I would love to see sparkfun do an all in one solution for a 11.1V battery pack.
I mean have a breakout board that will charge it say from a 12v plug pack (wall wart), but also have 3.3v and 5V outputs and maybe even a battery management chip that reports low power and cuts off power when the battery is critical.
Batteries for RC cars are very cheap on ebay. See item number 250401770515 as an example.
They are just out of reach for other projects because of their finnicky requirements.
Those are interesting looking packs. Do you know how they are wired internally? How the “balanced charge connector” works?
I have never opened one up but I can only presume that the wires are all tapped between each battery.
So a three cell pack will have a 4 wire charger, a two cell pack will have three wires.
Any other wires might be a temperature sensor for over charge protection.
RC LiPo packs generally have 2 connectors.
A “Discharge Connector” which usually has a “Deans” plug on it with a decent gauge wire.
A “Balance Tap” has N+1 connectors, where N is the number of cells in the pack. For a 11.1v (3S) pack, there will be 4 connections (GND, 3.7v, 7.4v, 11.1v).
These packs are a multitude cheaper than the cells SFE sells (no offence SFE). You can get a 2200mAH 3S pack for about $18usd, which works out to $6 per 2200mAH cell.
As far as the internals of a pack, its just simply the cells stacked on top of each other and then wrapped in heatshrink.
Interesting. So one could apply charge voltage to the output and use MOSFETs with a series resistor between tap points to shunt charge current around cells that are out of balance.
I’m sure I’m missing something there. I’d like a board like this as well. Maybe up to 4 cells.
LiPo packs for R/C are most common in 3S configurations, but 2S, 3S, 4S, 5S and 6S are common configurations also.
There are MANY products/designs avaiable for balancing the cells. Most R/C LiPo chargers do this while they charge the packs, balance the voltage in all the cells.
You can charge the packs either through the balance tap or through the discharge connector. These days cells don’t really go out of balance that much so its usually fine to charge all the cells at once.
When you say you would like a board like this, what exactly do you mean?
Well, the only products I have found are rather expensive. I would like a cheap solution ($20?) to deal with the problem. I have found a number of ICs that look to be capable of handling 3 and 4 cell configurations. Beyond that it looks to get significantly more complicated. They all seem to charge the entire pack at once and have no real consideration for balancing. Though it sounds like it might not be that big of a deal.
I’m more interested in charging, but a simple undervoltage shutoff would be nice to keep them from getting too discharged.
Yes, that’s almost exactly what I’m trying to find for an LED project - especially the undervoltage cutoff.
Same for a microcontroller project - although I’d like to incorporate a solar oscillating (joule thief) charger circuit with that one.
And yes, I also have some Sparkies LiPo cells which I’d like to find a balancer kit for as I’ve just bought an Imax B6 charger.
ttabbal:
Well, the only products I have found are rather expensive. I would like a cheap solution ($20?) to deal with the problem.…
I’m more interested in charging, but a simple undervoltage shutoff would be nice to keep them from getting too discharged.
Camera and cellphone batteries occasionally have surprisingly smart monitoring/metering/protection circuits like the [TI bq27200. Might be interesting to see a sparkfun battery pack with a chip like that for microcontroller projects.](http://focus.ti.com/docs/prod/folders/print/bq27000.html)
I have a single cell charger/supply I designed based on a LT LTC3556 USB power manager and one of the BQ series chips that I am about to redesign to have an LTC3567 and a BQ27541. How much would you be willing to pay for something like that?
It should be less then 1"x2" in size and charge off USB. It would have either an adjustable (via I2C) 3.3V or (possibly if the buck boost can do it) 5V output plus the battery output. I was thinking about working with SF to get it sold on the site if there was interest in it, or maybe eBay…
Another idea I had was to put an FT2232H on it and use the USB, it could be an all in one breadboard supply and serial/jtag port…
Mike
wiml:
Camera and cellphone batteries occasionally have surprisingly smart monitoring/metering/protection circuits like the [TI bq27200. Might be interesting to see a sparkfun battery pack with a chip like that for microcontroller projects.[/quote]](http://focus.ti.com/docs/prod/folders/print/bq27000.html)
I’m probably the wrong person to ask market research questions of (especially since what I want changes drastically for each project). But, off the top of my head, what I’d like is a board with a wider voltage input range (so it could charge off USB, or a solar panel, or a wall wart), a battery monitor / fuelgauge / protection circuit (like the bq chips), and ideally a switching regulator on the output to power my device. I’d be willing to pay, oh, $20+ for that? The bodhilabs vpack boards seem reasonably priced at $10, and this board would do roughly twice as much. 
The biggest issue is, what you want will cost more then twice what the vpack board does to make. First off, the battery monitor/fuel gauge/protection circuit parts alone cost more then 2x what just the BOM cost is of something like the vpack. That’s not even including the battery charger (which you definitely would want to be a switching charger if you wanted to charge off of a solar cell) which itself would cost another 2x of just the BOM cost of the vpack (good switching LiPO chargers are not cheap!). So it would more then likely be at least $30 if not more. I estimated my 5V (USB charge only) design as listed in the previous post with a 1500mAH battery would have to sell for at least $50 to make me any money.
I would want to sell it with a battery as opposed to letting the end user choose their own battery because the fuel gauge chip has to learn the characteristics of the particular battery before it has any accuracy at all. I would do this by creating a golden pack and program its values into all the new packs (see the TI app notes to understand what I mean here).
So basically a smart battery (which is what this would end up being really) is much much more complicated a product then just a simple switching regulator board. This is why it would cost much more to design, build, and sell then the vpack modules ever would.
wiml:
I’m probably the wrong person to ask market research questions of (especially since what I want changes drastically for each project). But, off the top of my head, what I’d like is a board with a wider voltage input range (so it could charge off USB, or a solar panel, or a wall wart), a battery monitor / fuelgauge / protection circuit (like the bq chips), and ideally a switching regulator on the output to power my device. I’d be willing to pay, oh, $20+ for that? The bodhilabs vpack boards seem reasonably priced at $10, and this board would do roughly twice as much.
Hmmm, I see what you are saying. Oh well, I guess everything can’t be super-cheap. My price assumption was partly based on the notion that there would already be highly-integrated LiPo monitor-and-charger chips developed for the cell phone / camera / etc markets which would bring the cost down, which is what you were talking about with the LTC3556/LTC3567, but I guess that’s not really adaptable to the solar power scenario.