Help! Li ion coin cell

[samlee]

I am new to this forum and don't profess to have any knowledge in this field so would appreciate any and all help. I want to replace a Li ion battery that is 7.4v and 540mAh with 2x 3.7v 75mAh Li ion coin cell set up. The current battery provides over 3000 miles and only requires charging every 6 months. I figure a coin cell setup would be good for around 400 miles which is more than acceptable. I have found some coin cells http://www.master-instruments.com.au/products/55002/PD2032.html that look like they would do the job. I need advice how to make a battery pack with these cells and more importantly how to make a charger (preferably USB). The coin cells require 4.2v and 37mA to charge http://www.powercellkorea.com/en/neowiz/board/up_files/files_8/pd2032-_specification-i-080729.pdf The battery pack needs to be totally waterproof and needs some sort of clip connection (like a radio control model car) that is also water proof and easy to use. I have a good friend who is an electrician who can do all the technical stuff for me. Thanks in advance.

 

[VidPro]

check out some of the stuff here http://www.batteryspace.com/cointestcellpreparation.aspx
for a start.
http://www.batteryspace.com/li-ionbuttoncells.aspx

i am just lasy and this happens to be a single place that has most of that Stuff.

cant Discharge these too low, they dont exactally have protection on them, so just like anything your . . .

skip all that, it is already done for you, mabey better. get a 9V li-ion rechargable, it wont be balanced but it already has a curcuit buit in.
http://www.batteryspace.com/li-ion9v400mah36whrechargeablebattery.aspx
what could be better, does it have to be smaller than that? because that IS a 7.4v (8.4v max) battery.

to charge, i think i might know just the thing. a "microvolt" is this tiny little anyvolt thing, tiny, really small feed the USB 5v in to a microvolt thing and adjust the 10turn pot to 8.4V exactally , its max power is 1amp at either side (input or output) so then you tack in a resister to limit the current. or make a dc-dc with current/voltage limiter. then (slightly) depend on the voltage and the built in protection on the cell.
that just leaves it wont be "balanced" so it will last about 2-3 years when treated well, then should be replaced. so snap on 9V connectors would just finish up the whole thing.

smaller smaller , ok, http://www.batteryspace.com/polymerli-ioncell37v170mah062wh034arate.aspx less metal, pack it tight easier
2 of them and one of these http://www.batteryspace.com/protectioncirciusmodulepcbfor74vli-ionbatterypack6-8alimit.aspx because you probably need the protection anyways
this one might work for 2 coin cells, http://www.batteryspace.com/pcbfor74vli-ion1865018500batterypacks7alimit2round.aspx a bit overkill.

similar thing finished more
http://www.amainhobbies.com/product...&utm_campaign=Product-Feeds&source=google_ext
they have this leetel hobby car that they race with, it uses 2xlipoly packs that are very small, even smaller than this. those would be cheap, even have proper balancing

Smaller, nooooooo
http://www.amainhobbies.com/product...ly-Battery-Pack-37V-70mAh-Cessna-210-Citabria
something like this often will not have protection. just dont drop it you will never find it again

some protection types can put a very small drain on the battery, sooo going way lower than the original battery in your device AND having it properly protected for low discharge , might not last as long as you wanted. it could be that some of the Extra size on your OEM device was to cover for things like that. and to cover for eventual reduced capacities, so going to low might not be fun.

The cells you listed are the same size weight as cells that are only 40miliamps for real in rechargables. unless you get some Fatter better cells your shrinkdown wont work when you cant make it 200miles and 2 weeks with the little mostly case 2032s , i can almost guarentee they cant shove no 75ma in there and have it Last and recharge many times, it is probably overrated.

go with the 3032 http://www.master-instruments.com.au/products/55000/VL3032-1GUF.html with solderable connections
the fat 2450 http://www.master-instruments.com.au/products/55006/PD2450.html
or the hard to connect to 3032 http://www.master-instruments.com.au/products/55007/PD3032.html

also how about just use lithium Primary cell items? they have 2-3 times as much power via these tiny sizes, they enjoy long wait times and slow discharges, they will handle to low of discharges, they wont recharge
if your only going to change it every month or so.