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Etmax
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Re: All About Batteries
Etmax   4/19/2014 4:33:31 AM
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@Duane, I read an article on charging Lithium chemistries and it seems that current limiting to less than the maximum charge current while restricting the maximum open terminal voltage is all they need. This means that if you have a load cell in parallel with each cell that bleeds current to avoid charging above ~3.6V per cell (depends on exact chemistry) is the way to go. Then you can put the cells in series. The current will either flow through the bleed circuit (above the cell voltage) or charge the cell (below the cell voltage. When the cell voltage == the charge voltage no current can flow so you can't overcharge the battery.

I used a precision low current opamp and reference driving a power transistor across each cell that would bleed at 3.7V (for my chemistry) and then charged with an 11.1V 5A supply.

Duane Benson
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Re: All About Batteries
Duane Benson   4/18/2014 12:00:56 PM
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Very nice looking chip. Not that I'll be using that many cells, but the chips can be daisy chained to make some quite large stacks.

Duane Benson
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Re: All About Batteries
Duane Benson   4/18/2014 11:57:45 AM
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I hadn't considered that type, but with a bit of looking I can see that they seem to have good availability and reasonable pricing. Not as dense as LiPoly, but still pretty good. I really like the fact that it's more robust. LiPoly make me nervous.

Duane Benson
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Re: All about batteries
Duane Benson   4/18/2014 11:54:37 AM
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Eetcowie - Thanks. I appreciate that. In the build vs. buy decision tree, the two sides pretty much end up as: does the idea intrigue me? Yes = build.  No = but. No, but can't find it = yes.

Parts-n-Pieces
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Rookie
Re: All About Batteries
Parts-n-Pieces   4/18/2014 11:07:33 AM
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Check out the ams AS8506C autonomous balancer: passive or active mode up to 7 cells. Integrated w/ temp sensor +2 temp sensor inputs, all necessary analog functions & switches built in. No error prone code or micro needed. Only 3 resistors needed: one for discharge 2 for high and low thresholds. Balancing & Monitoring now simplified in the real analog world.

BillWM
User Rank
Freelancer
Re: All About Batteries
BillWM   4/16/2014 10:54:48 AM
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Duane -- the LiFePo4 Batteries are used in Power Tools for Motor Drive (Will take more abuse than Li-Poly Battery)  -- Something to think about for a mobile robotics device -- also they will take about 350C heat rather than 100-150C heat(failure point for LI-Poly)

eetcowie
User Rank
Blogger
All about batteries
eetcowie   4/16/2014 10:50:47 AM
I will be designing a charger for a series stack of LiFeO4, to reach a high voltage. I plan to make individual, Isolated chargers for each sub-pack. This is for my regular day job. However, I will share my general approach, so you all can have some fun.

BillWM
User Rank
Freelancer
Re: All About Batteries
BillWM   4/15/2014 8:12:32 PM
Duane -- might have to shop arround and look over the Multi-Cell offerings -- There were some in articles on Planet Analog, as well as a variety of others.

Duane Benson
User Rank
Blogger
Re: All About Batteries
Duane Benson   4/15/2014 5:06:38 PM
That it would. I just want to make sure it waits until I'm not near before the fireworks start.

Max The Magnificent
User Rank
Blogger
Re: All About Batteries
Max The Magnificent   4/15/2014 5:02:49 PM
@Duane: The search for a non-explosive solution is back on.

You don't think the fact that your robot avatar might spontaneously burst into flames adds a certain air of excitement to the project?

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