Li-Po battery pack info?

[The Boston Dangler]

i was hoping you guys could help me out with some research. google comes up with rubbish, but searching AT yielded this gem: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
i'm looking for further reading, and recommendations for quality battery packs, chargers and stores. i expect to use 2s2p or 2s3p packs.

extraneous rambling:
my (potential) project is in the earliest of stages: a 12 - 15 inch tall humanoid robot. ultimately, i'd like it to be autonomous and incorporate a samsung galaxy or better smartphone (through usb) for higher-brain functions, communication, et cetera.

i'm thinking of getting a robonova kit (video, video) since the very tight packaging gives a (stock) height of just 12.5 inches. however, since i know i'll end up upgrading and modding everything to the point of building 2 robots, i might skip that and go all in on a custom bot built from the lynxmotion servo erector set. it would be bigger, but i'll buy most parts only once.

i expect to end up using a 7.4v 2s2p or 2s3p if space allows, into a regulator of unknown flavor, feeding 6v to the servos. the controller may or may not be powered by the big battery pack. sensors will get 5v or 3.3v from the controller. total draw is unknown, but will be way up there: up to 24 beefy servos, camera, sensors and so forth.

thanks for your help

 

[William Gaatjes]

What is it that you seek ? I have problems filtering out what you need.

Chargers /regulators can be found from for example linear technology.

http://www.linear.com/

Complete with free application notes and free LTSPice to simulate your circuit and optimize efficiency.

 

[The Boston Dangler]

sorry, maybe i can clarify the OP. i'm looking for links to comprehensive info on li-po battery packs and chargers, and purchase recommendations. opinions and experiences would also be welcome 😎

 

[wirednuts]

cost can be a huge factor. even if you buy high grade batteries, theyre still only going to last an average of 24 months before they start to lose overall capacity.

thats why a lot of hobbiests buy the cheapest lipos they can find, as long as those packs dont have a horrible reputation. the part where you really can get screwed is buying old batteries. batteries that werent used, theyre brand new but they were sitting on the shelf for years before you bought them. those will work fine for a few months, but wont have near the life expectancy of fresh batteries.

but like i said, cost is a factor. especially when its possible to buy packs like these- http://cgi.ebay.com/2X-7-4v-4000mAh...Control_Parts_Accessories&hash=item3cb2da0367

 

[TheNiceGuy]

You would do well to go on to one of the RC specific sites. Guys here would give you all the info you need for your whole project:
http://www.rcgroups.com/forums/index.php
As far as lipos, chargers, servos, et al, HobbyKing is famous for best prices and selection:
http://hobbyking.com/hobbyking/store/uh_index.asp

 

[Patmage]

I'm currently using an iCharger 208b. http://www.progressiverc.com is the US retailer for them. I found mine on sale at http://www.hobbyking.com. It is a really nice little charger that is usually highly recommended on R/C forums and will do a variety of different battery types. I also have had great luck with the Turnigy 2s1p 5000mAh packs. I've only ever used them for r/c boating and trucking. They are only 2c, but I normally charge 2 in parallel so I hit the 20amp max charge rate of the 208b anyhow. The iChargers require a power supply/car battery though. Personally I just used an old PSU I had collecting dust with a good 12v rail.

If you have never used LiPo's before, definitely read up all you can about them first. They are very specific about how they are charged and discharged. You can find some cheap warning devices that will let you know when the cells are getting low. Dropping below 3v per cell at rest is dangerous. Also protect them. I had a friend total his boat when his motor punctured a pack. The boat basically burst into flames.

 

[The Boston Dangler]

it looks like a lot of these multi-chargers are coming from the same factory. the devil is in the details

 

[xalos]

Have you considered making (im not sure about buying) a fuel cell instead of a battery?

 

[Modelworks]

You really need to figure out how much current you need and then pick the cells. There are so many different cells and chargers it really isn't best to pick the battery first.

 

[The Boston Dangler]

You really need to figure out how much current you need and then pick the cells. There are so many different cells and chargers it really isn't best to pick the battery first.

i'm not picking the battery first, but calculating the total draw might be impossible before assembly and testing (external power with an ammeter).

http://www.societyofrobots.com/actuators_servos.shtml#current

Servo current operates the same as in a DC motor, except that you now also have a hard to predict feedback control system to contend with. If your DC motor is not at the specified angle, it will suddenly draw huge amounts of current to reach that angle. But there are other peculiarities as well. If you run an experiment with a servo at a fixed angle and hang precision weights from the servo horn, the measured current will not be what you expect. One would think that the current would increase at some fixed rate as the weights increased linearly. Instead you will get unpredictable curves and multiple rates.
In conclusion, servo current draw is very unpredictable.

the best info i can get on my potential servos:
http://www.robotshop.com/PDF/HSR5990TG.pdf

which states:

@ 6V
idle current: 8mA at stopped
running current: 300mA / no load running
stall current: 4200mA

the 4.2A is the absolute worst-case scenario, likely to be reached at the positive stops at the limit of the servo's range. i think i can avoid this simply by putting a soft limit on the range.

 

[imabiggles]

You need to do some reading. Why do you think you need a 2P pack? You need a voltage regulator to drive the servos and if thats all you are feeding, you amperage will be low from the pack. Keep with a 2S1p pack and losses are low. Charging and balancing is also much easier.

Anyway, goto rcgroups.com and read all about batteries and chargers.

 

[The Boston Dangler]

You need to do some reading. Why do you think you need a 2P pack? You need a voltage regulator to drive the servos and if thats all you are feeding, you amperage will be low from the pack. Keep with a 2S1p pack and losses are low. Charging and balancing is also much easier.

Anyway, goto rcgroups.com and read all about batteries and chargers.

there is no way a 2s1p pack can supply enough power or discharge rate for all those servos. i'd be lucky not burn down the house with it.

as a reference point to start with, the stock robonova has 16 servos and a 6V 1000mAh ni-mh battery. with constant action the runtime is at least 1/2 hour. i reckon that's a 2A draw average, and power for the controller, sensors, etc is negligible.

i'm shooting for a servo count in the high 20's, at 3x the draw per servo at least. 75% more servos would be 3.5A and 4x draw yields 14A. assuming that 14A is correct-ish, that would be after a switch, low-voltage cut-off and 6V regulator. the regulator has an efficiency of about 85%, so the battery pack needs to discharge 16A or more.