Small Project to get a Grip

Sean Milligan

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So I am starting a small project first before tackling a power wall in order to get a better grip on 18650 usage. AND, I'm all ready confused.

I'm building an external starter (one you hold in your hands to crank a motor). The company recommends 2 deep cycle lead acid batteries in series for 24V with 900 cold cranking amps, so i wanted to do with 18650 harvesting and going through the whole process to learn.

So my question is, do I need to build a battery with 900 Ah rating, or can I use the C rating (max discharge) of the cell since it will only be a short couple second burst? OR is it something that as long as I have the equivalent watts, it should be good? For example run a 36 V battery instead of 24V?

Thanks for the help. Watched tons of YouTube videos and can't seem to get this answered. Love the projects!
 

Korishan

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I believe you will be able to do this using the C rating. However, I would recommend using high drain cells, the kind found in power tools. Their capacity is generally lower, but they are designed from the ground up to be used 10C, 15C, 25C or even higher.
If you do use laptop style cells, then you'd need to have far more of them to handle the amp draw. They were designed with low draw long life in mind. So going over C rating is like putting poison in their vanes.
 

Sean Milligan

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So I unintentionally posted this twice in another section, sorry....but what I think my options are..

1. Build a 24 volt battery with 900 Ah. Seems unlikely due to the size and volume of cells.

2. Build a 24 volt battery with 450 Ah since cells have a 2C max discharge rate. Seems likely since the battery would only operate for couple seconds at a time, but i could be at the limit of the cells and burn them up.

3. Build a 36 volt battery with 300 amp hours to get the same wattage just more volts for better cranking.

I tested this with my truck starter and two regular car batteries in series and it wasn't powerful enough.

Thinking of just using these at 36 V and hooking 10 in parallel. Looks like I "should" be able to get 10 A out of each cell, so 20A per pack, or 200 Amps Total but at 36 V.

https://www.ebay.com/itm/10-LG-36V-...138726?hash=item2f0323c4e6:g:EhIAAOSw8b1aBLZj
 

daromer

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Hmm didnt I answer this?

I cant write the answer again because it was rather long
First of all its not Ah its A when talking about current.
18650 cells are generally not good for high current that you want. Go for LiPo instead for. 18650 you would need heaps of high current cells to do it. Lipo does it 10x better.
 

Sean Milligan

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Ya sorry you did, I just didn't move your response over.

I agree it's Amps, but I was referencing Amp Hours to imply the series parallel configuration. The cells themselves are rated at 10A max each, so 5S.

Finding LiPo batteries isn't as easy for me, and I've read so much that past few days about 18650's, but I will take a look around for them.
 

DarkRaven

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The capacity of the battery isn't that important to start an engine. Only very little capacity is used from the battery to start the engine. It is just a matter of a big current over a very short time. The only reason to have a bigger battery might be its ability to supply enough amps. In case of used 18650s you might need a 900Ah battery (or even bigger!) to have enough cells to share the current between, but not because of the capacity. Stable voltages under heavy load is much more important than capacity.

Cranking amps differ from "normal" amps as they will only be needed for a very short time. Also, they are defined at certain temperatures. 900CCA means the battery needs to supply 900A over a few seconds at -18C.

Most 18650s are a really bad choice for this application, especially if you take the specification seriously. 900A at -18C with used LiIon / ICR 18650 cells means you need something like 12.600 cells in 7s1800p configuration. Even with rather bad 1500mAh cells this would be a 2700Ah battery and it would weigh 580kg.
With new high drain 18650s you are looking at ~ 7s40p, still a ~13kg battery, but already an improvement over a lead acid battery.

What you really want are probably lithium iron phosphate cells for this sort of thing. LiPos are good as well, but not when they are frozen at -18C. Generally you need something with very low internal resistance so huge currents aren't a problem. In lithium world that is either LiPo or LiFe.
 

