Building my own battery pack

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Dave-s
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I am planning on building my own lithium battery pack for my scooter from RC LIPO cells.
The cheapest cells I could find: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6499 ($18 per 5Ah cell).
I plan on taking this pack apart and building a 7s3p pack.
I will also slap on a BMS: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3247
I was thinking of buying a Ping pack, but they have low discharge rates, and much heavier and bigger.
Any input would be appreciated.

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reikiman
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Re: Building my own battery pack

Advice: LIPO's are among the variety which can blow up. (Youtube: Lipo Fire) Make sure to have a good BMS.

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Dave-s
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Re: Building my own battery pack

I know this.
But if I have a good charger and BMS then there is little chance that this will happen. There are also special LIPO sacks for charging, that can contain the fire and avoid the fire from spreading.
To date I think that LIPO is still the best solution if I take into account weight/power density/size/price/discharge rate/etc.
If I would have liked to use A123 cells, but they are still very expensive, and I need 8s6p to get the same pack capacity.

Dave-s
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Re: Building my own battery pack

I also plan in the future to design a PIC based BMS that not only monitors each cell's voltage, it also monitors cell temp.
In the end it will not only act as the BMS, but also as the motor controller.

reikiman
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Re: Building my own battery pack

Dave-s wrote:

I know this.
But if I have a good charger and BMS then there is little chance that this will happen. ...
If I would have liked to use A123 cells, but they are still very expensive, and I need 8s6p to get the same pack capacity.

Good.. I agree if there's a good BMS the chance of fire is small. The laptop fires from a couple years ago were to the best of my knowledge BMS failures. We're in agreement because clearly you're taking the precautions I had in mind.

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andys
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Re: Building my own battery pack

I was told the main drawback with RC Lipos is that they do not last very long. I guess with their low mass, high output, and relatively low price, there has to be a downside somewhere. I have a 11.1V 10 AH pack I am using as a booster pack and am happy with it so far

deronmoped
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Re: Building my own battery pack
Dave-s
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Re: Building my own battery pack

The problem with these cells is that I can only build a 10Ah pack or 20Ah.
To build a 24v10Ah pack I need 8 cells = 8*$39 = $312
The same pack with the LIPOs is 2*7*$17.5 = $245
This seller on e-bay is not a power seller, and I do not like to buy items from sellers that do not have this rating.

LinkOfHyrule
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Re: Building my own battery pack

True, but the LiFe should last much longer than the LiPo. LiPo is good for, what, 500 cycles? Decent LiFe is good for 1000. Better LiFe is good to 3000. The best are something insane like 10,000. It's the main reason I don't use LiPo, myself. That and Ping's packs are cheaper per Wh.

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Dave-s
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Re: Building my own battery pack

I also think that LIPO has the highest power density. i.e. smallest and lightest package per Wh.
And where did they measure such numbers? 10,000 cycles? This is a new technology. They need 10-20 years of charges and discharges to verify these claims.
LIPO technology has evolved, the cells have become safer, higher charge and discharge rates and bigger capacity.
The only problem I have with ping is the ducttape, I would like a nicer finished pack and the low dischrge rate.

Dave-s
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Re: Building my own battery pack

One more thing.
The LiFePo cells here have metal casing and metal terminals that make the cell heavier. LIPO has aluminum tabs and no casing.
When using them for RC applications the LIPO packs are abused. The discharge current can exceed 100A whereas the controller on my scooter limits the current to 30A.
Therefore I think that LIPOs will last longer.

dogman
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Re: Building my own battery pack

You are on the right track if high discharge rates are what you need. I love my duct tape battery from Li Ping, but I only need about 30 amps. The BMI cells look good, but the format can make the pack a bit awkward, and most custom pack builders have a lightweight, odd shaped pack in mind to tuck away somewhere on the frame. I need the big pack just to make it home. The downside of the rc packs is that they certainly won't last as long as lifepo4 is expected to. However, with the duct tape packs, we still don't have much real world data yet. I wish some 3c tests could be done on the new cells Ping is using. We do know the duct tape batteries can go 2 or 3 thousand miles, but I don't know anybody with 30,000. That is what I figure my pack should go and still have 70% left, unless I get a big motor.

Back to the BMI cells, even if they only went 1000 cycles, and they should go a lot more, they would be slightly more than half the cost of the rc pack in the long run. A member of the sphere has bought some and had no problems with the seller. It's definitely not eastjimmycammydaisylau. The same economics would apply to the A123's where saving a hundred now, may cost you several hundred later. Maybe this won't matter to you, a lot of folks ride a season, and then go back to the car, or get a gas scooter or get a job and a suv. When I bought my duct tape pack, I thought it was real expensive. One worn out bike and a burnt motor later, I find the battery costs less than anything else in the system.

