better battery for my robot, help needed

[Juha]

Hello everyone,

I'm working on a new controller for my bioloid, and one of the many things on the list is to replace the nimh battery with a better solution sometime in the future.

I noticed some people in other threads were talking about how they've replaced original batteries with LiPo cells on other robokits. I posted the topic over here, since it's a general one, not related to particular robokit.

The ideal solution would be based on LiPo, so that I could regulated the voltage to 10V. Of course, integrated protection circuit & charger would be awesome, but I don't know if my hopes are too high.

So if those people who have been working with LiPo batteries could give me some tips, perhaps tell about the solutions they're using and also point me to where they ordered the cells / voltage cutters / protection circuits /chargers etc (inside Europe prefered), I would really appreciate.

thanks,

Juha

[Robo1]

Hi,

I'm using lipo for my KHR-2 it's a 950mah and that last about 30-40 min. what you need is a 3 cell lipo, think of them like normal AA each of them are 3.75V max so a 3 cell would be 11V. but in reality there charge up to 14V.

So you got your self the battry and now you want to makeit 10V. what you use for the job is a voltage regulator. this is what I use. their very easy to use and cheap, this one is 12V but you cn find other that are 10V. the last thing you need is a lipo saver this is a little circuit with a led, you plug it in to the power (I use a spare servo pin) and it turns the LED on when the power is running low.

You can get all the lipo stuff from a RC shop as they use them for cars and plans. The voltage regulator you can get from any electric shop. I think there's a post somewhere in the RN thread that has a link to a shop on the web that sells ajustable voltage regs for RC hels that some people on the forum have used.

the last thing to think about are that there are dangerous. I haven't had a problem with them but if there shorted they can catch on fire, and if there discharged lower then 9V then there can't be recharged, I got through a couple by using my robot to long.

bren

[Juha]

Hi Robo1, and thanks for your reply.

Yeah I know about the voltage with the li-po/li-ion cells, regulators etc, the main problem that I have right now is the find a good place to buy the cells + cell-related items. I guess in the rc world these are mainly used in planed etc, most of the ones I've seen are typically too long, since I'm looking for something to store >2500mAh (which is how many Ah you get from the regular bioloid battery). I will be building a new torso for the robot so that I can fit my controller inside, so the battery can be larger / different shape then the original one.

Regulators I can get just from the store next door.

I'd be especially interested in integrated systems, like you mentioned I need 3 cells, and the problem is that li-po/li-ion cells need to be charged individually. But there are (I think) some integrated solutions which combine lets say 3 cells, and you just plug in the charger 'cause inside the battery there's logic which handles the charging of individual cells.

Also, automatic logic for cutting the current when voltage drops would be nice, specially if you have multiple batteries in different places like the legs. But I was just thinking... isn't the voltage when you should cut the power (to prevent damage to the cells) something like 2,7V (could be higher, can't remember..) ? So if I use only 3 cells, I'm not able to get all the juice out at 10V?

Also, yes I'm aware of the dangers with these... even if I believe li-po is safer then li-ion since it does not have the hard metal cover which can cause li-ion batteries to explode with _a lot of_ power.

Another thing, about charging.. I've seen some protection circuits for li-ion, and I'm not sure if same ones work with li-po. The kind which you can use for building an on board charger, so you just plug in the power for the robot and cells get charged.

Has anyone done anything like this on their robots? Or do you always use an external charger?

[Robo1]

For the charging, what you use is a balance charger adaptor. this basical splits the one input to three so each cell gets charged/balanced. this normally is external and just plugs in the end of the battery (most li-po I've seen have a 4 wire plug for charging).

your right about the 2.7V for the curt off but the warning comes on at 9V 3V per cell. If you go to 2.69V the cells dead.

from all the li-po that i've seen each cell is the same hight so they make them longer and a bit wider. A 2500mah is a bit big and heavy, it would last hours but wouldn't two 1250mha be better just change the battry when dead. I looked at the weigth of them a 2500 is abiout 130 - 150g where as a 950 is about 35 - 50g. I even thought about using nokia battery's you can get them of ebay for £10 with a charger or £5 on there own and there 950mha and weigh 19g put three of these and you have a cheap and light battery set up. just search for nokia battery.

bren

[limor]

I ran a search on ebay as you suggested and indeed there are dozens of cobo offers for Nokia chargers + li-po batteries for next to nothing shipping worldwide mostly from reliable 'powerseller' Hong-Kong stores.

So would you just connect 3 of these in serial to get 11V?
do the batteries offer any intrinsic low-voltage-cutoff mechanisim?

[Robo1]

I haven't bought ny yet as i'm still using a standard li-po. But I looked into it for a foot mod, because of the size and shape you could easily add two to both feet. Looking at them there have three connectors and using a multi meter you can see that the two big ones are the pos and neg. if you connected them uo in series there should out put what ever voltage you want. I'm going to use 4 and usea reg to get it down to 12V. I think it would be easy up to have a slot so that you can eject them to recharge. as they so cheap I was going to get 8 so hopefully continues power.

I just have to get my ass in gear and make the new feet to hold them.

bren

[rep001]

Guys
dont get to carried away with looking at cheap mobile cells.These are NOT going to have the discharge rate that you will be asking of them.The mobile technology has not needed to move on in terms of C ratings or discharge rates and therefore that is the reason you will find all the R/C style cells selling on that strength alone.I know from experience that this is the case.You may find it useful info that Thunderpower cells are always smaller and lighter and generally better shapped than Flight Power for example so it pays to look at the options.hope it helps.......

[Juha]

Hi guys,

I've also been thinking about mobilephone batteries at some point... but like rep001 pointed, they have some problems.

Also, I don't know how wise it is to use these batteries for something they're not ment for.. like we all probably know about the possible risks.

Someone posted this link to another thread:
http://www.duralitebatteries.com/index.html

They seem to have quite nice solutions. I think best candidates I've seen so far.

EDIT: one nice think is that they have a seperate connector for charging. So you could just make some plug for your robot for charging, no need to take the battery out.

EDIT2: btw, does anyone know, for example with these duralite batteries, if it's a problem to use the battery to operate the robot at the same time your charging it? Does it mess up the monitoring the charger does?

[rep001]

NOOOooo !
Never ever use and charge a Lipo especially not in this application.The charger will just keep ramping (if it is a good quality one) the amps to maintain voltage and you will end up with trouble.Mobiles again can be treated this way as they are dealing in 10's of mah not many 100's.