Building 8s LiFePo4 pack, need advice.

[puddleglum]

This is my first attempt at this so I hope you guys will be patient with me asking questions that have probably been answered here already. I've bought a stand up scooter that I want to convert to a folding mobility scooter. I want to go LiFe as it seems to have the longest cycle life. I'm restricted by the size of the battery box ( 9.3"x6"x3") I can maybe gain another .5" on the height if needed, but can't go wider so I think the pouch cells are out. I'm thinking 10-12Ah, 3C max. load and 1-2C continuous. Headways look like they would fit. I found some packs on ebay with 18650 cells that would fit also, but am I asking for trouble going that route? Is there another better option, or are either one of the above the way to go? Can you fold the edges of A123 pouch cells to make them narrower? Is it better to go with a BMS and bulk charge or could I use a balancing charger and hook up a Cellog8 to monitor the pack? I know there are pros and cons and this has been the subject of much debate. I've been trying to read through some of it but I get more confused. Which is safer, most economical, most reliable, easiest? Your help would be appreciated.

 

[dnmun]

do not fold the pouch cells, do not bend the edges, they need to be held flat in compression with plates and banding. don't mash the edges of the pouches either since all that just causes the separator and electrodes to be distorted out of the tight fit they need.

the cylindrical cells like the headways hold the separator and electrodes in compression so they last longer than the pouch cells.

you can buy a 8S signalab BMS from mr ping, and the charger too.

 

[puddleglum]

Are you saying to go with Headway batteries and a Ping BMS and Charger? Are the Ping packs 18650 cells? I passed them over as an option because they are too tall? Anyone know if the Headway DIY kits are any good?

 

[dnmun]

ping packs are made with lifepo4 pouch cells. the headways are cylindrical steel jacketed lifepo4. they weigh more because of the steel sleeve that holds the cell together in compression and protects it from damage. that's why they last longer than pouch cells which will swell up eventually.

a lotta people use the signalab BMS from mr li ping. simple and cheap and it will protect your pack from over charging and over discharging.

 

[999zip999]

Ask cellman to build you a pack out of a123 cells to your size. Plug and play. Can save you time and money.

 

[puddleglum]

I contacted Li Ping and found he can build me a 15ah pack that will fit the scooter for about the same price as a 10ah Headway pack. Do the Ping packs still have a long life if they aren't worked too hard? I doubt it would see more than 1C very often. I emailed Cellman but he hasn't responded yet. Li Ping responded right away.

 

[dnmun]

yep, they would do ok for that kinda current draw. even 2C, but if you have a box they will fit in to protect them, that is the most important part. falling on a bike with pouch cells can result in damage to the pouches if they tear on something they hit.

double check the dimensions and think about where the wires wires will have to run. the BMS should be out in the open, if that is on top of the pack where you can reach it, and the other thing that will help is if you can keep the pack in compression by putting shims of wood on each end against the sides of the box to keep it squeezed in compression.

 

[dogman dan]

1c discharged, a ping should last quite a while. 3-4 years perhaps. Less than 100% discharges increase lifespan as well.

The main issue is will it fit in the box. You may need to locate the bms outside of the box if it's really tight. it's about a half inch thick, so if it lays on one side of the battery, that side will need more space. Ask Ping to lengthen some wires if the bms has to go out side of the box.

 

[puddleglum]

Ping says he will build a flat pack to fit my box for a $5 upcharge, and that I will be able to mount the BMS remotely. If the BMS is mounted outside the box, does it need its own enclosure or is it weatherproof enough on its own. If it needs an enclosure, I might be able to put a window in the box to see the lights. Can I trust his BMS to keep the cells well balanced or should I still be monitoring cell voltages? The complaints about his packs seem to be regarding the BMS. I'm also a little confused about how his BMS works. If one cell was weak, would it be the one not lighting up or the one to light up first? I guess I like meters, warning lights scare me. I'm starting to lean more toward going this route, though for a first project. Emissions-free never responded to my e-mails, sent him three. I like the Headways but they will be harder to fit, if I go over 10ah. EVassemble has some pretty good prices on them, but are they a trustworthy vendor? I like that Li Ping has responded so quickly.

 

[dnmun]

the leds that light up are not warning lights. they are just a visible way of monitoring how the cells balance. if you can keep that on top of the metal box where you can see it, then it makes it simple to check to see how the pack balances. but you have to think layout in advance, so you can avoid shorts, and keep the BMS in an area of the box, such as on top where the lid is, so that the BMS can have air circulate around it to cool it. for high power some of us put heat sinks on the output mosfets, but i doubt if you need that for low power demands, just enuff space for air circulation is ok.

 

[puddleglum]

I"m still deciding on a pack. Cellman finally got back to me. His pack doesn't come with a charger and shipping is higher. If I add in a charger, plus some extra freight, cost is going to be about 20-25% more. Is the A123 pack likely to last that much longer to make it worth the extra up front cost? Has anyone had dealings with EVassemble? Their pricing on Headway cells is pretty good. If I bought cells and a charger from them, added a celllog8 to monitor the pack and balanced it with a 1s charger once in a while, I could save about 25%. Am I asking for trouble going that route?