Sean Milligan

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Ya the cold part of the cranking amps isn't a requirement. It's just what they put as a reference. That does bring up an interesting question cause I thought 18650 were LiFe batteries. Hmmmmm.....

I do agree with the 7s40p and had planned something close to that, but are you referencing that if I can find LiFe batteries, I can get that even lower?


Obviously these are not used laptop batteries, but just getting capacity vs, discharge current straight.

http://www.a123systems.com/Collateral/Documents/English-US/18650_2017_2 page.pdf

These cells are only 1.1 Ah, but they have a discharge of 30A! So that'd be more what I need right? The capacity doesn't matter, just the current.
 

Korishan

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Especially if the current draw is only for a short period of time and not repeated within a short period of time.

Most 18650 cells are Lithium Cobolt or Lithium Magnesium. Lithium Iron Phosphates are a different type of cell, they aren't in the 18650 format. Daromer knows more of the specs on those than I do. But I'm sure you can find lots of information about them.

But if you choose to stay with the 18650 format, there are plenty of high drain cells that can handle that abuse without issue, like the one you posted. Against, these cells, if gotten from recycled parts, will predominately be in power tool battery packs.
 

Sean Milligan

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Nice! Thanks!

I also found the prismatic cells from A123, one cell at 3.3V has a max discharge of 500 Amps. Dayum!
 

Wattsup

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Korishan said:
Especially if the current draw is only for a short period of time and not repeated within a short period of time.

Most 18650 cells are Lithium Cobolt or Lithium Magnesium. Lithium Iron Phosphates are a different type of cell, they aren't in the 18650 format. Daromer knows more of the specs on those than I do. But I'm sure you can find lots of information about them.

But if you choose to stay with the 18650 format, there are plenty of high drain cells that can handle that abuse without issue, like the one you posted. Against, these cells, if gotten from recycled parts, will predominately be in power tool battery packs.

You can get LiFe cells in18650 format. I think that's what are in daromers batteries.
I harvested 120 of them from a LiFe camping battery.
 

daromer

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Yeap LiFe exist in 18650. I have that and they have max current of 3A i think and nominal 1.5A :) My battery bank can give out 4500A easily but it also weigh like 800kg.

So that depends on what type of cells you find. But as said most of them are useless in low temperature conditions. Most Lithium are frankly.

Sean where do you live?
check out for instance Hobbyking. They sell bunch of LiPo if that is even a consideration. Then you will get high current battereis for very cheap.
 
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Most electric motorbikes use Li-po to get the current and weight mix right dude. Got a feeling your stuck searching for these! Check YouTube for an diy electric super moto bike done by an Australian guy! This might be the solution your after.

 

Korishan

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DarkRaven said:
Korishan said:
Most 18650 cells are Lithium Cobolt or Lithium Magnesium.

It's Cobalt and Manganese :D :D

I blame it on being cloudy headed from the meds to get over this cold :p
 

Sean Milligan

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I live in Virginia, but this hand held external starter is actually for starting a drag race motorcycle, so it won't ever be "cold." I just need to come up with the right combination of amps from the right battery to crank the motor. I'll check out HobbyKing, but just a brief search on the internet for the amps and 24v, gets really freaking expensive. I'm thinking of going with the 26650 A123 cells, if I build an 8s8p battery with the 26650 A123 cells, that should get me 24v with a max amp discharge of 960 A. Thoughts? Bad math?

Also, the point of this project was to start small with these batteries to get the tools and stuff together to build a powerwall, but it quickly seems to be not a viable option without building a LARGE battery with the recycled 18650's.
 

daromer

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Or just get high current 3s + 4s LiPo. If you just need 1000s of A you could use above with some super cap. The supercap with do the inital push :)
 

DarkRaven

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You can also take a look at the Headway 38120HP. HP being the High Power (red) version of the usual Headway cells (blue). They are rated for 200A peak. These are bigger, but you need less, and they have M6 screw terminals which is very convenient!
 
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