# Multi cell lipo charging or boost converter

I'm working on a prototype for a project that needs to drive a stepper motor at 12V and 2A. It needs to be powered by lipo batteries, have all of the charging circuitry in the device, charge through a DC jack, and everything needs to fit in a fairly small space. I've been doing a lot of research, but I'm a bit of an EE noob. Originally I was thinking I would have 3 3.7v lipo cells, but after tons of research, I can't find any off the shelf board to do balance charging of a 3S pack.

It appears there are lots of cheap single cell charger boards. Does it make more since to just use a large cell (8000mah), and then use a boost converter to get to 12V/2A? The problem I ran into there is that it looks like most of the boost converters I found don't go up to 2A when going from 3.7 to 12v. (The efficiency loss is another concern, though for the prototype, I will take simplicity and a larger battery over complexity)

I'm sort of out of ideas at this point. I'm looking for something I can use off the shelf. Preferably something that costs under $30. Does anyone have any ideas on the best approach here? Update: just to clarify. Ideally I would like the batteries to be self contained inside of the device and not need to be changed out. (The actual amount of time the device gets used will be very minimal, so battery life expectancy is not a big concern) Thanks! • Have you looked at RC battery packs/chargers? Semiconductor company evaluation modules for the battery charger and/or boost? How much of the actual engineering do you want to do yourself? – M D Jan 26 '16 at 0:18 • First, thanks for the welcoming to the community. Nice to see EE's are as friendly as software developers. I looked at RC battery chargers, but the ones I have seen are fairly large and usually need to dissipate quite a bit of heat (with a fan) Though I might have missed something. – Ryan Jan 26 '16 at 6:14 • I checked out evaluation modules on many IC's, but most of the evaluation boards are around$130. (I did find one at \$70, but thats still too much, even for the prototype: microchipdirect.com/… <- see the first item) My goal for the prototype, which we would like to product around 15 of is to keep the price down and not need to design our own circuit.
– Ryan
Jan 26 '16 at 6:14

Sounds like a high-discharge rate application where quick battery replacement would be ideal. Have you looked into power tool batteries? All the safety and cell balancing is built into the packs and the pin outs are well documented on forums.

If you are looking to minimize cost, weight, and maximize efficiency stacking cells wins hands down every time. The only thing paralleling cells does is eliminate the cell balancing... which at that voltage can be done with a single IC.

• Thanks for the info. So ideally, I wanted all of the charging circuitry to be contained inside of the device. (~6"x6"x3") With this are you suggesting using it with the charger it comes with? (sorry if I'm misunderstanding)
– Ryan
Jan 26 '16 at 3:19
• I have a Dewalt screw driver, 12V, whose charger is about that size. You can take apart the enclosure and keep the electronics and connector. You can discharge the pack with it connect to the battery management system (BMS). I did that once with a laptop pack. Jan 26 '16 at 3:44
• Thanks. I just checked out my drill charger. It looks like that one is nicad. @18v. I should have mentioned that the 6x6x3 is before we put everything else we need in it. Actual available space (not including batteries) is probably ~2"x2"x1" (might be able to make 2"x3"x1" work) Also, interfacing with a DC jack instead of a C14 connector is important.
– Ryan
Jan 26 '16 at 4:00

Your better off with single cell if your going to buy an off the shelf lipo charger, that means you'd better be albe to find a dc/dc converter to boost the output from the charger to 12V. Here is an example, you should investigate all the major suppliers like TI, Analog, and some other companies on digikey to see if there is a better way. You can get dev boards for most of these converters. You may even be able to find a module on digikey. Parameteric search is your friend. Engineering is full of tradeoffs, so don't marry yourself to one particular design or way to do something. I see two tradeoffs, you can get a multi lipo charger (I didn't find any in a quick search) and use a step down DC/DC. Or use a step up DC/DC and a single cell lipo charger. You may do some searching and find one is easier than the other, or one is cheaper, or too complicated. Good luck

http://www.linear.com/parametric/Switching_Regulator#!cols_1038,2167,1107,1367,1035,1646,1033,1032,1034,1105!s_0,1!gtd_!1032_%3C=3.5!1107_%3E=2!1034_%3E=14

• Sparkfun has some good single cell stuff, ebay is usually a good place to get cheap DC/DC's from china also, but I have only used step down DC's Jan 26 '16 at 0:20
• Thanks for the info. I checked out most of the data sheets on the boost converters on the link you to. It doesn't look like any of them will go all of the way from 3.7v to 12v at 2A. Most will do 3.7 to 12v, but they won't do it at 2A.
– Ryan
Jan 26 '16 at 3:54