View Full Version : [Question(s)] Power Management

08-16-2012, 01:42 AM
Hello All!
I'm new to this forum, names Mark.
I have been working on robotics for a few years now. I'm not by any means a skilled programmer but I try.

I have a question, I'm about to start on building my largest robot to date, its going to be 4.5ft tall.
What I'm having trouble with is power.
Mainly the lack there of, I was planing on buying 3 of these http://www.trossenrobotics.com/store/p/6309-3S-11-1V-8000mAh-Pro-Lite-Military-Spec-LiPo-Battery.aspx
It will take me a while to get the funds for them.

I want to use them in Parallel, I need a way manage the batteries completely, ie charging, balancing, checking levels. I prefer not to disconnect the robot and would opt for an Auto-docking situation.

If there are no out of box solutions then I will have to look at building my own. So any pointers would be great.
Any guides to management would also be great.
I plan to have this entire project open source, as the OS i'll be using will also be Open Source.

Thanks for any and all help.

08-16-2012, 03:08 AM
Been working on the same thing and I haven't seen any complete out of the box solutions to date, although some come close. Here's what I'm planning, hopefully some of it will be useful to you.

For charging and checking the levels I'm looking at this board (http://www.batteryspace.com/CMBfor14.8VLi-IonBatteryPack16Alimited.aspx) (this one's for 4S, should have a 3S version somewhere though), but it's got several limitations:

* No charge balancing
* Maximum output current is too low
* Maximum charging current is too low

The first can be solved with a charge balancing board. For the second I'll probably just hook up the motors directly, I might muck around with the current sense resistor but then it's a question of what the tracks can handle (I haven't seen the board yet, purely going off the written specs). As for the third, well, I'll learn to be more patient?

An additional hack would be taking the lines that indicate power levels and control the charge and discharge FETs, send them off to a micro or similar and use those so the robot knows when to return to base or when it's finished charging. Will need to see what voltage is going through them first though.

That said, I get this sneaking feeling that I'm missing something, and I don't know what it is. Anyhow, hope this helps.

08-16-2012, 09:45 AM
I can help with the money part... http://www.hobbyking.com/hobbyking/store/__16225__ZIPPY_Flightmax_8000mAh_3S1P_30C_.html

IMO, you should only buy the MilSpec if weight is a critical issue. I have a hard time imagining an extra 500g, if you use the above instead, to be an issue in a 4.5 ft robot.

Is there a reason you're not doing other battery types, e.g. lead acid, which come in much greater capacities? My impression with LiPo's is that if you're doing large batteries, you want a different supplier than the hobby level for robots/quadcopters/rovers/etc. Not sure who that supplier is. It might also be the point where you buy the cells directly and make your own battery pack.

08-16-2012, 10:44 PM
Thats awesome those batteries are a little cheaper.
Honestly I don't even have a clue as to how much this robot is going to weigh once its done. The main reason I'm going with Lipo is because of the weight, 24-30ah of power is quite a bit, the nearest Lead Acid weighs 33lbs. Where as the lipos will weigh in around 4lbs. That's a huge difference. I think for the weight save, its worth the extra work to get some additional electronics figured out.

Thanks, it helps a lot. The nearest out of box solution I've seen is a mass charger, I forget the manufacturer but you can hook up up to 4-6 Lipos all at once and it both balances and charges them all. Its an awesome system, especially for only being around $300. I think the name started with an X. I'm hoping to use or build a similar BMS.

08-16-2012, 11:04 PM
Will be interested if you can remember the name. I've seen plenty of hobby chargers that both charge and balance, and personally have an iCharger 106B+. Problem that I've found with them and automated recharging is that they've always got buttons to start the process rather than just being able to plug it in and have it charge automatically, and it's annoying to pull them apart and tap into data lines so the robot knows when charging has finished. That said, using one (and a control/interface board of some kind) along with a protection circuit is still an option, albeit a rather expensive one.

08-17-2012, 09:00 PM
My robot uses LiFePO4 batteries from all-battery.com (which seems to be connected to Tenergy USA AFAICT -- same physical address)
LiFePO4 have slightly less capacity than LiPo per weight, but has the benefit of not dramatically exploding (or at least catching fire) when mis-treated. They can also take at least 5x as many charge cycles as LiPo before they wear out.
I use 4S of these guys: http://www.all-battery.com/32v10ahlifepo4lithiumironphosphaterechargeablebatt ery.aspx
T (http://www.all-battery.com/32v10ahlifepo4lithiumironphosphaterechargeablebatt ery.aspx)hey also have raw LiPo cells: http://www.all-battery.com/37volt-8000mah15cli-polycell.aspx
N (http://www.all-battery.com/37volt-8000mah15cli-polycell.aspx)ote: These are cells, not packs. You have to buy a charger/balancer/protection circuit, and you have to provide physical protection yourself. The Trossen pack is more of a complete product.

08-18-2012, 12:35 AM
I'm still looking for the name of the company, however I did remember just now that I emailed them not to long ago, I will see if I can find it.

Ooooooo, this looks promising. I will have to read more up on battery management so I get a better understanding, but this looks very promising.
Thanks for the info

08-18-2012, 01:35 AM
Found it

08-18-2012, 02:19 AM
Thanks for the link. Looks like it works the same way as most other hobby chargers, although some of the extras look interesting. Still has the issue of not being automatic though, need to go through the menus to start the charging process. I guess automatic charging isn't desirable from a design point of view though, given the general tendency of batteries to do bad things if they're treated wrong, especially regular Li-ions/Li-pos, LiFePO4s excepted. Options to set and so on.

08-18-2012, 01:36 PM
The Tenergy "protection circuits" are automatic the way you want I think. I use one for my 4S pack, and it charges until full without needing to be "started." Also does balancing, and prevents over/under dis/charge.
Main drawback is that, keeping the charger connected but not powered will drain the battery pack.
This one is for LiPo batteries, 3S, and supports 9A continuous power draw (15A immediate cut-off): http://www.all-battery.com/protectioncircuitmodulepcbfor111vli-ionbatterypack3cellswith12alimit-pcb111v12a.aspx
I (http://www.all-battery.com/protectioncircuitmodulepcbfor111vli-ionbatterypack3cellswith12alimit-pcb111v12a.aspx)f you need more draw than that, you'll need to either build packs in parallel, or roll your own from higher-power MOSFETs and BMS ASICs. You can buy those parts off the shelf, but drawing out a board and then surface mount soldering to build it may be somewhat intimidating depending on your background. At high currents, anything that goes wrong can create quite a remarkable fireworks display! (I welded right through my pocket knife blade when accidentally shorting terminal wires I was stripping, for example...)

08-18-2012, 10:58 PM
Hmm... maybe? My understanding is that the board you've linked is more for protection rather than controlling charging, does it follow a CC/CV charging algorithm and what does it limit the charging current to? Interesting that you say it balances the board, no mention of that on the website.

In the related links below I do see a "universal fast smart charger" that looks like it'll charge as soon as you hook the battery up to it, might be useful in conjunction with a protection & balancing board.

08-19-2012, 11:46 AM
The board I have cuts power when any one cell goes below the protection voltage. It does charge the different cells differently when getting to 3.65 volts. Note that this is for LeFePO4 cells -- I couldn't find as good a good description of the LiPo version.
When using the "smart charger," beware that it will likely load the batteries if it's connected but not powered.