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Dave-s
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Re: Building my own battery pack

I had a thought about charging the pack.
Instead of having a cell ballancer for charging I would have a relay/switch that disconnects the cells serial connection, and puts all of them parallel to each other.
I was thinking of doing this with FETs, but even the best FET has ON resistance that is near the resistance of a cell.
In a regular ballancer the cells that charge faster than the rest are discharged via a resistor to get their voltage down to match the other cells.
This means energy is wasted.
When charging all the cells in parallel, the cell voltage cannot go over the rest so no energy is wasted. Each cell gets exactly the right ammount of charge.
Any thoughts?

reikiman
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Re: Building my own battery pack

Aren't those BMI cells the same as what you'd buy from LifeBatt? It even says in the eBay listing lifebatt.com.au ...

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deronmoped
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Re: Building my own battery pack

Here is a thread from a guy that went from lead to a Ping to a BMI. Re raves about the amount of power the BMI battery puts out. We need someone with access to the different batteries out there that can do a shoot out.

"I definately noticed a speed improvement in several ways. I was at 48V 12AH SLAs and I was averaging about 20mph for my 13.9 mile commute. My top (fresh off the charger speed) was about 25mph but I could only do that for a few miles. I also have this huge hill near the end of my commute and I could barely do 7mph up that hill. This made my commute about 40minutes. The biggest issue was I had to really throttle back to avoid zapping the batteries completely before I got to work. I had to walk the bike up the hill several times pushing >40lbs of lead. That really sucked.

When I went to Ping LIFEPO4, my average speed went up to around 23mph. I could really hold the top end speed of 25-27 with no problem (about 400watts of continous power), but I did get some really warm batteries doing that. I was using about 5.6-6.0AH to get to work each day. Climbing that big hill was done at about 9-10mph. My commutes regularly took about 35minutes. Sometime I'd hit the LVC while climbing the big hill.

Finally, I upgraded to a 48V 10AH BMI pack. That battery is insanely more powerful. Now, I really do hold the throttle wide open all the way and never worry about voltage sag or overheating. The packs a couple pounds heavier than the ping pack (about 22lbs) much much lighther than lead. I average more than 27mph on my commutes now and can make it in 30minutes! My top speed on the flats is about 30mph. Climbing the big hill is now done at full throttle (the controller limits me to 20amps) and at a speed of > 12mph. I've actually seen locals gasp as I fly up the hill :lol: I'm thinking about upgrading to a faster motor because now I use only about 50% of the batteries capacity on my commute. I figure a faster motor could use more power and add even more speed. BTW, anyone got suggestions on a faster more power motor and controller than my currend WE BL-36 that could give more speed at a cost of more current?"

http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=6215&start=0&st=0&sk=t&sd=a

Deron.

LinkOfHyrule
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Re: Building my own battery pack

Dave-s wrote:

I also think that LIPO has the highest power density. i.e. smallest and lightest package per Wh.
And where did they measure such numbers? 10,000 cycles? This is a new technology. They need 10-20 years of charges and discharges to verify these claims.
LIPO technology has evolved, the cells have become safer, higher charge and discharge rates and bigger capacity.
The only problem I have with ping is the ducttape, I would like a nicer finished pack and the low dischrge rate.

Yeah, they're something like half the size of LiFe for a given number of Wh.

I forget who measured the 10,000 cycles thing, but it was a lab test (i.e. they were constantly being discharged and recharged). Obviously, this wasn't to 100% DOD, (I think it was around 60% DOD), but still impressive.

LOL, that's everyone's beef with the duct tape. Everyone has to work out some sort of case for it. The discharge rates aren't that great, but they're fine when you get the bigger packs (which also equates to greater range).

Quote:

One more thing.
The LiFePo cells here have metal casing and metal terminals that make the cell heavier. LIPO has aluminum tabs and no casing.
When using them for RC applications the LIPO packs are abused. The discharge current can exceed 100A whereas the controller on my scooter limits the current to 30A.
Therefore I think that LIPOs will last longer.

True, but unless you're exceeding the discharge rates, peak current draws don't seem to have much of an effect on cycle life. Plus, LiPo only lasts a few years whether or not you're using them.

Quote:

I had a thought about charging the pack.
Instead of having a cell ballancer for charging I would have a relay/switch that disconnects the cells serial connection, and puts all of them parallel to each other.
I was thinking of doing this with FETs, but even the best FET has ON resistance that is near the resistance of a cell.
In a regular ballancer the cells that charge faster than the rest are discharged via a resistor to get their voltage down to match the other cells.
This means energy is wasted.
When charging all the cells in parallel, the cell voltage cannot go over the rest so no energy is wasted. Each cell gets exactly the right ammount of charge.
Any thoughts?

I don't know anybody who does this, but I did found this page which might be useful to you.

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milo0105
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Re: Building my own battery pack

lifepo4 is abvious bettter opinion than lipo as safety is the most important issue. You can consider about hipwoer lifepo4 batteries.

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