2013

S.L.A

Sealed Lead Acid

The most common rechargeable battery on the planet.

Lead Acid BAttery - Wikipedia

 

I started my ebike journey with lead acid, but have since moved on and have not used them on my own bikes for many years now, if you read my projects page (* phew.. long trip ) .. it's obvious I have moved to Lithium.

However, sometimes, comes a need for a low cost, safe, battery option ...

This led me to the local battery supply in town, below, a test batch from " Interstate Batteries "

8, 10, and 12 Ah

Ah = Amp Hour

Example : an 8 Ah 12v battery, can supply 8 amps for 1 hour .. more Ah = more capacity = more weight and bigger

After charging each 12v battery ... one at a time.

Wired them in series to produce 36v

Ran some discharge tests using my LBD

Above, the 8ah batteries deliver 4ah of usable energy at 5 amps.

Whoa.. hold on .. you might say.. rated 8ah but only delivers 4ah ?... that's 50% of their label rating... ???

Well.. it's called the Peukert Effect

if.. " IF " .. this battery was used at low power, and discharged over the course of 20 hours... it would deliver it's full capacity... but not when drained in 1 hour or less.

Because i'm discharging it at 5 amps, in one hour it's given all it can..

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Below, the 10ah batteries, deliver 6ah ....

The best example i've read compares this " Peukert Effect " to pouring beer from a bottle into a glass.. if you pour it gently and slowly, you get lots of beer with just a bit of foam at the top.. however if you were to shake the bottle upside down trying to force it out, you would get lots of foam and not much beer..... in this example.. the foam is the portion of energy lost during high power discharges.

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*** Geek alert... for those who care to know the details.

Tested at 5 amp discharge

8ah = 4ah
10ah = 6ah
12ah = 8ah
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Also weighed them.

8ah
no.1= 2425g
no.2= 2435g
no.3= 2502g

36v 8ah SLA= 7.3kg ( 16 lbs )


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10ah
no.1= 3326g
no.2= 2891g
no.3= 3346g

36v 10ah SLA= 9.5kg ( 21 lbs )
notice the no.2 cell was quite a bit lighter than the other two, this is typical of SLA brands, some weigh more than others for the same spec, basically, i got 2 batteries from one batch and one from another batch... 3300g is typical for a 10ah battery )

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12ah
no.1= 3738g
no.2= 3780g
no.3= 3800g

36v 12ah SLA = 11.5kg ( 25 lbs )

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Ok, so what does all that mean to ebike users ?

Lead is HEAVY... !!!

To put things in perspective, let's compare this to a 36v 8ah Lithium ( Lipo ) battery:

1.9kg = 4.1 Lbs

That means a 2kg lithium pack is equal to 12 kg SLA battery being drained at only 5 amps.!

*( not counting connectors, tape, wires, etc... just bare packs )

Most ebikes have 20 amp controllers, see this page for more info..

Lithium will deliver it's entire rated capacity, at 1 hour or 20 hours, it makes no difference, but lead acid delivers less and less, the faster you drain them..

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Also, the SIZE of lithium vs SLA:

Above, you can see the the massive difference between lead and lithium, both these packs are 36v and have 8ah of usable energy ( at 5 amps ) ..

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Now how about price $$$

Using the 12ah sla batteries shown above, each 12v bricks is aprox 35~50 $, they can be found for much less online or via other means, but as of this writing ( May 22, 2013 ) it's what they sell for if you buy new, and only 3 at a time from most local battery suppliers..

The 8ah Lipo pack shown above cost 70 $ each, you need 2 of them for a 36v pack + shipping costs.

Summary :

SLA 36v = 100~150 $

Lithium 36v = 140+ shipping = 200 $

*( charger cost not included.. lithium chargers and BMS increase cost.. more on that later )

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And we also need to consider Safety

This is the one factor in wich SLA wins over lithium, lead acid batteries are well known and common.. if you abuse lead or do bad things to them, they will simply stop working...

Lithium on the other hand, has a bad reputation for causing fires, the LiPo ( Lithium Polymer ) pack shown above is made from light weight foil pouches, if you puncture, drop, overdischarge, overcharge, short circuit etc... these packs can and will release a whole lot of energy, very quickly, and in some cases create a large fire .. so precautions must be taken to prevent this.. with some care and attention they can be used relatively safely...

 

If you are not willing or able to learn how to treat Lithium batteries properly, it's likely best that you use SLA. For example, I personally never.. EVER.. charge any battery while I am not home to keep an eye on things.. and I never go to bed while charging a battery pack.. of any chemistry. However, some people do.. SLA is reasonably safe enough that you " could " charge overnight ...

Lipo should be charged while supervised, not to say you need to sit there and stare at it while it's charging but since lithium can be charged in 30 minutes or less, pick a time when you are going to be around and charge it.. once charged and disconnected from the charger, lipo is very unlikely to go ballistic.

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Charging...

SLA : buy 3 batteries, 12v each, in a perfect world they should all be fully charged when you buy them, however, idealy they should be charged individually before being assembled in series for 36v. If you have a 12v charger, charge them before assembly, if you do not and you are buying them locally, you could call in advance and tell them you are making a 36v pack and to please.. pretty please.. charge them fully before you pick them up.

That being said, lead will tolerate overcharge to an extent, you can take 3 x 12v batteries and charge them at 36v, after a number of charge cycles they will eventually ballance out, but if one is way out of line it's possible to destroy a 12v battery with a deep cycle.. (details to be added )

Lithium : Much care and attention MUST be taken to keep every cell between 3.0v and 4.20v.. this is very important and critical.. Lithium cells do not tolerate overcharge and over discharge at all.. period.

When using a " 36v " pre-assembled pack with a BMS ( Battery Management System ) .. you should charge your battery pack regularly, specially during months of non-use ( winter ) .

If you opt for LiPo form RC toys, capable of huge power and lightest weight, an RC specific charger is a must, when you receive the packs you need to bring all cells to equal voltage and test the packs for bad cells..

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Summary..

SLA - heavy, large, cheap, safe

Lithium - Light, smaller, expensive, more dangerous, charge monitoring.

 

 

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