Hey,
I am looking to construct a high capacity 6V battery pack, and came up with a simple construction, but I need someone to confirm that everything looks properly configured before I go out start a fire with it.
The cells are two Li-Ion 10A 3.7V packs, which will be run in series to get the voltage up to 7.2. This pack will then be connected to a protection circuit module rated for max 9A discharge rate in order to prevent over/under charge/discharge. The voltage will then be stepped back down to 6V with a ultra low dropout regulator rated for 10A.
I plan on using this with my hexapod project, and am hoping that 9A will be enough to keep everything in working order. This should be an awesome mix since the entire setup is 156 x 118 x 9 and weighs only 375g.
when you say “Li-Ion 10A 3.7V packs” what exactly are you talking about?
The reason I ask, unless you are talking about a li-poly cell (or parallel stack thereof) it is quite possible it already has a protection circuit built in. Thing is, unless it is specifically designed as a high-discharge pack or a military battery, most off the shelf “packs” have a limit of about 3A to 4A output current, even if they are rated 8 to 10 AHr. Some of the new cordless drill packs might also be high discharge, but usually those are pretty high voltage (relatively speaking) too.
The phrase “ultra-low dropout” frequently refers to linear regulator designs, which when delivering 6V from a 7.2V source at 9A would dissipate roughly 10W. Something to consider unless portable space heater is one of your design objectives. Seriously though, it’s wasting energy from your battery which means less run time from a charge.
You might consider looking into an R/C BEC regulator, such as the castle creations BEC recently referenced in another forum thread here as an example, as an alternative to your ultra-low dropout regulator. Using a BEC like the one referenced, you would need 2 of them with one powering each VS input of the SSC-32 (removing the VS1=VS2 jumpers), as they are only rated 5A continuous w/10A peak. An important detail to note, because they are a “buck”-type switching regulator they are inherently low-drop out and once there is not enough input voltage to maintain the desired output voltage the output pretty much just follows the input down.
Thanks for the response. The cells are Li-Polys, the official name being “Polymer Li-Ion”, sorry for the confusion. They are rated for 10A discharge rate, but I know for certain that they do not come with built in protection circuitry. Thanks for checking me on that though, that was the type of mistake I was worried I might not pick up on and blow something up.
Your point about the regulator is well noted. I noticed that Lynxmotion in their battery section offers a “6V Regulator” but doesn’t include any specifications or diagrams so I’m not sure if this would work any better…? Can anyone shed some light?
I looked at the specs for CC BEC and it looks like a very nice unit, almost perfect for my situation. However, I saw a few issues: I will be using a custom board, not the SSC-32 to control my servos which could get confusing trying to figure out how to wire the thing up, especially two of them. Another issue is cost, $50 for two modules - How would that be configured? They had a special diagram for hooking up with 2 battery packs…not sure how to reconfigure to use two at once… Then since it comes out of the package with an output of 5.1V, I would have to purchase their Castle Link package for $25 to reconfigure the packs to output @ 6V. $75 for a confusing DC step-down system seems a little pricey to me.
Any suggestions on a similar product or DIY design? I suppose it boils down to can anyone find a high ampere switching regulator… Seems that there are very few 10A ones available, might have to run in parallel… Do anyone know how to construct their own switching power regulator given an IC?
Thanks for the help,
Peter
P.S. I found this little switching device, but again the cost! Product Link
Other than the obvious control benefits of the SSC-32 it also breaks the servo outputs into two banks (technically 4 but for the sake of power it’s only 2) and has provisions to power each bank from a separate supply. This is the VS1 and VS2 input ports I have referred to. In this way you can use a single battery going to two BECs and wire the output of each BEC to an input port VS1 and VS2. If you go down the path of trying to parallel the BECs you wind up using shottkey diodes in series with each output to keep the BECs from fighting with each other. Each diode has to be rated for the output current of it’s attached BEC, and dissiaptes power accordingly… certainly not on the order of a linear regulator but efficiency lost is just that none the less.
I’m not really sure what to tell you about the whole cost thing except when you start adding up what it costs for servos, frame, legs, electronics, etc and come to the realization that a lousy power system design can thrash your entire performance goals then you need to follow through and allocate resources for it appropriately.
If you are already spinning custom PCBs for your controller then perhaps you can find a good looking design in someone like linear technology or national semiconductor applications notes and implement the design onto your PCB directly. If you take some design apsects of the SSC-32 output into consideration and arrange your servo outputs similarly then you may be able to use 2 or more lower power buck-regulators to accomplish the same objective as using two purchased BECs into the SSC-32. A regulator like the Linear Tech LT1074 could probably be adapted to work in that application and if you study their AN19 you should have a reasonably good shot at a passable/working layout on your first pass. On a one-off basis I doubt it will be any less expensive than just using purchased modules but you gain the knowledge and experience for the effort so… meh… it’s one of those disscussion points people argue over all the time.
You make some good points Eddie. I only just recently made the decision to construct my own PCB, so why not just go ahead and do everything.
I was looking through some of those manufacturers catalogs yesterday, but couldn’t find many “how to apply” type of documents. Another issue I considered was how do I get just one or two modules since they intend for you to buy in bulk. If you know of any good documentation on implementing the bucks in custom designs please let me know. Otherwise I may be taking a stab in the dark with some internal “documentation”.
hmm, well just to continue my example of the LT1074, which I’m not saying is the right part but rather an option to consider, goto linear.com and enter LT1074 in the search window. You should get a few hits, the second one down will be to the product page. in the right hand column there are links to the data sheet, 4 application notes, and two design notes… all of which may or may not have any useful information but the FAE guys at linear tech think do.
if you dig flash product trees go to national.com and click select | product table from the menu bar across the top of the page (below the logo area) and that will open a flash menu of thier product tree. under power management, buck converters, there are a number of choices… say buck sync switching… go for the LM25116 because it looks like it can do 8A or so comfortably and its output range includes 6V… it’s product page has links to data sheets, application notes, and at the bottom even an app note about the Eval board national sells for it… now this is a nice feature because even if you don’t use the eval board the app note can sometimes answer questions about how to layout the power and ground paths… which can be almost critical in high frequency sync switching designs.
the big difference between the two parts I listed is the LT part has an integrated MOSFET switch while the national part uses external MOSFETs. the LT part is almost trivial to layout if you read the app notes, the national part has much more opportunity for problems but can both handle more power and use smaller parts due to it’s higher operating frequency.
as for buying the parts in small quantity, well many manufacturers will send samples out to a bussiness address, sometimes to universities, but failing that you have places like digikey, mouser, newark, jameco, and farnell if you are european based (plus some others I’m sure). actually even arrow and future will sell small orders although they are sometimes expensive, and if you want to pay the utmost possible in shipping fees there is always heilind electronics.
Thanks for the references! I searched and searched and searched and came down to one chip: Linear Technology’s LTC3850
Spits out two rails which I plan to configure in a 3.3V@2A and 6V@8A configuration for my electronics and servos respectively. Great chip, but the tech sheet on it makes my head spin. They also had available for download a SwitcherCAD file that contained the chip’s configuration diagram - which is a god send for figuring out where each lead is supposed to go. They don’t seem to give any information in their tech sheets about how to select the proper resistors, diodes, inductors or capacitors - Of which there are many to be configured. Is there any documentation detailing these steps in particular in detail or do you have to be an electrical engineer to hook it up?
The cells are Li-Polys, the official name being “Polymer Li-Ion”, sorry for the confusion. They are rated for 10A discharge rate, but I know for certain that they do not come with built in protection circuitry. Thanks for checking me on that though, that was the type of mistake I was worried I might not pick up on and blow something up. 
If you know of any good documentation on implementing the bucks in custom designs please let me know. Otherwise I may be taking a stab in the dark with some internal “documentation”.
They also had available for download a SwitcherCAD file that contained the chip’s configuration diagram - which is a god send for figuring out where each lead is supposed to go. They don’t seem to give any information in their tech sheets about how to select the proper resistors, diodes, inductors or capacitors - Of which there are many to be configured. Is there any documentation detailing these steps in particular in detail or do you have to be an electrical engineer to hook it up? 